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Flash photolysis of (η 3 -N-diene)W(CO) 4 in chlorobenzene (CB) solution gives rise to an intermediate in which the rapidly solvated, coordinatively unsaturated site is cis to the coordinated internal olefin. This intermediate reacts via CB displacement to form an (η 4 -N-diene)W(CO) 4 complex in which both the π-bonds are coordinated to the metal. In triethylsilane solution E- and Z-hydrosilylation products arising from regiospecific 1,2-addition to the diene are isolated. An agostic interaction between the Si-H bond of the silane and the coordinatively unsaturated metal is shown to be consistent with the results

The radical anions of a series of para -silyl substituted N,N -dimethylanilines; p 0SiH n Me 3− n C 6 H 4 NMe 2 ( n = 0−3) have been studied by electron spin resonance spectroscopy. When n = 0 or 1 the primary radicals may be generated by either electrolytic or metal reduction. For n = 2 or 3 reduction with potassium immediately gives secondary paramagnetic species resulting from loss of hydride from silicon. Primary radicals for the n = 2 or 3 compounds may be obtained electrolytically. The ESR spectra show increasing spin delocalization into the silyl group as n increases, to the point that the spin distribution in the p -SiH 3 compound approaches that of p -nitro- N,N -dimethylaniline radical anion. A tentative analysis of the secondary and tertiary spectra is given. The results, coupled with CNDO/2 calculations for the radical anions, indicate that the primary interaction between silicon and the aromatic system involves the σ * levels of the silyl substituents and the π * orbitals of N,N -dimethylaniline.

The fragmentation characteristics of the trimethyl-Group IV cations have been studied using mass analyzed ion kinetic energy (MIKE), V 1 2 / E (linked) metastable and consecutive reaction monitoring techniques. Both unimolecular (low energy) and collision induced (high energy) decompositions were studied. These methods give information on the ion of interest without interfering contributions from the precursor ion of the trimethyl cation. The results have been interpreted in terms of the decreasing metal—carbon bond strength and the increasing preference for the +2 oxidation state going down Group IV from carbon to lead.

Minimum energy geometries for silirane, 1; 1,1-difluoro-, 1; 1;1-difluorotetramethyl-, 3; 1,1-dimethyl-, 4; hexamethylsilirane, 5; 7-siladispiro[2.0.2.1]heptane, 6; and its 7,7-dimethyl derivative, 7, have been calculated using the CNDO/2 approximation. Good agreement has been obtained between the predicted structure for 7 and an experimental structure for a related compound. The calculations indicate that the enhanced stability observed for the spirocyclopropyl compounds such as 6 or 7 is due for the most part to increased overlap in the σ framework of the silirane ring resulting from increased contributions of the s orbitals of the spiro carbon atoms to the bonding. Hyperconjugation between filled Walsh orbitals of the spirocyclopropyl rings and acceptor orbitals on silicon does not contribute as much to the stabilization of 6 and 7 as does the increased σ overlap. The calculations predict that the 1,1-difluorosiliranes, 2 and 3, should have thermodynamic stability comparable to or greater than the other siliranes, but much higher reactivity due to charge separation.

Six bis(silyl)acetylenes (XMe2SiCCSiMe2X) with the following varied silicon substituents X were prepared: 1 (Me, Me); 2 (H, H); 3 (C1, H); 4 (CI, CI); 5 (MeO, H); 6 (MeO, MeO). While 1 and 2 may be prepared by the reaction of dilithio- or bis(bromomagnesium)-acetylide with the appropriate chlorosilane, similar reactions designed to give 3–6 yielded oligomers, XMe2Si(CCSiMe2)nX, 7, X=CI, MeO, as the major products, indicating that the acetylenic functionality on silicon activates the chlorosilane towards nucleophilic substitution. Compounds 3 and 4 were prepared by free radical chlorination of 2. Methanolysis of 3 and 4 gave quantitative yields of 5 and 6 respectively. Compounds 1–6 undergo a Diels–Alder reaction with α-pyrone to produce, after loss of carbon dioxide, bis(silyl)-substituted benzene derivatives. The order of reactivity has been determined to be: 4=6>3=5>1>2, indicating that chloro or alkoxy substituents favor the cycloaddition with 2- pyrone. The adducts formed by compounds 3–6 undergo an unusually facile hydrolysis or elimination to give 1,1,3,3-tetramethyl-1,3-disila-2-oxaindane.

The regioselectivity of the hydroboration of the methylchlorovinylsilanes, Cl n Me3− n SiCH CH2 ( n = 0 − 3), has been investigated using BH3←THF, 9-BBN, disiamyl- and dicyclohexylborane. Methylation of the adducts with methylmagnesium bromide is complicated by formation of tetraalkylboronates. Alkaline hydrogen peroxide oxidation of the boronates gives reasonable yields of the corresponding α- and β-trimethylsilylethanols for n = 0 and 1. For n = 2 and 3, conversion of the adducts to the corresponding α- and β- deuteroethylsilanes by treatment with excess sodium methoxide in methanol- 0 - d provides a more effective means of derivatization. Addition of the alkenes, n = 2 or 3, to excess BH3←THF gives ca . 90% of the α-boro-organo-chlorosilanes. For all of the alkenes, the dialkylboranes give ca . 80% of the β-boron adducts.

The destructive distillation or pyrolysis of borane 9-borabicyclo[3.3.1]nonane, or dicyclohexylborane adducts to vinylsilanes which have been treated with sodium methoxide in methanol gives significant yields of products resulting from protonolysis of the CB bond. Both α- and β-boro adducts undergo the reaction, which is favored when silicon has methoxy substituents. Quantitative incorporation of deuterium in the saturated products when methanol-O-d is used as the solvent shows that the hydroxyl group of methanol is the proton source. The absence of isomerization of the alkyl groups attached to silicon in the protonolysis of the adducts to propenyltrichlorosilane or isopropenyltrichlorosilane indicates that cyclic intermediates or free carbanions are not involved in the reaction.

