COMPARISON ON THE SELECTIVITY OF THE FIRST AND 2ND ATTACK OF SELECTEDLITHIUM ORGANYLS AS NUCLEOPHILES ON THE CARBONYLS IN THE BIOCTAHEDRONCOMPLEXES M-2(MU-PPH2)(2)(CO)(8) (M = MN, RE) - IS THERE A NEIGHBOR GROUP EFFECT FOR THE 2ND ATTACK

In thf solution the bioctahedron complex Re-2(mu-PPh2)(2)(CO)(8) 1 rea cts with one equiv of LiR (R = Ph, Me, nBu, tBu) at temperatures betwe en - 90 and - 50 degrees C to the acyl monoanion Li[Re-2(mu-PPh2)(2)(C O)(7)(ax-C(R)O)] 3 and with a second equiv of LIRI (R-1 = Ph, Me, nBu) at 20 degrees C to the diacyl dianions i-2[Re-2(mu-PPh2)(2)(CO)(6)(ax -C(R)O)(ax-C(R-1)O)] 2 as trans or cis isomers. Both nearly quantitati ve reaction steps run selectively: First, both lithium nucleophiles at tack exclusively axially coordinated carbonyl ligand. Second, the mono anion with R = Ph forms exclusively the cis diacyl dianions (R-1 = nBu , Ph). Through this reaction an anchimeric effect was proved. Third, t he monoanions with R = nBu, Me react only to the trans diacyl dianions . In general such mono- and dianions have been reacted with O-alkylati ng agens to the corresponding carbene and dicarbene derivatives (yield 50-70%) to get single crystals for X-ray structure analyses. All prod ucts were identified by means of H-1, P-1 NMR and v(CO)IR data. Additi onally the molecular structures of the Fischer type carbene complexes Re-2(mu-PPh2)(2)(CO)(7)(ax-C(t Bu)OMe) 5b, cis -Re-2(mu-PPh2)(2)(CO)(6 )(ax-C(Ph)OEt)(2) 14 and trans -Re-2(mu-PPh2)(2)(CO)(6)(ax-C(n Bu)OMe) (ax-C(Ph)OMe) 15b are presented. Opposite to 1 affords the homologous dimanganese complex 2 under the same reaction conditions at - 50 degr ees C with one and two equiv of lithium organyl an unselective nucleop hilic addition to axially and equatorially coordinated monoacyl monoan ions and diacyl dianions. Our attempts to separate such isomers of the mono- and dianion type as lithium salt led to their decomposition. Ho wever, the carbene and dicarbene derivatives Mn-2(mu-PPh2)(2)(CO)(7)(a x-C(Ph)OEt) 9ax and trans-Mn-2(mu-PPh2)(2)(CO)(6)(ax-C(nBu)OEt)(2) 16 could be crystallized as air-stable, yellow solids. Their molecular st ructures are presented. (C) 1998 Elsevier Science S.A. All rights rese rved.