Chem. Pharm. Bull. 50(11) 1502—1506 (2002)

In the case of the nitrosation of the methyl or ethyl group of carbonyl compounds, such as acetone and 2-butanone, the E-form of the hydroxyimino compound was predominantly obtained. On the other hand, a E-form/Z-form (E/Z) ratio of 2.3 was observed for the nitrosation of 3-methyl-1phenylbutan-1-one (PhCOCH2CH(CH3)2) (5) with tert-butyl nitrite in THF. Thus the E/Z ratio decreased with increasing bulkiness of the R and R groups of RCOCH2R . As shown in Fig. 1, alkyl nitrite R ONO exists as either the synor anti-conformer. The proportion of the anti-form of R ONO was found to increase in the order R CH3 primary secondary tertiary. The E/Z ratio of the hydroxyimino compound increased when methyl nitrite (3) was used in place of tert-butyl nitrite in the nitrosation of 5, which indicated that the E/Z ratio is affected by the conformation of R ONO. Our experimental and theoretical investigations on the mechanisms of nitrosation have shown that the E/Z ratio varied significantly with the participation of the counter cation M of the base catalyst. Consequently, two types of transition state models (TS) during the C–N bond formation process were proposed to elucidate the variation of the E/Z ratio in various solvents; specifically, the TS models were 1) metal-chelated pericyclic transition state (TSCHELATED) and 2) open-chain transition state without metal (TSOPEN). 4,7) Previous calculations of the nitrosation of sodium enolate [CH3COCH2] Na (1) with the anti-form of 3 (anti-3) have shown that Z-1-hydroxyimino-2-oxo-propane (4Z) was obtained predominantly, with the nitrosation proceeding via TSCHELATED. 4) In the case of naked enolate [CH3COCH2] (2), the reaction with anti-3 afforded not only 4Z but also 4E via TSOPEN. 7)