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An efficient method is presented for the synthesis of novel donor-acceptor silole chromophores through selective monohalogenation of 2,5-dimetallosiloles followed by Negishi alkyne cross-coupling reactions. The electronic properties and crystal packing of these new siloles can be controlled through judicious combinations of peripheral functional groups.

Increasing interest in p-conjugated compounds containing silole rings (1-silacyclopentadiene) [1] is to a large extent due to recent exploitation for applications such as electrochemiluminescent sensors and light-emitting diodes. [2][3][4][5][6][7][8][9][10] Their unique photophysical and electronic properties [11] arise from the particularly low lying LUMO owing to s*-p* conjugation between the s* orbital of two exocyclic s bonds on the silicon atom and the p* orbital of the butadiene moiety. [12,13] We recently reported the synthesis of donor-acceptor siloles [14] and oligomeric ethynyl siloles, [15] and the electrogenerated chemiluminescence (ECL) [16] of several silole-based chromophores.[17] We also reported a series of 2,5-bis(arylethynyl) siloles in which a curious improvement of photoluminescence (PL) quantum efficiency from 9 to 63 % was achieved by increasing the steric bulk on the silicon atom and 2,5-substituents.[18] It seemed plausible that the enhanced luminescence resulting from the highly improved quantum efficiency was due to increasing the energy barriers for nonemissive decay processes. However, these electronic improvements did not translate to ECL, and the efficiencies of some ethylene-and ethynyl-substituted siloles [17] were only in the range of 0.001 to 0.1 relative to 9,10-diphenylanthracene (DPA), [19] owing to the instability of their radical cations needed for ECL generation.[20] Herein we report that successful tuning of the electrochemical potentials and of silole-thiophene hybrid chromophores results in higher stability of the radical cations and ultimately in improved ECL efficiency.Considering that 1,1-dimethyl-2,5-bis(2-thienyl)-3,4-diphenylsilole (3 a) and 1,1-dimethyl-2,5-bis[(2,2'-bithiophen)-5-yl]-3,4-diphenylsilole (4 a) are efficient electrontransporting materials, [21] we expected that replacement of the methyl substituents with larger isopropyl, tert-butyl and nhexyl groups would augment the energy barriers for nonemissive decay processes, stabilize the radical cations generated electrochemically, and thus result in enhanced photoluminescence and ECL. Therefore the target chromophores, 3 b-d and 4 b-d, were prepared as shown in Scheme 1. Intramolecular cyclization of bis(phenylethynyl) dialkyl silanes 1 with an excess of lithium naphthalenide followed by reaction with ZnCl 2 afforded 2,5-dizinc intermediates 2, which were used in situ to prepare silole chromophores 3 and 4 by Negishi cross-coupling reactions catalyzed by [PdCl 2 -(PPh 3 ) 2 ]. [15,22] Good yields were obtained by carrying out the entire reaction sequence, consisting of cycloreduction, transmetalation, and cross-coupling, from silane 1 to silole 3 or 4 in one pot. Figure 1 shows cyclic voltammograms (CVs) of 0.78 mm 4 a (a) and 1.34 mm 4 c (b) in dichloromethane solution containing 0.1m tetrabutylammonium perchlorate as supporting electrolyte. Electrochemical data for the other six siloles are summarized in Table 1. As the potential is scanned from zero to the positive region, 4 a loses an electron to bec...

The synthesis of a second-generation cobalt catalyst for the formation of trans-THF products via the Mukaiyama aerobic oxidative cyclization is reported. Two procedures have been developed with the new water-soluble catalyst that give superior yields and greatly simplify purification compared to the previous catalysts.