2019 Volume 2 Issue 3

INEOS OPEN, 2019, 2 (3), 84–98  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF DOI: 10.32931/io1913r

Rare-Earth and Alkaline Earth Metal Complexes in Catalysis of Intermolecular Hydrophosphination of Multiple Carbon–Carbon Bonds
 

  

The present review highlights recent advances in the field of intermolecular hydrophosphination of alkenes and alkynes catalyzed by rare-earth and alkaline earth metal complexes. Addition of a P–H bond across multiple С–С bonds is an efficient and atom-economic method for obtaining organophosphorus compounds. Owing to the unique reactivity, the complexes of low toxic and naturally abundant lanthanide and alkaline earth elements can provide an inexpensive and efficient alternative to the compounds of d-block transition metals in catalysis of С–Р bond forming reactions.

Key words: phosphines, organophosphorus compounds, lanthanides, calcium, homogeneous catalysis, hydrophosphination.

 

1. Introduction

Table 8. Hydrophosphination of para-substituted styrenes with Cy₂PH and Ph₂PH catalyzed by complexes 8–12

Precat.	R2PH	X	Time, min	Conversion (%)a
10	Cy2PH	H	1080	31
11	Cy2PH	H	1080	41
12	Cy2PH	H	1080	50
8	Cy2PH	H	1080	35
10	Ph2PH	H	15	42
11	Ph2PH	H	15	92
12	Ph2PH	H	15	>96
8	Ph2PH	H	15	45
9	Ph2PH	H	15	93
10	Ph2PH	CF3	10	92
10	Ph2PH	Cl	10	80
12	Ph2PH	CF3	5	80
12	Ph2PH	Cl	<10	95
12	Ph2PH	Me	15	79
12	Ph2PH	tBu	30	44
12	Ph2PH	OMe	30	27

Reaction conditions: 10.0 μmol of the precatalyst, [precatalyst]₀/[styrene]₀/[phosphine]₀
= 1/50/50, neat substrates, T = 60 °C;
^a conversions were determined by NMR spectroscopy.

Analogously to the above-described example, an empirical law for the rate of hydrophosphination of styrene with diphenylphosphine catalyzed by Sr, Ba, and Yb(II) complexes can be described by the following equation: v_HP = k∙[Ph₂PH]⁰∙[styrene]¹∙[catalyst]¹. Because the reaction has the zero order by the phosphine, it was assumed that the insertion of the alkene into the metal–phosphide bond is a rate-limiting step of the process. The activation energy was defined by the Arrhenius method in the temperature range of 30–70 °C and composed: E_a = 70.5(5.4) (Ca), 43.3(6.0) (Sr), 34.2(2.4) (Ba), and 60.7(5.4) (Yb) kJ∙mol^–1. The values of ΔH^≠ and ΔS^≠ were calculated from the corresponding Eyring plots (Table 9).

Table 9. Activation energies (E_a), enthalpies (ΔH^≠), and entropies (ΔS^≠) of the hydrophosphination
of styrene with PhPH₂ catalyzed by complexes 8 and 10–12

Precatalyst	Ea [kJ∙mol–1]	ΔH≠ [kJ∙mol–1]	ΔS≠ [J∙mol–1К–1]
10a	70.5(5.4)	67.7(5.4)	–101.8(16.5)
11b	43.3(6.0)	40.6(6.0)	–180.7(18.4)
12c	34.2(2.4)	31.5(2.4)	–212.1(7.3)
8a	60.7(5.4)	58.0(5.4)	–115.4(22.3)

^a 10.0 μmol of the precatalyst; [precatalyst]₀/[styrene]₀/[Ph₂PH]₀ =1/80/8, C₆D₆ + Ph₂PH + styrene = 0.6 mL;
^b 3.0 μmol of 11; [11]₀/[styrene]₀/[Ph₂PH]₀ = 1/270/27, C₆D₆ + Ph₂PH + styrene = 0.6 mL;
^с 2.3 μmol of 12; [12]₀/[styrene]₀/ HPPh₂]₀ = 1/700/70, C₆D₆ + Ph₂PH + styrene = 1.2 mL.

A tentative catalytic cycle for the hydrophosphination of styrene with diphenylphosphine catalyzed by divalent lanthanide and alkaline earth metal complexes is depicted in Scheme 6.

Scheme 6. Tentative catalytic cycle for the hydrophosphination of styrene derivatives with diphenylphosphine
catalyzed by rare-earth and alkaline earth metal complexes 8–12.

Recently Cui et al. suggested readily available and efficient catalysts for hydrophosphination based on the amide complexes of divalent ytterbium with N-heterocyclic carbene ligands Yb[N(SiMe₃)₂]₂(NHC)₂ (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene, IMe₄ (13), 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene, IⁱPr (14)) (Scheme 7) [58]. It was found that a significant increase in the catalytic activity of hydrophosphination can be achieved owing to the substitution of coordinated THF molecules in Yb[N(SiMe)₂]₂(THF)₂ for neutral carbene ligands.

