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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.
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.
Although secondary amides with γ-branching (i.e., 2b and 2d) undergo ROMP, tertiary amides, in which branching is at the β-position, yield only ring-opened monomer. We have reported that monomer 4a undergoes ROM but not ROMP with catalyst 1.19 Here, we describe the ROM of a second ester and the regiochemistry of the ROM reaction.
ROM of 3a and 3b exhibits the same regioselectivity as that of the secondary amide monomers 2, and only resonances consistent with Pathway I were observed in the 1H NMR spectra (Figure S3).
An NMR time course of the reaction of 10 equiv of monomer 4a with 1 equiv of catalyst 1 is shown in Figure 3.
We hypothesize that the increased steric bulk around the ruthenium in RP-1 hinders the binding of subsequent monomers to the tertiary amide-substituted carbene (see below). As seen with monomer 4a, incubation of ester 4b with 10 mol % of catalyst 1 resulted in the ring-opening metathesis (ROM) of approximately 10 mol % of the monomer with no polymerization (Table 1, Figure S2); that is, the remaining 90% of the monomer does not react. 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. 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. 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. For the ROMP chain extending reactions, R1 ) R2 and R3 ) the polymer chain. In the optimized structure of the ruthenium carbene, the N-methyl group blocks the face of the metal carbene to which the incoming cyclobutene must bind (Figure 9a). Our initial hypothesis for the lack of polymerization of enoic carbene was that the electron withdrawing nature of the ester deactivated the cyclobutene olefin to further metathesis reaction. Neither cyclohexene carboxylic acid esters nor tertiary amides ROMP. Synthesis of 1-Substituted Cyclobutenesa a (i) 3a: H-Gly-OMe, EDC · HCl, DMAP, DIEA, CH2Cl2, rt; 3b: BOPCl, piperidine, DIEA, CH2Cl2, rt. (iii) 4a: N, N′-diisopropyl-O-methylisourea, Et2O, rt; 4b: Br CH2CO2CH3, KI, DIEA, rt. (v) Pivaloyl chloride or acetyl chloride, DIEA, DMAP, CH2Cl2, 0 °C to rt. Two Possible Reaction Pathways and Their Corresponding Regio- and Stereoisomeric Intermediates and Products in the Ruthenium-Catalyzed Ring-Opening Reactions of 1-Cyclobutene Derivativesa a For the ROM reactions and the ROMP initiating reaction, R2 ) H, and R3 ) Ph. To test our hypothesis, geometry optimization of the N, N-disubstituted carbonyl ruthenium carbene formed from monomer 3a was performed with B3LYP/LANL2DZ in Gaussian 03W. In the case of more electron-rich 1-substituents, the inverse rank order of energies between the intermediates and the π-complexed starting materials strongly suggests that the corresponding activation energies to reach the transition states may be close enough in magnitude to make ROMP of carbinol ester monomers 5 neither regio- nor stereoselective. As a function of time, the integration of the peaks a, b, and c (corresponding to protons in monomer 2b) decreased as the integration of peaks d and e (attributed to protons on the chain of the ring-opened ruthenium carbene) increased. In addition, the overall estimated activation energies for carbinol esters 5 are smaller than for those of the secondary amides 2; this result is consistent with the higher rates of propagation observed for monomers 5 (Table 1). Geometry of ROM Intermediate from Tertiary Amide Monomers 3. Derek Middlemiss for his consultation on calculations. The 1H NMR spectra of the ROMP reaction of monomer 2b illustrate the method used to follow the kinetics of the reaction and the assignment of stereochemistry in the products (Figure 2). Thus, poor regio- and stereoselectivity are observed. James Marecek for his assistance with NMR spectroscopy and Dr.