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Synthesis of highly hindered oxepins and an azepine from bis-trityl carbenium ions: structural characterisation by NMR and X-ray crystallography

Lookup NU author(s): Dr Kieren Carey, Emeritus Professor Bill CleggORCiD, Dr Mark Elsegood, Emeritus Professor Bernard Golding, Dr Maurice Hill


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Reactions of the bis-carbenium ion 2,2'-bis[bis(p-methoxyphenyl)methyl]biphenyl ditetrafluoroborate with ammonia, n-propylamine and benzylamine have been studied with the aim of developing an acid-labile protecting group for primary amines that masks both hydrogen atoms of the NH2 group. Although the parent 5,5,7,7-tetrakis(p-methoxyphenyl)-5,7-dihydrodibenzo[c,e]azepine was isolated and fully characterised, the corresponding azepines could not be obtained in a pure state from the reactions with the primary amines. The bis-carbenium ion was prepared from the treatment of 2,2'-bis[bis(p-methoxyphenyl) hydroxymethyl]biphenyl with fluoroboric acid. Attempts to prepare the corresponding diphenyl analogue were unsuccessful using either fluoroboric acid or boron trifluoride. The only product isolated from these reactions was 5,5,7,7-tetraphenyl-5,7-dihydrodibenzo[c,e]oxepin. The oxepin and its phenyl-p-methoxyphenyl and bis(p-methoxyphenyl) analogues were efficiently obtained by dehydration of the corresponding diol, e. g. 2,2'-bis[ bis( p-methoxyphenyl) hydroxymethyl]biphenyl, in the presence of 3 Angstrom molecular sieves or Amberlite IR-200 resin. The compounds 2,2'-bis[bis(p-methoxyphenyl)hydroxymethyl] biphenyl, (R,R/S,S)-2,2'-[bis(phenyl-p-methoxyphenyl)hydroxymethyl] biphenyl, 5,5,7,7-tetraphenyl-5,7-dihydrodibenzo[c,e]oxepin, 5,5,7,7-tetrakis(p-methoxyphenyl)-5,7-dihydrodibenzo[c,e]oxepin and 5,5,7,7tetrakis(p-methoxyphenyl)-5,7-dihydrodibenzo[c,e]azepine were characterised by crystal structure analyses. 2,2'-Bis[bis(p-methoxyphenyl)hydroxymethyl]biphenyl and 5,5,7,7-tetrakis(p-methoxyphenyl)-5,7-dihydrodibenzo[c,e] oxepin exhibited dynamic NMR properties, and free energy barriers have been determined.

Publication metadata

Author(s): Carey KA, Clegg W, Elsegood MRJ, Golding BT, Hill MNS, Maskill H

Publication type: Article

Publication status: Published

Journal: Journal of the Royal Chemical Society: Perkin Transactions 1

Year: 2002

Issue: 23

Pages: 2673-2679

ISSN (print): 1472-7781

ISSN (electronic): 1364-5463

Publisher: Royal Society of Chemistry


DOI: 10.1039/b207108h


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