 Scheme 7. Synthesis of complexes 13–15.

Bis(phosphide) complex Yb(PPh₂)₂(IMe₄)₃ (15) bearing N-heterocyclic carbene ligands was obtained by the reaction of complex 13 with two equivalents of Ph₂PH in toluene. Complexes 13–15 were tested as precatalysts for the hydrophosphination of styrene with diphenylphosphine in C₆D₆ at room temperature. All of them appeared to be active in hydrophosphination of alkenes, alkynes, and dienes; the best results were demonstrated by complex 13 (Table 10).

Table 10. Hydrophosphination of styrene with diphenylphosphine catalyzed by complexes 13–15

Precatalyst (mol %)	Time, h	Conversion (%)a
Without a precatalyst	10	4
Yb[N(SiMe)2]2(THF)2 (5)	2	10
13 (5)	2	98 (96)b
13 (3)	2	70
14 (5)	2	65
15 (5)	2	58

Reaction conditions: Ph₂PH (0.25 mmol, 46.6 mg), styrene (0.3 mmol, 31.2 mg),
and the precatalyst in C₆D₆ (0.4 mL), r.t.; 
^a conversions were determined by ¹H and ³¹P NMR spectroscopy;
^b bracketed value refers to the yield of the isolated product (%). 

In contrast, the complex bearing THF (Yb[N(SiMe)₂]₂(THF)₂) showed very low activity. In all cases the catalytic reactions led to the anti-Markovnikov product. Interestingly, amide complex 13 exhibited higher catalytic activity than phosphide analog 15, which is supposed to serve as the real catalytically active species in the reaction mixture. This observation was explained by a possible formation of more active low-coordinate phosphide intermediate Yb(PPh₂)₂(IMe₄)₂ during the catalytic cycle.

A series of catalytic experiments on the hydrophosphination of different styrene derivatives using complex 13 showed that the substrates bearing electron withdrawing substituents at the para-positions are more reactive and undergo quantitative conversion even at room temperature (Table 11). For styrenes bearing electron donating groups at the para-positions (Ме or МеО), the high conversions were achieved only at 60 °C. The use of vinylnaphthalene and vinylpyridine (о-, р-) also required prolonged heating.

 Table 11. Hydrophosphination of monoaryl-substituted ethylenes with Ph₂PH catalyzed by complex 13

Ar	T, °C	Time, h	Conversion (%)	Yield (%)
4-MeC6H4	60	5	97	94
4-MeOC6H4	60	20	62	57
4-FC6H4	r.t.	6	94	93
4-ClC6H4	r.t.	1	96	94
4-BrC6H4	r.t.	1	96	94
4-MeO2CC6H4	r.t.	4	97	96
4-NCC6H4	r.t.	2	69	67
2-naphthalene	60	20	100	99
2-pyridine	60	20	46	40
4-pyridine	60	20	61	57

Complex 13 also promoted the addition of diphenylphosphine to compounds with 1,1-disubstituted C=С bonds, internal С=С bonds, terminal and internal C≡C bonds as well as conjugated dienes (Table 12).

Table 12. Catalytic hydrophosphination of alkenes, alkynes, and dienes with diphenylphosphine catalyzed by complex 13

Substrate	T, °C	t, h	Conv. (%)	Selectivity
	r.t.	4	100 (98)a	–
	60	20	25 (22)a	–
	60	20	6	–
	r.t.	1	100	Z/E = 85/15
	r.t.	2	100	Z/E = 36/64
	60	2	94	Z/E = 39/61
	60	4	92	Z/E = 86/14
	r.t.	2	100	Z/E = 90/10
	r.t.	5	100	1,4/1,2 = 40/60
	70	20	25	1,4/1,2 = 70/30

Reaction conditions: HPPh₂ (0.3 mmol), alkene or alkyne (0.25 mmol),
and complex 13 (0.0125 mmool, 5 mol. %) in C₆D₆ (0.4 mL); 
^a bracketed values refer to the yields of the isolated products. 

The authors emphasized that an advantage of precatalyst 13 is the possibility to recycle it [58]. Due to the low solubility in C₆D₆ and toluene, complex 13 can be recovered from the reaction mixture by filtration after a catalytic reaction and used again without additional purification at least 4 times without loss of activity. The hydrophosphination of 1,1-diphenylethylene with diphenylphosphine had the zero order by the phosphine and the first order by the alkene and the precatalyst. An empirical rate law can be described by the following equation: v = k∙[catalyst]¹∙[alkene]¹. The activation energy of hydrophosphination of styrene catalyzed by complex 13 composed E_a = 34.2(1.2) kJ∙mol^–1. The enthalpy and entropy of activation were determined by the Eyring method: ΔH^≠ = 31.6(1.2) kJ∙mol^–1 and ΔS^≠ = –178.9(3.9) J∙mol^–1 K^–1.

