The intramolecularly stabilized mono- and dialkylaluminum complexes 1a, 2, 3, 4a, 5a, 5c, 6a, 6c, 7, 8, and 9 in the presence of palladium catalysts, cross-alkylate aryl, vinyl, and benzyl bromides and iodides under mild standard laboratory conditions. Aryl bromides with carbonyl substituents or benzylic halides are converted partially into dialkyl compounds. Under similar conditions, the analogous stabilized dimethylgallium complexes 1b, 4b, 5b, 6b, and 10 methylate aryl and vinyl bromides and iodides in a highly selective manner. Substituted bromobenzenes XC6H4Br, where X = CHO, COPh, CO2Et, CN, NO2, Cl, CH2Br, or CHCHCOPh, are methylated by the organogallium reagents usually only at the aromatic ring halogen atom to give substituted toluenes as single products. The methylation rates were shown to depend on the nature of the chelating ligands, on the solvent, and on the type of palladium catalyst employed.

Two methods for palladium-catalyzed cross-methylation of aryl chlorides by intramolecularly stabilized dialkylaluminium and -gallium complexes 6-13 have been studied. In one method, in which either tetrakis(triphenylphosphine)palladium (1) or dichlorobis(triphenylphosphine)palladium (2) is used as the catalyst at 80-90°C, the activation of the chlorine atom is affected by introduction of strong electron-withdrawing groups into the aromatic moiety. The second method is based on the application of either [1,3-bis(diisopropylphosphino)propane)]palladium (4) or homologous electron-rich palladium complexes as catalysts. Although 4 promotes smooth cross-alkylation of aryl chlorides it fails to activate simple aryl bromides.In a previous communication 1 we reported that intramolecularly stabilized alkylaluminium complexes can substitute Grignard and organolithium reagents in alkylation of many carbonyl compounds. Further studies revealed that at 80-90°C these aluminium reagents, as well as their gallium analogs, successfully cross-alkylate aryl bromides, 2 iodides 2 and trifluoromethanesulfonates 3 in the presence of either Pd(Ph 3 P) 4 (1) or PdCl 2 (Ph 3 P) 2 (2), as illustrated in Scheme 1. Aryl chlorides, however, react under these conditions extremely slowly or not at all. Scheme 1In this paper we report two methods by which cross-methylation of aromatic chlorides becomes feasible: (i) activation of the halogen atom by introduction of the strong electron-withdrawing nitro moieties into the aromatic skeleton and (ii) replacement of the above palladium catalysts by electron richer phosphine complexes such as [1,2-bis(diisopropylphosphino)ethane]palladium [Pd(dippe) 2 , 3], [1,3-bis(diisopropylphosphino)propane]palladium [Pd(dippp) 2 , 4], or [1,4-bis(diisopropylphosphino)butane]palladium [Pd(dippb) 2 , 5] 4 (Figure). Thus, although chlorobenzene reacts only sluggishly with dialkylaluminium and dialkylgallium complexes 6-13 (Figure) in the presence of 1, and does not react at all in P P P P Pd P P P Pd P P P 4 5 P 3 P Pd M N Me Me Me Me O M O Me Me O M O Me Me 7: M = Ga 6: M = Al Me Me 8: M = Al 9: M = Ga O M N Me Me Me Me O M N Me Me Me Me O Al O R R Me O Al O R R Me 10: M = Al 11: M = Ga 12: R = Me 13: R = Et Figure Structures of Pd catalysts 3-5 and dialkylaluminium and dialkylgallium complexes 6-13 Downloaded by: University of Illinois at Chicago. Copyrighted material.

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