NNNS Chemistry blog
Prevous: Chloroform chemical storage
More abnormal NHC carbenes
23 June 2012 - Orgo
The Bertrand lab continues to churn out carbenes by the dozens. In a latest variation a NHC carbene has its reactivity pattern tuned not only to behave as a nucleophile (the default behavior) but as an electrophile as well (DOI).
In a previous episode here one of the amino groups was replaced by a methylene group which enhances electrophilicity but also nucleophilicity. The new carbene has one of the amino groups pyramidalized which also increases electrophilicity (restricted N lone pair donation to carbene) but leaves nucleophilicity the way it was (no change in electronic contributions of the substituents).
Deprotonation of intermediate 2 turned out to be tricky: at -30°C carbene 3 attacks the base KHMDS by N-H insertion. Hence the -70°C. Nucleophilicity was then demonstrated by reaction with carbon dioxide and electrophilicity by reaction with cyclohexylisocyanide (otherwise an unknown reaction mode for NHC carbenes).