Stereoselective Aldehyde Addition to Rhenium-Coordinated Furans

Dihapto-coordinated furan complexes of the form TpRe(CO)(t-BuNC)(2,3-η2-furan) (Tp =
hydridotris(pyrazolyl)borate) react with aldehydes in the presence of BF3·OEt2 to yield
dihydrofuran complexes. Depending on the reaction conditions, these products take one of
three forms:  a dihapto-coordinated trans-2-alkyl-3-acyl-2,3-dihydrofuran, trans-2,3-dialkyl-2,3-dihydrofuran, or 4a,7a-dihydro-4H-furo[2,3-d][1,3]dioxine. The first two are formed
exclusively as trans-disubstituted products, whereas the last exhibits cis stereochemistry
at the ring fusion. These are isolated as mixtures of coordination diastereomers, which are
separable by chromatography. Of the two diastereomers of the trans-2-alkyl-3-acyl-2,3-dihydrofuran product obtained in each case, only the major isomer undergoes an interfacial
isomerization under ambient conditions. Attempts to decomplex the dihydrofurans were
unsuccessful.