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 1H­{31P} NMR spectrum of the dimethylphenylphosphine
complex in dichloromethane-d2 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 1H 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 1H­{31P} NMR spectrum of the closely
related [2-(methoxymethyl)­phenyl]­dimethylphosphine–arsenium
complex in dichloromethane-d2 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,RP diastereomer of the phosphine–arsenium complex confirmed
the absolute configuration of the resolved P-chiral
phosphine.