[2+2+1]cycloadditions of bis(dialkylamino)acetylenes with SiI 2(Idip): Syntheses and reactivity studies of unprecedented 2,3,4,5-tetraamino-1 H-siloles

Abstract

A novel method for the synthesis of 1H-siloles is presented. It involves a [2+2+1] cycloaddition of the ynediamines R2N-C≡C-NR2 (R=Me, Et) with SiI2(Idip) (Idip=1,3-bis(2,6-diisopropylphenyl) imidazolin-2-ylidene) to afford the orange-colored, highly water-sensitive 1,1-diiodo-2,3,4,5-tetraamino-1H-siloles SiI2{C4(NR 2)4} (1-I: R=Me; 2-I R=Et). Treatment of 2-I with an excess of SiBr4 afforded after I/Br exchange the 1,1-dibromo-1H- silole SiBr2{C4(NEt2)4} (2-Br). The 1H-siloles 1-I, 2-I, and 2-Br were fully characterized and their molecular structures determined by single-crystal X-ray diffraction. The compounds feature a slightly twisted five-membered silacyclopenta-2,4-diene ring and a double/single C-C bond alternation in the diene fragment. Reaction of 2-I with the N-heterocyclic carbene IMe4 (IMe4=1,3,4,5- tetramethylimidazolin-2-ylidene) leads, after displacement of the iodide groups, to the unprecedented diiodide salt [Si(IMe4)2{C 4(NEt2)4}](I)2 (3), containing a 1H-silole dication with a four-coordinate SiIV center. The crystal structure of 3 reveals similar bonding characteristics for the dicationic 1H-silole to those of the neutral 1H-siloles 1-I-2-Br. Two-electron reduction of 3 with C8K affords, after elimination of one IMe4 group, the thermolabile, carbene-stabilized 1-silacyclopentadien-1-ylidene Si{C 4(NEt2)4}(IMe4) (4), which was characterized by elemental analysis and 1H, 13C{ 1H}, and 29Si{1H} NMR spectroscopies. 2,3,4,5-Tetraamino-1H-siloles SiI2{C4(NR2) 4} (1-I: R=Me; 2-I R=Et) were synthesized by a [2+2+1] cycloaddition of R2N-C≡C-NR2 (R=Me, Et) with NHC-stabilized SiI2 (L1=1,3-bis(2,6-diisopropylphenyl)imidazolin-2- ylidene). Reaction of 2-I with the N-heterocyclic carbene IMe4 (IMe4 (L2)=1,3,4,5-tetramethyl-imidazolin-2-ylidene) leads to [Si(IMe4)2{C4(NEt2) 4}](I)2 (3), containing a 1H-silole dication with a four-coordinate SiIV center. Two-electron reduction of 3 with C 8K affords the carbene-stabilized 1-silacyclopentadien-1-ylidene Si{C4(NEt2)4}(IMe4) (4). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.