Results of ab initio calculations on the reaction pathways for the hydrogen addition to methanediazonium ion (CH,N=N+) and methyl isocyanide (CH,N=C) at both central and terminal atoms as well as for the stereomutation of a-adducts are reported. Structures of points on the energy surfaces were determined at the UHF/3-21 G level while their relative energies were estimated at UMP4SDQ/6-31 G"".Thermochemical properties were also computed. The a-addition of H' either to the terminal nitrogen of CH,N=N+ or to the carbon atom of CH,NrC is not stereospecific. Only a single transition structure can be located in each case. At the transition state, the substrate is only marginally bent. Soon after the transition state is passed, a bifurcation of the addition pathway occurs leading to the formation of both cis-and trans-isomers of the adduct. In both cases, the trans-adduct is more stable than the cis; the cistrans stereomutation takes place via nitrogen inversion of the methyl group and requires only 5-6 kcal mol-' therefore the trans-isomer should be the major adduct. The P-addition to central nitrogen exhibits, in both cases, an appreciably larger energy barrier which precludes the formation of the P-adduct.Recently, new synthetic routes to isocyanides (1) have resulted in increased interest in the use of this functional group in synthesis and further study of the often facile rearrangement to cyanides (2). ' Aliphatic diazonium ions (3), which are isoelectronic with cyanides and isocyanides, are less stable but are formed as intermediates in a number of reacti~ns.''~ Owing to the extremely good leaving ability of N,, they usually undergo rapid dediazoniation to give the corresponding carbocations R + . In some cases however, the diazonium ions can be trapped by nucleophiles in a diazo-coupling reaction giving an azo compound (4).3c,4
ab initio Calculations o n the mechanisms of conversion of 1 -ethynyl-2-hydroxyethene (I 7a) t o furan (17e) and of 1 -cyano-2-hydroxyethene (18a) to oxazole (18e) are reported. Structures of stationary points on the reaction potential surfaces were located at the HF/3-21G level while relative energies were estimated at the M P4SDQ/6-31 G" level plus zero-point vibrational corrections. The alkylidenecarbene (17c) was found not to be a local minimum whereas the isonitrile (18c) was found t o be a plausible intermediate during the cyclisation process. The latter lies comfortably in a potential well, making it detectable by spectroscopic means. Again its rearrangement to the cyclic product is rate determining.
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