1,3-Disilacyclobutane

Disilacyclobutane, 1,1,3,3-tetrachloro-physical properties, 1, 590 Disilacyclobutanes synthesis, 1, 587, 598... 

Experiments reported in 1982 " by tbe same group provided the first example of heteroallene dimerization. In this work, head-to-head dimerization of 1-silaallenes lla-c (Scheme 4) was observed, forming 1,2-disilacyclobutanes 12a-c with two exocyclic double bonds. 

This section deals with the most important control experiments to be considered when molecular complexes or NPs want to be proved as true catalysts. But in some cases both types of catalysts can be present in the same reaction. For example, in the ring opening polymerisation of l,l,3,3-tetramethyl-l,3-disilacyclobutane catalysed photo-chemically by Pt(acac)2, the co-existence of both homogeneous and colloidal catalytic species has been proved, giving each of them different type of polymers [10]. 

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... 

The second example of a bis-silene involved the thermolysis of the benzo-l,2-disilacyclobutane 2, which formed the bis-silene 3 that subsequently dimerized in the unusual manner shown in Eq. (3).95,96... 

References to the silenes so prepared since 1985 are listed in Table I. None of these silenes was stable, most undergoing head-to-tail dimerization to give 1,3-disilacyclobutanes in the absence of trapping reagents. Some interesting spontaneous silene-to-silene rearrangements were observed,52 which will be described in Section IV.E. 

The rate of pyrolysis of 1,3-disilacyclobutanes 44 (see Table III) was shown by the same authors to be dependent upon the substituents on... 

Dimerization is a special case of [2+2] cycloaddition with silenes it has been observed to occur in both a head-to-tail and in a head-to-head manner, yielding 1,3- or 1,2-disilacyclobutanes. These two cases will be discussed separately below. 

When phenyl (Ph) groups replaced both Me3Si groups, again a rather unstable 1,2-disilacyclobutane dimer appeared to be formed,90 as shown by NMR data but when f-butyl replaced a Me3Si group, the silene failed to dimerize.87 Thus, it is evident that whether or not head-to-head [2 + 2] cyclodimerization occurs depends on the bulk of the substituents on both sp2-hybridized silicon and carbon. 

Another example of ring closure involving a 1,5-H shift appears to be that provided by Jung,119 who reported that the heteroatom-substituted silenes 144 rearranged to give 1,3-disilacyclobutanes 145 via a diradical intermediate (Eq. 51). When R = Cl the yield was 30%, and with R = MeO the yield of the disilacyclobutane was 44%. 

In the sixties, such reactions were of greater interest as a source of 1,3-disilacyclobutanes (eq. 4) [9]. 

Tetramethyl-l,3-disilacyclobutane had been prepared earlier by Knoth and Lindsey [10], but a multistep synthesis was involved which was not generalizable to the synthesis of Si-functional 1,3-disilacyclobutanes. The reaction shown in eq. 4, provided it is carried out in the right way, represents a good, general route to 1,3-disilacyclobutanes. This reaction was reported first by Muller and his coworkers [11]. In this work, diethyl ether was used as reaction solvent and the product yield was only around 4%. Somewhat better yields were obtained" by Russian workers [12], but it was the detailed studies of the (chloromethyl)chlorosilane/ magnesium reaction by Kriner [13] which provided a good synthesis of... 

When starting (chloromethyl)chlorosilane contained more than one Si-Cl bond, lower yields of the 1,3-disilacyclobutane were obtained due to side reactions resulting from the availability of more Si-Cl functions (in the case of CH3Si(CH2Cl)Cl2, 2 was produced in addition to cyclo-[CH3(Cl)SiCH2]2) and to formation of higher yields of polysilmethylene [13, 14]. 

Another procedure for the synthesis of 1,3-disilacyclobutanes is the pyrolysis of monosilacyclobutanes (eq. 5), [9, 14, 16], but this method has difficulties and disadvantages [14]. One of these is that polysilmethylene formation is a side-reaction when it is carried out in the gas-phase. 

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. 

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. 

In accord with the proposed mechanism, copyrolyses of la or lb with 2,3-dimethyl-1,3-butadiene (DMB) or isoprene lead to silacyclopentene derivatives via a formal [4+1] cycloaddition of the silylenes (Scheme 2). The simultaneous existence of the silaethenes 2a/2b and the resulting silylenes 4a/4b in the gas phase is proven by the formation of the corresponding 1,3-disilacyclobutanes (5) and - in case of isoprene as the quenching partner - of the two isomeric silacyclohexenes 7 (Scheme 2) [2]. 

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