Silacyclobutanes

Silacyclobutanes as well as silacyclopropanes undergo aldehyde insertion under catalysis by /-BuOK (Equation (77)).292 The reaction of silacyclobutanes with lithium carbenoids such as dihalomethyllithium and oxiranyllithium gives 2-substituted silacyclopentanes (Equation (78)). Treatment of l-(l-iodoalkyl)- and 1-oxiranyl-silacyclobutanes with a stoichiometric amount of an alkali alkoxide leads to silacyclopentanes by anionic 1,2-shift of the ring carbon adjacent to silicon. These ring-expansion reactions proceed probably via a pentacoordinate silane intermediate. 

The strained silicon-carbon bonds of silacyclobutanes are subject to activation by Pd and Pt complexes. This reactivity has been used for a catalytic carbon-carbon bond formation.294,295 

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Aikynes insert into the silacyclobutane 168 to form the silacyclohe.xene 169[93]. Also, the silacyclopropene 170 is expanded to the silacyclopentadiene 171 by the insertion of an alkyne[94]. The insertion product 173 was obtained by the Pd-catalyzed reaction of the neopentylidenesilirane 172 with acety-lene[95]. 

The strained-ring compound 1,1-dimethyl-l-silacyclobutane (which may be regarded as an olefin of organosilicon chemistry) reacts with diiron nonacarbonyl in benzene at 6°-20°C as shown in Eq. (100) (89). (There is here some analogy with the reactions of transition metal complexes with strained hydrocarbons, which often produce valence tautomerization.) The... 

Fig. 5. Mechanism of the transition metal-catalyzed polymerization of a silacyclobutane.

The first evidence for the formation of silenes came from the thermolysis of silacyclobutanes, which resulted in a retro-[2+2] process leading to the silene Me2Si=CH2 and ethylene1 ... 

Generally, only simple silenes having small groups (H, Me, CH2=CH) are obtained as transient species from the thermolysis of silacyclobutanes. In part this is due to the high temperatures (usually above 450°C) required for the ring cleavage. Substitution on the carbon atom adjacent to silicon in the ring can lead to carbon-substituted silenes. 1,3-Disilacyclobutanes do not readily revert to silenes under thermal conditions, but examples... 

While the decomposition of silacyclobutanes as a source of silenes has continued to be studied in the last two decades, the interest has largely focused on mechanisms and kinetic parameters. However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon. Two cases of particular interest involve the apparent formation of bis-silenes. Very low-pressure pyrolysis of l,4-bis(l-methyl-l-silacyclobutyl)ben-zene94 apparently formed the bis-silene 1, as shown in Eq. (2), which formed a high-molecular-weight polymer under conditions of chemical vapor deposition. 

Conlin and co-workers have also studied the fragmentation of a siletane (silacyclobutane). In this case, both the E- and Z-isomers of 1,1,2,3-tetra-methylsilane 45 were prepared and thermolyzed (Scheme 8).144 Both E-and Z-isomers of 45 led to the same products in slightly different ratios the major products were propene with silene 46, and E- and Z-2-butenes with silene 47. Silene formation was inferred from detection of the disila-cyclobutane products. During these processes, the stereochemical integrity of the compounds was largely preserved. 

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

A more useful thermolytic polymerization which produces linear polysilmethylenes is that of 1,3-disilacyclobutanes carried out in the liquid phase. Such polymerization of l,l,3,3,-tetramethyl-l,3-disilacyclobutane was reported first by Knoth [17] (eq. 7). This process was studied in some detail by Russian workers [18]. l,l,3,3-Tetramethyl-l,3-disila-cyclobutane is more thermally stable than 1,1-dimethyl-l-silacyclobutane. 

Aryl and, more so, chlorine substituents on silicon enhance thermal stability of silacyclobutanes. The rate of the first-order thermal decomposition of silacyclobutanes varies inversely with the dielectric constant of the solvent used. Radical initiators have no effect on the thermal decomposition and a polar mechanism was suggested. Thermal polymerization of cyclo-[Ph2SiCH212 has been reported to occur at 180-200°C. The product was a crystalline white powder which was insoluble in benzene and other common organic solvents [19]. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

Review of ring opening reactions of silacyclobutanes N. S. Nametkin and V. M. Vdovin, Izv. Akad. Nauk SSSR, Ser. Khim. (1974) 1153. 

Functionalized silacyclobutanes 16 result from photochemical decomposition of [azido-, isocya-nato- and isothiocyanato-bis(tert-butyl)silyl]diazoacetates 15. They undergo a remarkably facile ring-expansion reaction to cyclic O-silyl ketene acetals 17 even at 60°C. 

With l,3>5-cycloheptatriene 2 can be trapped to yield four isomeric [2+2] adducts and the exo/endo isomeric [6+2] compound 16. Heating this mixture to 110°C leads to the complete transformation of the silacyclobutanes into 16 via a dipolar intermediate. The attempted synthesis of the diphenyl derivative of the [2+2] products leads to the stereospecific formation of endo-Yl which could be characterized by X-ray diffraction analysis [4]. 

All results so far obtained prove the high synthetic potential of differently substituted neopentylsilenes. Especially silicon dichloro substituted silenes are useful for the preparation of a wide variety of new silacyclobutanes and -butenes. These SiC four membered ring compounds can be utilized as pre-... 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Penta-alkylphosphorane intermediates are also inferred from the products of the reaction of these ylides with silacyclobutane, from which hydrogen is eliminated.44 However, the cyclobutane ring is left intact when the reaction is carried out with more bulky ylides (Scheme 10).46 Silicon- and germanium-substituted allenes have been prepared by the reaction of monometallated ketens with stable ylides, e.g. (43).46... 

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