Through-Shell Alkyllithium Additions and Borane Reductions

The through-shell borane reduction and methyllithium addition to benzaldehyde (1), benzocyclobutenone (2), and benzocyclobutenedione (3) incarcerated inside a hemicarcerand (4) with four
tetramethylenedioxy bridges are reported. All guests could be reduced and methylated. Selective
monoreduction and monomethylation were observed for 3. In the methyllithium addition to 4⊙3,
the initially formed lithium alcoholate underwent a Moore rearrangement. The reactivity of the
incarcerated guests toward methyllithium increased in the order 1 < 2 ≪ 3 and toward borane in
the order 1 ≪ 2 ≈ 3. Guest reactivity was correlated with the inner-phase location of the reacting
carbonyl group in the preferred guest inner-phase orientation. The latter was determined from
the X-ray structures of 4⊙1, 4⊙2, and 4⊙3, from molecular mechanical calculations, and from the
hemicarcerand-induced upfield shift of the guest proton resonances. In the methyllithium and
n-butyllithium addition to 4⊙1 and 4⊙3 at elevated temperatures, selective cleavage of a host's
spanner or tetramethylenedioxy bridge, respectively, was observed. The cleavage of one spanner
also took place in the methyllithium addition to the 1-methyl-2-pyrrolidinone hemicarceplex. These
scission reactions are initiated by the initially formed lithium alcoholates, which show enhanced
basicity and nucleophilicity in the inner phase as compared to the bulk phase. Mechanisms for the
host scission reactions are discussed.