A series of heteroleptic Ln(II) (Ln = Sm, Yb) and Ca amide complexes coordinated by bi- and tridentate amidinate and 1,3,6,8-tetra-tert-butylcarbazol-9-yl ligands were synthesized by the amine elimination under action of equivalent amounts of the protonated ligands on corresponding bis(amides) M[N(SiMe)₂]₂(THF)₂ (Scheme 8) [59, 60]. Complexes 16–22 were used as precatalysts for the hydrophosphination of styrene with diphenylphosphine (Table 13), which was chosen as a model reaction. All the reactions appeared to be regioselective and led to the exclusive formation of the anti-Markovnikov product.

Scheme 8. Synthesis of complexes 16–22.

Table 13. Hydrophosphination of styrene with Ph₂PH catalyzed by complexes 16–22^a

Precatalyst	Time, h	Conversion (%)b
16	2	58
17	2	100
18	2	100
19	2	100
20	2	26
21	2	100
22	2	71
[(Me3Si)2N]2Yb(THF)2	2	24

^a Reaction conditions: neat substrates, [styrene]₀/[Ph₂PH]₀/[precatalyst]₀ = 50/50/1, T = 60 °C;
^b conversions of styrene were determined by ¹Н NMR spectroscopy.

Complexes 17–19 and 21 displayed the highest efficiencies and afforded the quantitative conversions over 2 h at 60 °C. The activity of the whole series significantly exceeded that of their bis(amide) precursor [(Me₃Si)₂N]₂Yb(THF)₂, thus, indicating the importance of an auxiliary ligand and the design of a catalytic center in providing high catalytic performance. Complex 22 led to 92% conversion over 4 h at 60 °C even at the precatalyst loading of 1 mol %. It should be noted that calcium complex 17 exhibited better performance than isostructural Yb analog 16 (100% vs. 58% over 2 h). Samarium amide complex 18, bearing the same amidinate ligand but deprived of coordinated THF molecule was also more active than ytterbium complex 16. In a pair of isostructural complexes 19 and 20, the samarium compound was much more efficient in the hydrophosphination of styrene with diphenylphosphine than ytterbium analog 20 (100% vs. 26%). In a series of ytterbium complexes 16, 20, 21, the tridentate amidinate ligand bearing a Ph₂P=O pendant donor group provided the highest catalytic activity (58%, 26%, and 100% conversions, respectively) [60].

The detailed kinetic investigations on the hydrophosphination in the presence of complex 22 revealed the zero order of the reaction by the phosphine, the first order by styrene, and 0.5 order by the precatalyst [59]. Complexes 16–22 were used as precatalysts for the hydrophosphination of styrene with primary phosphine PhPH₂. The catalytic experiments were carried out either in neat substrates ([styrene]/[PhPH₂] = 1/1) or in C₆D₆ at 60 °C in the presence of 2 mol % of the precatalyst (Table 14). Complexes 16–22 catalyzed the addition of PhPH₂ to styrene [60]. Surprisingly, despite the larger volume of diphenylphosphine compared to that of phenylphosphine, the addition of the former to styrene occurred much more rapidly. All the reactions catalyzed by compounds 16–22 appeared to be very chemoselective and led to the formation of the product of a single addition to secondary phosphine (PhCH₂CH₂)PhPH with the selectivity above 95%. Furthermore, all the catalysts enabled regioselective hydrophosphination, leading only to the anti-Markovnikov product (2,1-addition). The catalytic activity of the complexes was affected both by the nature of a central metal atom and the nature of a stabilizing ligand environment. In a series of the complexes bearing the same amidinate ligands, the catalytic activities reduced in the following order: 17 ≥ 18 > 16. The values of TOF were within the range of ≈ 0.3–0.7 h^–1. However, the application of the tridentate amidinate ligand allowed one to essentially increase the catalytic activity: the value of TOF for complex 19 composed 8.3 h^–1.

 Table 14. Hydrophosphination of styrene with PhPH₂ catalyzed by complexes 16–22

Precatalyst	Time, h	Conversion (%)c	sec-P/tert-Pc
21a	2	26	98/2
21b	2	92	98/2
20b	70	86	95/5
19a	2	63	97/3
19a	6	100	95/5
19b	4	99	97/3
17b	70	100	96/4
16b	70	41	99/1
18b	70	94	95/5
22a	60	61	19/1

^a Reaction conditions: C₆D₆, [styrene]₀/[PhPH₂]₀/[precatalyst]₀ = 50/50/1, [precatalyst]₀ = 17.5 mmol;
^b reaction conditions: neat substrates, [precatalyst]₀ = 80.5 mmol;
^c styrene conversions and selectivities were determined by NMR spectroscopy.

