Studies on enamides . Part-71 : a novel photochemical reaction pathway of N-aroyl-N-1-cyclohexenylanilines

UV irradiation of N-aroyl-N-1-cyclohexenyl-anilines involves a cleavage of vinyl nitrogen bond under oxidative condition, whereas 6H-5-phenyl/aryl-1,2,3,4-tetrahydrophenanthridine-6-ones are obtained when the reaction is performed non-oxidatively.

But surprisingly, we found that the UV irradiation of the title compounds in the presence of iodine as oxidant (Scheme 1), led to a novel and unexpected cleavage of the vinyl nitrogen bond 8 of the parent enamides (4); and the product was found to be anilides [ 5 (a-e)].
However, the same experiment, when carried out with 4a in the absence of iodine for 25 h, afforded 6H-5-phenyl-1, 2,3,4-tetrahydrophenanthridin-6-one (6a) (19%), mp 153 o C (acetonelight petroleum, 60-80 o C), IR (KBr) н max 1655 cm -1 .Similarly, while the substrate [4 (b-g)] gave the phenanthridones [6 (b-c), f, g] (Table ) during photo-irradiation, the compound (4d) appeared to be inert.under photochemical condition and 4e produced some intractable oily material.The apparent lack of reactivity of 4d and 4e may be explained as follow: i) the excited state of 4 is analogous to an azadiene system (4р-electron) and a dienophile unit (2р-electron) present in the cyclohexne moiety.The intramolecular photo-cyclisation of such type of compounds is a case of reverse electron demand Diels-Alder reaction.Thus, whenever an electron-withdrawing group such as nitro-(4d) or chloro-(4e) is present, the HOMO energy of the diene component increases and the LUMO energy of the dienophile unit also increases.This, therefore, causes the photocyclisation reactions very difficult, and as result of which the compounds 4d and 4e do not give any photocyclised products; ii) during photochemical cyclisation reaction of the parent enamide (4), the presence of orthosubstituent (e.g., nitro and chloro in 4) offers a considerable steric hindrance at the site of carbon-carbon bond formation in the transition state and the reaction, therefore, fails.
Incidentally, this is the first observation of б-cleavage for the proposed formation of anilides, and in order to substantiate the pathway for the formation of anilides, we have independently prepared the intermediate (7) as described in Scheme 2.

Scheme 2
As the anilides are only formed in the presence of iodine, we, therefore, reasoned the reaction probably involved an iodine radical induced isomerism of the double bond of the cyclohexane ring of 4, followed by a fission of C−N bond to give the anilides 5.This was established by irradiating 4a in the presence of N-bromosuccinimide (NBS) in methanol, whence we could isolate 5a in 53% yield.Furthermore, when 3-bromo-N-benzoyl-N-1-cyclohexenyl-Nphenylamine (7), prepared from 4a by the reaction of NBS in dark, was irradiated in methanol only for 3 h., it afforded 5a in 73% yield, showing the same to be a true intermediate in the reaction profile (Scheme 2).
The reaction thus proceeded through an allylic bromination or iodination to produce (8) and the loss of halogen radical during the reaction (Scheme 3) generated the radical (9).This further gave rise to a new radical (10) and thence to the anilides (5).
It is known [10][11][12] that for enamide photocyclisation reaction the involvement of the lone pair of the nitrogen atom is necessary.In the present instance, such participation of the lone pair of the nitrogen atom is difficult as the same is experiencing a conjugative effect of the phenyl group attached to the nitrogen atom.Thus, the phenyl group in 4 is acting as a sink for the lone pair of nitrogen atom and hence, behaves as an electron-withdrawing group (EWG).This EWG effect of the phenyl ring makes the lone pair relatively unavailable for a smooth [2+2+2]-cycloaddition reaction.However, when the phenyl ring is replaced by a 4-methoxyphenyl group, this mesomeric effect on the lone pair of the nitrogen atom is nullified and the photocyclisation becomes easier and higher yields of hexahydrophenanthridones (6f and 6g) are obtained (Table ).In the presence of only phenyl ring in the parent enamides (4a-e) as the lone pair is withdrawn, the photocyclisation becomes competitive with other alternate pathways leading to the anilides.In the absence of any radical initiator the enamide photocyclisation is the only available course of the reaction save Type I cleavage, to afford the highly conjugated hexahydrophenanthridones [ 6 (a-c, f, g].][15] It is also known in the literature the attachment of an electron-releasing group such as alkyl, allyl or benzyl to the nitrogen atom of an enamide enhances its reactivity towards the photochemical [2+2+2]-cycloaddition reaction (Reviews 2).On the other hand, when an an electron-withdrawing group (carbonyl or an aromatic ring) is linked with the nitrogen atom as the third valent unit, a reluctance in the photocyclisation is the consequence and the photochemical rearrangement becomes the major pathway. 2,16,17 Hwever, the present study shows that even in the presence of an EWG (phenyl or aryl) on the nitrogen atom, a photocyclisation reaction occurs instead of a photo-rearrangement.This is notably a new observation and hence, a remarkable effect of the presence of EWG 18 on the nitrogen atom of enamides on the course of their photochemical reactions is observed, and to synthesize the hexahydrophenanthridones from N-phenyl/aryl substituted enamides, a non-oxidative procedure should be employed.

Experimental Section
General Procedures.The melting points recorded in sulfuric acid bath, or boiling points are uncorrected.The IR spectra were obtained in a Perkin-Elmer 297 Infrared spectrophotometer, 1 H NMR spectra, unless otherwise stated, were recorded in a Brьcker 300 MHz AM spectrophotometer in CDCl 3 with TMS as internal standard and the microanalyses were performed in a Perkin-Elmer 240 C Elemental analyzer.The photolysis experiments were carried out in a quartz vessel (immersion type) with a low-pressure mercury lamp (16 W, > 90% 254 nm, Model 3016) manufactured by Applied Photophysics Ltd., England.The solvent abbreviations are: A -Acetone, E -Diethyl ether, H -Hexane, MC -Methylene chloride, P -Light petroleum, 60-80 o C. The yield denotes the combined amounts of crystallized or sublimed material obtained after chromatography (silica gel, 60-120 mesh, Glindia).