Silacyclobutene

To see if silacyclobutenes (28) would react in the same manner as the alkynyl-polysilanes (1) and respond similarly to steric differences, compounds 28a and b were heated in the presence of NiCl2(PEt3)2 and PTMSA (Scheme 9). " Compound 28a gave silole 20a in 94% yield,but the only isolable products from the thermolysis of 28b were 26 (51%) and 27b (36%). The products from the thermolysis of silacyclopropene 28b were very similar to that for alkynylsilane lb, but for some reason there were many fewer products for the thermolysis of 28a than for alkynylsilane la. These results suggest that the more sterically hindered lb... 

Nowadays silenes are well-known intermediates. A number of studies have been carried out to obtain more complex molecules having Si=C double bonds. Thus, an attempt has been made to generate and stabilize in a matrix 1,1-dimethyl-l-silabuta-l,3-diene [125], which can be formed as a primary product of pyrolysis of diallyldimethylsilane [126] (Korolev et al., 1985). However, when thermolysis was carried out at 750-800°C the absorptions of only two stable molecules, propene and 1,1-dimethylsilacyclobut-2-ene [127], were observed in the matrix IR spectra of the reaction products. At temperatures above 800°C both silane [126] and silacyclobutene [127] gave low-molecular hydrocarbons, methane, acetylene, ethylene and methylacetylene. A comparison of relative intensities of the IR... 

A quite pronounced similarity is observed between 2 and (Me3SiO)2Si =CHCH2Bul the latter may be considered as the best synthetic equivalent to dichloroneopentylsilene. Although it does not react with naphthalene and furans, it readily prefers the [2+2] addition to numerous dienes and forms silacyclobutenes with alkynes [12]. 

Scheme 122) (78JA1630). Silacyclobutenes also result from the FVP of l,3-bis(trimethyl-silyl)-3-methoxysilylpropenes through /3-elimination of the methoxysilane (Scheme 123) <81JOM(216)C5>. 

What restricted chemistry is known of the silacyclobutenes indicates them to be susceptible to nucleophilic ring opening, as illustrated by (79) with basic alumina or phenyllithium (Scheme 124) (65JA2678) and (80) with lithium aluminum hydride (Scheme 125) (64JA5589). 

A direct dynamics technique has been employed312 to study the thermal isomerization of (5VV)-tra .v-cyclopropane-1,2-ch and cyclopropane-1,2,3-d in an attempt to examine the possibility of stereochemical control in the isomerization. The stereochemistry of the ring expansion of various 2-isopropenylcyclopropane-l-carbonyl compounds (276) to the corresponding methyldihydrooxepins (277) has been studied,313 and the rearrangement of cyclopropylsilylene to silacyclobutene has been examined314 by ab initio MO theory. 

The thermolysis of neat acylpolysilanes usually gives mixtures of compounds. However, thermolysis in the presence of a scavenger such as an alcohol or an acetylene is much cleaner. Brook and coworkers showed that thermolysis of pivaloyltris(trimethylsilyl)silane 147 in the presence of 1-phenylpropyne yields the 2-silacyclobutene 148 in 72% yield, indicating the thermolytic generation of 14986. (Note, however, that originally a different regiochemistry for the cycloaddition product 14886... 

Photolytically generated 1-silabuta-l,3-dienes undergo a thermal reverse reaction to 2-silacyclobutenes. Thus 2-phenylsilacyclobut-2-ene 360 is easily opened to the 2-phenylsilabuta-1,3-diene 361 by irradiation in 3-methylpentane matrix at 77 K or by flash photolysis at ambient temperature (equation 97)183. The rate for the thermal reverse reaction was measured at room temperature and the activation energy for the 1 -siladiene ring closure was estimated to be 9.4 kcalmol-1183. 

The photolysis and flash photolysis of 1,1 -dimethyl-l-sila-2-phenylcyclobut-2-ene, 35, in the inert solvent cyclohexane have been studied by Conlin, Fink and coworkers26. The reaction yielded the siladiene 36, as shown in equation 5 the kinetics of its reversion to the precursor silacyclobutene 35 were also studied. 

Silacyclobutene (57) is obtained in high yield on acidification of (56), which is readily prepared from dimethyl-bis-(phenylethynyl)silane (95JA2665). 

Restricted Hartree-Fock ab initio calculations have been performed on various isomers of silacyclobutenes <1988JPC3037> and silacyclobutadiene <19870M1977>. 

Oxidative addition of the Si-aryl carbon bond in the silacyclobutene ring to Pt gives the optically active intermediate Pt-complex. Further coordination of (+)-l-methyl-l-(l-naphthyl)-2,3-benzosilacyclobut-2-ene to the complex and cr-bond metathesis will provide the cyclic dimer Pt-complex. Reductive elimination from the intermediate platinum complex gives cyclic polymers and oligomers. Preference of cr-bond metathesis over reductive elimination gives polymers of higher molecular weight. The presence of EtsSiH in the system results in the formation of linear products via cr-bond metathesis. 

Conlin and coworkers reported a rate constant for the thermal electrocyclic ring closure of the transient l,3-(l-sila)butadiene derivative 8a in cyclohexane at 25 C, k = (1.19 0.06) x 105 s l31. The rate constant corresponds to the inverse of the lifetime of the silene in the dry solvent, as measured by laser flash photolysis of the isomeric silacyclobutene derivative 7a, the stable product of the thermal ring closure reaction... 

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