Disilenes cycloadditions

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]. 

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). 

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. 

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 addition of t-Bu2>Si to 1,4-diaza-l,3-butadienes competes with dimerization of the silylene only when the concentration of t-BinSi is low170. Subtle steric effects must also be responsible for the addition of /-BinSi to the W-cyclohexyl mono-imine of benzil, while only the silylene dimer undergoes addition under similar conditions in the presence of the IV-methyl mono-imine171. It may be that t-Bi Si and its dimer t-Bu2Si=SiBu-t2, both formed simultaneously upon photolysis of cyclo (t-Bu2Si)3, are in equilibrium, and the steric effect is upon the (2+4) cycloaddition of the disilene. 

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. 

A number of formal [2+ 1] and [2 + 2] cycloadditions of disilenes were investigated before 1996 and discussed extensively in review OW as useful methods for the... 

Styrene and substituted styrenes react with tetramesityldisilene 1, tetra-tert-butyl-disilene 21, and tetrakis(tert-butyldimethylsilyl)disilene 22 to afford the corresponding disilacyclobutane derivatives.127,134 Similarly, [2 + 2] additions occur between the disilenes with a C = C double bond in an aromatic ring135 and acrylonitrile.136 Bains et al. have found that the reaction of disilene 1 with trans-styrene- provides a 7 3 diastereomeric mixture of [2 + 2] adducts, 201 and 202 [Eq. (95)] the ratio is changed, when czs-styrene-Ji is used.137 The formation of the two diastereomeric cyclic adducts is taken as the evidence for a stepwise mechanism via a diradical or dipolar intermediate for the addition, similar to the [2 + 2] cycloaddition of phenylacetylene to disilene ( )-3, which gives a 1 1 mixture of stereoiso-meric products.116,137... 

Cyclotrisilene 49 reacts with 2 mol of phenylacetylene to give unique bicyclic product 203 in good yield [Eq. (96)].138 On the basis of the results of deuterium labeling experiments, compound 203 is proposed to form via two consecutive [2 + 2] cycloadditions of 49 to phenylacetylene, i.e., the initial [2 + 2] cycloaddition giving 204 followed by the isomerization to disilene 206 via 205, and then the second [2 + 2] cycloaddition of 206 [Eq. (97)]. The reaction of 1-disilagermirene 50 with phenylacetylene proceeds in a similar way.139... 

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). 

A number of formal [2 + 3] and [2 + 4] cycloadditions of disilenes giving five- and six-membered ring compounds have been discussed in review OW, while their mechanisms have not been elucidated in detail. 

Stable aryl- and alkyl-substituted disilenes undergo the [2 + 4] cycloadditions with benzyl, acylamines, and 1,4-diazabutadienes but not with 1,3-butadienes.77 131 On the other hand, stable tetrasilyldisilene 22 reacts with 2,3-dimethyl-1,3-butadiene at rt giving the corresponding 4,5-disilacyclohexene 218 [Eq. (104)].127 The reason for... 

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>. 

Thermolysis of compound 29 at 150-180 °G in the absence of trapping agents presumably proceeds via the competitive cleavage of the Si-Si and C-C bonds, resulting in simultaneous transient formation of the silene 30 and the disilene 31, which undergo fast cycloaddition to produce trisilacyclobutane 34 (Equation 5) <20020M2049>. 

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... 

One of the most fascinating reaction modes of disilenes is the ready formation of disiliranes, three-membered ring systems containing two silicon atoms and one heteroatom in the ring, which are hardly accessible by other routes. They are mostly formed by [2 + 1] cycloadditions of atoms or molecular fragments to the Si=Si double bond. One of the few examples of other accesses to this ring system is the reaction of AMithio-2,4,6-trimethylanilide 53 with the disilane 52, which affords the azadisilacyclopropane 54 in modest yield (equation 8)75. 

The first structurally confirmed [2 + 4] adduct of a disilene and a 1,3-diene was compound 89, obtained from cyclopentadiene and 4174. The formation of the tricyclic compound 95 from furan and the cyclotrisilane 40 is probably initiated by a [2 + 4] cycloaddition of 41 to the five-membered ring to afford 94, which then undergoes a [2 + 1] addition at the newly formed double bond with the silylene 42 formed concomitantly in the photolysis of 40 (equation 16)74. 

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