Phosphine complexes of an enantiomerically pure, atropisomeric arsenium ion

The first enantiomerically pure secondary iodoarsine has been prepared by ring closure of (aR)-[{Li(TMEDA)}₂ {2,2′-bis(methylene)-1,1′-binaphthylyl}] with dichlorophenylarsine, followed by disproportionation of the resulting seven-membered (aR)-phenylarsepine with triiodoarsine in boiling toluene. The corresponding (aR)- chloroarsepine was prepared from the (aR)-iodoarsepine by halide metathesis with silver chloride in dichloromethane. The crystal structures of the two air-stable (aR)- haloarsepines have been determined. A series of phosphine-stabilized arsenium hexafluorophosphates has been prepared from the (aR)-iodoarsepine by reactions with trimethylphosphine, dimethylphenylphosphine, and [2-(methoxymethyl)phenyl]- dimethylphosphine in dichloromethane in the presence of aqueous potassium hexafluorophosphate. The crystal structures of the three complexes have been determined. The structure in each case revealed the significant twist in the (aR)- binaphthyl framework of the seven-membered arsepinenium ion and the coordination of the phosphine approximately orthogonal to the trigonal AsC₂ plane of the arsepinenium ion. The ¹ H{³¹P} NMR spectrum of the dimethylphenylphosphine complex in dichloromethane-d ₂ at 25 °C contains broadened singlets for the diastereotopic PMe groups and a multiplet for the methylene protons of the arsepinenium ring because of phosphine exchange. At the slow exchange limit (ca. −50 °C), the complex cation is devoid of symmetry, as indicated in the ¹ H NMR spectrum by the sharp resonances for the PMe groups and separate AB and A′B′ spin systems for the two pairs of methylene groups in the arsepinenium ring. The ¹ H{³¹P} NMR spectrum of the closely related [2-(methoxymethyl)phenyl]dimethylphosphine−arsenium complex in dichloromethane-d ₂ at 25 °C is sharp, however, which is in agreement with additional stabilization of the complex by coordination of the 2-methoxymethyl group to the phosphorus and arsenic atoms, as indicated in the crystal structure. The resolution of (±)-[2-(methoxymethyl)phenyl]methyl(2-naphthyl)phosphine was achieved by complexation to the enantiomerically pure (aR)-arsenium hexafluorophosphate auxiliary. The crystal structure of the less soluble aR,R_P diastereomer of the phosphine−arsenium complex confirmed the absolute configuration of the resolved P-chiral phosphine.