Carbon-carbon coupling of C(sp3)-F bonds using alumenium catalysis.
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abstract
Dialkylalumenium cation equivalents coupled with the hexabromocarborane anion function as efficient and long-lived catalysts for alkylation of aliphatic C-F bonds (alkylative defluorination or AlkDF) by alkylaluminum compounds. Only C(sp(3))-F bonds undergo AlkDF; C(sp(2))-F bonds are unaffected. Examples of compounds undergoing AlkDF include monofluoroalkanes, gem-difluorocyclopentane, and compounds containing a CF(3) group attached to either an aryl or an alkyl substituent. Conversion of C-F bonds to C-Me bonds is accomplished with high fidelity using Me(3)Al as the stoichiometric reagent. In reactions with Et(3)Al, hydrodefluorination of the C-F bonds is competitive with alkylation, indicative presumably of competitive hydride vs alkyl transfer from Et(3)Al. In a trialkylaluminum reagent, 1.1-1.4 alkyl groups per Al can be used to replace C-F bonds. Organoaluminum compounds efficiently remove water from the reaction mixture, obviating the need for rigorously dry solvents. Some organoaluminum compounds, especially methylaluminoxane, are capable of AlkDF with more reactive substrates, but catalysis by alumenium offers an advantage over the uncatalyzed C-F activation in terms of both increased rate and, in some cases, a dramatically increased selectivity.
