Disilacyclopentanes

Disilanes add to conjugated dienes by splitting their Si—Si bond. 1.1.2.2-Tetramethyl-1.2-disilacyclopentane (82) reacts with butadiene at 100 C to give l,l,5,5-tetramethyl-l,5-disilacyclotrideca-7,l 1-diene (83) in 8330 yield[77]. The six-membered carbodisilanes undergo a similar reaction to give 14-membered compounds. 

The disilanickela compound 21 is not effective in the nickel-catalyzed double silylation reaction with styrene. However, the stoichiometric reaction of 21 with styrene afforded 4,5-carboranylene-l,l,3,3-tetramethyl-2-phenylmethylene-1,3-disilacyclopentane 29. A key feature in the 111 NMR spectmm of 29 includes a singlet at 7.71 ppm assigned to the vinyl proton. A characteristic low-frequency 13C NMR resonance at 139.75 ppm provides evidence for a tethered carbon atom of the two silicon moieties. Unambiguous confirmation was provided by X-ray crystallographic analysis of 29. 

Anionic polymerization techniques were also critical for the synthesis of a model cyclic triblock terpolymer [cyclic(S-fo-I-fr-MMA)] [196]. The linear cctw-amino acid precursor S-fr-I-fr-MMA was synthesized by the sequential anionic polymerization of St, I and MMA with 2,2,5,5-tetramethyl-l-(3-lithiopropyl)-l-aza-2,5-disilacyclopentane as the initiator and amine generator, and 4-bromo-l,l,l-trimethoxybutane as a terminator and carboxylic acid generator. Characterization studies of the intermediate materials as well as of the final cyclic terpolymer revealed high molecular and compositional homogeneity. Additional proof for the formation of the cyclic structure was provided by the lower intrinsic viscosity found for the cyclic terpolymer compared to that of the precursor. 

The following cyclization took place readily to favor formation of a disilacyclopentane ... 

The discussion continues regarding the role of silanone and cyclodisiloxanes as reactive intermediates in the formation of Si-O-Si bond.25 In studies of the reaction of dimethyldichlorosilane, phenylmethyldichlorosilane, or diphenyldichlorosilane with dimethyl sulfoxide in the presence of 2,2,5,5-tetramethyl-l-oxa-2,5-disilacyclopentane, Weber and co-workers obtained products of the insertion of diorganosiloxy unit into the cyclic siloxane, accompanied... 

The selective cleavage of the silicon-silicon bond in the disilacyclopentane and -hexane is probably due to the concentration of internal angular strain at this bond in such smaller polygonal molecules. In case of the disilacyclo-heptane, however, it seems likely that the strain is smaller and uniformly distributed all over the ring, and hence cleavage occurs preferentially at the silicon-carbon bond by the accepted mechanism involving both an electrophilic attack on carbon and nucleophilic attack on silicon by the sulfuric acid molecule(s) 169). 

Hexaorganodisilanes are quite resistant to alcoholic alkali (Section II, B, 2). However, l,l,2,2-tetramethyl-l,2-disilacyclopentane undergoes cleavage exclusively at the silicon-silicon bond with evolution of an almost theoretical volume of hydrogen when heated at 90° C with 0.6 N sodium ethoxide in ethanol over an 80-hour period, giving products as depicted by the following equation (124). 

Scheme 4.1 Synthesis of oligopeptide conjugates. Reaction conditions and reagents (a) n-BuLi, THF, -78°C->0°C 1(3-bromopropyl)-2,2,5,5-tetramethyl-l,l-aza-2,5-disilacyclopentane THF/HCl (b) Fl2 (lOObar), Pd/C, toluene, 80°C, 3 days (c) Fmoc-Ala-OH, EDCI/HOBt, TEA, DCM, -40°C r.t. (d) piperidine, chloroform (e) Fmoc-Ala-Ala-OH, PyBOP, DIEA, DCM, r.t.

Tetramethyl-l, 3-disilacyclopentane 89 was obtained by the hydrolysis of the A1 compound. Addition of R2A1H to the acetylenic group must therefore lead to the cis-olefine compound besides other products. Only compound 89 conform the five-membered ring 87 was obtained in about 40% yield. 

In this reaction two Si compounds are obtained which are more volatile than the starting material. The nmr resonances at —25 Hz, 4.0 Hz and 39 Hz are assigned to Si2C7Hi8 (l,l,3,3-tetra-methyl-l,3-disilacyclopentane 89, which can also be synthesized in a different procedure (see Section 4). The signal intensities at 1.5 Hz, 12 Hz and 370 Hz are in a 2 12 2 ratio and represent the three different proton groups of the four-membered ring 88. 

Not very much had so far been reported on the 1,3-disilacyclopentanes. Small amounts of 1.1.3.3-tetramethyl-l,3-disilacyclopentane were found in the products of the pyrolysis of tetramethylsilane33) and trimethylsilane42. Furthermore, 1,1,3,3-tetramethyl (or tetraphenyl-)-2-methyl-1,3-disilacyclopentane is obtained in low yields by the reaction of HR2SiCH = CH2 (R = CH3 and CgHs) at platinum contacts43. According to the results of the synthesis of the 1,3-disila-pentanes 86 and 87 two cyclization reactions are expected to be suitable for the synthesis of 1,1,3,3-tetramethyl-1,3-disilacyclopentane a and b... 

The synthesis of the Si-functional derivatives of 89 can be accomplished in a similar way. All the Si-Cl-containing derivatives of 1,3-disilacyclopentane were obtained39 ... 

The nmr spectra of 1,3-disilacyclopentanes show higher bond orders. In all cases, the structures were confirmed by their characteristic spectra39. ... 

Palladium-catalyzed reactions of as- and /ra t-l,2-diphenyl-l,2-disilacyclopentanes 116 with phenylacetylene and diphenylacetylene were carried out in a sealed glass tube at 200 °C resulted in the formation of corresponding 1,4-disilacyclohept-2-enes 120 (Scheme 23). The stereospecific formation of 120 was explained in terms of the insertion of a palladium species into a silicon-silicon bond in disilacyclopentanes to give a 2-pallada-l,3-disilacyclohexane 117 with retention of the configuration, followed by coordination of an alkyne to the palladium atom in the intermediate... 

In this section, ring formation of silicon-containing metallacycles is described and classified according to the heteroatom involved in the reaction. Therefore, various syntheses of disilacyclopentanes, disilacyclopentenes, silaboracyclopentanes, azasilacyclopentanes, oxasilacyclopentanes, and finally thiasilacyclopentanes are described. 

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