Silylenes theoretical studies

We theoretically studied the reactions of stable West silylenes 32 and 73 with phosphorus ylide H2C=PMe3.74 Similarly to the simplest analogs of carbenes, these compounds can form betaines in which the negative charge is localized on the silicon atom and the positive charge is localized on the phosphorus atom. These betaines can thermally decompose to form silenes (direction A, Scheme 39) or be isomerized to ylides via direction B. 

II. Theoretical Studies of Silylene-Lewis Base Complexes. 2... 

THEORETICAL STUDIES OF SILYLENE-LEWIS BASE COMPLEXES... 

Only few theoretical studies have been devoted exclusively to the coordination of Lewis bases to silylenes. Most calculational evidence for the formation of silylene-Lewis base complexes was obtained from the computational investigation of the insertion reaction of HiSi into various H-X a bonds, where X is an heteroatom center possessing one or more free... 

Theoretical Studies of Silylene-Lewis Base Complexes. 

As mentioned above, the cyclic silylenes 75c and also 76c (or 79a and 79b) are dramatically less reactive than other known silylenes which are all transients. Thus, 75c has such a low Lewis acidity that, unlike other silylenes, it does not react as an electrophile. However, 75c reacts as a Lewis base, i.e. as a nucleophilic reagent980. The known reactions of Arduengo-type silylenes are summarized in References 2e, 98c, 98d and 99b and the references cited therein. The theoretical study of the possible reactions of Arduengo-type silylenes is quite limited and awaits the attention of computational chemists. 

For a theoretical study of amine-silylene donor-acceptor bonding, see R. T. Conlin, D. Laasko and P. Marshall, Organometallics, 13, 838 (1994). 

The nature of substituent effects on the position of absorption maxima corresponding to n — p transition in polyatomic silylenes have been considered in detail in theoretical studies of Apeloig and coworkers126 128. Unfortunately, there is no similar study for germylenes, stannylenes or plumbylenes. However, the available experimental data show that the main conclusions obtained by Apeloig and coworkers are also applicable to polyatomic germylenes, stannylenes and plumbylenes. 

Quantum-chemical calculations have been used to probe all the characteristic chemical reactions of CAs (at least in the case of silylenes and germylenes). The theoretical studies cover intramolecular rearrangements, insertions into 0-bonds, additions to double and triple bonds and dimerizations. Note that experimental data on the mechanisms of these reactions are still scarce and the results of theoretical studies are needed to understand the main trends in the reactivity of germylenes, stannylenes and plumbylenes. 

Figure 3.18 Theoretical study of ethylene insertion into the Zr-C bond in the silylene-biscyclopentadienyl(methyl)zirconium cation [H2Si(Cp)2Zr—Me]+. E — energy...

We have shown in this paper that molecular orbital calculations at the ab initio level can be used to predict reliably the spectral transitions in silylenes, to evaluate the effects of substituents on the Si=Si multiple bond, to shed new light on existing experimental data and to direct future work towards the synthesis of novel isomers of disilenes. Although carbon and silicon are isoelectronic, multiple bonds to silicon differ dramatically from multiple bonds to carbon and analogies from carbon chemistry might therefore be entirely misleading when applied to silicon compounds. We believe that our studies have demonstrated the enormous power of modem computational methods and hope that this paper will prompt future theoretical studies and more importantly, theoretical-experimental collaborations in the field of organosilicon chemistry. 

It has been theoretically and experimentally well established that silylenes have a singlet ground state [1]. Such species posses a free electron pair in a o-orbital and an empty orbital of Jt-symmetry therefore, they are a priori ambiphilic compounds, which can react either as an electrophile or as a nucleophile towards appropriate substrates. However, most silylenes have revealed a distinctive "electrophilic character". Dimethylsilyene, e g., adds to olefins and alkynes in the gas phase via a rate-controlling step that is accelerated by electron-donating substituents [2] these experimental results are in good agreement with a theoretical study of the reaction of SiH2 with ethylene, which shows that this cycloaddition proceeds via an initial electrophilic phase in which the silylene LUMO interacts with the 7t-electron system of the double bond [3]. Up to now, only some stable silylenes, such as recently described 1 [4] or silicocene 2 [5] have shown nucleophilic reactivity. 

A theoretical study of the intermediates involved in the formation of phospha-propyne from pyrolysis of vinylphosphirane has led to a new route to phospha-alkynes. Thus, pyrolysis of trimethylsilyl(l-phosphiranyl)diazomethane has yielded MeaSiC = P, via an intermediate 1-phosphiranylmethylene . Regioselec-tivity in the [3 + 2] cycloaddition reaction between phosphaethyne and diazomethane has been studied by theoretical techniques , and further examples of reactions of this type described . Cycloaddition of phospha-alkynes with silylenes has also been reported. The primary phosphine 324 has been isolated from the addition of diethylphosphite to t-butylphosphaethyne. The chemistry of phospha-alkyne cyclotetramer systems has been reviewed and the first examples of platinum(II) complexes of such cage systems described. Aspects of the reactivity of coordinated phospha-alkynes have received further study, and a remarkable metal-mediated double reduction of t-butylphosphaethyne to the complexed fluorophosphine 325 described Phosphorus-carbon-aluminium cage structures have been isolated from the reactions of kinetically stable phospha-alkynes with trialkylaluminium compounds and new phosphaborane systems have been obtained from the reactions of phospha-alkynes with polyhedral boranes . Further studies of wo-phospha-alkyne coordination chemistry have appeared . The reactivity of the ion 326 has been explored. ... 

There are examples of electron-deficient divalent metal derivatives called heavy analogs of carbene with the general formula R2M , including silylenes, "" germylenes, "" " stannylenes, "" "" and plumbylenes. When compared to carbenes, however, these compounds exhibit a significantly decreased reactivity in C-H insertion reactions. Theoretical studies have indicated that the heavier the metal, the larger the activation barrier and the less exothermic the reaction. Electropositive substituents, however, may increase reactivities. " ... 

Silylenes have attracted strong interest of both experimentalists and theoreticians and their chemistry has been reviewed1 3,262. The theoretical studies of silylenes have been reviewed most recently by Gordon and coworkers22. 

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