Abstract
The usefulness of flavin-based aerial photooxidation in esterification under Mitsunobu reaction conditions was demonstrated, providing aerial dialkyl azodicarboxylate recycling/generation from the corresponding dialkyl hydrazine dicarboxylate. Simultaneously, activation of triphenylphosphine (Ph3P) by photoinduced electron transfer from flavin allows azo-reagent-free esterification. An optimized system with 3-methylriboflavin tetraacetate (10%), oxygen (terminal oxidant), visible light (450 nm), Ph3P, and dialkyl hydrazine dicarboxylate (10%) has been shown to provide efficient and stereoselective coupling of various alcohols and acids to esters with retention of configuration.
Highlights
The usefulness of flavin-based aerial photooxidation in esterification under Mitsunobu reaction conditions was demonstrated, providing aerial dialkyl azodicarboxylate recycling/generation from the corresponding dialkyl hydrazine dicarboxylate
There are a few cases in which an acyloxyphosphonium intermediate predominates under Mitsunobu reaction conditions, subsequent SNAc substitution of which gives the product with retention of configuration.[4,5,6]
We have shown for the first time that the original Mitsunobu reagents, DIAD (1b) and DEAD (1a), can be regenerated from the corresponding hydrazines by photocatalytic and/or organocatalytic system,[25] allowing esterifications that are catalytic in 1
Summary
A systems of triphenylphosphine (Ph3P) or a related phosphine activated by an oxidant allow efficient esterification under mild conditions.[1] In the Mitsunobu reaction,[2,3] PPh3 is oxidized by a dialkyl azodicarboxylate (usually DEAD [1a] or DIAD [1b]) to form a betain species, which facilitates transformation of an alcohol to a reactive alkoxyphosphonium intermediate (Scheme 1). The latter undergoes SN2 substitution with carboxylate giving an ester with inversion of configuration. Scheme 2 Proposed mechanism of flavin (Fl)-mediated photocatalytic esterification with (A) and without (B) contribution from azodicarboxylate 1
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