Selones ketones

Consecutive nuclear Overhauser enhancement spectroscopy (NOESY) experiments allowed one to calculate the distance between the enolate proton and other protons in the octahedrally coordinated titanium enolate 13. The H -tf and H -H- distances calculated from NMR spectra are 0.2 A longer or 0.5 A shorter, respectively, than those in the model structures. Using modeling studies, the same prediction can be observed, indicating that one side of the Ti enolate complex is less sterically hindered than the other side. This is due to the efficient diffusion pathway formed by the enolate H (H ), the protons on the i-Pr group (H, Me) and the protons on the selone heterocycle (H, H ). These distances confirm that the enolate oxygen atoms are cis to each other and their orientation should promote a strong facial preference upon subsequent reaction with an aldehyde or ketone. 

Coupling of an appropriate ketone, thione, or selone with selone (90) using a trialkyl phosphite (P(0R)3 R = Me or Et) has been used extensively for preparation of fulvene systems. An example (91) from the dichalcogenafulvene series is shown in Scheme 18 hydrolysis and decarboxylation give the parent system (92) <88CC822,93JCS(P2)1815>. 

The thiadiazole route to l,3-dithiole-2-thiones and l,3-diselenole-2-thiones or selones is quite versatile as 1,2,3-thia- (184) or selenadiazoles (185) are readily prepared from an appropriate methyl ketone semicarbazone (183) (Scheme 48). 

Keto-enol-enolate rate and equilibrium constants have been reported for N-methylindoline-2-one (90), its 2-thione, and its 2-selone, i.e. a lactam, a thiolactam, and a selenolactam. As an example, the ketone tautomer of the thione has an acidity constant of 8.93, and that of its enoT is 4.05, giving a tautomerization constant, pACe. of 4.88. 

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