No abstract

No abstract

The effects of structural changes on the rates of hydrolysis of a series of thiochloroformate esters in water have been investigated. The reactivity is enhanced by increased electron donation by the hydrocarbon group. These results, the activation parameters for the hydrolysis of methyl thiochloroformate and the solvent deuterium isotope effect, are shown to be consistent with the operation of the SN1 mechanism.Canadian Journal of Chemistry, 48, 522 (1970) Introduction It is well known that alkyl chloroformates (1; X = 0) undergo solvolytic reactions much more slowly than other acid chlorides (1, 2) and also seem to find the S,1 mechanism unfavorable. Two factors are responsible for these characteristics.( i ) The initial states are stabilized by conjugation (2; X = 0) and the effect is enhanced by structural changes that increase electron donation by the alkyl group. This is shown by the rate sequence

The cyclobutene n-bond of dimethyl 2,3-diazabicyclo[2,2,0]hex-5-ene-cis-2,3-dicarboxylate (15) has been employed as a dienophile in the Diels-Alder reaction with 2,5-dimethyl-3,4-diphenylcyclopenta- 2,4-dienone (17). The reaction occurred with high stereoselectivity and led, almost exclusively, to the formation of the exo-fused adduct (18). A similar cycloaddition was performed between the dienone (17) and the bicyclo[2,2,0]hex-5-ene-cis-2,3-dicarboxylic anhydride (5) to yield a 90 : 10 mixture of adducts (6). These several adducts were used as precursors for the preparation of the related bicyclo[4,2,0]octa-2,4-diene derivatives (7) and (20), themselves used as photosubstrates for the 1,2-photoaromatization reaction. In this way the carbocyclic precursor (7) yielded cyclobut-3- ene-cis-1,2-dicarboxylic anhydride (8), and the hetero analogue (20) yielded the title diazetine (21). Hydrogenation of this heterocycle yielded the related 1,2-diazetidine (22) which was fully characterized. The reactivity of the diazetine, which formally contains six delocalized electrons, is considered especially as it relates to its potential aromatic character. In practice the facile ring- opening of the diazetine to the 1,4-diazabuta-1,3-diene (23) fairly reflects the lack of aromaticity of this ring system. An INDO MO SCF method has been used to evaluate the energetics of the nitrogen inversion process in the unsubstituted 1,2-diazetine ring. The results indicate that the planar form lies at a maximum on the energy surface and that there is little electron delocalization. The planar form is best described as being antiaromatic. PMO arguments are presented which support this conclusion.

A computational study of the conformational analysis of a series of C60 ball-and-chain bichromophoric molecules (1)-(6) has been carried out by means of the AM1 and HF/3-21G//AM1 theoretical methods. It is found that the AM1 method underestimates both the relative conformational energies and the magnitude of the energy barriers folded and extended conformers. In contrast, the HF/3-21G//AM1 method gives reasonable values for these quantities. The HF/3-21G//AM1 calculated energy differences between the folded and extended conformers of systems (2)-(5) are less than 5 kJ/mol and the barriers to their interconversion are c. 30 kJ/mol. The conformational bias in these systems may be modified by the judicious placement of methyl groups in the region of the hydrocarbon chain closest to the C60 cage. It is found that the length of the chain and the nature of the non-C60. chromophore have little effect on the conformational energetics.