Regioselectivity of the 1,3-dipolar cycloaddition of fluorinated fluoren-9-iminium ylides to heteroelement-containing dipolarophiles: Experimental and quantum-chemical study

N-Substituted 9H-fluoren-9-imines react with difluorocarbene to give the corresponding iminium ylides whose further transformations in the absence of active dipolarophiles depend on the substituent at the nitrogen atom and reaction conditions. N-Ethyl-, N-benzyl-, and N-(2-phenylethyl)-9H-fluoren-9-imines are thus converted in low yield into the formal cyclodimerization products and/or 9H-fluorene-9-carboxamides. N-Methyl-substituted fluoreniminium ylide readily adds at the C=N bond of initial N-(9H-fluoren-9-ylidene)-methanamine with formation of spiro-fused imidazolidine derivative; in the presence of fluorenone, acetaldehyde, or benzaldehyde, addition at the C=O group of the dipolarophile occurs to give the corresponding oxazolidine derivatives. The regioselectivity of the cycloaddition of iminium ylides having a fluorene fragment at a double carbon-heteroelement bond can be described by quantum-chemical calculations in terms of the density functional theory (DFT; local hard and soft acids and bases concept): the cycloaddition leads preferentially to the 2,2-difluoro-substituted adduct.