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Diastereoselective transformation of arenes into highly enantiomerically enriched substituted cyclohexadienes
|Published in||Synthesis. 2001, vol. 2001, no. 13, p. 2040 - 2054|
|Abstract||Sequential addition of a C-nucleophile and a C-electrophile to enantiomerically pure arene tricarbonyl chromium complexes 3a,b, 6, and 8b, containing aryl bound chiral oxazoline, SAMP-hydrazone and chiral imine auxiliaries affording substituted cyclohexadienes. C-Nucleophiles included alkyl-, vinyl-, and phenyl-lithium reagents. C-Electrophiles included methyl iodide, allyl bromide, benzyl bromide, and propargyl bromides. The 1,3-cylohexadienes were obtained with a 1,5,6-substitution pattern. The results are consistent with a diastereoselective exo-nucleophilic addition to an ortho position of the complexed arene, followed by addition of the electrophile to the metal center. With allyl, ben-zyl, and propargyl groups, direct reductive elimination then yielded trans-5,6-substituted products. With methyl iodide, reductive elimination was preceded by CO insertion and acetyl cyclohexadienes were formed exclusively whose in situ deprotonation/alkylation gave products in which three C-substituents had been added across an arene double bond with complete regio- and stereocontrol. The two path-ways reflect migratory aptitude to carbonylation. An X-ray structure determination of the phenyl oxazoline complex 3a allowed a rationalization of observed diastereoselectivity. Asymmetric induction was very high with the oxazoline and the SAMP-hydrazone complexes (>90% de) whereas the chiral benzaldehyde imine complex 8b afforded the substituted diene aldehydes in moderate enantiomeric purity (34-58% ee). Changing the reaction medium from THF to toluene in reactions with 8b resulted in products of the opposite chirality.|
|Keywords||Diastereoselective dearomatization — Cyclohexadiene — Arene Cr(CO)3 complexes — Imine — Oxazoline — SAMP-hydrazone|
|BERNARDINELLI, Gérald Hugues et al. Diastereoselective transformation of arenes into highly enantiomerically enriched substituted cyclohexadienes. In: Synthesis, 2001, vol. 2001, n° 13, p. 2040 - 2054. doi: 10.1055/s-2001-17704 https://archive-ouverte.unige.ch/unige:8457|