Silicon heterocycles

The major synthetic routes to transition metal silyls fall into four main classes (1) salt elimination, (2) the mercurial route, a modification of (1), (3) elimination of a covalent molecule (Hj, HHal, or RjNH), and (4) oxidative addition or elimination. Additionally, (5) there are syntheses from Si—M precursors. Reactions (1), (2), and (4), but not (3), have precedence in C—M chemistry. Insertion reactions of Si(II) species (silylenes) have not yet been used to form Si—M bonds, although work may be stimulated by recent reports of MejSi 147) and FjSi (185). A new development has been the use of a strained silicon heterocycle as starting material (Section II,E,4). 

Much cleaner and synthetically useful are, however, the rapid trapping reactions with dienes and heterodienes [4], carried out by the copyrolysis technique. In this way a variety of five membered unsaturated silicon heterocycles may be produced. 

The action of a catalytic amount of triethylborane on tris(trimethylsilyl)silane induces the formation of tris(trimethylsilyl)silyl radicals, which promote the ring-closure of 1,6-heptadiene to a mixture of the cis- and rirms-cyclopcntanc derivatives 115, together with a small amount of the silicon heterocycle 116 (equation 61)68. 

TABLE 10. Standard enthalpies of formation (AHf°/kJmol-1) of three-membered silicon heterocycles... 

In the chemistry of silicon, heterocycles are rings containing silicon atoms and one or more of another type of atom. Of the isocyclic compounds, four to seven-membered rings are known, while the existence of an eight-membered ring is probable. The same or similar ring sizes are expected for heterocyclic silicon compounds. 

Silicon-29 and carbon-13 spectra have been reported for the silicon heterocycles (27) and (31) and fully support the assigned structures <920M1009>. 

The silicon-heterocyclic bond of 2-trimethylsilylbenzothiazole (225) is unusually labile. It is cleaved smoothly by benzaldehyde, with formation of the silyl ether (226) which is hydrolyzed to the alcohol (227) the cleavage by benzoyl chloride affords the corresponding ketone (228 Scheme 155). 

Summary Copyrolytic gas-phase reactions of l,l,2,2-tetra(alkoxy)- and tetrakis-(dimethylamino)dimethyldisilanes with dienes, heterodienes, and heterotrienes in a flow-system furnish a number of unsaturated silicon heterocycles via silylene-intermediates with the exception of 1,4-diaza- and 1,4-oxazadienes, yielding 40-65 % of 1,3,2-diaza- and l,3,2-oxazasilacyclopent-4-enes, respectively, mixtures of diastereoisomeric 1-oxa- or 1-aza-2-silacyclopent-4-enes and -3-enes are formed. Conjugated oxatrienes were found to undergo additionally [6+l]-cycloadditions giving l-oxa-2-silacyclohepta-4,6-dienes. 

Molecular structure of Si,O-heterocycles in the gas phase 90MI4. Silicon heterocycles possessing Si-atoms with unusual coordination types 89UK353. 

Results of dehydrohalogenation studies are summarized in Table V. Conditions for chlorination with sulfuryl chloride have been reported only for silicon heterocycles. Ring dehydrohalogenation reactions of cyclic germanium compounds have been reported to yield tars 310). 

Silicon heterocycles with different ring sizes have been synthesized by variations of the organometallic Wurtz coupling reaction. The method requires synthesis of intermediate organosilicon derivatives that can involve several steps. 

Transannular HCl eliminations involving the heteroatom [cf. Eq. (102)] have achieved only moderate success. Although reactions of boron and silicon heterocycles with amine bases appear to proceed, monomeric products have not yet been isolated. The results are summarized in Eq. (102). 

There are several limitations on these methods with regard to the heteroatom involved. Reaction of the tin heterocycle with NBS results in cleavage of the aromatic-tin bonds to give o,o -dibromobibenzyl (81). Reaction of the tin heterocycle with DDQ resulted only in products of heteroatom elimination (81). Attempts to generate a silicon heterocycle with a functional group on the Si atom have failed to produce the unsaturated system (XVII). Dehydrobromination reactions of the silane afford highly colored reaction products but no isolable identifiable monomer. It is probable that the dehydrobromination reaction occurs not only at the ethylene bridge but also by transannular elimination. 

Summary The silylenes MeSiNMe2 and MeSiNH(iPr) can be thermally generated from disilanes. In rapid copyrolytic gas phase reactions between disilanes and trapping reagents the intermediate silylenes and dienes or heterodienes furnish a variety of five-membered unsaturated silicon heterocycles in 35-65 % yield. 

Four-membered silicon heterocycles, formation and properties of 79CRV529. 

New types of silicon heterocycles 86PS(28)91 79H(13)531. Organocyclodisilazanes, structure and properties 81UK2212. 

Silicon heterocycles, multiple bonds with Si participation in 85CRV419. Silicon heterocycles, oxidative scission of Si —Si bonds in 86UK1096. Siloxazolidines with pentacovalent Si atom 77KGS1299. 

Dimesityl-2,2-diphenylsilene (169) can be prepared by irradiation of the novel air-stable disilathietane (170). In contrast, the disilanylbenzoquinones (171) undergo photochemically induced conversion into the silyl ethers (172) in alcohols and into the silicon heterocycles (173) in the presence of ketones, in both transformations via the unstable sila-m-quinones (174). o-Silaquinone methides are intermediates in the photoreactions of... 

Silicon heterocycles as starting compounds, intermediates, and reaction products 00MI32. 

The regiospecific (3 -t- 2) cycloaddition of the bis(diazomethyl)phosphane (236) with (-butylphos-phaalkyne gives the phosphine (237), substituted by 1,2,4-diazaphosphole units. With dichloro-phenylphosphine (237) condenses smoothly to furnish compound (238), with a 1,3,2,5-diazadiphosphinine unit. An analogous condensation with SiCU results in the spiro-linked silicon heterocycle (239) with a 1,3,5,2-diazaphosphasilabenzene structural unit (Scheme 50) <88TL925>. 

Summary Monoaminosilylenes and bis(diethylamino)silylene are formed by thermolysis of amino-substituted disilanes and characterized by trapping with dienes. In the case of MeSiNMe2, cycloaddition reactions were extended to 1,4-diheterodienes, unsaturated ketones and imines allowing an easy synthetic access to functionally substituted unsaturated silicon heterocycles. The syntheses of an isolable, unsymmetric diaminosilylene and of related germylenes and stannylenes are described. The results are presented and discussed in relation to theoretical work on stabilization of donor-substituted silylenes and recent work on isolable diaminosilylenes done by or in cooperation with other groups. 

Keywords Gallium-Silicon Heterocycles / Hypersilyl / Main Group Elements / Clusters... 

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