ABSTRACT
The configurational stability of chlorophosphines is investigated. Several mechanisms involving chlorophosphine monomer, dimers, and adducts with HCl are evaluated by density functional theory calculations. The presence of HCl in the medium is found to catalyze the P-center chiral inversion at room temperature. The reaction involves a two-step mechanism with low transition states (10 kcal.mol-1) and a stabilized achiral intermediate (-2.6 kcal.mol-1). Further calculations and experiments on the halogen exchange with HBr corroborate this mechanism, with bromophosphines being formed instantaneously. Finally, to avoid the racemization, the borane is found to be a very promising protecting group for the configurational stability of the P-chirogenic chlorophosphines.
ABSTRACT
Chiral alpha- and beta-hydroxy acids such as (S)-lactic acid, (S)-phenyllactic acid, (S)-mandelic acid, or (3R)-3-hydroxybutyric acid have been used as tether groups for intramolecular and diastereoselective [2 + 2] photocycloaddition of 3-oxocyclohexene carboxylic acid derivatives. Total regiocontrol toward the straight adduct and high diastereoselectivities (up to 94%) were observed in the case of butenyl lactate 11. After separation of the two diastereoisomers, cleavage of the chiral tether under basic conditions afforded cyclobutane lactones in good yield and enantiomeric pure form. An X-ray structure has been recorded that confirmed the relative and absolute configuration of the three contiguous stereogenic centers assigned according to CD spectra.
