Cyclic disilenes

The method employed for the first synthesis of stable disilene l,1 the photolysis of linear trisilanes, has been the most widely used synthetic method for disilenes. However, photolysis of cyclic trisilanes and dehalo-genation of dihalosilanes and 1,2-dihalodisilanes are also good routes in some cases (Table I). 

The first synthesis of a disilene by photolysis of the corresponding cyclic trisilane was reported by Masamune et al. in 1982.14 This method has been adapted for the synthesis of a variety of stable and marginally stable disilenes (Eq. 6).15-20 More recently, Kira et al. synthesized the first example of a stable tetrakis(trialkylsilyl)disilene 22 by this method.21... 

Disilene 21 can react with the C=N bond in a cyclic imine 73 and cumulenes such as carbodiimide75 and keteneimine74 (Eqs. 18-21). The reactions with cumulenes are not straightforward, although the initial products are most likely [2 + 2] cycloadducts. 

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. 

At this time, although details remain to be elucidated, it seems that most of the main kinds of reactions of disilenes, have been discovered. These have led to numerous new classes of cyclic compounds, some with unprecedented and beautiful structures. In addition, the chemical bonding in disilenes has received serious study and is at least partially understood. 

The ladder polysilanes also show interesting photochemistry on irradiation with a high-pressure mercury lamp, the tricyclic ladder extruded the transient intermediate four-membered cyclic disilene which could be trapped using methanol, 1,3-butadiene, and anthracene, as shown in Scheme 38. 

Scheme 38 Photolysis of ladder polysilane and trapping products of extruded cyclic disilene (substituents /-Pr, except as noted)308...

Thus, with an increase in the number of silicon atoms in a cyclic structure, analysis, in terms of the aromaticity concept, of the structural types and stability loses its prognostic value. A striking example of this is given by the pyramidalization of the Si atoms, typical of disilene, in hexasilaben-... 

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Cathodic reduction potentials of disilenes were determined by cyclic voltammetry56. As shown in Table 18, tetraaryldisilenes are reduced at less negative potentials than dialkyl-diaryl derivatives. This is in sharp contrast to the fact that anodic oxidation potentials are similar for both types of these disilanes (see Table 13). 

Kira and coworkers recently described the preparation of the first cyclic disilene, 396210. On photolysis at 420 nm 1,2-silyl rearrangement occurred to yield the bicyclic[1.1.0] system, 397 (equation 52), which on standing in the dark reverted to the relatively stabler disilene. 

Cyclic siloxanes are important precursors in the silicon industry, being formally dimers or trimers of silanone (R2Si=0), a known intermediate. Cyclic siloxanes have been synthesized by four routes, the conventional methods being the condensation of silanediol or the hydrolysis of species such as halosilanes or aminosilanes (Scheme l)16-20. Alternatively, oxidation of disilene by triplet oxygen (equation l)21-27 or oxidation of oxadisiliranes by singlet oxygen (equation 2)28-31 may be utilized. 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

In review OW,7 only four types of synthetic methods are introduced for stable disilenes as shown in Scheme 1 (1) photolysis of linear trisilanes (method A), (2) photolysis of cyclic trisilanes (method B), (3) reductive dehalogenation of dihalosi-lanes (method C), and (4) reductive dehalogenation of 1,2-dihalodisilanes (method D). 

The first cyclic disilene, hexakisUerf-butyldimethylsilyljcyclotetrasilene (51), is synthesized in 13.6% yield by the reduction of a 1 2 mixture of tetrabromodisilane 86 and dibromosilane 87 with lithium naphthalenide in THF [Eq. (12)].23 Interestingly, the initial product of the reductive condensation is not cyclotetrasilene 51 but bicyclo[1.1.0]tetrasilane 88. Thus, reduction of a mixture of 86 and 87 with lithium naphthalenide at -78 °C followed by hydrolysis at 0 °C affords mainly a hydrolysis product of bicyclotetrasilane 88, 89 [Eq. (13)]. As discussed in Section IV.A.4 in more detail, compound 51 should be formed by the thermal isomerization of 88 during workup at higher temperatures. 

The four-membered cyclic disilene with mixed group-14 elements 56 is synthesized in 95% yield by the reaction of the dipotassium salt of tetrakis(di-tert-butyl-methylsilyl)-l,2-disila-3,4-digermacyclobutadiene dianion 74 with Me2S04 the two Me groups in 56 are bound selectively to Ge atoms 28... 

The reduction of dichlorodisilene 42 with lithium naphthalenide in THF at —78 °C in the presence of ethylene provides orange-red crystals of cyclic disilene 57 as a formal [2+2] adduct of disilyne 75 to ethylene 32... 

A similar reduction of 42 in the presence of excess 1,3-butadiene gives yellow-orange crystals of six-membered cyclic disilene 61, which would be formed by the [2 + 4] cycloadditon of disilyne 75 to 1,3-butadiene 32... 

Strain Energies (SE) of Cyclic Alkenes and Disilenes (in kcalmol-1)3... 

Peak Potentials of Disilenes Determined by Cyclic Voltammetry3 13... 

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