The effect of electronic properties of styrene substrates on the reaction rate was explored [60]. A series of catalytic experiments on the hydrophosphination of styrenes bearing different substituents at the para-position of aromatic rings with phenylphosphine catalyzed by complex 19 showed that the electron withdrawing substituents (Cl, F) do not affect the reaction rate, whereas the electron donating substituents considerably reduce it (Table 15). Unexpectedly, in the case of 4-Ме-styrene having an electron donating substituent, the reaction rate was still comparable with the rate of the reaction of unsubstituted styrene. The results obtained were in good agreement with the observation reported earlier by the groups of Prof. Hill [61] and Carpentier [57, 62], which explained this phenomenon by stabilization of a partial negative charge of the benzyl carbon atom in a polarized four-center cycle during a limiting step of the process owing to the electron withdrawing para-substituent. In contrast, in the case of an electron donating para-substituent at the aryl ring of a styrene substrate, the supposed transition state is destabilized.

Тable 15. Hydrophosphination of para-substituted styrenes with PhPH₂ catalyzed by complex 19^a

Time, h	Styrene	Conv. (%)b	sec-P/tert-P (%)b
4	4-H-styrene	99	97/3
4	4-F-styrene	88	98/2
4	4-Cl-styrene	99	97/3
4	4-Me-styrene	90	96/4
4	4-tBu-styrene	30	100/0
4	4-OMe-styrene	14	100/0

^a Reaction conditions: neat substrates, [styrene]₀/[PhPH₂]₀/[precatalyst]₀ = 50/50/1, [precatalyst]₀ = 80.5 mmol, T = 60 °C;
^b styrene conversions and reaction selectivities were determined by ¹H and ³¹P NMR spectroscopy.

Complexes 16–21 enabled the addition of PhPH₂ across the internal C≡C bond of tolane (Table 16). The high (84%) and quantitative conversions of the substrate were achieved upon application of complexes 17 and 21, respectively. Unexpectedly, in a series of complexes 16–18 bearing the same amidinate ligands, the samarium complex showed the lower activity, despite the larger ionic radius, (24% vs. 56% in the case of 16). All the complexes tested in tolane hydrophosphination led to the formation of a mixture of Е- and Z-isomers. The highest selectivity of the formation of Е-isomer was observed in the case of complex 18 (E/Z = 83/17).

Table 16. Hydrophosphination of tolane with PhPH₂ catalyzed by complexes 16–21^a

Precatalyst	Time, h	Conversion (%)b	E/Z (%)b
21	70	100	36/64
20	70	37	33/67
19	70	35	28/72
17	70	84	58/42
16	70	56	70/30
18	70	24	83/17

^a Reaction conditions: neat substrates, [tolane]₀/[PhPH₂]₀/[precatalyst]₀ = 50/50/1, T = 60 °C;
^b conversions of tolane and PhPH₂ as well as Z/E stereoselectivities of the products were determined by NMR spectroscopy. 

In 2016 the syntheses of Yb(II) and Sm(II) amide complexes {LO^(NO)2}Yb{N(SiMe₃)₂} (23), {LO^NO2}Yb{N(SiMe₃)₂} (24), {LO^NO4}Yb{N(SiMe₃)₂} (25), {LO^NO4}Sm{N(SiMe₃)₂} (26) as well as two amide and two alkyl derivatives of calcium and strontium {LO^(NO)2}Ca{N(SiMe₃)₂} (27), {LO^NO2}Ca{N(SiMe₃)₂} (28), {LO^NO2}Ca{CH(SiMe₃)₂}(THF) (29), {LO^NO2}Sr{CH(SiMe₃)₂} (30) bearing aminoether–phenolate ligands were reported (Scheme 9). The complexes were obtained by the stoichiometric elimination of the corresponding amine/alkane from bis(amide) or bis(alkyl) metal derivatives under action of the protonated ligands in toluene [63, 64]. The catalytic tests on hydrophosphination of styrene with phenylphosphine ([styrene]₀/[PhPH₂]₀/[precatalyst]₀ = 50/50/1) in the presence of complexes 23–30 were carried out in C₆D₆ at 25–60 °C (Table 17). These complexes were found to serve as very active precatalysts for hydrophosphination of styrene derivatives with phenylphosphine, providing TONs up to 2150, TOFs up to 30 h^–1, and chemoselectivities within the range of 95–99%. The reactions appeared to be fully regio- (anti-Markovnikov products) and chemoselective and led to the formation of a secondary phosphine (sec-P) with the selectivities >95%, i.e., less than 5% of a tertiary phosphine was formed as a result of double hydrophosphination. The Yb(II) catalysts, among which the most important complex was compound 25 ({LO^NO4}Yb{N(SiMe₃)₂}) bearing a phenolate ligand with the bulky crown ether substituent, significantly exceeded in the efficiency the corresponding calcium analogs. Complexes 25 (Yb) and 26 (Sm) having the ligands with the highest denticity ({LO^NO4}) were the most active precatalysts. It should be noted that in the case of the ytterbium complexes, the activity improved with increasing ligand denticity. Calcium alkyl and amide complexes 28 and 29 exhibited similar activities, which indicated the lack of a strong leaving group effect. Comparison of the results of these catalytic studies supports the necessity of an auxiliary ligand to provide catalytic activity.

