Abstract:forms adducts 3 . donor, the stability of which increases in the order of donor = E t 2 0 < Br-< THF < NEtMe, < F-, and combines with the reactants a-b (e.g. R-Li; R = H, Me, nBu, Ph) with insertion into the a-b bond, with a=b-c-H [e.g. O=CtBu-CH,-H; CH2=CR-CH2-H, R = Me, CMe=CH,, CH2SiPhzCH(SiMe3),] according to a n ene reaction, with a = b (e.g. CHz=CHOMe; PhzC=NSiMe3) or a = b = c (e.g. tBuzRSiN3, R = Me, tBu) or a=b-c=d [e.g. may be said that going from Me,Si=C(SiMe& to PhzSi=C-(SiMe,), there is only a gra… Show more
“…This trend is continued with 1,1-diphenylsilene, which exhibits slightly lower reactivity than 3b with alcohols under similar conditions . Wiberg and Link have recently reached a similar conclusion from a comparison of the reactivities of the transient silenes Ph 2 SiC(SiMe 3 ) 2 and Me 2 SiC(SiMe 3 ) 2 toward a wide range of silene trapping agents …”
The reactivities of a series of substituted 1-methylsilenes RMeSidCH 2 (R ) H, methyl, ethyl, t-butyl, vinyl, ethynyl, phenyl, trimethylsilyl, and trimethylsilylmethyl) in hydrocarbon solvents have been investigated by far-UV (193-nm) laser flash photolysis techniques, using the corresponding 1-methylsilacyclobutane derivatives as silene precursors. Each of these silacyclobutanes yields ethylene and the corresponding silene, which can be trapped as the alkoxysilane RSiMe 2 OR′ cleanly upon 193-or 214-nm photolysis in solution in the presence of aliphatic alcohols. UV absorption spectra and absolute rate constants for reaction of the silenes with methanol, ethanol, and t-butyl alcohol have been determined in hexane solution at 23°C. The rate constants vary from a low of 3 × 10 7 M -1 s -1 for reaction of 1-methyl-1-trimethylsilylsilene with t-BuOH to a high of 1 × 10 10 M -1 s -1 for reaction of 1-ethynyl-1-methylsilene with MeOH. In several cases, rate constants have been determined for addition of the deuterated alcohols, and for addition of methanol over the 0-55°C range. Invariably, small primary deuterium kinetic isotope effects and negative Arrhenius activation energies are observed. These characteristics are consistent with a mechanism involving reversible formation of a silenealcohol complex which collapses to alkoxysilane by unimolecular proton transfer from oxygen to carbon. Silene reactivity increases with increasing resonance electron-donating and inductive electron-withdrawing ability of the substituents at silicon and is significantly affected by steric effects within this series of compounds. This is suggested to be due to a combination of effects on both the degree of electrophilicity at silicon (affecting the rate constants for formation and reversion of the complex) and nucleophilicity at carbon (affecting the partitioning of the complex between product and free reactants). Two 1-methyl-1-alkoxysilacyclobutanes were also investigated, but proved to be inert to 193-nm photolysis.
“…This trend is continued with 1,1-diphenylsilene, which exhibits slightly lower reactivity than 3b with alcohols under similar conditions . Wiberg and Link have recently reached a similar conclusion from a comparison of the reactivities of the transient silenes Ph 2 SiC(SiMe 3 ) 2 and Me 2 SiC(SiMe 3 ) 2 toward a wide range of silene trapping agents …”
The reactivities of a series of substituted 1-methylsilenes RMeSidCH 2 (R ) H, methyl, ethyl, t-butyl, vinyl, ethynyl, phenyl, trimethylsilyl, and trimethylsilylmethyl) in hydrocarbon solvents have been investigated by far-UV (193-nm) laser flash photolysis techniques, using the corresponding 1-methylsilacyclobutane derivatives as silene precursors. Each of these silacyclobutanes yields ethylene and the corresponding silene, which can be trapped as the alkoxysilane RSiMe 2 OR′ cleanly upon 193-or 214-nm photolysis in solution in the presence of aliphatic alcohols. UV absorption spectra and absolute rate constants for reaction of the silenes with methanol, ethanol, and t-butyl alcohol have been determined in hexane solution at 23°C. The rate constants vary from a low of 3 × 10 7 M -1 s -1 for reaction of 1-methyl-1-trimethylsilylsilene with t-BuOH to a high of 1 × 10 10 M -1 s -1 for reaction of 1-ethynyl-1-methylsilene with MeOH. In several cases, rate constants have been determined for addition of the deuterated alcohols, and for addition of methanol over the 0-55°C range. Invariably, small primary deuterium kinetic isotope effects and negative Arrhenius activation energies are observed. These characteristics are consistent with a mechanism involving reversible formation of a silenealcohol complex which collapses to alkoxysilane by unimolecular proton transfer from oxygen to carbon. Silene reactivity increases with increasing resonance electron-donating and inductive electron-withdrawing ability of the substituents at silicon and is significantly affected by steric effects within this series of compounds. This is suggested to be due to a combination of effects on both the degree of electrophilicity at silicon (affecting the rate constants for formation and reversion of the complex) and nucleophilicity at carbon (affecting the partitioning of the complex between product and free reactants). Two 1-methyl-1-alkoxysilacyclobutanes were also investigated, but proved to be inert to 193-nm photolysis.
