Substituent Effects on the Reactivity of the Silicon−Carbon Double Bond. Mechanistic Studies of the Ene-Addition of Acetone to Reactive Arylsilenes

Bradaric, Christine J.; Leigh, William J.

doi:10.1021/om970875v

Cited by 30 publications

(62 citation statements)

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“…Silacyclobutanes are well known to yield products consistent with the formation of silene-reactive intermediates upon photolysis in solution or the gas phase, or upon pyrolysis at high temperatures (1)(2)(3)(4)(5)(6)(7)(8). These compounds have proven to be tremendously useful as photochemical precursors for transient silenes, in experiments aimed at detecting the silenes directly and studying the kinetics and mechanisms of their reactions with alcohols and other "silenophiles" (9)(10)(11)(12)(13)(14)(15)(16)(17). Photochemical silene formation proceeds with useful efficiencies from silacyclobutanes that bear alkyl, vinyl, ethynyl, silyl, and phenyl substituents at silicon, and possess UV absorption maxima between 190 and 250 nm (2,4,12,13,(17)(18)(19)(20).…”

Section: Introductionmentioning

confidence: 99%

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

Leigh¹,

Boukherroub²,

Bradaric³

et al. 1999

Can. J. Chem.

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Direct photolysis of 1-phenylsilacyclobutane and 1-phenyl-, 1-(2-phenylethynyl)-, and 1-(4'-biphenylyl)-1-methylsilacyclobutane in hexane solution leads to the formation of ethylene and the corresponding 1-arylsilenes, which have been trapped by photolysis in the presence of methanol. Quantum yields for photolysis of the three methyl-substituted compounds have been determined to be 0.04, 0.26, and 0.29, respectively, using the photolysis of 1,1-diphenylsilacyclobutane Φsilene = 0.21) as the actinometer. The corresponding silenes have been detected by laser flash photolysis; they have lifetimes of several microseconds, exhibit UV absorption maxima ranging from 315 to 330 nm, and react with methanol with rate constants on the order of (2-5) × 109 M-1 s-1 in hexane. Absolute rate constants for reaction of 1-phenylsilene and 1-methyl-1-phenylsilene with water, methanol, tert-butanol, and acetic acid in acetonitrile solution have been determined, and are compared to those of 1,1-diphenylsilene under the same conditions. With the phenylethynyl- and biphenyl-substituted methylsilacyclobutanes, the triplet states can also be detected by laser flash photolysis, and are shown to not be involved in silene formation on the basis of triplet sensitization and (or) quenching experiments. Fluorescence emission spectra and singlet lifetimes have been determined for the three 1-aryl-1-methylsilacyclobutanes, 1,1-diphenylsilacyclobutane, and a series of acyclic arylmethylsilane model compounds. These data, along with the reaction quantum yields, allow estimates to be made of the rate constants for the excited singlet state reaction responsible for silene formation. 1-Methyl-1-phenylsilacyclobutane undergoes reaction from its lowest excited singlet state with a rate constant 10-80 times lower than those of the other three derivatives. The results are consistent with a stepwise mechanism for silene formation, involving a 1,4-biradicaloid intermediate that partitions between product and starting material.Key words: silene, silacyclobutane, photochemistry, biradical.

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Section: Introductionmentioning

confidence: 99%

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

Leigh¹,

Boukherroub²,

Bradaric³

et al. 1999

Can. J. Chem.

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“…18,20,21 The two silenes exhibit lifetimes of >5 µs in hexane and 1-4 µs in acetonitrile under these conditions and decay with mixed firstand second-order kinetics, with 1a being the longer-lived of the two. Much different behavior is observed in THF solution, where the lifetimes of the two silenes are longer, 1b is the longer-lived of the two, and their spectra are broadened and/or red-shifted as compared to hexane solution because of complexation with the ether solvent.…”

Section: Resultsmentioning

confidence: 96%

“…This behavior is also analogous to what has been observed previously for the addition of oxygen-centered nucleophiles to these two silenes. 15,18,21 In the case of oxygen-centered nucleophiles, we have recently shown that these trends are due, at least to some extent, to the effects of weak complexation of the free silene…”

Section: Discussionmentioning

confidence: 99%

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

Leigh

2003

J. Phys. Chem. A

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Primary and secondary amines such as n- butyl-, t-butyl-, and N,N-diethylamine add across the SidC bond of transient silenes such as 1,1-diphenylsilene (1a) and 1,1-bis(4-trifluoromethylphenyl)silene (1b) to yield the corresponding amino(methyl)diarylsilanes as the only products of reaction. The kinetics and mechanism of reaction of these three amines with the two 1,1-diarylsilene derivatives have been studied in hexane, acetonitrile, and tetrahydrofuran (THF) solution by laser flash photolysis techniques, using the corresponding 1,1-diarylsilacyclobutanes as photochemical precursors to the silenes. The reactions proceed with clean secondorder kinetics and bimolecular rate constants in excess of 5 × 10 8 M -1 s -1 in hexane and MeCN, with 1b being up to four times more reactive than 1a. Arrhenius plots for reaction in hexane and/or acetonitrile solution show strong curvature over the 0-60°C temperature range, consistent with an addition mechanism involving the intermediacy of a zwitterionic silene-amine addition complex, which collapses to product by intramolecular proton transfer from nitrogen to the silenic carbon. The reactions are substantially slower in THF, where rate reductions on the order of 5-10-fold and 50-70-fold are observed for 1a and 1b, respectively, as compared to MeCN solution. This is due to the effects of complexation of the silenes with the ether solvent, the equilibrium constant for which enters the expression for the reaction rate constant in the complexing solvent. In contrast to the situation in hexane and MeCN, Arrhenius plots for reaction of n-BuNH 2 with 1a,b in THF solution are linear and lead to positive activation energies. Addition of amines in THF solution is proposed to occur predominantly via the free silenes but with minor contributions from a pathway involving nucleophilic displacement of the solvent from the silene-solvent complex.

