1,3-Disilacyclobutanes

By comparing the thermal and chemical stabilities of the Si-methylated cyclic carbosilanes (Me2Si—CH2)3 and (Me2Si—CH2)2 22, it is quickly recognized that the 1,3- 

3-disilacyclobutanes can be divided into a number of groups based on their behavior to HBr and Br2 [139] (Table 66). 

Of course, ring strain is a very important ground for the tendency of 1,3-disilacyclobutanes to be cleaved. This is supported by the fact that compounds such as (Mc2Si — 

Group Compound Reaction with HBr Reaction with Br, 

2 [(CX)HSi-CH,], (MeHSi-CH,), Ring cleavage no ring cleavage Si-substitution 

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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. 

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. 

Cycloreversions of a 1,2-disilacyclobutane (42) and a 1,2-digermacyclobutane (43) have been induced in solution both thermally and photochemically fragmentation of sterically congested (42) follows Scheme 5 paths a and b, respectively fragmentation of (43) yields (46) (which photodissociates to 48) in each case. " ... 

Pd and Pt atoms derived from (Ph3P)2PdCl2 and (Ph3P)2 Pt(CH2=CH2) are smoothly inserted into 1,2-disilacyclobutane (261) to give the corresponding five-membered heterocycles (262 M = Pd, Pt) <84JOM(27l)337>. Reaction of the heterocycle (263) with aldehydes in the presence of PhsP leads ultimately to the 1,2,5-oxadisilacyclopentane (264) <830M1846>. 

In contrast, 1,1-silylsilenes39,104 107,108 and 2-siloxysilenes 3-86-90-180 j e sjienes of the Apeloig-Ishikawa-Oehme and Brook type as well as some 1-silaallenes184 185, dimerize in a head-to-head mode yielding 1,2-disilacyclobutanes (equation 98). 

Relatively little is known experimentally about the mechanism of the reaction. A widely accepted mechanism originally suggested by Brook and coworkers starts with the formation of a Si—Si bond, giving a carbon centred 1,4-biradical86. This 1,4-biradical then combines in a second step to the 1,2-disilacyclobutane. This mechanism is favoured by the calculations and is also corroborated by experiments the relatively stable silene 149... 

Head-to-head dimers with bulky substituents at the C—C bond tend to be thermola-bile and they can be used as convenient sources for relatively stable or transient silenes. Examples are the already-mentioned dimer of the relatively stable silene 14986 and the 1,2-disilacyclobutane 164, which liberates the adamantylidene silene 165 smoothly upon heating to 70 °C in benzene (equation 106)39. 

In contrast, the cleavage of the 1,2-disilacyclobutane 378 needs more drastic conditions (250 °C) and yields the thermodynamically more stable linear dimer 379. It was suggested that this reaction proceeds via the 1,4 biradical 380 (equation 107)112. 

A silirene may also generate a silylene. In the absence of catalyst, thermolysis of a silirene can give various products resulting from its decomposition into a silylene and an alkyne50. Thus siloles are formed together with 1,2-disilacyclobutanes, 1,4-disilacyclohexadienes and other products. With silirenes bearing bulky substituents on the ring carbon atoms, siloles become the major products (Scheme 9). 

The heating of 1,2-disilacyclobutane 29 in hexane at 60-100 °C in the presence of trapping agents leads to the expected products of the silene 30 trapping. In benzene at 60 °C, 29 exists in equilibrium with 30 <20020M2049>. 

Disilacyclobutane can be obtained by the dimerization of a transient silene generated by the salt elimination method (Scheme 18) <1996JOM181>. Other examples of head-to-head dimerization of silenes have been published C1995CB143, 1995CB1083,1996CB15>. 

Zirconocene-mediated coupling of bis(methoxyethynyl)disilanes led to zirconacycles, which were converted into 1,2-disilacyclobutanes by protonolysis (Scheme 22) <2000CL1082>. 

Thermolysis of /ra t-2,3-dimethyl-l-tri-fert-butylsilyl-l-tri-isopropylsilylsilirane in the absence of trapping agents yielded 1,2-disilacyclobutane by intramolecular insertion into the C-H bond (Scheme 34) <2003OM2233>. 

The Brook silene 28, produced photochemically as shown in equation 18, dimerizes to yield a mixture of the 1,2-disilacyclobutane 29 and the acyclic ene-dimer 3064, the common mode of dimerization for the large majority of l,l-bis(trialkylsilyl)silenes that have been studied to date12. Conlin and coworkers determined the absolute rate constant for dimerization of 28 in cyclohexane solution, k, tm = 1.3 x 107 M 1 s 1 at 23 °C65. Arrhenius activation parameters for the reaction were determined over the 0-60 °C temperature range. The values obtained, a = 0.9 0.4 kJmol 1 and log(A/M 1 s 1) = 7 1, are consistent with the stepwise mechanism for head-to-head dimerization originally proposed by Baines and Brook (equation 19)64, provided that the rate of reversion of the... 

Originally synthesized by a ground state route, 32 is known to dimerize in head-to-head fashion to yield the 1,2-disilacyclobutane 33 (equation 20). The silene was generated and detected in hexane solution at 25 C along with t e e asi y su s i u e... 

Only four transient disilenes have been studied to date by fast time-resolved spectroscopic techniques l,l,2-trimethyl-2-phenyldisilene (103), ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (104) and tetrakis(trimethylsilyl)disilene (35). The first three compounds were generated by photolysis of the 7,8-disilabicyclo[2.2.2]octa-2,5-diene derivatives 101 and 102 (equation 76)148 while 35 was generated, together with 106, by photolysis of the 1,2-disilacyclobutane derivative 33 (equation 77)68. 

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