Synthesis of a water-soluble carbene complex and its use as catalyst for the synthesis of 2,3-dimethylfuran

1,1,3,3'-Tetrakis(p-dimethylaminobenzyl)-2,2'-biimidazolidinylidene, L-2(R) (R = CH2C6H4NMe2-p) which contains four peripheral NMe2 substituents, was obtained from 4-dimethylaminobenzaldehyde by a three-step reaction sequence , and was used for the preparation of imidazolidin-2-ylidene Ru(II) and Rh( I) complexes 1 and 2. Etherial HCl readily protonates the NMe2 functionalit y on the carbene ligand of 1 to give the corresponding salt, 1'; whereas th e reaction of 2 with HCl gave a hygroscopic and ill-defined rhodium species . In aqueous solution the salt 1' is an efficient and active catalyst for i ntramolecular cyclisation of (Z)-3-methylpent-2-en-4-yn-1-ol into 2,3-dimet hylfuran. The catalyst 1' could be recovered by simple phase separation and the catalytic reaction was maintained for five different runs. All of the new compounds were characterised by elemental analyses, IR and NMR spectros copy and the molecular structure of I was determined by X-ray crystallograp hy. (C) 2001 Elsevier Science B.V. All rights reserved.