Carbonylative silylcarbocyclizations

Extensively developed by Ojima and co-workers, SiCaT and carbonylative silylcarbocyclization (CO-SiCaC) represent a rapid entry into polycyclic molecules of interest.271 For instance, the rhodium-catalyzed intramolecular SiCaT of triyne 441 afforded tricyclic compound 442 in high yield, accompanied by a small amount of cycloadduct 443 (Scheme 111).270... 

Under a pressure (20 bar) of carbon monoxide, carbonylative silylcarbocyclization of enyne 92 was examined in the presence of a cationic rhodium-BINAP catalyst (Scheme 31).86 Although the enantioselectivity is low, the five-membered carbocycle functionalized with an alkenylsilane moiety and a formyl group was obtained with high selectivity. 

In the first rhodium-catalyzed carbonylative silylcarbocyclization (CO-SiCaC), which was reported in 1992 [12, 13], silylcyclopentenone 9 was isolated as a minor product in the silylformylation of 1-hexyne 8 (Scheme 7.4). Under optimized conditions using Et3SiH and ( BuNC)4RhCo(CO)4 as the catalyst at 60°C, 9 is formed in 54% yield [13]. A possible mechanism proposed for this intermolecular CO-SiCaC is shown in Scheme 7.4 [13]. In this mechanism, the formation of 9 is proposed to proceed via in-... 

The first Rh-catalyzed carbonylative silylcarbocyclization (CO-SiCaC) was reported in 1992 in which silylcyclopentenone 531 was isolated as a minor product in the silylformylation of 1-hexyne 530 (Scheme 2-79). 

Scheme 2-79. Rh-catalyzed carbonylative silylcarbocyclization (CO-SiCaC) of 1-hexyne 530.

The hydrosilylation of unsaturated carbon-rhodium-catalyzed silylcarbocyclizations. In the presence of Rli4(CO)i2 and triethoxysilane, a rigid triyne backbone can undergo a silylcarbotricyclization cascade reaction to yield [5,6,5]-tricycles (eq 16). Similar to the results observed by Sieburth for the hydrosilylation of enamines, the alkoxysilane functionality provides significant rate enhancement in comparison to silylcarbocyclizations using alkyl- and arylsilane reagents. The incorporation of carbonyl functionality as terminal electrophiles into these cyclizations has also been successful. Rhodium-catalyzed carbonylative silylcarbocyclizations proceed in the presence of carbon monoxide (10 atm) to incorporate a carbonyl unit, usually as the aldehyde. Both of these tandem ad-dition/cyclization strategies produce functionalized carbocycles with simultaneous incorporation of sUyl functionality as aryl- and vinylsilanes. These alkenylsilanes can then be exploited for further synthetic manipulations as discussed above. "" ... 

Recent advances in the cyclizations catalyzed by transition metals and their complexes are reviewed. The catalytic cyclizations discussed here include various carbocyclizations, for example, cycloisomerization, cycloaddition, reductive cyclization, and so on cascade carbocyclizations, for example, cyclotrimerization, silylcarbocyclization, and Heck reaction carbonylative carbocyclizations cyclohy-drocarbonylations intramolecular hydrosilylations intramolecular silylformyla-tions and aldol cyclizations. These reactions serve as efficient and useful methods for the syntheses of a variety of heterocycles and carbocycles that are important... 

Similar to the results discussed for the silylcarbocyclizations of carbon-carbon multiple bonds, reductive cyclizations in the presence of carbonyl compounds are readily achieved. Crowe has developed a titanium-catalyzed procedure for the intramolecular reductive coupling of i5, -unsaturated carbonyl compounds in the presence of triethoxysilane (eq 18).The electronic advantage of triethoxysilane is demonstrated by the lack of reductive coupling in the presence of less reactive silanes, such as triethylsilane and diphenylsilane. With this method, Mori has utilized nickel(O) catalysts to generate five- and six-membered carbocycles and pyrrolidine derivatives. Furthermore, coordination of a chiral phosphine ligand to the nickel catalyst renders the reaction moderately enantioselective. ... 

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