 Scheme 9. Synthesis of complexes 23–30.

Тable 17. Hydrophosphination of styrene with PhPH₂ catalyzed by complexes 23–30^a

Precat.	[styrene]0/[PhPH2]0/[precatalyst]0	T, °C	t, h	Conv. (%)b	sec-P/tert-Pc
23	50/50/1	60	24	1	n/d
24	50/50/1	60	24	52	99/1
25	50/50/1	60	0.5	100	94/6
25	50/50/1	25	1	74	97/3
26	50/50/1	60	0.5	80	97/3
26	50/50/1	25	1	63	97/3
27	50/50/1	60	24	24	97/3
28	50/50/1	60	24	40	99/1
29	50/50/1	60	24	34	99/1
30	50/50/1	60	24	15	100/0
25	100/100/1	25	3	72	99/1
25d	100/100/1	60	3	91	95/5
25e	200/200/1	60	3	82	96/4
25f	500/500/1	60	0.5	66	93/7
25f	500/500/1	60	15	100	93/7
25g	2500/2500/1	60	72	86	80/20
25	100/50/1	60	24	100	0/100
25h	100/100/1	25	3	100	99/1

^a If not mentioned otherwise, the reactions were carried out in C₆D₆ using 11.5 mmol of the precatalyst.
The regiospecific formation of 100% of the anti-Markovnikov secondary phosphine was established by NMR spectroscopy.
^b styrene conversions were determined by NMR spectroscopy;
^c product chemoselectivities were determined by ³¹P NMR spectroscopy;
^d [precatalyst]₀ = 9.35 mmol;
^e [precatalyst]₀ = 7.95 mmol;
^f [precatalyst]₀ = 6.99 mmol;
^g [precatalyst]₀ = 1.62 mmol;
^h reaction performed in a Schlenk flask with continuous stirring.

Complex 25 appeared to be stable enough and retained the high level of activity even upon loading of 2500 equivalents of each substrate, providing 86% conversion in 72 h (TON = 2150, TOF = 30 h^–1). In the case of hydrophosphination at the molar ratio [styrene]₀/[PhPH₂]₀ = 2/1, the selective formation of a tertiary phosphine proceeded with a quantitative conversion.

Complex 25 was used for single-reactor two-step synthesis of unsymmetrical tertiary phosphines bearing three different substituents at the phosphorus atoms (Scheme 10). At the first step, the hydrophosphination of equivalent amounts of a styrene derivative and phenylphosphine gave rise to a secondary phosphine. Then the second equivalent of the styrene derivative was added to the reaction mixture. A series of the experiments were carried out with the styrene substrates containing different substituents at the para-positions (X = Cl, F, ^tBu, OMe). For unsubstituted styrene (X = H) as well as for 4-Сl- and 4-F-substituted derivatives the second step of the process proceeded quantitatively over 3 h, resulting in unsymmetrical tertiary phosphines. In the case of 4-tert-butyl-substituted styrene, the reaction was slower, affording 56% conversion over 20 h, whereas for the styrene with a strongly electron donating OMe substituent, it did not proceed at all. Hence, the reaction rate is highly sensitive to the electronic effects of styrene substrates and increases in the following series: OMe < ^tBu < H, F, Cl.

Scheme 10. Synthesis of tertiary phosphines via single-reactor two-step processes catalyzed by complex 25.

For the hydrophosphination of 4-^tBu-styrene with PhPH₂ catalyzed by complex 25, the reaction rate equation was found to be as follows: v = k·[4-^tBu-styrene]·[25]. The kinetic studies also showed that the reaction rate increases upon introduction of an electron withdrawing substituent at the para-position of a styrene substrate. It was assumed that the catalytic hydrophosphination proceeds through a limiting step of the alkene insertion into the Yb–Р bond. The activation parameters of the reaction, ΔH^≠ = 29.7 kJ·mol^–1 and ΔS^≠ = –191.2 J·mol^–1·K^–1, were determined by the Eyring method and corresponded to the value of ΔG^≠ equal to 86.5 kJ·mol^–1 at 25 °C. These values indicate that the reaction proceeds by the associative mechanism and is controlled by entropy; a transition state is highly ordered.

 

The kinetic studies of the reaction between two equivalents of 4-^tBu-styrene and PhPH₂ catalyzed by complex 25, which leads to secondary and tertiary phosphines (mono- and dihydrophosphination products), showed that the formation of the secondary phosphine is chemospecific, and the alkylation of the second Р–Н bond and the formation of a tertiary phosphine start only when all PhPH₂ is consumed. It was revealed that despite the close rates of reactions for primary and secondary phosphines with 4-^tBu-styrene, the σ-bond metathesis of a reaction intermediate in the case of the secondary phosphine is considerably slower than that of an analogous intermediate in the case of the primary phosphine. The hydrophosphination of 1-nonene with phenylphosphine catalyzed by complex 25 afforded only 3% conversion upon heating at 60 °C for 70 h.

