In Pathway I, the Ru-cyclobutane ring is formed with a 1,3-relationship between the substituent-bearing carbons, whereas in Pathway II, there is a 1,2-relationship between them. The ROMP kinetics of monomers 2a-2e were measured by monitoring the ROMP reactions in CD2Cl2 at 25 °C.
When a 1-substituted cyclobutene coordinates to the ruthenium center of the catalyst, there are four Ru-cyclobutane intermediates that may be formed (cis- and trans-IM-1 and cis- and trans-IM-2, Scheme 4).
Here, we present the kinetic data for the metathesis transformations, the computed energies of key reaction intermediates and products, and the correlation between these energies and the observed reactivities. The known cyclobutene-1-carboxylic acid 6 was derivatized to provide all of the monomers (Scheme 3).10,15 It was converted to amides 2 by EDC coupling.
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.
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. 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. 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. However, internally heterogeneous mixtures are obtained because the polymerization reactions lack complete stereochemical and regiochemical control. 10.1021/ja1037098 2010 American Chemical Society polymers with accurate molecular weight control and low polydispersities (PDIs). Selectivity in formation of the metallocyclobutanes determines the regiochemistry and stereochemistry of chain extension (IM f RP). We have previously shown that secondary amide cyclobutenes provide at least 50-mer polymers with excellent PDIs, if required.10 Here, we studied (16) Griffin, R. 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 ROMP reaction in the context of the preparation of shorter polymers so that NMR analysis would be more informative. The consequence is that trans- and cis-π-SM-1 react at a comparable rate to trans- and cis-π-SM2. The NBO charge and energy calculations suggest that trans-π-SM-2 and cis-π-SM-2 are lower in energy than the π-SM-1 regioisomers. As these steric interactions develop in the transition state, the difference in TSa energies between isomers is much smaller than in either the π-SM or IM species.