HANS CERFONTAIN, COR KRUK, ROEL REXWINKEL, and FRANK STUNNENBERG. Can. J. Chem. 65, 2234 (1987). This study describes a method using 170 nmr to determine the intercarbonyl dihedral angle in a homologous series of seven 1,2-diketones. It is shown that the 170 nmr shifts of the 1,2-diketones are linearly dependent on both the electron density at the oxygen atoms and the n-n * excitation energy of the 1,2-diketo chromophore. Reduction of the overlap in the .rr system of the 1,2-diketo moiety leads to a gradual upfield shift of the 170 nmr signal, due to the increase in both the T-electron density at the oxygen atoms q6 and the mean transition energy AE.HANS CERFONTAIN, COR KRUK, ROEL REXWINKEL et FRANK STUNNENBERG. Can. J. Chem. 65, 2234 (1987). Cette Ctude dCcrit une mkthode qui fait appel B la rmn du 170 pour dkterminer l'angle dikdre intercarbonyles dans un sCrie homologue de sept dicttones-1,2. On dCmontre que les deplacements des dicktones-1,2 en rmn du 170 dipendent d'une fa~on IinCaire A la fois sur la densit6 Clectronique des atomes d'oxygkne que sur l'energie d'excitation n-n* du chromophore dictto-1,2. La rtduction dans le recouvrement du systkme-.rr de la portion dicCto-!,2 conduit a un dCplacement graduel vers les hauts champs du spectre rmn du 170; ce rksultat provient d'une augmentation tant de la densit6 des Clectrons-.rr au niveau des atomes d'oxygkne (96) que de l'inergie de transition moyenne (AE).[Traduit par la revue]
The photochemistry ( h 300 nm) of the a-0x0 oxime ethers l a and 2a has been studied with the objective of detecting any y-H abstraction. Direct irradiation of ( 2 ) -l a results only in (E)-(Z) isomerization, whereas direct irradiation of the isomeric u-0x0 oxime ether l b and the corresponding a-diketone Ic leads mainly to cyclobutanol formation. The absence of cyclobutanol formation with l a is ascribed to the higher angle of twist of the intercarbonyl-iminyl angle of the reacting excited state of la, as compared with those of both l b and lc. The photodeuteration of the 7-methyl hydrogens of 2a, using C2H,0ZH as (co)-solvent, is a very inefficient process ((D 7.0 x lo-'), and that of the benzylic hydrogens of the related ketone 3a is even more inefficient ((D d 0.5 x lo-'). The inefficiency is ascribed mainly to the relatively low rate of y-H abstraction in the excited triplet (nrc*) state as well as to the very short life-times of the ground state (Z)-enols resulting from the relatively high rate of reketonization. The higher photodeuteration yield of the a-0x0 oxime ether 2a, as compared with the benzylic hydrogens of 3a, is ascribed to stabilization of the triplet 1 ,4-diradical formed by y-hydrogen abstraction of the excited triplet (Z)-2a, which results from electron delocalization and intramolecular hydrogen bonding.Part 8, T.
Abstract. With the objective of understanding the photodissociation of cyclic s-cisoid 2 -0 x 0 oxime ethers upon irradiation at i , 254 mi, an ub-initio study has been performed using .r-c~ii-ethanedial nionooxinie as model compound. I n accordance with previously presented experimental obscrvations, the calculations demonstrated (i) that the initial step in the photodissociation is N -0 homolysis and (ii) that N -0 scission takes place with the iEj, but not Lvith the / Z ) isomer, due t o the presence of a relatively low-lying doubly excited (nrr*)(n7t*) state with the ( Z ) isomer. causing this isomer to decay via Z-E isomerization.
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