Pyridine reaction with singlet

METHOX (22) was synthesized in three steps from the commercially available pinocarvone (47) using the Krohnke annulation (47 + 48 —> 50). Pinocarvone itself, commercially available only in one enantiomeric form, can be readily obtained from the much cheaper oi-pinene (46) via an ene-reaction with singlet oxygen. Since both enantiomers of oi-pinene are commercially available, this procedure can be utilized for the preparation of both enantiomers of METHOX (22) [28b], The Krohnke reagent 48 [52] is readily obtained in one pot via iodination of the corresponding acetophenone (49) in pyridine [28bj. 

In 1997 the electronic absorption spectra of phenylnitrene" and its perfluorosubstituted analogues " were detected. Recently the kinetics of bimolecular reactions of the singlet fluoro-substituted arylnitrenes were studied using direct spectroscopic methods. The absolute rate constants of reaction of singlet perfluoroarylnitrene 16f and 16g with amines, pyridine and dimethylsulfoxide are presented in Table 11. 

Singlet fluorocarbenes 51 substituted with diphenylphosphoryl, phenylsulfanyl, and TMS groups were generated from OT /o-10-fluoro- o-10-substituted tricyclo[4.3.1.0]decadienes 50 under photolysis conditions to produce the ultraviolet-visible (UV-Vis) active ylides 52 on reaction with pyridine (Scheme 8) <2004PCA1033>. 

Whereas most reactions of singlet arenes and aryl olefins with secondary amines result in N-H addition, there are exceptions to this generalization. Cookson et al. reported that irradiation of 1-phenylcyclohexene in isopropylamine yielded a mixture of N-H and a-C-H adducts. Gilbert and co-workers ° found that the reaction of benzene with dimethylamine yields N-H adducts, whereas pyridine reacts with diethylamine to yield the substitution product 11, which presumably is formed by aromatization of the a-C-H adduct. Irradiation of stilbene and indole in mixed crystals yields both N-H and C-H adducts, product ratios being dependent upon the reactant ratio. Investigation of the reactions of 9-cyanophenanthrene with primary and secondary... 

Thus, heating under reflux a mixture of 10 and 12 in pyridine afforded, in each case, one product whose H NMR spectrum showed the absence of the methylthio (6 2.55-2.65) and amino (6 5.0-5.03) proton signals present in the spectra of the respective 4-amino-3-methylthio-l,2,4-triazinones 12. Instead, the spectra of the products isolated revealed in each case a characteristic one proton singlet (N-H) in the region S 9.0-9.4. These findings indicate that route A seems to be the most plausible mechanism for the studied reactions of 10 with either 8 or 12 <2000JPR342>. 

Because of the presence of a lone pair and a vacant orbital, singlet carbenes are supposed to be able to react with both Lewis bases and acids. Transient electrophilic carbenes are known to react with Lewis bases to give normal ylides (Scheme 8.19). For example, carbene-pyridine adducts have been spectroscopically characterized and used as a proof for the formation of carbenes,and the reaction of transient dihalogenocarbenes with phosphines is even a preparative method for C-dihalogeno phosphorus ylides. Little is known about the reactivity of transient carbenes with Lewis acids. 

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