Photochemical and Thermal Rearrangements of Allyl and Propargyl Ethers of Vicinal Hydroxynaphthophenones and Scholl Reaction in Naphthol Derivatives

Abstract

Thermal rearrangement of aryl allyl ether with an unsubstituted ortho position gives rise to single product (the ortho-substituted phenol). The corresponding photochemical reaction leads to a mixture of products, viz. ortho- and para- substituted phenols along with the parent phenol. In some cases, even metal substituted phenol is also observed. This can be explained by a mechanism that involves a homolytic cleavage of the C-O bond as the primary process, followed by recombination of the radical pair at various positions of the aromatic ring with or without allylic inversion. This was supported by the findings from irradiation of isotopically labelled ethers. A remarkable product selectivity in favour of ortho was observed when photo-Claisen rearrangement of phenyl allyl ethers were carried out as solid complexes within zeolites and and#946;-cyclodextrin. Substituent effects, Arrhenius activation parameters, and rate constants for the photo-Claisen rearrangement of allyl aryl ethers has also been studied. Photochemical rearrangement of benzyl vinyl ethers leads to 1,3-migration of the benzyl group in contrast to the [3,3]-migration observed in thermal rearrangement. Influence of phenyl core substituent`s and vision on orthogonality in the photo-Claisen rearrangement of allyl phenyl ether in micro-flow has recently been reported. However, there are only a few reports on photo-Claisen rearrangement of naphthyl allyl ethers. Photolysis of allyl ether of 1-naphthol was reported to yield both the (2-allyl-) and (4-allyl-) 1- naphthols. While, 3-methyl-2-butenyl naphthyl ethers exhibited fluorescence upon irradiation in methanol-d4 at 298 K, the methyl naphthyl ether analogues were inactive. 1-Halo-2-naphthyl allyl ether and 2-halo-1-naphthyl allyl ether do not undergo photo-Claisen upon irradiation but leads to 2-halomethyl-2,3- dihydronaphthofurans. In contrast, 3-halo-2-naphthyl allyl ether is photo stable and does not undergo any photoreaction.