3‐Mercapto‐4‐phenyl‐3‐cyclobutene‐1,2‐dione

Abstract

is much more shielded. The presence of the C-phenyl group in (1) instead of the C-methyl group in (2) has the same effect, since the delocalized aromatic z-electron system of the phenyl group interacts with the vacant p-orbital of the carbene carbon, whose electron deficiency can thus be more effectively lowered than by the methyl group, which only acts inductively[31. Similarly, a dependence between degree of shielding and nature of heteroatom is observed in compounds (3), ( 4 ) , and ( 5 ) . The less electronegative nitrogen in ( 5 ) clearly confers more negative charge on the carbene carbon than does the more strongly electronegative oxygen in (3).In the case of ( 5 ) measurements were carried out on a mixture of the cis-trans isomers [41 (65:35). The isomers, which differ from each other only in the orientation of their C-and N-methyl groups, exhibit different I3C-NMR spectra. The signals for cis-and trans-(5) in the table were assigned on the basis of intensities. The I3C-NMR signals of the two isomers appear in the same sequence as do the N-methyl signals in the 'H-NMR spectrum, oiz. the signal of the cis isomer appears at higher field than that of the trans isomer. -4-phenyl-3-cyclobutene-1,2- We wish to report on the synthesis and properties of 3-mercapto-4-phenyl-3-cyclobutene-1,2-dione (PCB-SH) ( S ) , the first thio analog of the well-known PCB-OHr1I. 3-MercaptoBecause of its expected high acidity and its capability of being transformed into the novel 4-phenyl-3-thiolatocyclobutenediylium 1,2-diolate systemC2], compound ( 5 ) proves to be a very important representative of the phenylcyclobutenedione series.Dropwise addition of a benzene solution of (1) into pyridines (2) saturated with hydrogen sulfide leads to formation of the pyridinium salts ( 3 ) ['I, which have been characterized chemically and spectroscopically (see Table).In agreement with the salt structure, no peaks for the molecular ions appear in the mass spectra of (3a)-(3c). On the other hand, a peak due to the anion whose mass is increased by one unit (m/e 190) and the peak of the cation both appear with high intensity.-The absence of an IR -