pubs.acs.org/IC Published on Web 10/08/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 10255–10263 10255 DOI: 10.1021/ic101235e Lithium Salts of [1,12-Dialkyl-CB 11 Me 10 ] - Anions Michal Val  a  sek, † Jan  Stursa, † Radek Pohl, † and Josef Michl* ,†,‡ † Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, 166 10 Prague 6, Czech Republic, and ‡ Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215 Received June 21, 2010 We report the syntheses of several [1-R-CB 11 -Me 11 ] - and [1-R-12-R 0 -CB 11 -Me 10 ] - anions (R, R 0 = alkyl) and the solubilities of their lithium salts in cyclohexane. These solutions are of interest as Lewis acid catalysts. The new anions are not directly accessible by methylation with methyl triflate because of intervening triflyloxy substitution on one or more boron vertices. The difficulty has been circumvented in two ways. Either (i) an iodo substituent is first introduced into position 12, permitting a clean decamethylation, and then replaced with a methyl by reaction with trimethyl- aluminum or (ii) the offending triflyloxy substituents are replaced with methyls by reaction with trimethylaluminum. Introduction Lithium salts of polymethylated carborane anions 1-3 are soluble in solvents of low polarity, such as benzene and 1, 2-dichloroethane. In these solutions, the lithium cation acts as a strong Lewis acid and catalyzes pericyclic reactions 4 and radical-induced polymerization of terminal alkenes. 5,6 The catalytic activity might be higher in solvents with even lower ability to solvate the Li þ cation, such as cyclohexane. However, extremely nonpolar solvents fail to dissolve salts such as Li[CB 11 Me 12 ] and Li[HCB 11 Me 11 ] to a sufficient extent. A remedy is now being sought in the replacement of one or more of the methyl groups with a longer alkyl. Positions 1 and 12 are obvious candidates because substitu- tion at these sites preserves 5-fold symmetry and is least likely to be plagued by the formation of mixtures. The introduction of a longer alkyl group on the carbon vertex 1 by deproton- ation followed by alkylation is facile. 7 Ordinarily, the boron vertices are subsequently methylated with strong electrophiles such as methyl triflate, 1,2 methyl bromide, 8 or a trimethyl- aluminum/methyl iodide mixture. 9 The ease of these reactions depends strongly on the substituents that are already present, and it is unfortunate that 1-alkyl groups other than methyl greatly promote a side reaction in which one or more of the substituents carried by the boron vertices are replaced with triflyloxy groups. This is especially likely to occur at the higher temperatures that often need to be employed for exhaustive alkylation. Therefore, such direct methylation does not represent a viable route to the desired anions. Presently, we describe two routes to the previously un- known peralkylated anions [1-R-CB 11 Me 11 ] - and [1-R-12- R 0 -CB 11 Me 10 ] - . We have converted some of the most prom- ising candidates to their lithium salts and find that they are indeed several times more soluble in cyclohexane than that of [CB 11 Me 12 ] - . Results Synthesis. Alkylation of the lithiated cesium salt of [CB 11 H 12 ] - (1) with alkyl iodides to yield the 1-alkylcar- *To whom correspondence should be addressed. E-mail: michl@eefus. colorado.edu. (1) King, B. T.; Janou sek, Z.; Gr€ uner, B.; Trammel, M.; Noll, B. C.; Michl, J. J. Am. Chem. Soc. 1996, 118, 3313. (2) King, B. T.; K€ orbe, S.; Schreiber, P. J.; Clayton, J.; N emcov a, A.; Havlas, Z.; Vyakaranam, K.; Fete, M. G.; Zharov, I.; Ceremuga, J.; Michl, J. J. Am. Chem. Soc. 2007, 129, 12960. (3) K€ orbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106, 5208. (4) Moss, S.; King, B. T.; de Meijere, A.; Kozhushkov, S. I.; Eaton, P. E.; Michl, J. Org. Lett. 2001, 3, 2375. (5) Vyakaranam, K.; Barbour, J. B.; Michl, J. J. Am. Chem. Soc. 2006, 128, 5610. (6) Volkis, V.; Mei, H.; Shoemaker, R. K.; Michl, J. J. Am. Chem. Soc. 2009, 131, 3132. (7) Jelı´nek, T.; Baldwin, P.; Scheidt, W. R.; Reed, C. A. Inorg. Chem. 1993, 32, 1982. (8) Tsang, Ch. W.; Xie, Z. Chem. Commun. 2000, 1839. (9) Jiang, W.; Knobler, C. B.; Mortimer, M. D.; Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1995, 34, 1332.