A Combined Experimental and Theoretical Study of the Kinetics and Mechanism of the Addition of Alcohols to Electronically Stabilized Silenes:  A New Mechanism for the Addition of Alcohols to the SiC Bond

Abstract

The stabilized silene 1,1-bis(trimethylsilyl)-2-adamantylidenesilane (4) has been generated by photolysis of a novel trisilacyclobutane derivative in various solvents and studied directly by kinetic UV spectrophotometry. Silene 4 decays with second-order kinetics in degassed hexane solution at 23 degrees C (k/epsilon = 8.6 x 10(-6) cm s(-1)) due to head-to-head dimerization. It reacts rapidly with oxygen [k(25 degrees C) approximately 3 x 10(5) M(-1) s(-1)] but approximately 10 orders of magnitude more slowly with methanol (MeOH) than other silenes that have been studied previously. The data are consistent with a mechanism involving reaction with the hydrogen-bonded dimer of the alcohol, (MeOH)(2) (k = 40 +/- 3 M(-1) s(-1); k(H)/k(D) = 1.7 +/- 0.2). The stable analogue of silene 4, 1-tert-butyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilane (5), reacts approximately 50 times more slowly, but via the same mechanism. The mechanism for addition of water and methanol (ROH; R = H, Me) to 4, 5, and the model compound 1,1-bis(silyl)-2,2-dimethylsilene (3a) has been studied computationally at the B3LYP/6-31G(d) and MP2/6-31G(d) levels of theory. Hydrogen-bonded complexes with monomeric and dimeric methanol, in which the Si=C bond plays the role of nucleophile, have been located computationally for all three silenes. Reaction pathways have been characterized for reaction of the three silenes with monomeric and dimeric ROH and reveal significantly lower barriers for reaction with the dimeric form of the alcohol in each case. The calculations indicate that 5 should be approximately 40-fold less reactive toward dimeric MeOH than 4, in excellent agreement with the approximately 50-fold difference in the experimental rate constants for reaction in hexane solution.

Addition Reactions: Polar Addition

“…Reaction pathways have been characterized for the reaction of the three silenes with monomeric and dimeric ROH, which reveal significantly lower barriers for the reaction with the dimeric form of the alcohol in each case. The calculations indicate that t-butyldimethylsilyl-1-trimethylsilyl-2-adamantylidenesilane should be ∼40-fold less reactive toward dimeric MeOH than 1,1-bis(trimethylsilyl)-2-adamantylidenesilane, in excellent agreement with the ∼50-fold difference in the experimental rate constants for reaction in hexane solution 126. 11 Addition Reactions: Polar Addition…”

“…178 A direct asymmetric reductive Mannich-type reaction that allows for the formation of three contiguous stereocentres with high chemo-, diastereo-, and enantio-selectivity ee (10:1 to 50:1 dr; 96-99% ee) has been presented (Scheme 4). The reaction commences with the formation of the corresponding iminium ion from aldehyde (122) and prolinol de catalyst (125), followed by conjugate reduction with Hantzsch ester (123) to generate an enamine, which then undergoes Mannich reaction with imine (124) to produce (126 Nucleophilic addition of α-halo-4-tolylsulfonyl methyl anions to quinone methides has been reported to afford three kinds of products as a result of domino reactions. Two of them were identified as rearrangement products and one as the vicarious nucleophilic substitution (VNS) product.…”

Theoretical study on the addition reactions of MeOH, PhOH and PhSH to silastannenes

“…In the meantime, inspection of E r of the first or the second transition state for the monomer, dimer and trimer reactions indicates that when n changes from 1 to 2 and then from 2 to 3 the values of E r decrease consecutively. For the process after Lewis adduct the strainless geometries and hydrogen-bond interactions in TS5a(b)-2 and TS6a(b)-2 should be responsible for this result, which is similar to the case in reactions of dimeric hydrogen fluoride with ethylene [22] and reactions of methanol dimer with metallenes [11,15] or dimetallenes [13,16]. The changes of E r mean that if only the energy factor is considered the trimer reactions are more favorable than corresponding monomer and dimer reactions kinetically.…”

“…The utility of DFT-B3LYP [17,18] method has been proved in the computational studies of water or methanol addition to metallenes [11,15] and dimetallenes [13,14,16]. In this article, the molecular geometric structures of reactants (R), complexes (COM), transition states (TS) and products (P) for addition reactions of ROH (R@Me, Ph) and PhSH to silastannenes were fully optimized using B3LYP method.…”

“…The theoretical studies show that the addition reactions of ROH monomer to silenes [5][6][7] or disilenes [6,[8][9][10] start generally by the formation of an intermolecular complex which is converted to the final product through a four-membered transition state. In subsequent studies [11][12][13] it was found that the reaction of silenes or disilenes with the ROH dimer is much more favorable than with the monomer. More recently, Su et al [14][15][16] investigated the mechanisms and potential energy surfaces of water addition to 1,2-dimetallacyclohexene (R 2 E@ER 2 , E = C, Si, Ge, Sn, and Pb) and methanol (monomeric and dimeric) addition to adamantyl-substituted metallenes with a C@E (E@C, Si, Ge, Sn, and Pb) double bond and the fused tricyclic dimetallenes (R 2 E@ER 2 , E@C, Si, Ge, Sn, and Pb), which show that the dimeric methanol is the better addition reagent than the methanol monomer.…”