The conversion of widely available carboxylic acids into versatile boronic esters would be highly enabling for synthesis. We found that this transformation can be effected by illuminating the -hydroxyphthalimide ester derivative of the carboxylic acid under visible light at room temperature in the presence of the diboron reagent bis(catecholato)diboron. A simple workup allows isolation of the pinacol boronic ester. Experimental evidence suggests that boryl radical intermediates are involved in the process. The methodology is illustrated by the transformation of primary, secondary, and tertiary alkyl carboxylic acids as well as a diverse range of natural-product carboxylic acids, thereby demonstrating its broad utility and functional group tolerance.
By reviewing the recent progress on the elucidation of the structure of gold carbenes and the definitions of metal carbenes and carbenoids, we recommend to use the term gold carbene to describe gold carbene-like intermediates, regardless of whether the carbene or carbocation extreme resonance dominates. Gold carbenes, because of the weak metal-to-carbene π-back-donation and their strongly electrophilic reactivity, could be classified into the broader family of Fischer carbenes, although their behavior and properties are very specific.
Cationic gold(I) promotes the retro-Buchner reaction of 7-substituted 1,3,5-cycloheptatrienes, leading to gold(I) carbenes that cyclopropanate alkenes.
The fate of the aryl
gold(I) carbenes generated by retro-Buchner
reaction of ortho-substituted 7-aryl-1,3,5-cycloheptatrienes is dependent
on the constitution of the ortho substituent. Indenes and fluorenes
are obtained by intramolecular reaction of highly electrophilic gold(I)
carbenes with alkenes and arenes. According to density functional
theory calculations, the gold-catalyzed retro-Buchner process occurs
stepwise, although the two carbon–carbon cleavages occur on
a rather flat potential energy surface.
The intermolecular gold(I)-catalyzed
reaction between arylalkynes
and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which
takes place stepwise, first by formation of cyclopropyl gold(I) carbenes,
followed by a ring expansion. However, 1,3-butadienes are also formed
in the case of ortho-substituted arylalkynes by a
metathesis-type process. The corresponding reaction of alkenes with
aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively
to cyclobutenes. A comprehensive mechanism for the gold(I)-catalyzed
reaction of alkynes with alkenes is proposed on the basis of density
functional theory calculations, which shows that the two pathways
leading to cyclobutenes or dienes are very close in energy. The key
intermediates are cyclopropyl gold(I) carbenes, which have been independently
generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa[a]naphthalenes.
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