In 2015 the first example of intermolecular hydrophosphination of alkenes catalyzed by trivalent alkyl and hydride complexes of lanthanides was reported [65].

It was found that neutral and ionic alkyl complexes [2,6-ⁱPr₂C₆H₃NC(Me)=C(Me)NC₆H₃ⁱPr₂-2,6)]Y(CH₂SiMe₃)(THF)₂ (31), {Li(THF)₄}⁺{[2,6-ⁱPr₂C₆H₃NC(Me)=C(Me)NC₆H₃ⁱPr₂-2,6]Y(CH₂SiMe₃)₂}⁻ (32) and hydride complex {[2,6-ⁱPr₂C₆H₃NC(Me)=C(Me)NC₆H₃ⁱPr₂-2,6]Y(THF)(μ-H)}₂(μ-THF) (33) coordinated by dianionic en-diamide ligand [2,6-ⁱPr₂C₆H₃NC(Me)C(=CH₂)NC₆H₃ⁱPr₂-2,6]₂ (Schemes 11 and 12), аs well as parent bis(alkyl) complex of yttrium [2,6-ⁱPr₂C₆H₃NC(Me)C(=CH₂)NC₆H₃ⁱPr₂-2,6]Y(CH₂SiMe₃)₂(THF) (34) [65] are efficient precatalysts for the hydrophosphination of styrene and 4-vinylpyridine with phenyl- and diphenylphosphines and afford quantitative conversions of the substrates over 72 h, resulting exclusively in the anti-Markovnikov products (Table 18).

Scheme 11. Synthesis of alkyl complexes 31, 32.

Scheme 12. Synthesis of hydride complex 33.

Table 18. Hydrophosphination of alkenes with Ph₂PH catalyzed by complexes 31–34

Precatalyst	Alkene	Conversion (%)
31	A	100
32	A	100
33	A	100
34	A	100
31	B	100
32	B	100
33	B	100
34	B	100
31	C	0
32	C	17
33	C	0
34	C	0

Reaction conditions: neat substrates in a 1:1 ratio, precatalyst concentration 2 mol %, 70 °C, 72 h.
Conversions were determined by NMR spectroscopy.

It is important to note that complex 32 is able to catalyze the addition of Ph₂PH across the C=C bond of usually inert 1-nonene with the conversion of 17%. Before that, the only one example of a catalyst for hydrophosphination of aliphatic 1-alkenes was metallacyclic alkyl complex of zirconium reported by the group of Waterman [66].

In all the catalytic experiments with PhPH₂, complexes 31–34 demonstrated excellent chemoselectivities: at a 1:1 ratio of the substrates, only the product of a single P–H bond addition was formed (Table 19). The interaction of PhPH₂ with two styrene equivalents in the presence of complexes 31–34 led to the production of only tertiary phosphine PhP(CH₂CH₂Ph)₂ in 76–97% yields (Table 20). Complexes 31–34 catalyzed sequential alkylation of phenylphosphine with styrene. They offer an opportunity for a single addition of the Р–Н bond to styrene, resulting in secondary phosphine PhPH(CH₂CH₂Ph), whereas the addition of the second equivalent of styrene leads to the formation of tertiary phosphine PhP(CH₂CH₂Ph)₂.

 Table 19. Hydrophosphination of alkenes with PhPH₂ catalyzed by complexes 31–34

Precatalyst	Alkene	Conversion (%)
31	A	100
32	A	100
33	A	100
34	A	100
31	B	100
32	B	100
33	B	100
34	B	100

Reaction conditions: neat substrates in a 1:1 ratio, precatalyst concentration 2 mol %, 70 °C, 72 h.
Conversions were determined by NMR spectroscopy.

Table 20. Double addition of styrene to PhPH₂ catalyzed by complexes 31–34

Precatalyst	Conversion (%)
31	76
32	97
33	81
34	83

Reaction in neat substrates, styrene:PhPH₂ = 2:1, precatalyst concentration 2 mol %, 70 °C, 96 h.
Conversions were determined by NMR spectroscopy.

Complexes 31–34 promoted the addition of phenylphosphine to triple bonds of tolane. Diphenylphosphine appeared to be less active in the hydrophosphination of tolane; the quantitative conversion was achieved only in the case of complex 32, with predominant formation of Z-isomer (Tables 21 and 22).

Table 21. Hydrophosphination of tolane with PhPH₂ catalyzed by complexes 31–34

Precatalyst	Conversion (%)	Z/E
31	100	45/55
32	100	42/58
33	100	44/56
34	50	40/60

Reaction conditions: neat substrates in a 1:1 ratio, precatalyst concentration 2 mol %, 70 °C, 72 h.
Conversions were determined by NMR spectroscopy.

