Sequential synthesis of main-chain NLO oligomers which contain 4-dialkylamino-4'-(alkylsulfonyl)azobenzene chromophores

Abstract

A protected ω-secondary amino carboxylic acid monomer which contains the 4-dialkyl- amino-4'-(alkylsulfonyl)azobenzene nonlinear optical (NLO) chromophore has been prepared by a convergent synthesis. The carboxylic group is protected as a benzyl ester, while the N-benzyl secondary amino group is protected as a 9-fluorenylmethyl carbamate (Fmoc). Selective deprotection of the protected monomer benzyl-OOC-monomer-N(benzyl)-Fmoc was carried out. Treatment with piperidine selectively removed the Fmoc-protecting group to give benzyl-OOC-monomer-NH(benzyl). Alternatively, treatment with HCl-acetic acid removed the benzyl-protecting group to yield HOOC-monomer-N(benzyl)-Fmoc. The mono-protected monomers were coupled with dicyclocarbodiimide (DCC) to afford a protected dimer, benzyl-OOC-dimer-N(benzyl)-Fmoc. In the same way, selective deprotection followed by DCC coupling of the mono-protected dimers yielded a protected tetramer, benzyl-OOC-tetramer-N(benzyl)-Fmoc. Monomers, dimers and tetramer have been fully characterized.

Keywords:

• Main-chain NLO oligomers
• selective deprotection
• oligo-amides