Photochemistry of α‐oxo oximes. Part 9. The γ‐hydrogen abstraction in α‐oxo oxime ethers

Abstract

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.

Electron-Transfer-Induced Tautomerization in Methylindanones: Electronic Control of the Tunneling Rate for Enolization

“…The mixture of 5-methyl- and 7-methylindanone that was obtained in this way was separated by column chromatography on silica 60F with methylene chloride to give MI in about 38% yield (mp 52−54 °C, lit . mp 54.7−55.2 °C; 1 H NMR, IR, and UV spectra in accord with literature reports , ).…”

2‐(Hydroxyimino)aldehydes: Photo‐ and Physicochemical Properties of a Versatile Functional Group for Monomer Design

“…In this case, significant, if not quantitative, formation of a single new product was observed. Based on the evidence presented in this paper and on the behavior of α‐oxo‐oximes documented by Buys et al., we identified this product as a cyclobutanol oxime (CBO) derivative (Scheme ) formed by Norrish–Yang cyclization …”

“…[19] Metal coordination complexes and polymers have been obtained by using oximes as ligands. Specifically,t he oxime bond dissociation enthalpy (BDE) and the structure and stability of the resulting > C=NOC radical may in-fluence reactions interfering with radicalp olymerizations, such as addition to the C=Nb ond or hydrogen abstraction.W ea lso observed that the E/Z switch in the photostationary state [25] is very limited in the case of the polymer in Figure 1b and we surmise that it may be enhanced by inserting ad ifferent spacer between the HIA group andt he polymer backbone. [21] Croitor et al [22] obtained av arietyo fc oordination networks through am ixed ligand approach, employing oximes with different coordinationc apacities and preferences.…”

“…As shown in Table , the measured p K a values for all the HIAs fall within the narrow range (2.2 p K a units) of approximately 7.7–9.9, so they are weaker acids than α‐arylcyanoximes (p K a range 4.5–6.2) . Their acidity is, instead, comparable to that of hydroxamic acids, which range approximately from 7.9 for C 6 H 5 C(O)N(OH)CH 3 to 9.9 for (CH 3 ) 3 C(O)N(OH)CH 3 .…”

“…[1] Oxo-oxime ethersh ave been extensivelyi nvestigated by Cerfontain and co-workers (see Ref. Based on the evidence presented in this paper and on the behavior of a-oxo-oximes documented by Buys et al, [35] we identified this product as ac yclobutanol oxime (CBO) derivative (Scheme 4) formed by Norrish-Yang cyclization. [34] In the presentw ork, we carriedo ut an NMR-based investigation of the photo-and thermo-isomerism of several HIAs.…”