Disilenes cycloaddition reactions

Compounds with a Si=Si or Ge=Ge bond (i.e., disilenes and digermenes) can be isolated when the double bond contains bulky substituents. Only a few cycloaddition reactions with diazo compounds are known, and two reaction modes have been observed. One of the paths leads to the formation of disiliranes 184 (242) and digermiranes 185 (243) (Scheme 8.42), probably by ring contraction of an initially formed [3 + 2] cycloaddition product. The other path involves a 1,1-cycloaddition of the diazoalkane to give disilaaziridine 186 (244) and digermaazir-idine 187 (245). This nitrene-like reactivity is rather uncommon although some intramolecular examples are known (see Section 8.6.1). 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

Stepwise radical mechanisms have been proposed for the apparent [2 + 2] cycloaddition of disilenes with ketones by Baines et a/.140,141 They have found that the reactions of tetramesityldisilene 1 with trara-2-phenylcyclopropane carbaldehyde (208a) and fra/M,fra/M-2-methoxy-3-phcnylcyclopropane carbaldehyde (208b), a mechanistic probe developed by Newcomb et al.,142 undergo characteristic cyclopropane ring-opening as shown in Eq. (99). 

Cycloaddition reactions of transient or isolable disilenes with heterocumulenes such as CX2 (X = S, Se) produce heterocyclic carbenes, for example, carbene 59, which has a disilane backbone. These carbenes are only transient species and were not isolated but were either trapped with C6o. or dimerization of the carbenes occurred to give the tetrathiafulvalene or tetraselenafulvalene analogues 28 <2002CEJ2730, 2005AGE7567>. 

The [2+2] cycloaddition of the Si=Si double bond of disilenes across a hetero double bond belongs to the most typical reactions for the preparation of disiletanes. Reaction of the supersilyl stabilized disilene 90 with PhHC=0 and Ph2C=S gave oxa- and thiadisiletanes 91 and 92, respectively (Scheme 15). The use of heterocumulenes 0=C=0 and 0=C=S in a similar cycloaddition reaction yielded oxa- and thiadisiletanes 44 and 31. The isolated disiletanes are colorless and oxygen, water, and thermostable compounds <2002CEJ2730>. 

The [2+2] cycloaddition reaction of the unsymmetrically substituted disilene with benzophenone proceeded with a high degree of regioselectivity to yield the 1,2,3-oxadisiletane 45 (Scheme 16) <1995CB935>. 

Small amounts of 1,2,3-oxadisiletanes were generated in reactions of disilenes with methyl- and phenyloxiranes. Their identity was established by comparison of their spectral data with authentic samples obtained by known [2+2] cycloaddition reactions of the appropriate aldehydes <1996JOM(521)363>. 

The 1,2,3-azadisiletidine 47 and 1,2,3-azadisiletine 48 are, in a formal sense, the products of a [2+2] cycloaddition reaction between nitriles and disilene (Scheme 20). It can be assumed that the latter is the crucial intermediate formed during the thermolysis of hexasubstituted cyclotrisilane <1995TL8187>. 

SCHEME 1. Some addition and cycloaddition reactions of disilene 16... 

Ene addition products have been isolated from reactions with various alkenes containing allylic hydrogen atoms compounds 49 and 50 are shown here as examples. Analogously, the reaction with a 1-alkyne furnishes the adduct 47 while styrene, in contrast, reacts to afford the [2 + 2] cycloaddition product 51. The latter mode of reaction, however, is no longer considered to be unusual since the tetraalkyldisilene 41 also forms [2 + 2] cycloadducts with various C=C double bond systems71-73. On the other hand, until very recently [2 + 2] cycloadditions of the tetraaryldisilene 9 were unknown. It has now been shown that 970, as well as 4171, can undergo cycloadditions with the C=C double bonds of styrene and 2-methylstyrene. [4 + 2] Cycloaddition reactions of disilenes with... 

