Heterocyclic Silanes

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. 

We are familiar with heterocyclic ring systems with various elements, ring sizes, and numbers of hetero atoms. Here only systems with Si-Si bonds are discussed. The number of possible systems is very great in this field with the frequent possibility that different isomers are present. Only a small number of such ring systems have so far been synthesized. 

A relatively large number of heterocycles are known in which carbon is the hetero atom. Within this group there are various ring sizes. 

A four-membered ring of this type is formed by reacting SiF2 copolymers with ethylene631 or acetylene397)  

A Five-membered ring with three carbon atoms and one Si-Si bond was found by some Chinese chemists654) who reacted l-(dimethylchlorosilyl)-3-(methylphenylchlorosilyl) propane with metallic sodium  

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This review would be incomplete without a summary of the properties of the Si—Si bond in cyclic and heterocyclic silanes and polymeric compounds containing Si—Si bonds. Important results were obtained, e.g. the reason for the color of these compounds was discovered. The bibliography is very comprehensive and will lead the reader to a more detailed study of the subject. 

Detailed investigations of the hydrolysis of disilanes to form heterocyclic compounds were carried out by Kumada et al.360) (for more details, see Chapter 9.2 Heterocyclic Silanes). 

The first heterocyclic silane with boron as a hetero atom was recently prepared280- 281> ... 

U.v.-induced addition of secondary phosphines to chlorovinylsilanes and allyl(chloro)silanes leads to the silylphosphines (36). A similar addition to vinyl-substituted heterocyclic silanes has given compounds such as (37). ... 

The new heterocyclic silane (93) can be prepared from 1,4-dilithio-octa-phenyltetrasilane and dichloro(dimethylamino)borane. It is a white. 

It should be emphasized that the results of these kinetic studies do not represent only the TBAF-promoted coupling of alkenylsilanols. Analyzed in the context of the spectroscopic studies and reported observations - which reveal that sile-tanes, silanols, and fluorosilanes, silyl hydrides, and heterocyclic silanes all form related species when mixed with TBAF - the mechanism deduced by the kinetic studies likely represents the mechanism of cross-coupling of all of these species. These mechanistic results are therefore relevant to a significant body of work, encompassing everything from the early fiuorosilane cross-coupling systems to the more recently developed TBAF-promoted coupling of pyridyl-, thienyl-, and benzylsilanes. 

K. Abersfelder and D. Scheschkewitz (2010) Pure Appl. Chem., vol. 82, p. 595 - Synthesis of homo- and heterocyclic silanes via intermediates with Si=Si bonds . 

The silicon containing (5 5 5) heterocycle 32 has been efficiently synthesized starting from. Y-prolinc 375 via silanes 376, 378, and 379. The final step has been achieved via radical cyclization. The amine 377 was treated with COCl2 and the resulting carbamoyl chloride 378 reacted with PhSeNa to give the radical precursor 379 (84% overall yield from 376). Compound 379 when refluxed with Ph3SnH or Bu3SnH in the presence of AIBN afforded the desired heterocycle 32 in 75% yield (Scheme 81). 

Silanes with heterocyclic residues were also found to be reactive (Equation (20)), although the most likely reason is the in situ cleavage of the heterocyclic residue in the presence of hydrated TBAF to form silanols.290-292 Similarly, among various arylgermanes, only those containing 2-furyl groups were reactive in cross-coupling reactions.293... 

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. 

Radialenes 86 were obtained when bis(l-diazo-2-oxoalkyl)silanes 84 were decomposed with copper or palladium catalysts60,61. The assumption, that the heterocyclic [3]cumulene 85 is the immediate precursor of 86, is corroborated by its trapping in a Diels-Alder reaction with furan. 

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

A number of P-N ligands has been reported as efficient ligands for the asymmetric hydrosilylation of ketones. We mention the phosphinooxazolines developed by Helmchen, Pfaltz, and Davis, we have seen before and mixed ligands containing planar-chiral heterocycles such as ferrocene [31] (Figure 18.17). For several ketone and silane combinations e.e. s in the high nineties were obtained. 

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

Silathietanes were first introduced as a new heterocyclic system in 1976 by a Soviet research groups who prepared them by addition of substituted silanes to divinyl sulfide (Scheme 19). The two possible intermediates 288... 

The intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

The synthetic plan was to assemble both the dihydropyran 3 and the cyclopentane 4 in enantiomerically-pure form, then to effect Lewis acid-mediated coupling of the ally silane of 4 with the anomeric ether of 3 to form a new stereogenic center on the heterocyclic ring. A critical question was not just the efficiency of this step, but whether or not the desired stereocontrol could be achieved at C-3. 

Amino acids continue to be useful starting materials for the preparation of enantiomerically-pure heterocycles. Henk Hiemstra of the University of Amsterdam and Floris Ruljes of the University of Nijmegen report (J. Am. Chem. Soc. 2004,126,4100) that cyclization of the ally silane 9 followed by ring-closing metathesis leads to the highly-functionalized quinolizidine 11. 

Irradiation of an imlntum salt with toluene (or with benzyltrimethyl-silane) yields a benzyl-substituted amine (5.16), and this reaction also can be adapted to produce multicydic nitrogen heterocycles by intramolecular reaction. 

Reaction with dimethylphenylsilane is catalyzed at room temperature under 250 psi of carbon monoxide. Other silanes tested, triethyl- and triphenylsi-lane, are not effective reagents in this system. A variety of aldehydes are good substrates for the reaction, including benzaldehyde, substituted benzaldehydes, and heterocyclic aldehydes. Aliphatic aldehydes also yield a-siloxy aldehyde products, but the reaction must be run at higher CO pressure (1000 psi) to avoid hydrosilylation. The reaction does not tolerate substrates bearing strong electron-withdrawing substituents, such as p-nitrobenzaldehyde. 

Several aromatic and heterocyclic acyl trimethylsilanes have been used as acyl anion equivalents by treatment with fluoride ion (Scheme 81, path a)23 133 154b160191192. Provided that the acyl substituent is electron-withdrawing, and that there are no aryl substituents on the silicon atom, acyl anions can be trapped by various electrophiles in moderate to good yields indeed, acyl anions and pentacoordinate silicon anionic species have both been detected in gas-phase reactions of acyl silanes with fluoride ion193. 

Heating 4- and 5-bromoacyl silanes at 100 °C in a polar aprotic solvent induces cyclization through the enol forms to give 2-silyldihydrofurans and 2-silyldihydropyrans, respectively195. Similar transformation of 4- to 7-halothioacyl silanes, prepared from the corresponding haloacyl silanes by reaction with hydrogen sulphide, but induced by sodium hydroxide, gave the 2-silylated sulphur heterocycles in excellent yields (Scheme 84)196. Intermolecular enolate reactions of acyl silanes are also known (vide infra, Section IV.A.6). 

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