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

Abstract

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.

Evidence that Additions of Grignard Reagents to Aliphatic Aldehydes Do Not Involve Single-Electron-Transfer Processes

“…Our approach to discovering whether radical intermediates were present during the carbon–carbon bond-forming step of the addition of an allylmagnesium reagent to a nonaromatic aldehyde involved the use of a radical clock. The 2,2-diphenylcyclopropylcarbinyl system (Scheme ), one of the fastest radical clocks known, was chosen to maximize the chance of observing products derived from single-electron-transfer reactions. , The 2,2-diphenylcyclopropylcarbinyl radical undergoes ring opening at a rate of 5 × 10 11 s –1 , − and oxygen-containing substituents have been shown to have little effect on this rate. , Consequently, if a radical intermediate were formed, it should undergo ring opening at a rate that is competitive with the rate of geminate radical pairs undergoing recombination. , Cyclopropane-derived radical clocks have been used to provide evidence for radical intermediates. , These radical clocks have been used to identify ketyl radicals as intermediates in reactions of reagents such as SmI 2 and tributyltin hydride with aldehydes, ,, ketones, − esters, − amides, and carboxylic acids …”

Synthetic methods : Part (iii) Heteroatom chemistry

“…A study of the reaction mechanism between silenes and aldehydes was carried out by Baines (Scheme 34). 83 It was discovered that the mechanism proceeds through either a zwitterionic or radical intermediate, depending on the polarity of the SiQC bond. More polarised Couret silenes 52 proceeded through a zwitterionic pathway.…”

Addition of Aldehydes to Germenes: The Influence of Solvent

“…We have utilized trans -(2-phenylcyclopropyl)carboxaldehyde ( 2 ) as a probe to examine the nature of any reactive intermediates that may form during the cycloaddition of aldehydes to a variety of group 14 (di)metallenes. − The probe is able to distinguish between biradical and cationic intermediates. Rapid and regioselective ring opening occurs upon the formation of an α-cyclopropyl radical ( 2b ); in contrast, the analogous α-cyclopropyl cation ( 2a ) does not rearrange (Scheme ).…”

“…Using this approach, we have found that the mechanistic pathway is highly dependent on the nature of the (di)metallene. For silenes, the addition of aldehydes to the naturally polarized neopentylsilene Mes 2 SiCHCH 2 t -Bu proceeds via a zwitterionic intermediate . In contrast, the addition of aldehydes to the relatively nonpolar Brook silene (Me 3 Si) 2 SiC(OSiMe 3 )R, the nonpolar disilene Mes 2 SiSiMes 2 , , or the relatively nonpolar germasilene Mes 2 GeSiMes 2 , proceeds by way of a biradical intermediate.…”