Table 22. Hydrophosphination of tolane with Ph₂PH catalyzed by complexes 31–34

Precatalyst	Conversion (%)	Z/E
31	10	100/0
32	100	97/3
33	21	92/8
34	50	68/32

Reaction conditions: neat substrates in a 1:1 ratio, precatalyst concentration 2 mol %, 70 °C, 72 h.
Conversions were determined by NMR spectroscopy.

In 2018 the detailed investigation on the catalytic activity of a series of heteroleptic amide complexes of Yb(II) and Ca coordinated by amidine-amidopyridinate ligands L1–L4 in hydrophosphination was reported (Scheme 13) [67].

Scheme 13. Synthesis of complexes 35–39.

Complexes 35–39 were studied as precatalysts for the hydrophosphination of styrene with different phosphines. The results of catalytic experiments allowed for evaluation of the effect of a phosphine nature on the reaction rate and selectivity (Scheme 14).

Scheme 14. Hydrophosphination of styrene with various phosphines catalyzed by complexes 35–39. 

Complexes 35–39 demonstrated high catalytic activity and regioselectivity in the hydrophosphination of styrene with PhPH₂ and Ph₂PH. In the case of PhPH₂, the addition to styrene catalyzed by complexes 35, 36, and 37 was chemoselective and led to the formation of secondary phosphine Ph(PhCH₂CH₂)PH, whereas the use of complexes 38 and 39 as precatalysts afforded mixtures of secondary and tertiary phosphines. Furthermore, complexes 35–39 promoted the hydrophosphination of styrene with mesityl- and 2-pyridylphosphine. Surprisingly, despite the close ionic radii of catalytic centers, the reaction of 2-C₅NH₄PH₂ with styrene catalyzed by complex 35 proceeded nonregioselectively, whereas the analogous calcium complexes (36–39) provided regioselective anti-addition of the phosphine. Isostructural complexes 35 and 36, bearing catalytic centers with the close ionic radii, exhibited different catalytic activities: the ytterbium complex (35) appeared to be more active precatalyst for the hydro phosphination of styrene with Cy₂PH, 2-C₅NH₄PH₂,and PhPH₂, but less active in the case of more sterically hindered 2,4,6-Me₃C₆H₂PH₂. Furthermore, a series of catalytic experiments on the hydrophosphination of a range of different alkenes with phenyl- and diphenylphosphine was also described (Tables 23 and 24).

 Table 23. Hydrophosphination of styrenes with PhPH₂ and Ph₂PH catalyzed by complexes 35–39

Precatalyst	Substrates	Time, h	Conv. (%)	Chemoselectivity
35	styrene PhPH2	3	100	94/6
36		3	33	91/9
36		10	100	97/3
37		3	55	95/5
38		3	100	80/20
39		3	76	86/14
35	4-F-styrene PhPH2	16	72	94/6
36		3	82	91/9
37		3	61	92/8
38		3	100	85/15
39		3	67	86/14
35	4-Cl-styrene PhPH2	3	80	90/10
36		3	86	96/4
37		3	71	88/12
38		3	79	90/10
39		3	80	91/9
35	4-Me-styrene PhPH2	16	96	98/2
36		16	96	90/10
37		16	100	68/32
38		16	100	83/17
39		16	88	84/16
35	4-tBu-styrene PhPH2	16	28	100/0
36		16	96	91/9
37		16	100	83/17
38		16	100	83/17
39		16	85	87/13
35	4-OMe-styrene PhPH2	70	55	95/5
36		40	100	89/11
37		40	100	77/23
38		40	100	83/17
39		40	100	90/10
35	α-Me-styrene PhPH2	40	62	100/0
36		40	82	100/0
37		40	90	100/0
38		40	100	100/0
39		40	88	100/0

Reaction conditions: neat substrates, [phosphine]₀/[styrene]₀/[precatalyst]₀ = 50/50/1, [precatalyst]₀ = 87.0 mmol, T = 70 °C;
styrene conversions and chemoselectivities were determined by NMR spectroscopy. 

 Table 24. Hydrophosphination of alkenes with PhPH₂ and Ph₂PH catalyzed by complexes 35–39

Precatalyst	Substrates	Time, h	Conv. (%)	Chemoselectivity
35	2,3-dimethyl- 1,3-butadiene PhPH2	40	40	100/0
36		40	100	100/0
37		40	100	100/0
38		40	90	40/60
39		40	95	82/18
35	1-nonene PhPH2	96	0	–
36		72	12	100/0
36		96	28	100/0
37		40	3	100/0
38		40	23	100/0
39		40	25	100/0
35	1-nonene Ph2PH	96	0	–
36		40	4	100/0
37		40	40	100/0
38		40	34	100/0
39		40	13	100/0
35	cyclohexene PhPH2	40	0	–
36		40	0	–
37		40	0	–
38		40	0	–
39		40	0	–
35	trans-stilbene PhPH2	40	0	–
36		40	0	–
37		40	0	–
38		40	0	–
39		40	0	–

Scheme 15. Synthesis of complexes 40–45.

