pubs.acs.org/IC Published on Web 10/11/2010 r 2010 American Chemical Society Inorg. Chem. 2010, 49, 10247–10254 10247 DOI: 10.1021/ic101234p Microwave-Assisted Alkylation of [CB 11 H 12 ] - and Related 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 A total of 19 permethylated derivatives of substituted [CB 11 H 12 ] - anions have been prepared using alkylation with microwave assistance. The reactions proceed much faster and more cleanly than under ordinary conditions. Microwave assistance is especially convenient for the permethylation of carborane anions carrying electron- withdrawing groups in positions 1 and/or 12. Even [1-F-CB 11 H 11 ] - can be undecamethylated, whereas under ordinary heating, it has only been hexamethylated. Introduction Many derivatives of the [CB 11 H 12 ] - anion 1 have been reported over the years, 2-6 and polyalkylated anions have been of particular interest in our laboratory. 7,8 Highly alkylated carborane anions have remarkable properties, related to their high lipophilicity. They have a striking solubilizing ability for large organic 9 and “naked” alkali-metal 10 cations in solvents of low polarity. In poorly ligating solvents, their lithium salts strongly catalyze pericyclic reactions 11 and the radical-induced polymerization of terminal alkenes, such as isobutylene. 12,13 The relatively electron-rich nature of the BH vertices in [CB 11 H 12 ] - and similar anions, particularly in position 12 and only a little less in positions 7-11, is reflected in their reactivity toward electrophilic reagents. Methyl triflate, 7,8 methyl bro- mide, 14 or a trimethylaluminum/methyl iodide mixture 15 are capable of replacing all boron-bound hydrogen atoms with methyl substituents. However, alkylation of these anions with alkyl triflates often proceeds very slowly at room temperature 16 and yields triflyloxylated byproducts at higher temperatures. We now report that microwave heating permits a much easier synthesis of several known highly methylated carborate anions and affords previously inaccessible ones. Results The carborane anions 1a-19a were converted to the permethylated anions 1b-19b by the standard methylation procedure with methyl triflate in sulfolane in the presence of CaH 2 , 17 but with microwave assistance (Scheme 1 and Table 1). The base is needed to neutralize the triflic acid byproduct, which would otherwise tend to protonate the methyl groups to produce methane, placing triflyloxy substituents on the cage. The permethylation of [1-I-CB 11 H 11 ] - (3a) illustrates the difference between the conventional thermal (Figure 1) and microwave (Figure 2) conditions. Ordinary methylation of this anion proceeds very slowly at room temperature and is only complete after 6 weeks. 16 We now find that increasing the temperature to 70 °C shortens the reaction time to 4 days, and microwave-assisted reaction under the same reaction conditions, including temperature, provides the fully methyl- ated product in 90 min. The methylation reaction employed a smaller excess of the methylation agent than was found desirable in earlier work (20 equiv per 1 equiv of the carborate anion). The time needed for permethylation depends on the substituents X and Y present in positions 1 and 12 of the CB 11 cage, respectively (Scheme 1). Permethylation proceeds best for carborane *To whom correspondence should be addressed. E-mail: jessica@ eefus.colorado.edu. (1) Knoth, W. H. J. Am. Chem. Soc. 1967, 89, 1274. 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