Comparative Study of the Reactivity of Brook and Couret Silenes:  Aldehyde Addition

The addition of the mechanistic probe trans-2-phenylcyclopropanecarbaldehyde (10) to the naturally polarized Couret silene Mes2SiC(H)(CH2 -t-Bu) (2) and the relatively nonpolar Brook silenes (Me3Si)2SiC(R)(OSiMe3), where R = t-Bu (1a), 1-Ad (1b), respectively, was examined. The addition of aldehyde 10 to silene 2 produces vinylsilane 17, siloxetanes 18a−c, and ester 19. Upon chromatography, siloxetanes 18a−c were found to undergo conversion to the alkenes trans-20 and cis-20 and (Mes2SiO)2. The formation of siloxetanes 18a−c and ester 19 is consistent with the formation of a 1,4-zwitterionic intermediate during the course of the addition of the aldehyde to silene 2. The addition of the cyclopropyl aldehyde 10 to Brook silenes produces the cis,trans-dienes 23 and 26, siloxetanes 24a,b and 27a,b, and acylsilanes 25a,b and 28a,b derived from silenes 1a and 1b, respectively. The formation of the cis,trans-dienes and the siloxetanes provides compelling evidence for the formation of an α-cyclopropylcarbinyl radical during the course of the addition of the aldehyde to the Brook silenes. This work provides a unique comparison of the reactivities of the naturally polarized Couret silene 2 and the relatively nonpolar Brook silenes 1a,b toward aldehydes. It is evident from the results that the polarity of the SiC bond of the silene has a profound influence on the mechanism of the addition of aldehydes to silenes.