ANHYDROUS MONOMERIC GLYOXAL PYROLYTIC GENERATION AND STABILIZATION BY THE CHIRAL RHENIUM FRAGMENT [ETA(5)-C5H5)RE(NO)(PPH3)]+
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Reaction of [5-C5H5)Re(NO)(PPh3)(ClCH2Cl)]+BF4 (5+BF4) and a CH3OH/H2O glyoxal solution gives the complex [(5-C5H5)Re(NO)(PPh3)(2-O=CHCH=O)]+BF4 (1+BF4) in 17 % yield as a 95:5 mixture of RS,SR/RR,SS diastereomers. The trimeric dihydrate of glyoxal and P2O5 are pyrolyzed under vacuum at 160 C. Pure anhydrous monomeric glyoxal can be trapped at 80 C, as assayed by 1H and 13C NMR. Subsequent reaction with 5+BF4 gives 1+BF4 (33%; 95:5 RS,SR/RR,SS). The NMR and IR properties of 1+BF4 are studied in detail, especially with regard to the free and bound O=CH moieties. A 1H NOE experiment suggests that an s-cis O=CC=O conformer dominates in solution. The crystal structure of (RS,SR) 1+BF4 also shows an s-cis conformer, with free/bound O=C bond lengths of 1.206(7)/1.331(5) . Reaction of 1+BF4 and NaI in CH3CN gives 1+I (75%), which when placed in CH3OH gives (5-C5H5)Re(NO)(PPh3)(I) (69%). Reaction of 1+BF4 and CD3CN at 80 C gives [(5-C5H5)Re(NO)(PPh3)(NCCD3)]+BF4. However, in none of these substitutions is free glyoxal detected. Well-defined additions to the glyoxal ligand could not be effected. 1994, American Chemical Society. All rights reserved.
