Wednesday, May 9, 2007

Nucleophilicities of Amines in Water

picture taken from Brotzel, F., Chu, Y.C., Mayr, H. J. Org. Chem. 2007 72, 3679-3688. Copyright 2007 American Chemical Society

A lab-mate alerted me to this article in the current issue of JOC, and if you are at all interested in comparing the nucleophilicities of amines, then it is worth taking a look. In the past, nucleophilicity has been measured by investigating the kinetics of nucleophilic additions and substitutions. For instance, the rate of reaction of primary or secondary amines with CH3I/CH3Br or stabilized carbocations can be measured and directly related to nucloephilicity using a general formula:Bunting [1, 2] and coworkers have characterized the nucleophilicities of over 70 amines to date using a variation of this method (reactivity towards 1-methyl-4-vinylpyridinium cation as well as methyl 4-nitrobenzenesulfonate).

More recently, researchers from the Mayr lab have utilized benzhydrylium ions (electrophiles) to study the nucleophilicity of various non-amine nucleophiles; as it would be useful to have methods for comparing nitrogen bases in order to design new organocatalytic reactions, they have expanded these methods to include 26 amines. While many of the observations are completely logical (increased branching decreases the nucleophilicity of primary amines, ammonia is less nucleophilic than methyl amine which is less nucleophilic than dimethylamine--which tracks nicely with basicity), others were quite unexpected (at least for me, and I'm certainly not an expert). According to this research, aniline, which is quite a bit less basic than ammonia, is a much stronger nucleophile than ammonia.

As chemists, we all know that basicity certainly doesn't equal nucleophilicity, but this article really drives that fact home.

Update: Thanks to Handles for pointing out this website where Mayr has put all of the nucleophilicity data online :o)


PMP said...

Aniline is a very interesting nucleophile. You might check out some of the early papers by Jencks on the use of aniline as a nucleophilic iminium catalyst (this is much before MacMillan "discovered" iminium catalysis)

Handles said...

In case you didnt notice, Mayr has kindly published everything online in nice tables. Very useful.

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