A one pot nucleophile/electrophile additiodhydrogenation sequence was applied to [(benzene)Cr(CO)j] to give predominantly the 4,5-trans-disubstituted cyclohexene. Several asymmemc modifications of the sequential addition of C-nucleophiles and C-elecuophiles to (arene)Cr(CO), complexes are discussed. A mechanistically intriguing route involves the use of chiral phosphorous ligands to control the diastereoselectivity in the migratory CO insertion step and/or the reductive elimination step in the sequence. Ephedrine and norephedrine derived ligands gave product ee of up to 69 9%. New C2-chiral bidentate ligands (L*) which emulate some of the bonding characteristics of CO were synthesized and briefly considered for this application. The main interest in these ligands concerns their potential in catalytic C-C bond forming reactions; a first application to the Lewis acid [CpFeL*]+ catalyzed Diels-Alder reaction between enals and dienes was successfully realized.The readily accessible, air stable (arene)Cr(CO)g complexes 1. react with carbanions by addition to the arene exo-face to afford anionic 'q5-cyclohexadienyl complexes 2. In sifu reaction with the appropriate electrophile gives access to aromatic and alicyclic products as shown in Scheme 1 (1-5). Scheme 1. Products resulting from the sequential addition of a C-nucleophile and an electrophile to an (arene)Cr(C0)3 complex.
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Two new C2 chiral bidentate phosphorous ligands have been prepared in enantiomerically pure form. The two phosphorous centers bear electron-withdrawing groups ((CF3),CH-0, C6F5) and are linked by a trans-cyclopentane-1,2-diol-derived bridge. Photolysis of [Cr(q6-C6H6)(Co)3] in the presence of these two new ligands and of two previously reported bidentate phosphites, and fluorophosphinites (L) afforded [Cr(q6-C6H6)(CO)L] complexes. IR Spectral comparison of the complexes shows the new ligands to be intermediate in their bonding properties between alkyl phosphites and CO.
A series of [Cr(benzene)(CO)2L] complexes with L = PPh3, P(OMe)3, PPh2 ((–)-menthyl), P(OPh)2(O-(–)-menthyl), P(O-(–)-menthyl)3 were subjected to a nueleophile addition/acylation sequence to give trans-5,6-disubstituted cyelohexadienes. Low-to-moderate asymmetric induction was observed with the chiral ligands. Experimental and theoretical evidence for an alkylation at the metal center trans to the P ligand is presented, and a crystal structure determination of a [Cr(η5–cyclohexadienyl)(P(OMe)3)(CO)2SnPh3] complex is included.
New C2-Chiral Bidentate Ligands Bridging the Gap Between Donor Phosphine and Acceptor Carbonyl Ligands. -Starting from the enantiomerically pure cyclopentanediol (+)-(I) the new C2 chiral bidentate phosphorus ligands (III) and (V) are prepared. Their enantiomers are analogously obtained from (-)-(I). Photolysis of η6-benzene-tricarbonyl-chromium in the presence of the two new ligands affords the complexes (VI) which show the new ligands to be intermediates in their bonding properties between alkyl phosphites and CO. -(KUENDIG, E. P.; DUPRE, C.; BOURDIN, B.; CUNNINGHAM, A. JUN.; PONS, D.; Helv. Chim. Acta 77 (1994) 3, 421-428; Dep. Org. Chem., Univ. Geneva, CH-1211 Geneva 4, Switz.; EN)
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