Petasis vinyl boronic acids

Petasis reported an efficient addition of vinyl boronic acid to iminium salts.92 While no reaction was observed when acetonitrile was used as solvent, the reaction went smoothly in water to give allyl amines (Eq. 11.54). The reaction of the boron reagent with iminium ions generated from glyoxylic acid and amines affords novel a-amino acids (Eq. 11.55). Carboalumination of alkynes in the presence of catalytic Cp2ZrCl2 and H2O affords vinylalane intermediates, which serve as nucleophiles in the subsequent addition to enantiomerically enriched... 

Allylic amine from the three-component reaction of a vinyl boronic acid, a carbonyl and an amine. Also known as boronic acid-Mannich or Petasis boronic acid-Mannich reaction. Cf. Mannich reaction. 

Vinyl boronic acids were among the most reactive boronic acids used in the Petasis borono Man-nich reaction, but aryl boronic acids and heterocyclic derivatives (thienyl and pyridyl boronic acids, for instance) can also be used [54]. Mechanistically, this three-component reaction type is characterized by a complex equilibrium among all the three components involved, and several intermediates are formed, including a reactive imine (Scheme 6.39). 

Many other fascinating examples of organocatalysis have been presented in a recent monograph and reviews [6, 11, 12], In its original form, the Petasis reaction was developed as a three-component, non-catalytic condensation of amine, aldehyde and vinyl boronic acid (Scheme 8.2) [13, 14],... 

In 1993, Petasis and Akritopoulou described the addition of vinyl boronic acids to formaldimine, leading to tertiary allylic amines [78]. Owing to the rruld reaction... 

Scheme 6.82 Proposed reactive complex of the Petasis reaction utilizing a-hydroxy aldehydes, amines, and organic boronic acids (A) and bifunctional mode of action of chelating thiourea catalyst 65 in the enantioselective Petasis-type 2-vinylation of N-acetylated quinolinium ions (B).

The mechanism of the Petasis boronic acid-Mannich reaction is not fully understood. In the first step of the reaction, upon mixing the carbonyl and the amine components, three possible products can form iminium salt A, diamine B, and a-hydroxy amine C. It was shown that preformed iminium salts do not react with boronic acids. This observation suggests that the reaction does not go through intermediate A. Both intermediate B and C can promote the formation of the product. Most likely, the reaction proceeds through intermediate C, where the hydroxyl group attacks the electrophilic boron leading to an ate -complex. Subsequent vinyl transfer provides the allylic amine along with the boronic acid sideproduct. 

M.G. Finn and co-workers developed a procedure for the preparation of 2/-/-chromene derivatives that includes a Petasis three-component reaction between salicylaldehyde, vinylic- and aromatic boronic acids, and dibenzylamine. The hydroxyl group of the salicylic aldehyde is essential for the activation of the boronic acid. The initially formed allylic amine undergoes a cyclization upon ejecting the dibenzylamine, thus rendering the process catalytic in the amine. 

R.A. Batey and co-workers developed a modification of the Petasis-boronic acid-Mannich reaction that occurs via N-acyliminium ions derived from A/-protected-2,3-dihydroxypyrrolidine and 2,3-dihydroxypiperidine derivatives. This method was utilized in the total synthesis of (+)-deoxycastanospermine. The formation of the A/-acyliminium ion was achieved by treating A/-Cbz-2,3-pyrrolidine with BF3-OEt2. ° Subsequent vinyl transfer from the alkenylboronic ester provided the product with excellent yield and diastereoselectivity. 

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