The chemistry of endo-tricyclo[6.2.1.0 (2.7)]undec-9-en-11-ylidene (10), an archetypal foiled carbene, has been investigated. The intermolecular reactions of 10 are most conveniently performed with oxadiazoline 6 because the corresponding diazirine can be obtained only in very low yield. Furthermore, the aziridinyl imine is difficult to decompose and the tosylhydrazone sodium salt poorly soluble in common organic solvents. Photolysis of 6 in diethylamine leads merely to a reduction of the diazo group and regeneration of acetyl hydrazone 5, whereas thermolysis cleanly affords tertiary amine 12(anti) in 77% yield. Calculations show that even stabilized-nucleophilic carbenes react with amines through an ylidic pathway and not by a concerted insertion into the N-H bond. Nevertheless, in the gas phase, norbornen-7-ylidene (13) is predicted to be stabilized by one molecule of NH3 more efficiently through a hydrogen bond than by ylide formation.
Bicyclo[3.2.1]oct-6-en-8-ylidene (1) can assume either the conformation of "classical" carbene 1a or that of foiled carbene 1b in which the divalent carbon bends toward the double bond. Oxadiazoline precursors for the generation of 1 were prepared, followed by photochemical and thermal decomposition as well as flash vacuum pyrolysis (FVP) of a tosyl hydrazone sodium salt precursor, to give a number of rearrangement products. Matrix isolation experiments demonstrate the presence of a diazo intermediate and methyl acetate in all photochemical and thermal precursor reactions. The major product from rearrangements of "classical" bridged carbene 1a is bicyclo[3.3.0]octa-1,3-diene as a result of an alkyl shift, while dihydrosemibullvalene formed from a 1,3-C-H insertion. In contrast, thus far unknown strained bicyclo[4.2.0]octa-1,7-diene formed by a vinyl shift in foiled carbene 1b. The experimental results are corroborated by density functional theory (DFT), MP2, and G4 computations.
Rhodium acetate-catalyzed decomposition of methyl 2-diazo-2-phenylacetate in the presence of substituted N-methylbenzylideneamines possessing an activated alkenyl fragment (dipolarophile) in the side chain gives products of intramolecular cycloaddition of intermediate Z,E-and E,Z-azomethine ylides. The cycloaddition is regioselective, and the products are hexahydrochromeno[4,3-b]pyrrole derivatives. The stereoselectivity of the process depends on the temperature. In the temperature range from 20 to 80°C, the major stereoisomer is that with cis junction of the tetrahydropyran and pyrrolidine rings. N-Phenylazomethine ylides generated from methyl 2-diazo-2-phenylacetate and alkyl 4-[2-(phenyliminomethyl)phenoxy]-2-butenoates at 40°C undergo cyclization to aziridines at a higher rate, as compared to the rate of cycloaddition to the internal dipolarophile. N-Phenylazomethine ylides generated by thermolysis of the corresponding aziridine or by the "deprotonation" method react with equal regio-and stereoselectivity to give intramolecular cycloaddition products, hexahydrochromeno[4,3-b]pyrrole derivatives with trans-fused tetrahydropyran and pyrrolidine rings. Analysis of the experimental and calculation data suggests preference of the endo transition state in the cycloaddition of the examined azomethine ylides. Intramolecular 1,3-dipolar cycloadditions of azomethine ylides underlie effective procedures for the synthesis of various fused, bridged, and cage-like polycyclic nitrogen-containing compounds [1]. An important problem is the stereoselectivity of cycloaddition of ylide intermediates, for it determines the efficiency of this approach as applied to a particular synthetic task. Low stereoselectivity often results from configurational heterogeneity of ylide intermediate owing to fairly severe conditions of its generation [2][3][4]. The stereoselectivity problem may be solved by development of such methods for generation of azomethine ylides which would allow their geometry to be controlled. One of these methods is based on reactions of carbenes or carbenoids with compounds possessing a C=N bond and is referred to as "carbene" technique. These reactions occur as a rule under mild conditions, and they make it possible to generate ylides which cannot be obtained by other methods [5].The synthetic sequence intermolecular carbene generation of azomethine ylide-intramolecular cycloaddition almost was not studied as a method of synthesis of nitrogen-containing polycyclic compounds. Only recently, we described examples of such syntheses with the use of difluorocarbene and dichlorocarbene [6][7][8].The goal of the present study was to develop approaches to tandem synthesis of fused heterocycles via the above reaction sequence involving carbenoids generated from diazo compounds. As model system we used azomethine ylides generated by addition of carbene-like species derived from methyl 2-diazo-2-phenylacetate to N-methyl-and N-phenylbenzylideneamines I-IV possessing an activated alkenyl fragment (dipolarophile) in the ortho po...
Base: Intramolecular 1,3-Dipolar Cycloaddition. -Intermediate azomethine ylides undergo intramolecular cycloaddition to form fused chromenes, e.g. (III). However, substrate (V) does not give the desired cyclized product [no yield given for (IV)]. -(KHLEBNIKOV, A. F.; NOVIKOV, M. S.; BESPOKOEV, A. A.; KOSTIKOV, R. R.; KOPF, J.; STARIKOVA, Z. A.; ANTIPIN, M. Y.; Russ. J. Org. Chem. 41 (2005) 6, 922-932; St. Petersburg State Univ., St. Petersburg 198504, Russia; Eng.) -K. Schneider 05-128
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