Mechanism for Nagata Alkylation

Question

I am looking into the mechanism for the Nagata Alkylation; however, I cannot find anything more specific than the mechanism listed below.

I am only refering to the formation of the first intermediate, not the cyclization of the ortho-quinone methide. I would assume that the phenylboronic acid first forms an ester with the phenol and then the lewis-basic carbonyl attacks the boron. This might then make an electrophillic carbon that can undergo an alkylation reaction. I am not very sure about this proposal of mine, and I was wanting to ask if anyone had any suggestions as to how this mechanism might be occuring.

Answer 1

This reaction of ortho-alkylation of phenols was first indroduced by Nagata and coworkers in 1979 (Ref.1). Accordingly, for example, reaction of a phenol with a large excess of an aldehyde and a slight excess of benzeneboronic acid in refluxing benzene (or toluene) in the presence of $\pu{0.1 mol}$ equivalent of propanoic acid (or trichloroacetic acid) gave about 97% of the expected product, a 2-phenyl-4 H-1,3,2-benzodioxaborin, as a crystalline product (Ref.1):

Note that the presence of propanoic acid is critical in the reaction (no propanoic acid, no expected product). The authors have suggested that mechanistically, the reaction would have taken place by coordination of an aldehyde molecule to the boron atom of a probable intermediate I (i.e. a substituted phenyl benzeneboronate), followed by [3,3]-sigmatropic rearrangement (as indicated in II) to give the product, a substituted benzodioxaborin.

References:

1. Wataru Nagata, Kyo Okada, Tsutomu Aoki, “ortho-Specific $\alpha$-Hydroxyalkylation of Phenols with Aldehydes. An Efficient Synthesis of Saligenol Derivatives,” Synthesis 1979, (5), 365-368 (DOI: 10.1055/s-1979-28683).