Intramolecular CH Oxidative N‐Allylation of C‐Homoallylhydrazones to 5‐Vinyl‐2‐pyrazolines Catalyzed by the [Pd(OAc)₂]/BIMINAP System

Abstract

A catalytic procedure is described for intramolecular benzoquinone‐mediated CH oxidative N‐allylation of C‐homoallylhydrazones to 5‐vinyl‐2‐pyrazolines. Depending on the steric and electronic demand of the substituents at the allylic moiety of the substrate, other six or seven membered cyclic hydrazones were obtained in moderate yields. As compared to other Pd(OAc)2/phosphine combinations, the optimal catalytic conditions are based on the Pd(OAc)2/BIMINAP system [BIMINAP=formal contraction of the acronyms BIMIP=2,2′‐bis(diphenylphosphino)‐1,1′‐bibenzimidazole and BINAP=2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl in a 10 % ratio. In a preliminary attempt, the use of the enantiopure (R)‐BIMINAP ligand was shown to provide the corresponding 5‐vinyl‐2‐pyrazoline in 75 % yield and 30 % ee in the (+)‐enantiomer, whereas the analogous (R)‐BINAP ligand afforded the same product in only 47 % yield and 10 % ee. The results highlight the specific value of the electron‐poor atropochiral BIMINAP ligand in catalysis and, in particular, oxidative catalysis.

Palladium catalyzed oxidative aminations and oxylations: where are we?

“…When an alkene coordinates to a PdX 2 complex, the electronic character of the olefin π-system changes enabling two main and mutually excluding reaction paths, namely nucleopalladation [1][2][3][4][5][6] or C-H allylic activation [7][8][9][10][11][12] to provide an electrophilic η 3 -allylpalladium complex. Other reactions and intermediates are also accessible depending upon the nature of the starting alkene and/or the reaction conditions (Scheme 1) [13].…”

Stereospecific Intramolecular C–H Amination of 1-Aza-2-azoniaallene Salts

“…This unexpected result is, to our knowledge, the first example of heteroallenes such as 2a reacting by a nitrene-like manifold to provide the product of an intramolecular C–H amination reaction. Although pyrazolines can be readily prepared by reaction of hydrazines with α,β-unsaturated aldehydes and ketones or by 1,3-dipolar cycloaddition of diazoalkanes with alkenes, , the development of new methods to prepare these compounds is still of interest − due to their prevalence in biologically active compounds. ,, The novelty of this C–H amination reaction encouraged us to examine this new reactivity more thoroughly.…”