Rh(II)-carbenoid mediated 2H-azirine ring-expansion as a convenient route to non-fused photo- and thermochromic 2H-1,4-oxazines

The Rh2(OAc)4-catalyzed domino reaction of 2H-azirines with 2-acyl-2-diazoacetates provides a unique synthetic approach to non-fused 2H-1,4-oxazines. The reaction proceeds via sequential formation of a rhodium carbenoid, an azirinium ylide, and a 1-acyl-1-(alkoxycarbonyl)-2-azabuta-1,3-diene to give, after 1,6-π-electrocyclization, a 2H-1,4-oxazine derivative in good yield. The observed stereoselectivity of 2-azadiene formation is consistent with DFT calculations of the azirinium ylide–2-azadiene isomerization, providing evidence for the ylide mechanism of the reaction. The substitution pattern and configuration of the carbon–carbon double bond in 2-azadienes has a dramatic influence on their cyclization to oxazines: the CO2R group on C4 stabilizes an open-chain form and, as the result of this, a new class of stable 2-azadienes, 1-acyl-2-azadiene-1,4-dicarboxylates, could be isolated. The 1,4-oxazines obtained by this method are the first representatives of monocyclic 2H-1,4-oxazine derivatives, which exhibit photo- and thermochromic activity.