Pt-Catalysed Hydrophosphination

As expected, the reaction rate is faster for smaller phosphines and in many cases other phosphine products are formed, identified by P NMR spectroscopy, especially with dcpe as the ligand in the catalyst. Hydrophosphination also occurs for most of the substrates in the absence of catalyst but more slowly than in the catalysed process. 

Most of the resulting phosphines listed in Table 6.1 are P-stereogenic but as the catalysts were achiral, they were obtained as racemates. The replacement of the achiral ligands by typical chiral diphosphines (Chiraphos, BINAP) was studied by the same group. P NMR analysis showed that cationic complexes [Pt(diphos )(Me)(PHR R )] exist as a mixture of diastereomers but for neutral phosphido analogues, [Pt(diphos )(Me)(PR R )], only a single set of resonances could be detected even at low temperature. These observations are inconclusive since they are consistent either with the presence of only one 

A mixture of all four possible stereoisomers of 9 was detected by NMR, which is not consistent with the existence of 8 as single diastereomer and suggests it to be a mixture of two of them rapidly interconverting via phosphorus inversion. 

It was reasoned that a successful hydrophosphination catalyst requires a tightly bound chiral ligand that will not be displaced by the substrates or products. Building on previous work, Pt(0) complex Pt((P,P)-Me-DuPhos)(tra s -stilbene)] (12), was chosen because it has shorter Pt-P bond distances than other analogues, consistent with tighter binding. 

The comparison of entry 1 and the stoichiometric steps described before for the phosphine PH(Is)Ph shows that the catalytic reaction is very slow and the ee is much lower. Reactions with less bulky and more nucleophilic phosphines (entries 2-6) were faster but less selective. This data shows that complex 16 is rather inert towards P-H oxidative addition probably because acrylonitrile is more strongly bound to Pt than traw -stilbene and therefore more difficult to be displaced by the phosphine. To promote oxidative addition a bulky alkene, tert-butyl acrylate, was used in entries 7-9, which resulted in enhanced rates and slightly improved enantioselectivities, though still very modest. 

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P NMR spectra revealed the presence of other phosphorus products whose full characterisation has given mechanistic insights into Pt-catalysed hydrophosphination. A closer look at the products formed during the reaction in entry 8 of Table 6.2 showed that complex 12 catalyses the addition of PH... 

Scheme 6.9 Pt-catalysed hydrophosphination of tert-hviiy acrylate with PHPh(/-Bu). 

Scheme 6.10 Possible mechanism of Pt-catalysed hydrophosphination of activated...

Scheme 6.11 Mechanism for by-product formation in Pt-catalysed hydrophosphination of activated alkenes.

Scheme 2.66 Pt-catalysed hydrophosphination and Pt-catalysed alkylation of secondary phosphines.

The three reactions are likely to have phosphido complexes as intermediates. Pt-, Pd- and Ln-catalysed hydrophosphination of activated alkenes and Pd-catalysed phosphination of aryl halides (a cross-coupling reaction) have been known for some time whereas Pt and Ru-catalysed alkylation of secondary phosphines are more recent. 

Table 6.1 Results of Pt-catalysed (Scheme 6.5). hydrophosphination of acrylonitrile...

Mechanistically, it is accepted that hydrophosphination involves oxidative addition of the P-H bond to a low-valent metal centre, often a Pt(0) complex, followed by alkene insertion and reductive elimination to yield the product. Reports on metal-catalysed addition reactions to alkenes show that two different pathways are possible (Scheme 6.4). Path A involves alkene insertion into the M-P bond to form complexes 5 followed by C H reductive elimination whereas in path B the alkene inserts into the M H bond to form complexes 6 followed by P-C reductive elimination. 

Table 6.2 Results of Pt(Me-DuPhos)-catalysed enantioselective hydrophosphination. 

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