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“…The 1 H NMR spectrum of the compound exhibited singlets at δ 0.41 (SiCH 3 ), 2.46 (o-tolyl CH 3 ), and 3.41 (OCH 3 ) and resonances assignable to the n-propyl group (δ 0.90 (t, 2H), 0.95 (t, 3H), 1.38 (m, 2H)), and the following mass spectrum: m/z (I) = 208 (4), 193 (2), 165 (100), 151 (12), 135 (33), 119 (12), 105 (18), 91 (11), 59 (20). Both the photolysis and dark reaction mixtures also contained a second minor isomer in the same yield relative to 15 (1:8) which was tentatively identified as benzylmethylmethoxy(1-propyl)silane (16, 21%) on the basis of its mass spectrum: m/z (I) = 208 (17), 165 (15), 151 (10), 133 (12), 117 (90), 91 (30), 75 (100), 59 (30)). The latter clearly shows the fragmentation pattern expected for a benzylsilane.…”

Section: Methodsmentioning

confidence: 99%

“…Silacyclobutanes are well-known photochemical and thermal precursors to transient Si=C containing compounds ("silenes") (1)(2)(3)(4)(5)(6)(7)(8), and are tremendously useful in experiments aimed at detecting these reactive intermediates directly and studying the kinetics and mechanisms of their various characteristic reactions (9)(10)(11)(12)(13)(14)(15)(16)(17). Photochemical silene formation proceeds with useful efficiencies from silacyclobutanes bearing a wide range of substituents at silicon, ranging from compounds in which the silacyclobutane ring is the primary chromophore (e.g., 1,1-dialkylsilacyclobutanes) to those in which the lowest excited state is localized more in the substituent (e.g., 1-arylsilacyclobutanes).…”

Section: Introductionmentioning

confidence: 99%

The one- and two-photon photochemistry of benzylsilacyclobutanes, acyclic benzylsilanes, and 1,1,2-triphenylsilacyclobutane

Leigh¹,

Owens²

2000

Can. J. Chem.

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The photochemistry of several α-silylbenzyl compounds has been investigated in hexane and in methanol solution. Direct photolysis of 1-benzyl-1-methylsilacyclobutane (1) in methanolic hexane solution produces 1-propyl-1-methyl-2,3-benzosilacyclobutene (6) in quantitative yield, by a sequential two-photon process involving the photoactive isotoluene derivative 1-methylene-6-(1-methylsilacyclobutyl)-2,4-cyclohexadiene (13a), which has been identified on the basis of its 1H NMR and UV absorption spectra. In contrast, direct irradiation of 1-benzyl-1-phenylsilacyclobutane (2) under similar conditions results in the formation of a complex mixture of products consistent with the competing formation of 1-benzyl-1-phenylsilene and benzyl- and 1-phenylsilacyclobutyl radicals. The silene is a transient which has been detected directly by laser flash photolysis of 2 (λmax = 315 nm, τ ~ 4.5 µs). Free radical formation is shown to be due to secondary photolysis of a second primary product, 1-methylene-6-(1-phenylsilacyclobutyl)-2,4-cyclohexadiene (13b), which has also been detected and identified by static UV absorption (λmax = 335 nm) and 1H NMR spectroscopy. In a reaction with some analogy to the acid-catalyzed desilylation of allylsilanes, both 13a and 13b can be intercepted in neutral or acidic methanol solution to yield toluene and 1-methyl- or 1-phenyl-1-methoxysilacyclobutane, respectively. Direct photolysis of benzyldimethylphenylsilane (4) also leads to the formation of the corresponding isotoluene derivative, while benzyltrimethylsilane (3) exhibits negligible photoreactivity. The endocyclic benzylsilane 1,1,2-triphenylsilacyclobutane (5) is shown to undergo competing [2 + 2]-cycloreversion and [1,3]-silyl migration to yield a bicyclic isotoluene analogue, which reacts rapidly with methanol to yield the acyclic methoxysilane reported previously to be the main product of photolysis of this silacyclobutane in methanol solution. Relative quantum yields for isotoluene formation from photolysis of 1-4 and absolute rate constants for methanolysis of several of these compounds under neutral and acidic conditions have also been determined.Key words: photochemistry, organosilicon, benzylsilane, silacyclobutane, silene, kinetics, isotoluene.

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Substituent Effects on the Reactivity of the Silicon−Carbon Double Bond. Mechanistic Studies of the Ene-Addition of Acetone to Reactive Arylsilenes

Cited by 30 publications

References 48 publications

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

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

The one- and two-photon photochemistry of benzylsilacyclobutanes, acyclic benzylsilanes, and 1,1,2-triphenylsilacyclobutane

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