“…The stereochemistry of the reaction is consistent with either a concerted mechanism or a stepwise one involving the initial formation of a Lewis acid−base complex in which a high degree of double-bond character persists in the silenic SiC bond, which proceeds to product by 1,3-migration of the trialkylsilyl group from oxygen to carbon (eq 2). A similar mechanism was first proposed by Wiberg for the addition of alcohols and other polar reagents to silene 1 on the basis of relative reactivities, ,, and detailed product and/or kinetic studies have since provided compelling evidence that stepwise mechanisms involving initial nucleophilic attack are common to many of the polar addition reactions that silenes undergo, including addition of alcohols, − carboxylic acids, − ketones, ,, and amines. , Clearly, there is good reason to suspect that the addition of alkoxysilanes should proceed by this mechanism as well. …”
Absolute rate constants have been measured for the reaction of methoxytrimethylsilane with a series of transient, para-substituted 1,1-diphenylsilenes (H 2 CdSi(C 6 H 4 X) 2 , where X ) H, Me, F, Cl, and CF 3 ) in hexane solution at room temperature. The data correlate with Hammett substituent constants, affording the reaction constant F ) +0.9 ( 0.2. For the parent compound and the 4,4′-bis(trifluoromethyl) derivative, rate constants have been determined over the 0-60 °C temperature range in hexane, 1,2-dichloroethane, and acetonitrile solution. The rate constants for reaction of the parent compound increase in the order hexane ≈ 1,2-dichloroethane < MeCN, but are relatively insensitive to solvent in the other case. However, differences are revealed in the Arrhenius activation energies for reaction, which are negative for both compounds in all three solvents. Those for the 4,4′bis(trifluoromethyl) compound in particular change to more positive values with increasing solvent polarity. The data are consistent with a two-step mechanism involving reversible preassociation of the silene and the alkoxysilane to form a Lewis acid-base complex, which collapses to product by intramolecular transfer of trimethylsilyl from oxygen to the silenic carbon.
“…There is a good deal of speculation in the literature regarding the mechanism for the addition of carbonyl compounds to silenes: the intermediacy of both zwitterions and biradicals has been proposed; 10b,− however, little evidence, experimental or theoretical, has been presented.…”
Section: Introductionmentioning
confidence: 99%
“…Wiberg proposed that the addition of benzophenone to the silenes R 2 SiC(SiMe 3 )(SiMeR 1 2 ), where R = Me and R 1 = t -Bu or R = Ph and R 1 = Me, forms the zwitterionic complexes 7a , b as intermediates during the cycloaddition (Chart ). 1h, The proposal is reasonable, given that the THF complex of a related silene has been isolated and characterized by X-ray crystallography ( 8 ·THF; Chart ) . Similarly, the molecular structure of a zwitterionic adduct between benzophenone and a silanimine has been determined ( 9 ; Chart ) …”
The addition of the mechanistic probe trans-2-phenylcyclopropanecarbaldehyde (10) to the naturally
polarized Couret silene Mes2SiC(H)(CH2
-t-Bu) (2) and the relatively nonpolar Brook silenes (Me3Si)2SiC(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 SiC bond of
the silene has a profound influence on the mechanism of the addition of aldehydes to silenes.
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