RHODIUM CARBENOID MEDIATED CYCLIZATIONS OF ORTHO-ALKYNYL-SUBSTITUTED ALPHA-DIAZOACETOPHENONES

o-Alkynyl-substituted alpha-diazoacetophenones undergo facile cyclization to indenone derivatives upon treatment with catalytic quantities of Rh(II) carboxylates. A variety of structural influences were encountered by varying the nature of the substituent group attached to the alkyne carbon atom. The cyclization reaction involves addition of a rhodium stabilized carbenoid onto the acetylenic pi-bond to generate a vinyl carbenoid. The vinyl carbenoid was found to undergo both CH and CC migration as well as delta-CH insertion into the alkyl backbone. Different catalysts were shown to result in significant variation in the product ratios for these reactions. Treatment of o-(5-(allyloxy)-1-pentynyl)-alpha-diazoacetophenone with rhodium(II) mandelate afforded 2-(2-propen-1-yl)-2-(1-oxo-1H-indenyl)-2,3,4,5-tetrahydrofuran in high yield. The formation of this compound involves initial formation of a vinyl carbenoid which reacts with the neighboring oxygen atom to give an oxonium ylide which subsequently undergoes a 2,3-sigmatropic rearrangement. When 2-ethynyl-alpha-diazoacetophenone is used, only products derived from 6-endo closure are observed. Substituted o-alkynyl alpha-diazoacetophenones give products derived from 5-exo cyclization. The mode of ring closure is controlled by both steric and electronic factors.