Reported here is a palladium catalyzed intramolecular acylcyanation of alkenes using α-iminonitriles. Through this method, highly functionalized indanones are synthesized in moderate to high yields using Pd(PPh3)4, without need for any additional ligands, and a common Lewis acid (ZnCl2). Additionally, the reaction tolerates substitution at various positions on the aromatic ring including electron donating, and electron withdrawing groups.
The first example of aryl-1-aza-2-azoniaallene salts undergoing a [4⊕ + 2]-cycloaddition reaction in which the azo bond and one aromatic π-bond make up the 4π component is described. This intramolecular reaction appears to be concerted and provides high yields of protonated azomethine imine products that contain a 1,2,3,4-tetrahydrocinnoline core. Substituted alkenes provided products that contain all carbon or nitrogen bearing quaternary centers in high yield.
We studied key aspects of the mechanism of Pd-catalyzed C–CN bond activation and intramolecular enantioselective alkene cyanoamidation. An Abboud–Abraham–Kamlet–Taft (AAKT) linear solvation energy relationship (LSER) model for enantioselectivity was established. We investigated the impact of Lewis acid (BPh3), Lewis base (DMPU), and no additives. BPh3 additive led to diminished enantioselectivity and differing results in 13CN crossover experiments, initial rate kinetics, and natural abundance 12C/13C kinetic isotope effect measurements. We propose two catalytic mechanisms to account for our experimental results. We propose that the DMPU/nonadditive pathway passes through a κ2-phosphoramidite-stabilized Pd+ intermediate, resulting in high enantioselectivity. BPh3 prevents the dissociation of CN−, leading to a less rigid κ2-phosphoramidite-neutral Pd intermediate.
Palladium Catalyzed Intramolecular Acylcyanation of Alkenes Using -Iminonitriles. -The title reaction affords functionalized indanones (II) in moderate to high yields and tolerates substitution at various positions on the aromatic ring including electron donating and electron withdrawing groups. -(RONDLA, N. R.; OGILVIE, J. M.; PAN, Z.; DOUGLAS*, C. J.; Chem. Commun. (Cambridge) 50 (2014) 64, 8974-8977, http://dx.doi.org/10.1039/C4CC04068F ; Dep. Chem., Univ. Minn., Minneapolis, MN 55455, USA; Eng.) -M. Paetzel 01-116
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