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

Fox, M.A.; Cameron, A.M.; Low, P.J.; Paterson, M.A.J.; Batsanov, A.S.; Goeta, A.E.; Rankin, D.W.H.; Robertson, H.E.; Schirlin, J.T.

Abstract

A high-yield preparation of the C-monoethynyl para-carborane, 1-Me3SiC[triple bond, length as m-dash]C-1,12-C2B10H11, from C-monocopper para-carborane and 1-bromo-2-(trimethylsilyl)ethyne, BrC[triple bond, length as m-dash]CSiMe3 is reported. The low-yield preparation of 1,12-(Me3SiC[triple bond, length as m-dash]C)2-1,12-C2B10H10 from the C,C′-dicopper para-carborane derivative with 1-bromo-2-(trimethylsilyl)ethyne, BrC[triple bond, length as m-dash]CSiMe3, has been re-investigated and other products were identified including the C-monoethynyl-carborane 1-Me3SiC[triple bond, length as m-dash]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, length as m-dash]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, length as m-dash]CCMe2OH. The molecular structures of the two C-monoethynyl-carboranes, 1-RC[triple bond, length as m-dash]C-1,12-C2B10H11 (R = H and Me3Si), were experimentally determined using gas-phase electron diffraction (GED). For R = H (RG = 0.053) a model with C5v symmetry refined to give a C[triple bond, length as m-dash]C bond distance of 1.233(5) Å. For R = Me3Si (RG = 0.048) a model with Cs symmetry refined to give a C[triple bond, length as m-dash]C bond distance of 1.227(5) Å. Molecular structures of 1,12-Br2-1,12-C2B10H10, 1-HC[triple bond, length as m-dash]C-12-Br-1,12-C2B10H10 and 1,12-(Me3SiC[triple bond, length as m-dash]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, length as m-dash]C-1,2-C2B10H11 and the ethene, trans-Me3SiBrC[double bond, length as m-dash]CSiMe3Br are also reported.

Citation

Fox, M., Cameron, A., Low, P., Paterson, M., Batsanov, A., Goeta, A., …Schirlin, J. (2006). Synthetic and structural studies on C-ethynyl- and C-bromo-carboranes. Dalton Transactions, 29, 3544-3560. https://doi.org/10.1039/b517538k

• Journal Article Type: Article
• Publication Date: 2006-04
• Journal: Dalton Transactions
• Print ISSN: 1477-9226
• Electronic ISSN: 1477-9234
• Publisher: Royal Society of Chemistry
• Peer Reviewed: Peer Reviewed
• Volume: 29
• Pages: 3544-3560
• DOI: https://doi.org/10.1039/b517538k
• Keywords: 1477-9226, 1477-9234