Abstract
C-H alkylation reactions using short chain olefins as alkylating agents could be operationally simplified on the lab scale by using quaternary ammonium salts as precursors for these gaseous reagents: Hofmann elimination delivers in situ the desired alkenes with the advantage that the alkene concentration in the liquid phase is high. In case a catalytic system did not tolerate the conditions for Hofmann elimination, a very simple spatial separation of both reactions, Hofmann elimination and direct alkylation, was achieved to circumvent possible side reactions or catalyst deactivation. Additionally, the truly catalytically active species of a rhodium(i) mediated alkylation reaction could be identified by using this approach.
Highlights
In recent years, the direct functionalization of C–H bonds has established itself as an increasingly important method for the formation of new C–C bonds.[1,2,3,4,5,6] A key contribution to this field was the acetyl directed alkylation of aromatic C–H bonds disclosed by Murai in 1993 7 (Scheme 1)
C–H alkylation reactions using short chain olefins as alkylating agents could be operationally simplified on the lab scale by using quaternary ammonium salts as precursors for these gaseous reagents: Hofmann elimination delivers in situ the desired alkenes with the advantage that the alkene concentration in the liquid phase is high
Since quaternary ammonium salts are typically prepared from the corresponding alkyl halides, we investigated a protocol which tried to use the alkyl halides directly as olefin precursors.[16]
Summary
The direct functionalization of C–H bonds has established itself as an increasingly important method for the formation of new C–C bonds.[1,2,3,4,5,6] A key contribution to this field was the acetyl directed alkylation of aromatic C–H bonds disclosed by Murai in 1993 7 (Scheme 1). We showed that these requirements were met with tetraalkylammonium salts as surrogates for olefins.[15] Upon addition of potassium hydroxide to the reaction mixture it was possible to generate the corresponding olefin in situ using Hofmann elimination.[31] During that work, we demonstrated ethylation and other n-alkylations (up to n-octylation) of N-benzyl-2-aminopyridines (1) via C–H activation using the corresponding quaternary ammonium salts and with moderate to good yields.
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