The relative stabilities of the cation radicals of several Group IV substituted anilines have been determined by cyclic voltammetry. They are p -tert-butyl- > m -trimethylsilyl- > p -trimethylsilyl- > p -trimethylgermyl- ⪢ N,N -dimethylaniline radical cation. The order of stability parallels that of the compounds towards protodemetalation. CNDO/2 calculations of the charge distribution in the ground states and cation radicals of the molecules resolve the apparent, anomaly in the rates of protodesilylation of m - and p -trimethylsilyl- N,N -dimethylaniline. A mechanism for the chemical reactions of the radical cations which is consistent with the experimental and theoretical results is proposed. It involves nucleophilic displacement of the metal from the cation radical prior to the coupling of the radicals. The relative stabilities of the radical cations of the silicon compounds are in the same order as the amount of π character of the SiC bond in the radical cation. The CNDO/2 calculations indicate that this π character is due to both ( p-p )π and ( p-d )π interactions in the neutral molecules and in the radical cations.

Minimum energy geometries for silacyclopropene, 1,1-difluoro-,1,1-dimethyl,2,3-dimethy1-, and tetramethylsilacyclopropene have been calculated using the CND0/2 approximation. The calculations predict a remarkably small ring CSiC bond angle of about 43.6°. The average ring CC bond length is predicted to be 1.380 A, slightly shorterthan the CC bond length in aromatic rings. Substantial stabilization of the compounds arises from π interaction between the CC double bond and acceptor orbitals on silicon giving significant aromatic character to the molecules.

When E - or Z -1-phenyl-1-methyl-2-neopentylsilene is generated by retro Diels-Alder flash vacuum thermolysis of their corresponding cyclopentadiene or anthracene adducts at temperatures between 400 and 650°C and trapped with 2,3-dimethyl-1,3-butadiene, the stereochemical distribution of the products is independent of the stereochemistry of the silene precursor indicating that the silene is not configurationally stable towards cis-trans isomerization at these temperatures. Evidence that intermolecular “ene” reaction and the [4 + 2] cycloaddition which occur between silenes and the diene are both concerted reactions is presented.

Summary The reaction of t-butyllithium with phenylmethylvinylchlorosilane at low temperature in hexane solvent gave a 48% yield of a mixture of the five isomers of the 1,3-diphenyl-1,3-dimethyl-2,4-dineopentyl-1,3-disilacyclobutanes, along with an acyclic isomer. The separation and determination of the stereochemistry of the diasteromeric 1,3-disilacyclobutanes by means of carbon-13 NMR is described.

The VUV spectra of methylvinylsilane and propylene are reported and corresponding transitions are assigned. The small red shift of the π→π* transition and the large blue shift of the σ→σ* excitation of the methyl group in methylvinylsilane may be attributed to the +I inductive effect of silicon without involving interaction between d orbitals and the π system. No evidence for a second allowed π→π transition is found in methylvinylsilane.

Etude de la pyrolyse du (E)-methyl-2 phenyl-2 neopentyl-3 dibenzo [5,6:7,8] sila-2bicyclo [2.2.2] octane en presence de methoxy trimethyl silane

Publisher Summary This chapter discusses electron transfer processes, properties of anion radicals, and alkenyl and alkynyl derivatives. Phenyllithium, n -butyllithium, or benzyllithium reacts with sterically hindered olefins––such as 1,2,3,4-tetraphenyl-butadiene or tetraphenylethylene––to form olefin radical anions. Some interesting electron-transfer reactions of sodium naphthalide have recently been reported. The trimethylsilyl radical produced either rapidly dimerizes or reacts with solvent so that very clean electron spin resonance (ESR) spectra of the radical anion––with minimum interactions with the counterion––can be obtained. Alkenyl and alkynyl derivatives of the Group IV elements react readily with alkali metals at low temperatures in donor-type solvents to give radical anions. Triphenyl silane and triphenyl-(1-propenyl)silane undergo coupling at the carbons β to silicon, whereas trimethyl(trans-β-styryl)silane couples at the carbons α to silicon, consistent with the anticipated relative stabilization of the intermediate radical anions. Aryl group IV derivatives are readily reduced in donor solvents, using either alkali metals or electrochemical reduction. The electron-exchange phenomenon is not limited to organometallic radical anions. As methyl groups on silicon are replaced by hydrogen, the electron-withdrawing effect of the silyl substituent increases to the extent that the spin distribution in the p -silyl- N , N -dimethylaniline radical anion approaches that for the p-nitro- N , N -dimethylaniline radical anion.

The reaction of two equivalents of vinyldimethylethoxysilane or vinyldimethylmethoxysilane with hydrocarbon soluble alkyllithium reagents; t -BuLi, s -BuLi, or n -BuLi; in hexane at low temperature gives high yields of 1,1-dimethyl-2-alkyl-4-(dimethylalkoxysilyl)silacyclobutanes. With methyl- or phenyllithium substituted vinylsilanes are obtained. The stereochemistry of the silacyclobutanes is assigned on the basis of Si-29 and H-1 NMR. For vinyldimethylethoxysilane the ratio of cis to trans silacyclobutane is about 57 43 , and is independent of the alkyllithium reagent used. In the reaction of vinyldimethylmethoxysilane with t -BuLi a sol70 30 ratio of the cis and trans silacyclobutane is obtained. A pathway is proposed which is consistent with the stereochemical results and with the products isolated in the reaction run in THF.