D. V. Ramana

Synthesis of 2-substituted benzoxazoles and benzimidazoles based on mass spectral ortho interactions

The ortho interactions of the hydroxy and amino groups with the amide function in N-(2-hydroxyphenyl)and N-(2-aminophenyl)-benzamides on electron impact lead to the elimination of H2O from the molecular ions resulting in the formation of 2-substituted benzoxazoles and benzimidazole radical cations, respectively. This mass spectrometric reaction has been successfully mimicked in the laboratory to synthesise these heterocyclic compounds in excellent yields by the thermolysis of the corresponding N-(2-hydroxyphenyl)- and N-(2-aminophenyl)-benzamides. The mechanisms and ion structures proposed in the mass spectral study are supported by high-resolution data, B/E and B2/E linked-scan spectra, Collision-Activation-Decomposition (CAD)-B/E linked-scan spectra, chemical substitution and 2H-labelling.

Electron-impact-induced 3,3-sigmatropic rearrangement and cyclization in phenyl allenylmethyl ethers

A 3,3-sigmatropic rearrangement in the M+· of phenyl allenylmethyl ether is proposed for the observed losses of CO, C2H4, and CH3. Direct cyclization in the M+· also leads to the [M−CH3] ion. The presence of sulfur as the heteroatom in phenyl allenylmethyl sulfide does not significantly influence the occurrence of Claisen rearrangement. Ortho interaction of the nitro group with the allenyl double bond in the side chain leads to characteristic fragment ions in 2-nitrophenyl allenylmethyl ether. Linked scans, high-resolution mass spectrometry, collision-activated dissociation-B/E linked-scan spectra, and D-labeling have been employed to support the proposed mechanisms and ion structures.

Electron-impact-induced fragmentation of long-lived C14H14·+ ions from dibenzyl sulphone and 1,2-diphenylethane

Abstract Gaseous dibenzyl sulphone radical cations ( 1 ·+ ) expel SO 2 , forming C 14 H 14 ·+ ions, whose fragmentation is identical with that of the radical cations of 1,2-diphenylethane ( 2 ·+ ). Metastable ions [ 1 − SO 2 ] ·+ and 2 ·+ eliminate predominantly benzene to give C 8 H 8 ·+ , in competition with the well-known benzyl cleavage. The [M − SO 2 ] ·+ ions from deuterium-labelled isotopomers of 1 react in exactly the same way as 2 ·+ ions bearing the label in corresponding positions (e.g. C 6 H 5 CD 2 SO 2 CD 2 C 6 H 5 ·+ and C 6 H 5 CD 2 CD 2 C 6 H 5 ·+ ). Benzene loss is initiated by migration of a benzylic H atom, but does not occur by simple 1,2-elimination; rather, the C 6 H 6 molecule incorporates an H atom from the other ring. Both fragmentation reactions are preceded by slow interannular hydrogen exchange, which involves preferentially the ortho positions of the rings; however, this exchange is coupled with a relatively fast intra-annular hydrogen ring-walk by which the hydrogens at the meta and para positions of the rings participate in the interannular exchange. The data suggest that the extrusion of SO 2 from ionized dibenzyl sulphone generates a complex of a benzyl cation and a benzyl radical, which recombine by bond formation between the benzylic α carbon atoms, but possibly also by electrophilic attack of C 6 H 5 CH 2 + at the various sites of C 6 H 5 CH 2 · .