In spite of their bulky substituents, the presence of which is necessary to prevent oligomerization reactions, disilenes undergo numerous addition and cycloaddition reactions these have been summarized in several review articles [1]. For example, stable or marginally stable disilenes react with the C=0 and C=S groups of ketones [2-4] and thioketones [5] as well as with the triple bonds of acetylenes [2, 3] and nitriles [6]. Surprisingly, however, no cycloaddition reactions of disilenes with simple alkenes have yet been reported [1]. 

Although the currently available results still do not provide a uniform scheme, they do clearly indicate that silylenes bearing bulky substituents such as 2, and also dimesitylsilylene [14], xmdergo [2+1]-cycloadditions to the double bonds of 1,3-dienes rather than [4+l]-cycloadditions. In contrast, the behavior of disilenes towards the C=C double bonds of alkenes and conjugated dienes is still not clear. While additions of the stable tetraaryldisilenes to such double bond systems are still unknown [1], the marginally stable disilene 3 is able, in individual cases, to take part in both [2+2]- and [4+2]-cycloaddition reactions. 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

In the reaction with thiophene 32, the disilathiirane 38 was unexpectedly formed in about 50 % yield by way of sulfur extraction. The X-ray structure analysis of the disilathiirane 38 revealed a very short Si-Si bond length of 230.5 pm and an almost planar environment at the silicon atoms (angular sum C-Si-C + C-Si-Si + Si-Si-C = 358 7°) [10]. These features are typical for related three-membered rings of this type and were also observed for other disilathiiranes [13]. As illustrated by the isolation of the 1,2-disilacyclohexadiene 39, sulfur abstraction from 32 seems to be initiated by [2+4]-cycloaddition of disilene 3. The bicyclic compound 40 is most likely formed by photoisomerization of 39 [14]. 

Since the isolation of the first molecular compound with an Si-Si double bond in 1981 [1] the chemistry of the disilenes has experienced an almost explosive development, as is reflected in numerous review articles [2]. Addition or cycloaddition reactions to this double bond provide an entry to a series of three- and four-membered ring compounds that are hardly accessible by other routes. Of particular interest among these systems are the disilaoxiranes and disiladioxetanes, both of which open up new questions about the bonding situation in small rings made up of main group elements. 

Another interesting method to obtain cyclotetrasilanes is the [2 + 2]cycloaddition of disilenes. Depending on the nature of R groups disilenes either are stable or undergo a dimerization reaction to a cyclotetrasUane. Tetrakis(trimethylsilyl)disilene, which forms either by thermolysis of methoxytris(trimethylsilyl)silane [104], by treatment of l,2-dipotassiotetrakis(trimethylsilyl)disilane with BrCH2CH2Br [96] or by reaction of the respective fluoride adduct with MgBr2 [105], dimerizes to octakis (trimethylsilyl)cyclotetrasilane (Scheme 6). 

Disilenes react with various types of reagents to afford novel three-membered cyclic compounds that are otherwise inaccessible. Even though some are not mechanistically [2+1] cycloadditions, all the reactions in which three-membered rings are formed from disilenes are summarized in Scheme 12. 

While disilene 5 does not undergo Diels-Alder reactions with 1,3-dienes, the [4+2]-cycloaddition products are formed with heterodienes, e.g. 1,4-diazabutadienes [17] or a-ketoimines [19]. It can be deduced that the electron deficient properties of such dienes cause them to readily take part in hetero-Diels-Alder reactions, which have inverse electron demands. This is corroborated by theoretical calculations which predict an inverse electron demand of the Si-Si double bond it is strongly electron donating rather than electron accepting towards butadienes and other compounds [24,25]. 

The formation of an intermediate difluorodisilene 25 (equation 8) was proposed by Jutzi and coworkers34 in the reaction of decamethylsilicocene with tetrafluoroboric acid. The disilene which was characterized by the 29Si NMR spectrum, then formed the isolable cyclotetrasilane by [2 + 2] cycloaddition. 

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