Treatment of the erythro-f3-hydroxyalkyldibenzophosphole oxide (6), derived from (E) -stilbene oxide and lithiodibenzophosphole, with 1,5-diazabicyclo[5.4.0] undec-5-ene (DBU) in dimethyl sulphoxide (DMSO) affords (Z)-stilbene with high stereospecificity.In an earlier paper we described the results of an investigation into the preparation of stilbene from the diastereoisomeric 1,2-diphenyl-2-diphenylphosphinoylethan-1-01s (1) and (2) in order to clarify some details of the work by Horner and Klink on this system. As expected for a syn elimination mechanism, treatment of the threo-isomer (2) with base gave (E)-stilbene stereospecifically. However under most conditions, base treatment of the erythro-isomer (1) also gave predominantly (E)-stilbene, together with variable amounts of benzyldiphenylphosphine oxide. This conversion of erythro-isomer (1) into (E)-stilbene was shown to occur via fragmentation to Ph,POCHLiPh and PhCHO followed by recombination to the anion of the threo-isomer (2) and then elimination. Apparently the combined influence of unfavourable Ph-Ph eclipsing interactions in the syn-elimination pathway from (1) and the relatively high acidity of benzyldiphenylphosphine oxide conspire to disfavour the formation of (2)-stilbene from (1). In contrast, erythro-l,2-disubstituted 2-diphenylphosphinoylethan-1-01s with an alkyl group at C-2 give (2)-alkenes stereospecifically on treatment with base.3In a similar vein, a recent paper dealing with the diethyl phosphonates analogous to (1) and (2) has shown that either diastereoisomer, on base treatment under a range of conditions, inevitably gives (E)-stilbene.'In order to achieve efficient formation of (Z)-stilbene from and erythro-precursor of type (1) it is thus necessary to favour the phosphinate elimination relative to the 'reverse aldol' fragmentation (kE) (Scheme 1).
It is well known that incorporation of Pv into a fivemembered ring can lead to markedly enhanced rates of reaction where a trigonal bipyramidal intermediate is involved; andH Ph Ph PhzPO;+ >=(" scbeme 1.there are a number of cases in the literature where, on starting from, for example, a metallated phosphonate or ylide and an aldehyde, enhanced ( Z ) : ( E ) ratios are obtained when the phosphorus atom is incorporated in a five-membered ring.6 However there do not appear to be any examples of an isolated P-hydroxyalkylphosphine oxide, P-hydroxyphosphonate, or phosphorus betaine where the possible influence of the fivemembered ring heterocycle has been explored. Accordingly we decided to investigate a P-hydroxyalkylphosphine oxide of this t Y P e a 5-Phenyldibenzophosphole (3) was prepared essentially according to the literature procedure.' Conditions for reductive cleavage of (3), hydroxyalkylation with an epoxide and oxidation to the P-hydroxyphosphine oxide were worked out using ethylene oxide. In our hands cleavage was best accomplished using lithium in liquid ammonia (at -78 "C) rather than lithium in tetrahydrofuran (THF); ethylene oxide was added at -35 "C. After oxidation, 2-hydroxy...