Main-Chain Chiral Polymers from .beta.-Citronellene via Tandem Diene Metathesis Cyclization/Ring-Opening Metathesis Polymerization

Abstract

The synthesis of non-naturally occurring optically active polymers that possess main-chain chirality remains a topic of current interest.' In this regard, the polymerization of enantiomerically-enriched chiral monomers in which stereogenic centers of the monomer become those of the polymer backbone guarantees that complete control of the absolute and relative configurations of centers within the main chain, and hence, the "handedness" and tacticity, respectively, of the new material, can be a~h i e v e d .~,~ Accordingly, through this strategy, the potential exists to produce not only new materials with maximum optical rotatory power but also those with well-defined backbone conformations, especially in cases where torsional interactions between backbone substituents limits the conformational space that can be probed by the polymer chain.4 Related to this, we have been interested in further delineating the factors which control the polymerizability of cyclic olefins via olefin metathesis, and in particular, of exoand endo-substituted cyclopentenes where we have previously shown that the relief of torsional strain can drive the polymerization process irreversibly f~r w a r d .~ Thus, as a natural extension of these studies, we were intrigued with the idea of generating new classes of optically active polymers and copolymers from readily available, torsionally-strained, chiral cyclic monomers through "living" ring-opening metathesis polymerization (ROMP).6 Herein, we now report the first results regarding the realization of this goal which include (1) the large scale syntheses of isomerically pure (3R)-and (3S)-3-methylcyclopentene (3-MCP) (1) via diene metathesis cyclization (DMQ7 of (-)-and (+)-b-citronellene (2), respectively, and (2) the first unequivocal demonstration of the ROMP of (3R)-and (3S)-3-MCP to yield polymers and copolymers with high optical rotatory power.3-Methylcyclopentene (1) is notoriously difficult to obtain in isomerically and optically pure form, and previously, this has only been accomplished by a lengthy multistep stereoselective synthesis which provided less than 100 mg of the final producte8 Given this, we envisioned the production of large amounts of (3R)-and (3s)-1 via the DMC of commercially available (-1-and (+)-/3-citronellene according t o Scheme 1. As this scheme indicates, the DMC of 2 could indeed be accomplished in high yield by employing Schrock's molybdenum imido alkylidene complex, [(CF3)2CH3COI&lo-[CHC(Me)zPh][ N(2,6-iPr2C6H3)10 (3), which has previously been used for the DMC of a variety of other a,wd i e n e~.~J l J~ In the present example, however, several important features can be noted. First, on a small scale, using 2 mol % of 3, this cyclization reaction, which is 0.75 M in (+)-2 (of 95% optical purity),12J3 proceeds quantitatively in toluene at room temperature within 30 min to provide isomerically pure (3S)-1 as followed by 'H NMR spectroscopy. More importantly, the DMC of technical grade (-)-2 (85% optically pure)12 of 90% 0024-9297/95/2228-0656$09.00/0 Scheme 1 ...

Ruthenium‐Catalyzed Cross Metathesis of β‐Myrcene and its Derivatives with Methyl Acrylate

“…Significant differences between the conversion and yield indicated polymeric side reactions, for example, the ring‐opening polymerization of cyclic diene 2 . This phenomenon was described by Sita in the polymerization of citronellene . Products derived from an acyclic diene metathesis of the substrate were not observed in all runs is in accordance with the results of Fogg et al.…”

Selective ring opening cross metathesis of cyclopropenone ketal: a one step synthesis of protected divinyl ketones

“…Unfortunately, the expected triene was not obtained and, instead, the reaction afforded 2h (in 52% yield) and 3-methylcyclopentene. To explain this result, which contrasts with the others, we believe that product 2h is obtained via a cascade reaction involving a ring opening cross metathesis 12 associated with a ring closing metathesis (Scheme 2). The above sequence overcomes the limitation imposed by the substitution of olefins and therefore, other 1,6-dienes could be use to introduce a disubstituted alkenyl fragment.…”

Ring closing diene metathesis in organic synthesis

“…More recently Sita has shown that the catalyst is compatible with the formation of optically active carbocycle 16. 34 Forbes et al showed that geminal disubstitution could assist in closing five-and seven-membered (e.g. 17, R = Me, Scheme 8) carbocycles by the Thorpe-Ingold effect, 30 and failures to close simple seven-membered (e.g.…”