Silacyclobutene derivatives

Conlin and coworkers reported a rate constant for the thermal electrocyclic ring closure of the transient l,3-(l-sila)butadiene derivative 8a in cyclohexane at 25 C, k = (1.19 0.06) x 105 s l31. The rate constant corresponds to the inverse of the lifetime of the silene in the dry solvent, as measured by laser flash photolysis of the isomeric silacyclobutene derivative 7a, the stable product of the thermal ring closure reaction... 

Typical Procedure for the Preparation of Silacyclobutene Derivatives (see Eq. 2.78) To a solution of Cp2ZrCl2 (1.25 mmol, 0.365 g) in THF (10 mL) at —78°C (dry-ice/methanol bath) in a 20 mL Schlenk tube, EtMgBr (2.5 mmol, 0.90 m solution in THF, 2.78 mL) was added dropwise by means of a syringe. After the addition was complete, the reaction mixture was stirred at — 78 °C for 1 h. Bis(phenylethynyl)dimethylsilane (1 mmol) was then added and the mixture was allowed to gradually warm to room temperature. After the reaction mixture had been stirred at room temperature for 1 h, it was quenched with 3 n HC1, and the resulting mixture was extracted with diethyl ether (3 X 70 mL). The combined extracts were washed with water and brine, and then dried over MgS04. Evaporation of the solvent in vacuo gave the desired products. 

Scheme 4.3. Ring-opening polymerization of an optically active silacyclobutene derivative.

A pseudo-silacyclobutene derivative, 2 3-benzo-l,l-diphenyl-l-silacyclobut-2-ene, was prepared by three independent synthetic procedures, and was found to exhibit the high reactivity of silacyclobutanes. The facile ring openings observed with this pseudo-silacyclobutene are... 

Hydrolysis of 1-21 with water affords their corresponding silacyclobutene derivatives 1-22 in high yields. Reaction of 1-21 with iodine results in demetallation and affords 1,3-butadiyne 1-23. 

Nagao et al. studied the zirconocene-mediated coupling of unsymmetrical bis (alkynyl)silanes with bulky substituents on silicon atom. When unsymmetrical bis (alkynyl)silanes 1-30 was applied, upon hydrolysis, the silacyclobutene derivatives 1-32 could be synthesized regioselectively (Scheme 1.17) alkyl or alkenyl group was selectively located at a-position of silacyclobutene, while aryl group was selectively located at -position of zirconacyclobutene [54]. 

Takahashi T, Xi Z, Obora Y et al (1995) Intramolecular coupling of alkynyl groups of bis (alkynyl)silanes mediated by zirconocene compounds formation of silacyclobutene derivatives. J Am Chem Soc 117 2665-2666... 

As described in Eq. 14 and Eqs. 26 to Eq. 28, and Fig. 2, along with formation of metallacycles, silacyclobutene derivatives and the phosphorus analogues are formed. These methods represent general routes for silacyclobutene derivatives. Interesting reaction chemistry of such silacycles can be expected. 

The only metalloid derivative of benzocydobutene contains a Si heteroatom. This is not surprising since the only derivative of a four-membered heterocyclobutene is the silacyclobutene discussed in Section... 

A direct approach to silacyclobutane (siletane) derivatives is the reaction of meth-ylsilenes with other unsaturated compounds. Thus, the silaethylene (silene) (326), formed from pivaloyl-tris(TMS)-silane (325) via thermolysis (10h/140°C), reacts with 1-phenylprop-l-yne to yield the silacyclobutene (silete) derivative 327 (equation 148)172. 

In recent years Ishikawa and coworkers have described some very complex photochemical rearrangements of compounds derived from nickel-catalyzed reactions of silacyclopropenes with silylalkynes. As depicted in equation 93, it was shown that the disilacyclohexadiene A could be photochemically converted to the same products as were formed from the silacyclobutene B. In the absence of a trapping reagent the silaindene C was formed as a major product, via the proposed intermediates shown141. In the presence of methanol, two adducts were isolated whose structures can be understood in terms of the reaction of methanol with one of the proposed intermediates. While the structures of the products are confirmed by the X-ray crystal structure data, the mechanisms of the reactions observed must be considered speculative at this time. 

The 1-silacyclobutene side of this type of equilibrium was also reached by ring expansion in phenylcyclopropylsilylene obtained by pyrolysis of a silabenzonorborna-diene precursor (equation 106)223. Photochemical or pyrolytic ring opening of a l-silacyclobut-2-ene derivative provides an entry to the 1-silabutadiene series151. 

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