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The stereochemical inversion of unbiased allylic alcohols using triphenylphosphine, diethyl azodicarboxylate, and benzoic acid, commonly known as the Mitsunobu reaction, was studied in three different solvents with specific attention toward the product composition. The results generated for the Mitsunobu reaction of (R)-3-deuterio-2-cyclohexen-1-ol and the cis and trans isomers of 1-deuterio-5-methyl-2-cyclohexen-1-ol, 1-deuterio-5-tert-butyl-2-cyclohexen-1-ol, and optically active cis and trans 5-isopropyl-2-methyl-2-cyclohexen-1-ol all gave similar product distributions with respect to inversion and retention at the carbinol center as well-as syn and anti S(N)2' type addition when THF or benzene was used as the solvent (CH(2)Cl(2) gave less selective product distributions). Interestingly, it was found that the quasi-equatorial and quasi-axial nature of the starting allylic alcohol does not appear to affect the product distribution for this reaction, nor does methyl substitution at the central carbon of the allylic alcohol. In all cases, significant amounts (8-28%) of non-S(N)2 type products were detected for these sterically unbiased allylic alcohols; only 72-77% of the product was from S(N)2 type reaction when sterically undemanding (R)-3-deuterio-2-cyclohexen-1-ol was subjected to Mitsunobu conditions.

The simple, straightforward 1 H NMR spectrum of the neutron capture therapy drug, -p-boronophenylalanine (-p-BPA), in D 2 O changes to a more complex one in DMSO-d 6 in which the ratio of new species observed is highly concentration dependent. The new species detected can only be explained by an additional stereocenter being formed at the boron center by intermolecular chelation of the amino acid of another molecule of -p-BPA. This gives rise to the presence of an oligomeric species as well as another whose aromatic protons appear as a pair of sharp AB quartets centered further upfield at 6.66 and 6.84. Due to these shifts and couplings observed between the benzylic protons and the proton at the stereocenter of the amino acid, it is proposed that this species is a paracyclophane dimer of -p-BPA in which one molecule of -p-BPA chelates head-to-tail with another. This cyclophane dimer predominates in low concentrations (<50 mM) while the oligomer predominates at higher (>90 mM) concentrations. The formation of these two species is completely reversible, the addition of D 2 O completely regenerating -p-BPA. Variable temperature 1 H NMR found that the two pairs of aromatic protons of the cyclophane dimer coalesce at T c = 141 ЊC, corresponding to a ∆G ‡ = 20.6 kcal mol Ϫ1 .

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