Time course of ROMP of monomer 2b followed by 1H NMR spectroscopy. The 10-mer polymers derived from secondary amides 2 are formed with high regioselectivity and stereoselectivity. This reactivity opens an avenue for preparing alternating polymers with unique functionality.19 Steric crowding prevents propagation for tertiary amide substituents. 18.0-19.1 ppm indicated that the ruthenium alkylidene ([Ru]d CH-CH2R) was not formed, and implied that an amidesubstituted ruthenium carbene (RP-1 in Pathway I, Scheme 4) Figure 2. Therefore, we conclude that severe steric crowding is responsible for a lack of propagation after the initial ring-opening occurs. However, the NBO calculations followed by AIM electron density analysis reveal that the charge density on the estersubstituted cyclobutene is not significantly different than the 10520 J. When an enoic carbene is formed from ester monomers 4, the formation of a ruthenium chelate with the ester oxygen traps the enoic carbene in a stabilized state that precludes the reaction of these esters with 1-substituted cyclobutene esters, for example, in homopolymerizations.
In contrast, both 1-cyclobutenecarboxylic acid tertiary amides (3) and 1-cyclobutenecarboxylic acid esters (4) undergo only a single ringopening metathesis cycle (ROM) without polymerization. Ring-Opening Metathesis Polymerization (ROMP) of 2a10 mixture. 2010, 132, 10513–10520 9 10513 ARTICLES Song et al. General Mechanism of Ring-Opening Metathesis Polymerization with Catalyst 1 for the Display of Bioactive Functional Groups Scheme 2. The peak at (18) Lapinte, V.; de Fremont, P.; Montembault, V. The secondary amides provide translationally invariant polymers (E-olefins). Although the carbinol esters yield stereo- and regiochemically heterogeneous polymers, the 1-cyclobutenecarboxylic acid esters and tertiary amides undergo ring-opening metathesis (ROM) but not ROMP. Kinetics, Regiochemistry, and Stereochemistry of 1-Substituted Cyclobutene ROMP. Reduction of ester 4a with DIBAL-H gave cyclobutene-1-carbinol, which was esterified with acetyl and pivaloyl chloride to provide 5a and 5b, respectively. The polymerization rates of substrates 2 are approximately 4 times slower than those of 1,2-unsubstituted, 3-substituted cyclobutenes,18 in which the olefinic bond is disubstituted and the substituents are one atom removed from the carbons that undergo metathesis. The ring-opening reaction requires 2 h to reach this 10% conversion; no polymerization is observed (Table 1, Figure S2). Reaction conditions: CD2Cl2, [2b] ) 0.1 M,  ) 0.01 M, 25 °C. The rates of consumption of monomers 2a-2e are very similar (t50 entries, Table 1, Figure S1). Reaction of tertiary amides 3a and 3b with 10 mol % catalyst 1 resulted in the ringopening metathesis (ROM) of approximately 10 mol % of the monomer. In conclusion, of the functionalized cyclobutenes studied, the secondary amides 2 exhibit the optimal level of reactivity and stereo- and regio- control for the generation of translationally invariant polymers. This research was supported by NIH grants R01HD38519 (N. In addition, the absence of a peak at Scope of the ROMP Reaction of 1-Substituted Cyclobutenes ARTICLES Scheme 3. We hypothesized that the second N-substituent may block the approach and binding of an incoming monomer at the ruthenium carbene center. Regioregular addition to the catalyst carbene is consistent with both the calculated charge distributions for the carbene and the cyclobutene and the minimization of steric interactions. This material is available free of charge via the Internet at During the course of the reaction of each monomer 2, a single broad peak appeared at 6.2 ppm in the 1H NMR spectrum, indicating the formation of an internal trisubstituted olefin with E-configuration.10 There was no evidence of disubstituted olefin (signals in the 5 to 6 ppm region). Kinetic studies with monomers 3a and 3b revealed that only ROM reactions occurred. 30, 2010 In summary, we have found that cyclobutenes undergo stereoand regioregular ring-opening metathesis when substituted with an electron withdrawing carbonyl at the 1-position. Supporting Information Available: Additional figures, experimental methods and results including spectroscopic data and coordinates of calculated structures.