Asymmetric Catalysis in the [2+2+2] Cycloaddition of Arynes and Alkynes: Enantioselective Synthesis of a Pentahelicene

Abstract

The use of BINAP-based palladium(0) catalysts in the reaction of 7-methoxy-1,2-didehydronaphthalene (2) and dimethyl acetylenedicarboxylate (DMAD) affords non-racemic 9,12-dimethoxypentahelicene 1 with reasonable ees, among other cycloaddition products. This is the first example of an enantioselective, metal-catalyzed cycloaddition involving arynes.

Stereocontrol in Palladium‐Catalyzed Propargylic Substitutions: Kinetic Resolution to give Enantioenriched 1,5‐Enynes and Propargyl Acetates

“…This complex exhibits a slightly distorted square-planar geometry in which the alkyne is placed almost parallel to the P-Pd-P plane and the acetate group is directed away from the metal. This is in agreement with previous DFT calculations, [23] spectroscopic, [24] and crystallographic [25] data for Pd-alkyne complexes with phosphine ligands. Such a binding mode maximizes the orbital overlap required for back donation from the filled Pd d-orbitals into the C1≡C2 π* orbital.…”

Pyridine‐Directed Palladium‐Catalyzed Phosphonation of C(sp2)H Bonds

“…The palladium(0) complex 7 [17] was isolated in 29 % yield together with 4 a (90 %). Thus, the mechanistic pathway of the phosphonation of 2-phenylpyridine, proceeding through cyclometallation, ligand exchange, and reductive elimination, was followed step-by-step on the basis of stoichiometric reactions, thus providing an experimental basis for the postulated mechanism depicted in Scheme 3.…”

Aryne Cycloaddition Reactions in the Synthesis of Large Polycyclic Aromatic Compounds

“…The reaction gave the substituted pentahelicene 24 , which has an estimated racemization barrier (Δ G ‡ ) of 25.3 kcal mol –1 at 298 K [B3LYP/6‐311G++(d,p)]. The best results were obtained with BINAP‐based catalysts, which afforded non‐racemic 24 in moderate yields and with enantiomeric excesses ( ee ) ranging from 37 to 66 % (Scheme ) 36. This was the first example of an enantioselective metal‐catalysed cycloaddition of an aryne and the second example of asymmetric catalysis applied to the synthesis of helicene‐like compounds 37…”