[reaction: see text] Vicinal stereocontrol during nucleophilic addition of tert-butyl lithiopropionate to eta(6)-anisole chromium tricarbonyl complexes with differing para substituents has been studied. Excellent vicinal double stereoinduction (>99:1) was observed when the para substituent was Si(CH(3))(3), and this has been applied to a stereoselective formal synthesis of (+/-)-erythro Juvabione. Asymmetric synthesis by chiral auxiliary directed nucleophilic addition is also discussed.
Asymmetric induction during enolate nucleophile addition to chromium tricarbonyl complexes of 4-alkoxy-1-(trimethylsilyl)benzene derivatives, in which the alkoxy group (R*O) is derived from an optically pure alcohol, is analyzed in terms of the effect of the chiral auxiliary (R*) on the conformation of the tricarbonylchromium tripod. Crystal structure determination on the three complexes 3a (R* ) 2-phenylisobornyl), 3b (R* ) 2-methylisobornyl), and 3c (R* ) 2-(3-[1,3]-dioxalanyl)isobornyl) reveals that the chiral group R* causes a rotation of the Cr(CO) 3 group from the normally preferred orientation, the extent and direction of which are dependent on the auxiliary. NMR studies on seven complexes indicate the presence of a single conformation in solution for each, which is assumed to be the same as that shown in the solid state. There is a correlation between the observed asymmetric induction and the Cr(CO) 3 tripod rotation, which is believed to be a result of the effect of this conformational distortion on the arene-centered FMO coefficients.
The tricarbonylchromium unit bound to the arene ring of the chiral title complex, [Cr(C19H26O3)(CO)3], is rotated by ca 25° in agreement with the proposed mechanism for 1,5‐asymmetric induction of nucleophilic attack.
All reactions involving chromium tricarbonyl complexes were performed using oven-dried (125 ºC) glassware under anhydrous, oxygen free argon atmosphere. All reactions were performed in freshly distilled (under nitrogen) solvents and monitored by TLC on silica gel. The TLC plates were visualized with UV light and/or with phosphomolybdic acid solution in ethanol. Reactions performed at -60 ºC were maintained at that temperature using ethanol bath and Neslab Cryotrol. Flash chromatography was performed on silica gel with mesh 170-400 under nitrogen pressure. NMR spectra were recorded on a Varian Gemini 200 (200 MHz) or Varian Gemini 300 (300 MHz) or Varian 600 (600 MHz) spectrometer. FTIR spectra were recorded as neat oils or KBr pellet on a Nicolet Impact 400 FTIR spectrometer. High resolution mass spectra (HRMS) of compounds were recorded in-house using a Kratos MS25A instrument by either EI (Electron Ionization) or FAB (Fast Ion Bombardment). Capillary GC was carried out using HP5890 Series II Gas Chromatograph. The melting points were measured on a Thomas Hoover apparatus and are uncorrected. The purity of new compounds was assessed from their 1 H and 13 C NMR spectra. 4-Methoxy trimethylsilylbenzene was prepared by the literature procedure. 1 Chromium complexes are previously reported in the literature and were prepared from the corresponding arenes by Pauson and Mahaffy procedure of refluxing the arene with chromium hexacarbonyl in dibutyl ether and THF mixture. 2 endo-2-Naphthyl isoborneol was prepared from D(+) Camphor by addition of naphthylmagnesium bromide in THF in the presence of anhydrous cerium chloride. 3 S2 Experimental parameters for Capillary GC on HP5890 Series II Gas Chromatograph: Column: 25 m HP 5MS capillary column Carrier gas: Helium at 45 psi Injector temperature: 200 ºC Detector temperature: 300 ºC Run time: 26 minutes η 6 -(4-methoxytrimethylsilylbenzene)chromium tricarbonyl (3)
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