Silicon compounds silacyclobutanes

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 However, a few reports are listed in Table I of the presumed formation of silenes having previously unpublished substitution patterns, prepared either thermally or photo-chemically from four-membered ring compounds containing silicon.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. 

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-... 

Difluoro-l-silacyclobutane is obtained by the exchange of two chlorine atoms for fluorine in the 1,1-diehloro analog using antimony(III) fluoride in w-xylene yield 46% bp 36 -38 C.64 Antimony(lll) fluoride is supposed to interact, through its lone electron pair, with coordinative-ly unsaturated silicon rather than with chlorine atoms at silicon. Because of this Si —Cl compounds are fluorinated slower than C — Cl derivatives in the presence of boron trifluoride.65... 

The reaction of l-allyl-l-(cyclohexyloxy)silacyclobutane with ct-hydroxy carbonyl compounds proceeded at a lower reaction temperature. The alkoxy group on silicon enhances the Lewis acidity of the allylsilacyclobutane and presents the possibility for ligand exchange to preceed the allylation event, which then occurs intramolecularly (Scheme 67). 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Summary From catalytic hydrogenation of olefinic silicon-functionalized compounds (chloro-Si, alkoxy-Si, alkyl-Si, and aryl-Si) the saturated products are available in good yield. In general, the chloro- and alkoxy substituents are unaffected and for silaheterocyclic compounds the cyclic or the bicyclic moieties, respectively, remain intact. Thus, the silanorbomenes 1, 2, and 3, compounds containing cyclopentenyl groups 13 and 15, and various carbon vinyl substituted silacyclobutanes 7, 8, and 11 were hydrogenated in a simple apparatus. The reactions were performed in ether and THF as solvents the hydrogenation catalyst Pd/C can be used several times. 

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the first reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [2]. Furthermore, five-coordinated silicon species are well described to be stable compounds [3]. 

The situation for strained silicon-containing ring compounds has become if anything, more confusing since the previous review. The only new experimental data, are those for a series of siletanes " (silacyclobutanes), shown in Table 8. From these, strain enthalpies in the range of 109-120 kJmol can be calculated. These are slightly larger than an... 

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