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The preparation and reactions of P-chiral phosphine oxides for asymmetric catalysis

Authors Au-Yeung, Terry Tin-Lok
Issue Date 2000
Summary Large-scale synthesis of racemic tert-butylphenylphosphine oxide 93 have been performed and the phosphine oxide was resolved into its individual enantiomeric components, (R)-(+)- and (S)-(-)-tert-butylphenylphosphine oxides 93, through sequential sulfurization, resolution and then desulfurization with Raney-nickel under ultrasound irradiation. Metallation of the enantiopure secondary phosphine oxide with lithium or sodium base followed by alkylation with alkyl halides, aldehydes or imines at -78℃ to room temperature provided tertiary (functionalized) phosphine oxide with retention of configuration at the phosphorus centre. In the case of aldehydes, a strictly stereoretentive [1,2] P-C to P-O bond rearrangement reaction took place at ambient temperature. Whilst it was not possible to isolate the intermediate in the case of ketones, the rearranged products, namely the phosphinates, were always isolated. With imines, no such reaarrangement was observed. An unprecedented stereoselective anti-[1,2] P-C to P-O shift was discovered in treatment of phosphinate 224 with LDA at -78℃ to give bis(tert-butylphenylphosphinoyl)methanol 283. (R)-(+)-tert-butylphenylphosphine oxide 93 was found to also react with aldehydes under neutral conditions to provide exclusively the alkylation product and the configurational integrity at phosphorus was maintained during the process. It has been demonstrated that the reaction is likely to proceed via open transition states. The suspected reactive PIII intermediate tautomer, phosphinite 179, of PV 93 was trapped with borane-tetrahydrofuran complex where retention adduct 210 was obtained. (R)- and (S)-tert-butylphenylphosphine oxides 93 were converted to the corresponding (S)- and (R)-tert-butylphenylphosphinobromidates 107. The latter were reactive towards hetereoatom nucleophiles and the reactions have been shown to proceed generally with inversion in cases of unhindered primary alcohol or aromatic alcohols and nitrogen nucleophiles to give optically pure phosphinates and amidates, respectively. Partial racemization has been noted in the case of relatively sterically bulky nucleophiles. Amidate 251 has been identified as a potential substitute for the bromidate 107. However, reaction of enantiopure 251 with oxygen nucleophiles was found to undergo partial racemization whereas with carbon nucleophiles, racemization appeared not to take place. Treatment of the aryl or vinyl phosphinates or their analogues 229-244 and amidate 253 with n-butyllithium or LDA induced a [1,3]-anionic P-O to P-C rearrangement to give α- or β-functionalized phosphine oxides. The reaction is stereoretentive in nature as supported by X-ray crystallographic studies of the rearranged products and the starting phosphinates. The rearranged products were tested as ligands in asymmetric diethylzinc additions to aromatic aldehydes and borane reduction of ketones but had shown poor performance against these reactions.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2000
Language English
Format Thesis
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