Azoalkenes as Intermediate Compounds in the Formation of Osazones from α-Acetoxy Ketones

Asymmetric synthesis of 2-(2-pyridyl)aziridines from 2-pyridineimines bearing stereogenic N-alkyl substituents and regioselective opening of the aziridine ring

The addition of chloromethyllithium to the imine derived from 2-pyridinecarboxaldehyde and (S)-valinol, protected as its O-trimethylsilyl ether, gave the 1,2-disubstituted aziridine with good yield and diastereoselectivity. The analogous reaction performed on the imine derived from (S)-valine methyl ester gave the product containing the aziridine ring and the alpha-chloro ketone group coming from the attack of chloromethyllithium to the ester function. Other stereogenic alkyl substituents at nitrogen gave less satisfactory results. Moreover, the aziridination protocol did not work on other aromatic imines which were not capable of bidentate chelation, e.g., 3- and 4-pyridineimine and benzaldimine. Preliminary studies showed the possibility to carry out regio- and stereospecific opening reactions of 2-(2-pyridyl)aziridines by attack of internally generated or external nucleophiles.

Cyclohexenylphenyldiazene: A Simple Surrogate of the Azobenzene Photochromic Unit

We have carried out an experimental and computational study on the ground- and excited-state photochemical and photophysical properties of (1-cyclohexenyl)phenyldiazene (CPD), a species formally derived from azobenzene in which one of the phenyl rings is replaced by a 1-cyclohexene substituent. The results show that CPD does substantially behave like azobenzene, but with a higher (approximately 70%) Phi(Z-->E) (npi*) photoisomerization quantum yield, calling for CPD as an effective alternative of azobenzene itself with new functionalization possibilities. By use of state-of-the-art ab initio CASPT2//CASSCF minimum energy path computations, we have identified the most efficient decay and isomerization routes of the absorbing singlet (pipi*), S1 (npi*), T1, and S0 states of CPD. The resulting mechanistic scheme agrees with experimental findings and provides a rationale of the observed photoisomerization quantum yields. Furthermore, this study provides a deep insight on the photophysical and photochemical properties of compounds based on the -N=N- double bond which supplies a general model for the photoreactivity of azobenzene-type compounds in general. This is expected to be a useful guideline for the design of novel photoreactive azo compounds.