The Polyhomologation of 1-Boraadamantane: Mapping the Migration Pathways of a Propagating Macrotricyclic Trialkylborane

Trialkyl and triaryl organoboranes undergo multiple, repetitive homologations upon reaction with
dimethylsulfoxonium methylide (1). This multiple homologation reaction, or polyhomologation, produces
polymethylene in a living reaction. Applying the polyhomologation reaction to cyclic and polycyclic
organoboranes permits the construction of unique oligomeric and polymeric architectures that are not readily
accessible by standard olefin polymerization. The polyhomologation of 1-boraadamantane·THF (2) by ylide
1 generates novel macrotricyclic trialkylboranes (3). The oxidation of these macrocyclic organoboranes
generates a three-armed star polymethylene polymer (4) incorporating a cis,cis-1,3,5-trisubstituted
cyclohexane core. Interestingly, only one-third of the initiators lead to product formation, resulting in an
observed degree of polymerization 3 times higher than expected. Close examination of the initial stages of
polymerization show that 1-boraadamantane·THF reacts with 1 equiv of 1 to afford a monohomologated
product. Subsequent homologations were found to contain branch points leading to isomeric tricyclic products
after the third, fourth, and fifth methylene insertions. At these stages of homologation, all of the propagating
species result in tricyclic trialkylborane cages with collapsed, inverted pyramidal boron centers that are
substantially less reactive toward ylide. Approximately two-thirds of the species discontinue polymerization
at these stages. However, one-third of these species continue to propagate and eventually result in the
formation of giant macrotricyclic polymers of narrow polydispersity. Molecular modeling and kinetic simulation
have aided in the analysis of the probable pathways through which the reaction proceeds.