Petasis arylboronic acids

J. Simon, S. Salzbrunn, G. K. S. Prakash, N. A. Petasis, G. A. Olah, Regiose-lective conversion of arylboronic acids to phenols and subsequent coupling to symmetrical diaryl ethers, Journal of Organic Chemistry 2001, 66, 633. 

Scheme 6.38 The Petasis reaction applying (a) arylboronic acids and (b) alkylboronic acids [52],...

Ionic liquids [61], were found to accelerate the Petasis boron Mannich reaction of 2-hydroxy aryl aldehydes, secondary amines, and arylboronic acids, under mild conditions, by the group of Yadav [62]. As far as we are aware, these authors were the first (and only ) to report the use of ionic liquids in this reaction. The remarkable features of this procedure were the improvement of yields in the synthesis of the desired products, as well as enhanced reaction rates and the possibility of recycling the ionic liquid for up to five times without apparent loss in enantioselectivity. 

Subsequent to this, there have been numerous reports of the use of this reaction using alkenyl, alkynyl and arylboronic adds or esters in reactions with a range of amines and aldehydes. This reaction has been variously named the boronic Acid Mannich , boronic Mannich , boro-Mannich , Petasis boronic acid-Mannich , Petasis borono-Mannich , and Petasis" reaction. The more indusive term of Petasis borono-Mannich reaction will be used throughout this chapter. 

Microwave-assisted Petasis borono-Mannich reactions of arylboronic acids, secondary amines and salicylaldehyde have been carried out in CH2CI2 at 120 °C for 10 min [40], The main problem encountered here was incomplete conversions, resulting in modest adduct yields (23-76%). The reaction of a primary amine (p-ani-sidine) under the same conditions failed to produce the desired adducts, giving instead only the imines (cf. reactions with glyoxylic acid). 

Microwave acceleration of reactions is a valuable tool for organic synthesis [39], and various specialized instruments are now commercially available. Tye and co-workers have reported the microwave-assisted Petasis borono-Mannich reaction of arylboron-ic acids and primary or secondary amines with either glyoxylic acid or salicylaldehyde [40]. Optimized reaction conditions employed didiloromethane as solvent, and microwave assisted heating at 120 °C for 10 min. Products were obtained in generally modest yields (10-83%), in part due to incomplete reaction conversion imder the reported conditions. 

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