Silicon-carbon bond reactivity

Computational investigations of vinylsilanes indicate that there is a groimd-state interaction between the alkene n oibital and the carbon-silicon bond which raises the energy of the n HOMO and enhances reactivity. Furthermore, this stereoelectronic interaction favors attack of the electrophile anti to the silyl substituent. 

Alkylsilanes are not very nucleophilic because there are no high-energy electrons in the sp3-sp3 carbon-silicon bond. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond that is followed by desilylation. Attack on alkenylsilanes takes place at the a-carbon and results in overall replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y-carbon and results in loss of the silicon substituent and an allylic shift of the double bond. 

Allylsilanes have been found to be more reactive than vinyl silanes toward electrophiles and they are being studied more intensively in recent years because they hold considerable promise in organic synthesis. In allylsilanes, the geometry of carbon-silicon bond can be more favourably be oriented for efficient stabilization of a developing positive change P to silicon. 

To develop new methods for organic synthesis, Woerpel and coworkers exploited the inherent reactivity of di -fc/ f-butylsilacyclopropanes to create new carbon-carbon bonds in a stereoselective fashion (Scheme 7.7).62 They discovered that transition metal salts catalyze the insertion of carbonyl compounds into the strained carbon-silicon bond to form oxasilacyclopentanes. The regioselectivity of insertion could be controlled by the identity of the catalyst. Copper promoted the insertion of croto-naldehyde into the more substituted C-Si bond of 52 to afford oxasilacyclopentane 53,63 whereas zinc catalyzed the insertion of butyraldehyde into the less substituted bond of 52 to provide the complementary product, 54.64 Oxasilacyclopentanes (e.g., 55) could be transformed into useful synthetic intermediates through oxidation of the C-Si bond,65 66 which provided diol 56 with three contiguous stereocenters. 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

The relative reactivities of organometallics containing group 14 elements are closely associated with the electronegativity and dissociation energy of the M—C bonds of the elements. For example, some organotin compounds such as allylstannanes are easily decomposed to form radical species whereas homolysis of the carbon-silicon bond of allylsilanes is rarely achieved, which reflects the dissociation energy of the respective M—C bonds. 

The carbon-silicon bond is also more reactive toward halogenation than is the carbon-carbon bond. This has been demonstrated by attempts to brominate or iodinate trimethyl-phenylsilane (Scheme V). 

Although the reactivity with different carbon electrophiles has been widely documented, there are not many examples of synthetically useful carbon-sulfur bond formation at the thiazole carbons. For instance, the regio- and chemoselective cleavage of the carbon-silicon bond of 2-TST (1) by trifluoromethyl-sulfenyl chloride was reported to take place at —78 °C. The expected 2-(trifluoromethylthio)thiazole was obtained with satisfactory 3uelds (eq 27). ... 

Propargylic trimethylsilanes, now readily accessible by reaction of lithium acetylides with (trimethylsilyl)methyl halides or triflate, are emerging as useful synthetic intermediates. The general pattern of reactivity involves attack by an electrophile with cleavage of the carbon-silicon bond. For example, treatment of propargylic trimethylsilanes with TFA or bromine leads to allenes or 3-bromoallenes respectively, " and reactions with acetals furnish 4-alkoxy-allenes" (e.g. Scheme 101). 

The carbon-silicon bond to saturated alkyl groups is not very reactive. Most of the valuable synthetic procedures based on organosilanes involve either alkenyl or allylic silicon substituents. The dominant reactivity pattern involves attack by an electrophilic carbon intermediate at the double bond. 

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