Silylformylations catalytic cycle

Insertion of unsaturated molecules into a transition metal-silyl bond has been suggested for the catalytic reactions related to hydrosilylation and silylcarbonylation. However, there is little direct evidence supporting such a process for unsaturated molecules to insert into a metal-silyl bond in organometallic complexes. " Thus, the fact that 108 is readily derived from 11 and 13 demonstrates the participation of this process in the catalytic cycle of silylformylation. 

All the starting compounds in Scheme 8 have a sufficient potential for both catalytic and stoichiometric silylformylation, when Me2PhSiH and 1-alkyne are present in a reaction vessel at the same time. Stable mononuclear complex, RhH(GO)(PPh3)3, is far inferior in catalyst efficiency at 25 °G, though the efficiency is improved under practical operation at 100 °C (Table 6). Though 7 and 11 are derived from Rh4(GO)i2 under controlled conditions and work as an active catalyst of silylformylation, their position in the catalytic cycle is still a precursor of truly active species, because it takes a far longer induction period for activation than that for silylformylation. 

The reaction was successfully extended to the hydroformylation of propargyl-type alcohols [154] and propargylamine [155], the silylative cyclocarbonylation of alkynes [156], silylcarbocyclization of alkenynes and diynes [157-160], and other transformations of C=C bonds in the presence of HSiRa and CO (e. g., [161]). A generalized catalytic cycle for the silylformylation of 1-alkynes catalyzed by rhodium-cobalt clusters is illustrated in Scheme 6. 

The Pauson-Khand Reaction and Silylformylation. Perhaps one of the earhest and most widely studied bimetallic alkyne activation processes is the Pauson-Khand Reaction (PKR), which involves the [2-1-2-1-1] cycloaddition of an alkyne, alkene and CO to yield a cyclopentenone product. The key intermediate in both the PKR and the related silylformylation reaction is a (p—r r )-bonded intermediate of structure E (Fig. 2). Extensive DFT modelling of the catalytic cycles has provided an excellent insight into the electronic changes that occur within the bimetallic unit during the reaction. 

The reaction is regio- and stereospecific and leads to the (Z)-l-silyl-2-formyl-1-hexenes, i.e. to the silylformylation products. In the most probable catalytic cycle, the insertion of alkyne occurs into a Rh-SiR3 bond leading to an intermediate... 

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