Polyboration Reaction of 1,1-Diethynyl-1-silacyclopent-3-ene with Boranes

2005 Bulletin of the Korean Chemical Society (Print)  
Conjugated polycarbosilanes through polymer backbone have received much attention for their potential applications as electronic and ceramic materials. 1 In this sense, considerable research efforts have been devoted to develop new polycarbosilanes containing novel conjugation moieties. Poly[(silylene)diacetylene] polymers showed conductivity values in the range of 10 −5~1 0 −3 S/cm when doped with FeCl 3 , and pyrolysis of the polycarbosilanes in inert atmosphere resulted in a β-SiC ceramic
more » ... erials in high yield. 2 Conjugated poycarbosilanes bearing arylethylene or acetylene-linked dithienosilole have been prepared and their electronic properties were also reported. 3 Oligomeric materials composed of alternating 2,5-bis(ethynylenedimethylsilylene)thienylene and arylene units in the main chain have been prepared by a cross-coupling reaction. 4 We have also reported the preparation and excited-state energy dynamics of polycarbosilanes containing 1,4-bis(thiophene or phenylene)buta-1,3-diyne in the polymer backbone. 5 Organosilacyclic compounds such as 1-silacyclopent-3-enes have received much concern in organosilicon chemistry. 6 1,1-Dimethyl-1-silacyclopent-3-ene undergoes anionic ringopening reaction to give poly(1,1-dimethyl-1-sila-cis-pent-3-ene). 7 Recently, we reported the preparation of polycarbosilanes containing diacetylenic and organosilacyclic groups along the polymer main chain such as poly(1,1-diethynyl-1silacyclopent-3-enes and 1,1-diethynyl-1-silacyclobutane) using oxidative coupling polymerization and their electronic properties. 8 Organoborons are useful reagents for the preparation of a variety of functional compounds in organic synthesis. 9 Chujo et al. have reported that various πconjugated organoboron polymers consisting of C-B bonds in the main chain were able to be synthesized by several polyaddition reactions between organic diynes and borane derivatives. 10 However, boration polymerizations between organosilicon diynes and borane derivatives have been scarcely reported. Very recently, we have demonstrated the boration polymerizations between 1,1-diethynyl-3-triethylsilyl-1-silacyclopent-3-ene and several borane derivatives. 11 Since we have successfully prepared 1,1-diethynyl-1-silacyclopent-3-ene as novel diyne-containing silacyclic monomer, 8,12 we have decided to extend this polyaddition technique to the preparation of polycarbosilanes containing silacyclopentene and organoboron groups along the polymer backbone by utilizing 1,1-diethynyl-1-silacyclopent-3-ene. Furthermore, the synthesized organoborane-bearing polycarbosilanes can be expected to include a novel π-conjugated moiety of C=C-B-C=C and the organosilacyclic group of silacyclopent-3ene along the polymer main chains. Here, we now report the polyboration reaction of 1,1-diethynyl-1-silacyclopent-3-ene with several organoborane derivatives along with the electronic and thermal properties of the obtained carbosilane materials. In particular, the fluorescence emission maxima of the prepared oligomers containing 1-silacyclopent-3-enes and boranes were observed at the range of 310-415 nm, whereas π-conjugated organoboron polymers containing heteroaromatic monomer such as thiophene or pyridine and mesitylborane showed those emission maxima at the visible region of 416-593 nm. 11, 13 Results and Discussion Synthesis of Oligomers. 1,1-Diethynyl-1-silacyclopent-3-ene (1) was synthesized by the reaction of 1,1-dichloro-1silacyclopent-3-ene with ethynylmagnesium chloride according to the previous method reported by our research group. 8 The monomer 1 was characterized using several spectroscopic methods such as 1 H, 13 C, and 29 Si NMR as well as IR spectroscopy. In the IR spectrum of 1, the characteristic acetylenic C ≡ C and ring vinyl C=C stretching frequencies are observed at 2040 and 1606 cm −1 , respectively. In the 1 H NMR spectrum of 1, the singlet resonance appears at 2.56 ppm for ethynyl (C≡CH) protons and the triplet at 5.91 ppm for vinyl (C=CH) proton on the ring. In the 13 C NMR spectrum of 1, four carbon peaks appear at 18.11, 84.12, 96.26, and 130.14 ppm, where the resonances assigned to the ethynyl carbons are observed at 84.12 and 96.26 ppm, and one assigned to the vinyl carbons at 130.14 ppm. In the 29 Si NMR spectrum of 1, silicon peak appears at -29.78 ppm. The polyaddition reactions of 1 with several borane derivatives 2a-e were carried out to give the novel materials 3a-e in moderate yields, respectively, which contain C=C-B-C=C moiety and organosilacyclic group in the polymer main chain, as shown in Scheme 1. These boration reactions proceed selectively in the manner of cis addition. 10a,14 The boron-bearing polycarbosilanes 3a-e obtained as dark brown powder or yellowish viscous gel were found to be soluble in common organic solvents such as CHCl 3 and THF. By running gel permeation chromatography (GPC) we measured average molecular weights of 3a-e which turned out to be oligomers with average molecular weights ranging
doi:10.5012/bkcs.2005.26.3.488 fatcat:gg4itvs4e5fqrfvg64v2kj3wma