The addition of PH₃ catalyzed by complexes 40–45 proceeded under mild reaction conditions with quantitative yields and afforded primary, secondary, and tertiary phosphines with high chemoselectivities. The authors performed the kinetic monitoring of the alkylation of PH₃, which revealed similar mechanisms of sequential alkylation of styrene with PH₃ and PhPH₂ in the presence of the amide complexes of calcium, ytterbium (II) and samarium(II) (Fig. 1). As well as in the earlier described example of sequential hydrophosphination [64], the reaction has the zero order by the phosphine and the first order by the alkene; the second step of the reaction starts only after full consumption of the starting phosphine (PhPH₂ in the case of complex 25 and PH₃ in the case of compounds 40–45). It was assumed that the exceptionally high chemoselectivity of this process is achieved not only owing to the difference in the rate constants of the first, second, and third steps, but also owing to the competitive metathesis of σ-bonds of intermediates, which take place in the presence of several phosphine substrates in a solution.

Figure 1. Kinetic profile of sequential alkylation of PH₃ catalyzed by complex 44.
Reaction conditions: C₆D₆, 25 °C, [PH₃]₀/[styrene]₀/[44]₀ = 20/160/1; [44]₀ = 30.0 mmol. 

Regardless of the ratio of the substrates, the addition of PH₃ to phenylacetylene catalyzed by complexes 40–45 led selectively to tris(Z-styryl)phosphines. It was also demonstrated that the alkylation of PH₃ with 2-vinylpyridine promoted by complexes 40–45 represents a convenient method for synthesis of tris(2-pyridylethyl)phosphine, which can serve as a tetradentate ligand. The significance of the coordinated Lewis base in the precatalyst structure for its catalytic activity in hydrophosphination was evidenced. The free N-heterocyclic carbenes were also shown to promote the addition of PH₃ to styrene, but the reactions rates were considerably lower than in the case of the metal complexes.

3. Conclusions

The last decade has witnessed a significant progress in the field of catalytic hydrofunctionalization of alkenes, which, in turn, reflects a global trend in the development of homogeneous catalysis that consists in the displacement of the compounds of expensive and toxic late transition metals by the complexes of less expensive and environmentally benign rare-earth and alkaline earth elements, more abundant in the crust. The growing number of reports on catalytic activity of complexes of rare-earth and alkaline earth metals in reactions of unsaturated substrates gradually expands our understanding about their potential and the mechanisms of activation of alkenes by these compounds, which principally differ from the activation mechanisms of d-block transition metal complexes. Rare-earth elements appeared to be one of the most promising candidates for addressing this problem, since they enable various functionalizations under mild conditions. Thus, the hydrophosphination of styrene substrates and alkynes, in particular, 1,1-disubstituted alkenes bearing terminal and internal multiple carbon–carbon bonds, with primary and secondary arylphosphines is now not a problem, since this reaction can be readily carried out under mild conditions with high product yields and regio- and chemoselectivities. The cascade alkylation of primary phosphines with different styrenes is of particular interest as an efficient and facile method for synthesis of unsymmetrical tertiary phosphines bearing three different substituents at the phosphorus atom. It is commonly accepted that the ion size of a catalytic center affects the catalytic activity; nevertheless, the comparison of isostructural Yb(II)/Ca(II) and Yb(II)/Sm(II) complexes showed that this relation is not trivial. Undoubtedly, the electronic and redox properties (in the case of Ln(II) complexes) of metal ions play an essential role, but the valent states of the metal ion is not a key factor that defines the catalytic activity. Of note is the importance of an auxiliary ligand, in particular, the comparative study of Yb(II) amide complexes coordinated by different ligand systems demonstrated that the character of the metal–ligand bond, denticity and basicity of the ligand are essential for the catalytic activity of these complexes.

Despite a great progress, there are still some challenges in the field of catalytic hydrophosphination of alkenes. The use of highly reactive alkyl and hydride complexes of lanthanides offers opportunities for solution of these issues. As it was mentioned earlier, organic derivatives of lanthanides can provide unique regioselectivity in the formation of a single anti-Markovnikov product, whereas the development of catalytic systems capable of promoting the formation of Markovnikov products is a serious problem. The scope of intermolecular hydrophosphination should be extended owing to the use of new substrates, such as di- and trisubstituted alkenes, aliphatic and sterically hindered phosphines. Hydrophosphination of functionalized substrates requires the improvement of catalyst tolerance to functional groups of substrates, and this is one of the most difficult problems which are yet to be solved. Furthermore, the physiological activity of phosphorus-containing compounds and their wide application as ligands in asymmetric catalysis inevitably dictate the necessity to develop enantioselective protocols for hydrophosphination of alkenes.

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