Kinetic Studies of the Reactions of Si:C and Si:Si Bonds

Abstract

Physical chemistryPhysical chemistry Z 0225 Kinetic Studies of the Reactions of Si:C and Si:Si Bonds -[164 refs.]. -(MORKIN, T. L.; OWENS, T. R.; LEIGH, W. J.; Chem. Org. Silicon Compd. 3 (2001) 949-1026; Dep. Chem., McMaster Univ., Hamilton, Ont. L8S 4M1, Can.; Eng.) -Lindner 19-273

Kinetics and Mechanism of the Addition of Aliphatic Amines to Transient Silenes

“…The nonlinear variations in log k amine with T -1 result from the superposition of a positive temperature dependence of k C on a negative temperature dependence of the complex partitioning ratio, k H /(k H + k -C ), which occurs because the reaction is strongly exothermic and the rate constant for the product-forming step (k H ) is dominated by entropic factors. 9 The temperature dependence of k amine , as defined by this mechanism, is given explicitly by eq 6, where A C and E C are the Arrhenius preexponential factor and activation energy for formation of the intermediate complex, A -C and A H are the preexponential factors for reversion of the complex to starting materials and collapse to product, respectively, and ∆E -C,H () E -C -E H ) is the difference in activation energies for the two processes. Integration of the logarithmic form of this expression with respect to T -1 leads to the prediction that k amine goes through a maximum value (i.e., E a overall ) 0) at the temperature where the partitioning ratio is equal to the quantity (1 -E C /∆E -C,H ).…”

“…18 This indicates that the Hammett F-value for amine addition to 1,1-diarylsilenes is small but positive, consistent with a reaction mechanism analogous to that for oxygen-centered nucleophiles. 9 This mechanism is shown in eq 4, while eq 5 gives the expression for the overall rate constant for reaction (k amine ), derived using the steady state assumption for the intermediate complex.…”

“…9 The 1,1-diarylsilene derivatives 1a-e are examples of silenes that show such behavior and have proven extremely useful in delineating the quantitative aspects of silene reactivity toward alcohols 18,19 and various other oxygen-centered nucleophiles. 9 The reaction of silenes with nitrogen-centered nucleophiles is also well-known, 1,3,8 but no detailed absolute kinetic studies have yet been reported. 7 More than 15 years ago, Wiberg and co-workers published the results of competition experiments on the addition of a comprehensive series of reagents to silene 2 and showed that primary amines are slightly more reactive than aliphatic alcohols toward addition to the SidC bond of this compound.…”

“…Silenes react regiospecifically with nucleophiles such as water, alcohols, carboxylic acids, alkoxysilanes, and amines by (1,2)-addition across the MdC bond. [1][2][3][4][5][6][7][8][9] The mechanism of alcohol addition has been extensively studied and is particularly well-understood (eq 1). 10,11 It is initiated by reversible nucleophilic attack at silicon to form a zwitterionic complex, 1,12 which proceeds to product by two competing, rate-limiting proton transfer pathwayssone intramolecular and one involving a second molecule of alcohol (see eq 1).…”

“…14,16,17 However, most of the transient silenes that have been studied to date are sufficiently reactive toward alcohols and water that only the second-order reaction pathway can be detected by these methods. 9 The 1,1-diarylsilene derivatives 1a-e are examples of silenes that show such behavior and have proven extremely useful in delineating the quantitative aspects of silene reactivity toward alcohols 18,19 and various other oxygen-centered nucleophiles. 9 The reaction of silenes with nitrogen-centered nucleophiles is also well-known, 1,3,8 but no detailed absolute kinetic studies have yet been reported.…”