Understanding The Chemo-selectivities Between Carbonyl And Hydroxyl Groups In The Rh(Ⅱ)-Azavinyl Carbene Involved Reactions

Metal carbenoids have been widely used as highly versatile important intermediates in organic synthesis.Transition metal-catalyzed reactions of diazo compounds,being precursor of metal carbenoid,have been studied extensively.Although synthetically useful transformations could be realized though transition metal-catalyzed reactions of diazo compounds to form metal carbene,diazo compounds are usually toxic,explosive and unstable,with a limited range of applications.Therefore,the development of novel approaches for the formation of metal carbenoids from nondiazo compounds has received significant research attention.1-Sulfonyl-1,2,3-triazole compounds,which are readily obtained by the copper-catalyzed alkyne-azide cycloaddition,could be employed as precursors to generate α-imino metal carbenoids promoted by dirhodium(II)catalysts.Computational studies were carried out to understand the chemo-selectivities between carbonyl and hydroxyl groups in the Rh(II)-azavinyl carbene(Rh(II)-AVC)intermediates involved reactions.For the Rh(II)-catalyzed tandem reaction of 1-sulfonyl-1,2,3-triazoles with salicylaldehydes to produce 2,5-epoxy-1,4-benzoxazepines,the nucleophilic addition of carbonyl group to the carbenoid of Rh(II)-AVC is more ready to occur than the nucleophilic addition of hydroxyl group and C-H group of phenyl ring,affording a formal [3+2] cycloaddition oxazoline intermediate.The detailed mechanistic pathway of subsequent conversion of oxazoline intermediate to the final product was revealed.The preferred chemoselectivity is mainly due to the insignificant steric hindrance and the formed H-bond between hydroxyl and imino groups.In contrast,for the Rh-catalyzed reactions of 1-sulfonyl-1,2,3-triazoles with 4-hydroxyacetophenone,the nucleophilic addition of hydroxyl group to the carbenoid of Rh(II)-AVC is more favorable than the nucleophilic addition of carbonyl group.Due to the absence of OH group at the ortho position in 4-hydroxyacetophenone,the stabilizing H-bonding interaction with the imino group is missing for the nucleophilic addition of carbonyl group to the carbenoid of Rh(II)-AVC.On the other hand,the destabilizing steric effect for the addition of hydroxyl group is less due to the absence of ketone group at the ortho position of OH group in 4-hydroxyacetophenone.Thus,the nucleophilic addition of hydroxyl group of 4-hydroxyacetophenone becomes more favourable than the addition of carbonyl group.On the other hand,for the Rh-catalyzed reactions of 1-sulfonyl-1,2,3-triazoles with α,β-unsaturated cyclic ketones bearing aliphatic hydroxyl group,the nucleophilic addition of hydroxyl group to the carbenoid of Rh(II)-AVC is also more favorable than the nucleophilic addition of carbonyl group.The main factors accounting for the chemo-selectivity are due to the geometrical flexibility to avoid significant steric hindrance and greater nucleophilicity of the aliphatic hydroxyl group in α,β-unsaturated cyclic ketones bearing aliphatic hydroxyl group.Experimental studies were explored to elucidate the chemo-selectivities between carbonyl and hydroxyl groups in the Rh(II)-AVC intermediates involved reactions.Interestingly,experimental studies showed that for β-amidone underwent N-H insertion in the presence of Rh(II)catalyst.We didn’t get the similar product reported by Li’s and Yu’s group.The preferred chemo-selectivity is mainly due to stronger nucleophilicity of N than O in carbonyl group.In addition,2-(1-alkynyl)-2-alken-1-ones have been used as a precursor of metal carbenoids in organic chemistry.Herein,Zn Cl2-catalyzed 2-(1-alkynyl)-2-alken-1-ones with 2,1-benzisoxazole was explored to synthesize corresponding imides.Detailed mechanisms of Zn Cl2-catalyzed 2-(1-alkynyl)-2-alken-1-ones with 2,1-benzisoxazole were investigated by DFT calculations.