Novel electrophilic aromatic iodine substitution of some naphtholic Schiff bases

During the ring closure trial of the naphtholic Schiff bases ( 1a-d ) we found that iodine was incorporated into the phenyl ring. The structures of the products ( 2a-e ) obtained with this novel substitution reaction, were determined by NMR ( 1 H-NMR, 13 C-NMR, 2D-NMR), mass, IR and UV spectra as well as chemical analysis


Results and Discussion
Naphtholic Schiff bases (1a-d) were synthesized according to the method of Sawich and coworkers. 32Mass spectra revealed that electrophilic substitution instead of aimed ring closure had occurred (3).There were two possibilities for the iodine substitution: (a) substitution of iodine in position 3 of the naphthalene ring yielding compound (4) (b) substitution of iodine in the phenyl ring yielding the compounds (2a-e).

Scheme 1
The structure of the Schiff base (1a-e) substrates prepared was determined by IR, UV, 1 H-NMR and 13 C-NMR spectra and compared with literature data.The naphtholic Schiff base 1a was reacted with DMSO/H + -I 2 for 12 hours.Preparative TLC (SiO 2 /toluene) of the crude product (2a) showed the presence of one new compound (R f =0.24) together with the unreacted Schiff base (1a).The new product was crystallized from alcohol yielding yellow crystals (mp=142-144 o C).Mass spectrum of the compound showed a molecular ion peak at 373.0 and a (M-HI) + peak at 245.0.In the IR spectrum there were (-CH=N-) and -OH groups at 1625 cm -1 and 3500 cm -1 .
Since we obtained the iodine substituted Schiff base 2a instead of ring closed product 3a we tried iodine substitution of Schiff base 1b, bearing an o-methyl group, with DMSO-I 2 -H + under the same reaction conditions.The crude mixture showed the presence of one new compound together with the unreacted Schiff base 1b.It was purified with preparative TLC (SiO 2 /toluene) (R f =0.26) and crystallized from alcohol (yellow crystals, mp: 136-138 o C).Mass spectrum of the purified compound 2b gave a molecular ion peak at 387.0 confirming iodine substitution.Chemical analysis of the compound was in agreement with the iodine substituted structure.In the IR spectrum there were -OH and -CH=N-peaks at 3500 and 1625 cm -1 .In 13 C-NMR spectrum of 2b there were C-I and -CH=N-peaks at 90.9 and 154.8 ppm respectively and together with the other peaks they confirmed the Schiff base structure 2b.From the 1 H-NMR spectrum of the product it was clear that the substitution occurred to the para-position of the phenyl ring with respect to the azomethine group.
Since in the o-methyl substituted Schiff base (1b) iodine was substituted to the para-position of the phenyl ring (2b), we investigated the effect of a m-methyl group in the reaction.When applying the same reaction conditions, two compounds (2c and 2d) were isolated by preparative TLC (SiO 2 /toluene).The compound with R f =0.25 was crystallized from alcohol delivering yellow crystals (mp: 96 o C). 1 H-NMR and 13 C-NMR spectra showed that iodine again was substituted in para-position of the phenyl ring with respect to the azomethine group (2c).For the compound with R f =0.46 (yellow crystals, mp: 146-148 o C) mass spectra and the chemical analysis confirmed the iodine substituted Schiff base (2d) with iodine substituted in the orthoposition with respect to the azomethine group and para-position relative to the -CH 3 group. 1H-NMR and 13 C-NMR spectra as well as the mass spectrum and chemical analysis were in agreement with this structure.
Subsequently, with a blocked para-position we wished to explore the effect on the substitution reaction.After heating of the Schiff base 1e with DMSO-I 2 -H + under the same reaction condition we obtained one product (2e) (mp: 142 o C) together with the unreacted Schiff base (1e).Mass spectrum of the isolated product was again 387.0 and chemical analysis confirmed the expected elemental composition of 2e.Analysis of 1 H-NMR and 13 C-NMR spectra showed that iodine was substituted in the ortho-position with respect to the azomethine group and meta-position relative to the -CH 3 group in the phenyl ring.For the mechanism of the iodine substitution reaction we suggest the mechanism in Scheme 2.

The synthesis of the Schiff bases (1a-d)
Schiff bases (1a-d) were synthesized according to the method of Sawich and coworkers. 31Their structures were determined by IR, UV and 13 C-NMR spectra.

Reaction of Schiff bases (1a-d) with DMSO-H + -I 2
Schiff bases (1a-d) (4 mmol) were dissolved in DMSO (16 ml) and the H 2 SO 4 was added until the pH was approximately 5.0 and then catalytic amount I 2 for (1a), for the others (1b-d) equivalent amount (4 mmol) I 2 was added to this solution.The reaction mixture was heated at 125-130 o C for 12 hours under N 2 atmosphere.The crude product was washed with iced water to remove DMSO and dissolved with 10% Na 2 S 2 O 3 to remove the excess iodine.Then the crude product was extracted with CH 2 Cl 2 (30 ml).After evaporation of the solvent in vacuo crude product was purified by preparative TLC (SiO 2 /toluene) and crystallized from ethanol.   C NMR (400 MHz, CDCl 3 ): δ 22.9 (-CH 3 ), 97.6 (C-4′), 108.9 (C-1), 118.9