Palladium-Catalyzed Cross-Methylation of Aryl Chlorides by Stabilized Dimethylaluminium and -Gallium Reagents

Abstract

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.

Mechanistic Aspects of Aryl–Halide Oxidative Addition, Coordination Chemistry, and Ring‐Walking by Palladium

“…Selective cleavage of a strong bond by a late‐transition metal in the presence of a weaker one is rare 4h. 5b, 24 In addition, no metal transfer was observed during the formation of complex ${{\bf 3}{{{\rm \bf Br}\hfill \atop = \hfill}}}$ from ${{\bf 2}{{{\rm \bf Br}\hfill \atop = \hfill}}}$ in the presence of PhBr …”

Palladium-Catalyzed Coupling Reactions of Aryl Chlorides

“…Blum et al have recently shown that aryl chlorides can be methylated by hypervalent aluminum and gallium complexes 165. [Pd(PPh 3 ) 4 ] is an acceptable catalyst for highly activated nitro‐substituted aryl chlorides, but the palladium complex of a bulky, electron‐rich, chelating ligand such as 1,3‐bis(diisopropylphosphanyl)propane (dippp) is required for electron‐neutral substrates.…”

Palladiumkatalysierte Kupplungen von Arylchloriden

“…Blum et al zeigten, dass Arylchloride durch hypervalente Aluminium‐ und Galliumkomplexe methyliert werden können 165. Für hochaktivierte nitrosubstituierte Arylchloride ist [Pd(PPh 3 ) 4 ] ein akzeptabler Katalysator, doch für elektronenneutrale Substrate ist der Palladiumkomplex eines voluminösen, elektronenreichen chelatbildenden Liganden wie 1,3‐Bis(diisopropylphosphano)propan (dippp) nötig.…”