Silylcarbocyclization cascade

Cascade Silylcarbocyclization of Enediynes, Triynes (SiCaT), and Diynals... 

Cascade silylcarbocyclization reactions tiave been developed based on the fact that it is possible to realize successive intramolecular carbocyclizations, as long as the competing reductive elimination is slower than the carbometallation. For example, the reaction of dodec-6-ene-l,ll-diyne 67 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at 50°C under 1 atm CO gives bis(exo-methylenecyclopentyl) 68 in 55% yield [44]. The reaction is stereo-specific that is, (6 )- and (6Z)-dodec-6-ene-l,ll-diynes, ( )-67 and (Z)-67, afford R, R )-68 and (S, R -68 respectively. A possible mechanism for this reaction is outlined in Scheme 7.20. It should be noted that none of the tricyclic product is formed even though a third carbocyclization in the intermediate III.2c is conceptually possible. 

The cascade silylcarbocyclization is applicable to diynals. For example, the reaction of undec-l,6-diyn-ll-al 72 with PhMe2SiH, catalyzed by Rh(acac)(CO)2 affords the corresponding silylated exo-silylmefhylene(hydroxy)bis(cyclopentylidene) 73 (Eq. 20) [48]. 

Cascade silylcarbocyclization of (E)- or (Z)-dodec-6-ene-l,ll-diyne (355) with HSiMe2Ph catalyzed by Rh(acac)(CO)2 gives the bis(exo-methylenecyclopentyl) (356) with complete stereospecificity (equation 144)342. The reaction of dodec-l,6,ll-triyne (357) under the same conditions affords a 3 1 mixture of 358 and 359 in 75% yield... 

The characteristic feature in the mechanism of cyclization reactions is the reductive cascade ring closure. The mechanisms of silylcarbocyclizations (SiCaC, SiCaT,f CO-SiCaC, = CO-SiCaB, and CO-SiCaT reductive... 

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... 

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. "" ... 

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