Effects of Structured Solids on Regioselectivity of Dibromination of Naphthalene

Abstract

The influence of structured solids (amorphous, lamellar and 3D crystalline) on the regioselectivity of the dibromination of naphthalene using bromine at room temperature has been investigated. The more acidic amorphous catalysts and an acidic clay give rapid reactions and lead to a large preponderance of 1,4-dibromonaphthalene over the 1,5-dibromo compound, while calcined KSF clay, a bentonite material, after a short reaction time gives a small predominance of the 1,5-isomer. Longer reaction times lead to the equilibration of reaction mixtures, which means that the 1,4-isomer eventually predominates in all cases. Based on these observations, it has been possible to devise synthetically useful preparative procedures, involving direct room temperature reactions of bromine with naphthalene, for either 1,4-dibromonaphthalene or 1,5-dibromonaphthalene based on the use of different solids, namely Synclyst 13 and KSF clay, respectively.

Polybromination of naphthalene using bromine over a montmorillonite clay and regioselective synthesis of 2,6-dibromonaphthalene

“…As reported previously, dibromination of 1 using two equivalents of Br2 in DCM over KSF clay that had been calcined by heating in air for 6 hours had given a small amount (up to 5%) of 1,4,6-tribromonaphthalene (6) as an impurity. 28 Reaction of 1 (7.64 mmol) with three mole equivalents of Br2 over freshly calcined KSF clay (4.0 g, based on our previous experience 26 ) in different solvents was attempted (Scheme 1) and the reaction time was varied from 30 minutes to 65 hours (Table 1). Scheme 1.…”

“…It is well known that α-positions are more reactive towards electrophilic substitution than β-positions and it is therefore not surprising that dibromination led to mixtures of 4 and 5. 28 Further bromination of 4 would be sterically hindered at the remaining α-positions due to the presence of bulky bromine substituents at the nearby peri positions, 1 and 4.…”

“…Therefore, it would not be surprising that further bromination of 4 might take place at one of the identical positions 6 or 7, leading to the production of 6. However, since 5 was also formed in a substantial amount in the reaction of two equivalents of bromine with naphthalene over calcined KSF clay, 28 it would be reasonable on the same basis to expect a significant amount of 1,3,5-tribromonaphthalene to be formed in the reaction with three equivalents of bromine, possibly with a smaller quantity of 1,2,5-tribromonaphthalene, which would require attack of bromine at a position adjacent to an existing bromine substituent. However, only small quantities of any tribromonaphthalenes other than 6 were seen in the reactions with three equivalents of bromine, even when the yield of 6 was as high as 66% (Table 1).…”

“…On the basis of the above analysis, involving successive further bromination steps on pre-formed bromo derivatives, it should be possible to enhance the selectivity towards particular polybromo isomers by starting with an appropriate pure dibromonaphthalene precursor rather than relying on the mixture of isomeric dibromonaphthalenes formed during dibromination of naphthalene itself over calcined KSF. Therefore, further brominations were attempted on a reaction mixture from dibromination over Synclyst 13 (an amorphous silica-alumina), which was virtually pure 1,4-dibromonaphthalene (4), 28 with two mole equivalents of additional Br2 in DCM, with and without added calcined KSF clay. Surprisingly, no further bromination occurred under these conditions in either case and 4 remained as the almost exclusive component of the reaction mixtures.…”

“…Following crystallization of the latter mixture, 5 was obtained in 40% yield. 28 The current work deals with polybromination of 1 using Br2 (three or four mole equivalents) over the same clay to produce tri-or tetrabromonaphthalenes, respectively. In addition, proto-debromination of tetrabromonaphthalenes using butyllithium (BuLi) was investigated in an attempt to produce the linear derivative, 2,6-dibromonaphthalene.…”