Chiral Bis-π-allylpalladium Complex Catalyzed Asymmetric Allylation of Imines:  Enhancement of the Enantioselectivity and Chemical Yield in the Presence of Water

Abstract

The chiral pi-allylpalladium complex 2a, prepared from exoethylidenenorpinane 7, catalyzed the allylation of diverse imines with allyltributylstannane in the presence of 1 equiv of water in good to high enantioselectivities. The catalyst prepared from a 1:1 mixture of (E)- and (Z)-7 was found to be consisting of two stereoisomers 2a and 2b in 1.3:1 ratio. On separation, 2a catalyzed the allylation of imines in much higher enantioselectivities than 2b, giving the same major enantiomer and thereby justifying the need to separate 2a free of 2b. We have achieved the highest separation ratio of >400:1 for 2a:2b by repeated recrystallizations. Isomerization of 2b to 2a during recovery of 2a from the filtrates was observed as more of 2a was recovered each time during recrystallization. Although dry THF was the best solvent, we tried various additives and found that addition of one equivalent of water gave the best results with respect to shorter reaction time, higher yields and enantioselectivities. Thus, we have developed a more general, reproducible, robust and a non-Lewis acid catalyzed procedure for catalytic asymmetric allylation of imines under essentially neutral conditions.

Nicht ganz unbeteiligt: der Einfluss von Olefinen auf übergangsmetallkatalysierte Kreuzkupplungen

“…Tatsächlich führte die Reaktion eines Phenylboronsäureesters mit 2-Cyclopentenon zu [90] Der Katalysezyklus wurde durch Kinetikstudien abgesichert und ist in Schema 74 dargestellt. [91] In [102][103][104] 6.1. Phosphanyl-Olefin-Liganden…”

Organic Synthesis with Bimetallic Systems

“…Catalytic asymmetric allylation of imines has been achieved using the chiral π-allylpalladium chloride dimer as a source of the palladium catalyst [43]. More recently, it has been reported that in the presence of water the chemical yields and enantioselectivities of catalytic asymmetric allylations of imines were enhanced, the latter up to 91% ee [44]. Not only allylstannanes but also allylsilanes can be used as a nucleophilic allyl source (Scheme 1.38) [45], and in the presence of a chiral palladium catalyst the asymmetric allylation takes place smoothly giving high yields and high ee values.…”

Allylation of Aldehyde and Imine Substrates with in situ Generated Allylboronates — A Simple Route to Enantioenriched Homoallyl Alcohols.

“…[13][14][15][16][17][18][19][22][23][24][25] In order to study the involvement of the palladium catalyses in the allylation step of the above reaction (Scheme 1), we reacted allylboronate 11 with nitrobenzaldehyde 3a and imine 4 under the usual reaction conditions but in the absence of palladium catalyst (Scheme 7). The reaction of 3a with 11 gave smoothly 5a, clearly showing that the allylation of the aldehyde compo-Scheme 6.…”