Synthesis and NMR-Study of the 2,3,4,5-Tetraethylsilole Dianion [SiC4Et4]2−•2[Li

Jang-Hwan Hong

doi:10.3390/molecules16098033

Abstract

The previously unknown silole dianion [SiC4Et4]2−•2[Li]+ (3) was prepared by the sonication of 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene [Cl2SiC4Et4, 2] with more than four equivalent of lithium in THF. 1H-, 13C-, and 29Si-NMR data of 3 are compared with those of the reported silole dianion [SiC4Ph4]2−. Trapping of 3 with trimethylchlorosilane gave 1,1-bis(trimethylsilyl)-2,3,4,5-tetraethyl-1-silacyclopentadiene [(Me3Si)2SiC4Et4, 4] in high yield. The silole of 2 was synthesized in high yield in three steps by a modified procedure using Cp2ZrCl2 via Cp2ZrC4Et4 and 1,4-dibromo-1,2,3,4-tetraethyl-1,3-butadiene.

Highlights

Since the first silole dianion, 2,3,4,5-tetraphenyl-1-silacyclopentadienide dianion, was prepared in 1990 by Joo and Hong [1], the aromaticity of the silole dianion [2] and germole dianion [3] was suggested by NMR study and it was confirmed by X-ray crystallography of the structures [4,5,6,7,8] and by theoretical study [9,10]
1,1-Dibromo-2,3,4,5-tetramethyl-1-silacyclopentadiene is synthesized from Cp2ZrC4Me4 and SiBr4 in very low yield [20]. 1,1-Dichloro-2,3,4,5-tetrametyl-1-silacyclopentadiene is synthesized from
7.83 In CDCl3, reference; external TMS as standard; b In THF-d8, reference = 25.30 ppm. These chemical shifts of 29Si and 13C resonances are consistent with delocalization of the negative charge into the silole ring, which is supported by the calculated negative NICS value of dilithiumsilole dianion [9,10]

Summary

Introduction

Since the first silole dianion, 2,3,4,5-tetraphenyl-1-silacyclopentadienide dianion, was prepared in 1990 by Joo and Hong [1], the aromaticity of the silole dianion [2] and germole dianion [3] was suggested by NMR study and it was confirmed by X-ray crystallography of the structures [4,5,6,7,8] and by theoretical study [9,10]. Only two silole dianions have been reported so far; [SiC4Ph4]2− (I) [1,2,4], [SiC4Me4]2− (II). [5] with the silafluorenyl dianion [SiC4(CH2)8]2−2[M]+ (III) [17,18] and the silaindenyl dianion [(CH2)4C2SiC2PhBu]2−2[M]+ (IV) [19] since the available 1,1-dihalosiloles are limited (Figure 1). Two silole dianions are reported since the synthetic methods for 1,1-dihalosiloles are limited to. The former is readily prepared from SiCl4 and 1,4-dilithio-2,3,4,5-tetraphenyl-1,3-butadiene, which is produced from diphenylacetylene and lithium, it is unable to exchange the phenyl groups with other groups [1]. 1,1-Dibromo-2,3,4,5-tetramethyl-1-silacyclopentadiene is synthesized from Cp2ZrC4Me4 and SiBr4 in very low yield [20]. 1,1-dichloro-2,3,4,5-tetraethyl-1-silacyclopentadiene [Cl2SiC4Et4] and an NMR study of the silole dianion [SiC4Et4]2−2[Li]+

Results and Discussion

General Procedures

Conclusions