A New Strategy for the Synthesis of Macrocycles. The Polyhomologation of Boracyclanes

Abstract

General strategies for the synthesis of macrocycles are quite limited. Aside from several very specialized reactions that include fragmentation of bicyclic rings, the basic approach relies on ring closure of an R,ω-difunctional linear precursor under conditions of high dilution (eq 1). 1 We report a new strategy for the synthesis of macrocycles. The approach draws upon our discovery of the polyhomologation of trialkylboranes. 2 Polyhomologation results in repetitive methylene insertions into the carbon-boron bond. When applied to boracyclanes, the reaction generates macrocyclic rings (eq 2). These macrocyclic organoboranes can be elaborated to carbocyclic rings and other derivatives.The polyhomologation reaction employs the ylide dimethylsulfoxonium methylide (1) 3 as monomer, and the polymerization is initiated by trialkylboranes. For simple trialkylboranes, all three alkyl groups on boron participate, giving rise to a star polymethylene organoborane 2 (Scheme 1). These oligomeric and polymeric organoboranes (2) are amenable to synthetic transformations that can be used to terminate the polymethylene chain with functionality. 4 For example, peroxide cleavage of the carbon-boron bonds in star 2 results in hydroxyl terminated linear polymethylene 3 in yields over 90% (Scheme 1). 2 The synthesis of macrocycles by the polyhomologation reaction requires boracycles as starting materials. These compounds are readily prepared by hydroboration of dienes. Since all three boron-carbon bonds undergo polyhomologation, we chose thexylborane 5 as the hydroborating agent. The thexyl group has been shown to exhibit a low migratory aptitude in rearrangements of organoboranes. 6 The suppressed tendency toward migration should permit exclusive

Cyclic polymers: Synthetic strategies and physical properties

“…246 Dimethyl sulfoxoniummethid served as monomer and the boracycloalkanes were finally transformed into cyclic ketones. Another example for a REP involving a noncyclic monomer concerns the radical polymerization of methyl methacrylate (Scheme 59).…”

“…[244][245][246][247][248] They are based on four different classes of cyclic initiators, four different classes of monomers, and four different reaction mechanisms.…”

Polyethylene‐Based Tadpole Copolymers

“…Polyhomologation, an organoborane initiated polymerization of dimethylsulfoxonium methylide, leading to hydroxyl-terminated polyethylene, recently developed by Shea, [34][35][36][37] is an attractive method toward well-defined PE-based polymers including cyclics. The only example of cyclic PE is given by Shea who used B-thexylborocane as initiator for the polyhomologation of dimethylsulfoxonium methylide followed by a stitching reaction to freeze the cyclic structure.…”

Topological polymer chemistry for designing multicyclic macromolecular architectures

“…However, the reactive propagating unit is inherently included in the backbone segment, and it is not readily removed by retaining the ring polymer structure. 41,42 To circumvent this problem, a ring-expansion polymerization has been combined with the end-linking technique. 43 Thus, as a typical example, a cyclic stannous dialkoxide has been used as an initiator for the ring-opening polymerization of e-caprolactone.…”