The (�)-benzyl(methyl)(4-methylphenyl)(naphthalen-1-yl)arsonium
cation has been synthesized and subsequently resolved by fractional
crystallization of monohydrogen [R-(R*,R*)]-2,3-bis(benzoyloxy)- butanedioate
salts. The separated diastereoisomers were converted into the corresponding
optically active arsonium bromides by ion-exchange column chromatography. The
absolute configuration of the arsonium cation exhibiting a positive rotation at
589 nm (sodium D line) has been established as (R) by single-crystal X-ray
analysis of both the bromide and hexafluorophosphate salts. The arsonium
bromide with [α]D + 54.8�(c, 0.62 in CH2Cl2)
crystallizes in the orthorhombic space group P212121
(D24, No.19) with a
22.472(8), b 15.724(7), c 12.585(5) � and U 4447(3) �3. The corresponding hexafluorophosphate with
[α]D + 19.3� (c, 0.5 in CH2Cl2)
crystallizes in the same space group with a
23.56(2), b 16.40(1), c 13.12(1) � and U 5067(6) � 3. Benzylidene transfer to benzaldehyde from
the arsonium ylide derived from either of the arsonium salts produced optically
pure (–)-(S)-methyl(4-methylphenyl)(naphthalen-1-yl)arsine, [α]D
- 115.9� (c, 0.593 in CHCl3), and partly resolved
[R-(R*,R*)]-2,3-diphenyloxiran.