Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes

Abstract

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me(3)SiC[triple bond]C-1,12-C2B10H11, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrC[triple bond]CSiMe(3) is reported. The low-yield preparation of 1,12-(Me3SiC[triple bond]C)2-1,12-C2B10H10 from the C,C'-dicopper para-carborane derivative with 1-bromo-2-(trimethylsilyl)ethyne, BrC[triple bond]CSiMe3, has been re-investigated and other products were identified including the C-monoethynyl-carborane 1-Me3SiC[triple bond]C-1,12-C2B10H11 and two-cage assemblies generated from cage-cage couplings. The contrast in the yields of the monoethynyl and diethynyl products is due to the highly unfavourable coupling process between 1-RC[triple bond]C-12-Cu-1,12-C2B10H10 and the bromoalkyne. The ethynyl group at the cage carbon C(1) strongly influences the chemical reactivity of the cage carbon at C(12)-the first example of the "antipodal effect" affecting the syntheses of para-carborane derivatives. New two-step preparations of 1-ethynyl- and 1,12-bis(ethynyl)-para-carboranes have been developed using a more readily prepared bromoethyne, 1-bromo-3-methyl-1-butyn-3-ol, BrC[triple bond]CCMe2OH. The molecular structures of the two C-monoethynyl-carboranes, 1-RC[triple bond]C-1,12-C2B10H11 (R = H and Me3Si), were experimentally determined using gas-phase electron diffraction (GED). For R = H (R(G) = 0.053) a model with C(5v) symmetry refined to give a C[triple bond]C bond distance of 1.233(5) A. For R = Me3Si (R(G) = 0.048) a model with C(s) symmetry refined to give a C[triple bond]C bond distance of 1.227(5) A. Molecular structures of 1,12-Br2-1,12-C2B10H10, 1-HC[triple bond]C-12-Br-1,12-C2B10H10 and 1,12-(Me(3)SiC[triple bond]C)2-1,12-C2B10H10 were determined by X-ray crystallography. Substituents at the cage carbon atoms on the C2B10 cage skeleton in 1-X-12-Y-1,12-C2B10H10 derivatives invariably lengthen the cage C-B bonds. However, the subtle substituent effects on the tropical B-B bond lengths in these compounds are more complex. The molecular structures of the ethynyl-ortho-carborane, 1-HC[triple bond]C-1,2-C2B10H11 and the ethene, trans-Me3SiBrC=CSiMe3Br are also reported.

Improved syntheses of bis(ethynyl)-para-carboranes, 1,12-(RCC)2-1,12-C2B10H10 and 1,10-(RCC)2-1,10-C2B8H8 (R=H or Me3Si)

“…The protected acetylenes 2 and 6 were likewise obtained from sequential reaction of 8 and 10 with methyllithium and chlorotrimethylsilane. The overall yields of 1, 2, 6 and 7 from para-carboranes 3 and 5 are 69%, 67%, 63% and 48%, respectively, which are superior to reported yields of up to 26% from previous single/two-step syntheses [6][7][8]10]. The convenience of a two-step route is also an obvious advantage in effort and cost over the more elaborate multi-step syntheses, which afford the ethynyl carboranes in comparable yield to those described here [9].…”

“…A low-yielding synthesis of 2 has been reported by Kaszynski et al [8] while we have also used 1-bromo-3-methyl-1-butyn-3-ol instead of 4 in a two-step synthesis of 1 in low yields [7]. Herzog et al reported multi-step syntheses of 1 and 2 from 3 in overall yields of 64% [9].…”

“…However, the expense of the 10-vertex precursor and the low yield of 6 have so far precluded the widespread use of 6 as a precursor of compounds to explore this point. It is noteworthy that not even the 10-vertex analogue of 1, 1,10-(HC"C) 2 -1,10-C 2 B 8 H 8 (7), was known prior to the present study, although calculations of this species have been reported at the HF/6-31G* level of theory [8,9].…”

“…The reaction of the 12-vertex para-carborane (3) with 1-bromo-2-(trimethylsilyl)ethyne (4) using our usual coppermediated cross-coupling procedure [5] gave 1,12-bis(trimethylsilylethynyl)-para-carborane (2) in low yields; subsequent desilylation of 2 affords the parent species 1, 12-bis(ethynyl)-para-carborane (1) (Scheme 1(a)) [6,7]. A low-yielding synthesis of 2 has been reported by Kaszynski et al [8] while we have also used 1-bromo-3-methyl-1-butyn-3-ol instead of 4 in a two-step synthesis of 1 in low yields [7].…”

Dicarba‐closo‐dodecaboranes with One and Two Ethynyl Groups Bonded to Boron

“…[19] In contrast, a number of dicarba-closo-dodecaboranes with ethynyl groups or other alkynyl substituents bonded to carbon were structurally characterized. [2,[27][28][29][30][31][32] In Figure 1 the diethynyl-substituted clusters 1a and 3a are depicted. The CϵC and B-C bond lengths of 1a and 3a are similar (Table 1) and they are also similar to bond lengths determined for ethynyl groups bonded to boron of different clusters, for example, in the anion [12-HCC-closo-1- [33] ( [34] The differences of the experimentally determined d(CϵC) of the ethynyl-functionalized boron clusters listed in Table 1 are small and not significant (Ͻ3σ).…”