Hydroformylation of Vinylsilanes

Catalyst Conversion [%] Yield (Selectivity) of linear product [%1 Yield of Flydrogenation [%1 

Reaction conditions 3.2 mmoi substrate, 1.8% catalyst (0.058 mmol), 1 M in dry distilled solvent, 100°C, 700 

5-2 equiv of AgBp4, pre-activation 160 C, Ih CsH /CHClj ° - isolated yield, 

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It is interesting to note that hydroformylation of vinylsilanes catalyzed by a Co or a Pt catalyst gives /J-silylaldehydes exclusively. A rhodium catalyst exhibits poor regioselectivity. However, in the presence of a large excess of PI13P, the regioselectivity improves to that of a normal aldehyde246-248. 

Fig. 2. Modified catalytic cycle for the hydroformylation of vinylsilanes (R=SiR 3) Intermediates la to Id are favored by the p-silicon effect (Fig. 3), small ligands L like CO, and higher temperature. Da to nd by a bulky ligand, e. g., L = PPhj.

A plausible picture of the P-silicon effect [5] on the hydroformylation of vinylsilanes R 3SiCH=CH2 with HRh(CO)(PPh3)3 as catalyst is shown in Fig. 3. 

Rhodium cationic and zwitteiionic complexes proved to be superior catalysts for the hydroformylation of vinylsilanes, producing either a- or ff-silyl aldehydes depending on the reaction conditions [162], On the other hand, carbonylation of vinylsilanes in the reaction related to hydrocarboxylation and hydroesterification afforded P- and a-silyl esters in high yields (eq. (14) [163]). 

To interpret the observed effects on the regioselectivity a modified model of the hydroformylation cycle is presented in Fig. 2 for the special case of vinylsilane precursors. 

Fig. 3. Schematic description of the P-silicon effect on the regioselectivity of vinylsilane hydroformylation. Conclusion...

As in hydroformylation, both linear and branched products can be obtained from RCH=CH2.The dehydrogenative silylation product, RCH=CHSiR3, is often present and can even predominate under some conditions (Eq. 9.21). The dehydrogenative path can only be explained on the modified Chalk-Harrod mechanism of Eq. 9.23, in which the alkene first inserts into the M-Si bond. 3 elimination of the intermediate alkyl now leads directly to the vinylsilane, the two H atoms thus released go on to hydrogenate the substrate leading to coproduction of alkane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eq. 7.21, also a reaction in which initial insertion of alkyne into the M-Si bond must... 

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