Factors Controlling Selectivity in the Ring-Opening Metathesis Polymerization of 3‑Substituted Cyclooctenes by Monoaryloxide Pyrrolide Imido Alkylidene (MAP) Catalysts Henry Martinez,* Marc A. Hillmyer, and Christopher J. Cramer Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States * S Supporting Information ABSTRACT: The origins of regio- and stereoselectivity in the ring-opening metathesis polymerization of 3-substituted cis-cyclo- octenes by monoaryloxide pyrrolide imido alkylidene (MAP) Mo- and W-based catalysts are determined at the M06-2X/SDD|6- 311+G(2df,p)//M06-L/SDD|6-31G(d)|MIDI! level of density functional theory. Considering cis-cyclooctene (COE) and 3- methyl-cis-cyclooctene (3MCOE) as monomers and W(N-t- Bu)(CH-t-Bu)(OHMT)(Pyr) (OHMT = hexamethylterphenox- ide, Pyr = pyrrolide) as a catalyst, all possible syn and anti combinations of alkylidene and cyclic olefin, relative to the imido ligand, are evaluated. The observed Z-selectivity for the ring- opening metathesis (ROM) of COE is due to the large size of the aryloxide ligand, which forces both the alkylidene and the incoming cyclic olefin to be syn relative to the imido ligand. As determined previously for Grubbs’ second-generation catalyst (G2), breakdown of the metallacyclobutane intermediate is the rate-limiting step for cyclic olefins having ring sizes exceeding five carbon atoms. Contrary to the G2 case, however, the ring-opening of 3MCOE by MAP catalysts prefers a proximal (3-substituent closest to the metal center) over a distal (3-substituent furthest from the metal center) approach. In all calculated paths, we observe inversion of catalyst configuration after each catalytic cycle. ■ INTRODUCTION Ring-opening metathesis polymerization (ROMP) of both substituted and unsubstituted cyclic olefins leads to polymers with tunable physical properties. 1-3 The relative ease with which functionalized monomers may be synthesized, together with the development of highly active and functional group tolerant catalysts, makes ROMP an attractive method for the preparation of functionalized polymers. The asymmetric nature of most functionalized monomers, however, can lead to the synthesis of regio-irregular polymers with head-to-head (HH), head-to-tail (HT), and tail-to-tail (TT) microstructures and mixtures of cis and trans stereochemistry. 3 However, more precise control over microstructure can have significant impact on the final material properties, including higher melting temperature, increased tensile strength, and increased modulus in some cases. 3-6 A number of Ru-, Mo-, and W-based compounds have proven to be effective ROMP catalysts. Of these three, Mo- and W-based catalysts demonstrate good control over stereo- and regioregularity, particularly in the ROMP of norbornene and norbornene derivatives. 7-11 The first generation of metal imido alkylidene bisalkoxide catalysts developed between 1986 and 1996 (Figure 1a) yielded trans syndiotactic polymers upon ROMP through a chain-end control mechanism. 9 Subsequently, metal alkylidene diolate catalysts were developed between 1996 and 2006 (Figure 1b). 9 The enantiomorphic site at the catalyst (e.g., with bisphenolate or binaphtholate ligands) yielded cis isotactic polymers. Since 2006, a new generation of Mo- and W-based catalysts have been designed (Figure 1c); 9,12 these metal imido alkylidene monoalkoxy pyrrolide (MAP) catalysts are the first to have a stereogenic metal center, yield cis syndiotatic polymers upon ROMP, and are the focus of the present work. Both experiments and calculations demonstrate that these asymmetric catalysts are more metathesis-active than those of prior generations, 13,14 which has been attributed to the difference in σ-donor strength between the two ligands (aryloxide vs pyrrolide): this causes the olefin to selectively approach the metal center trans to the better σ-donor pyrrolide Special Issue: Mechanisms in Metal-Based Organic Chemistry Received: July 29, 2014 Figure 1. Different catalyst generations (M = W or Mo) used in the synthesis of polymers with controlled microstructure via ROMP. R, R′, and R″ are, in most examples, alkyl or aryl groups. Article pubs.acs.org/joc © XXXX American Chemical Society A dx.doi.org/10.1021/jo501732q | J. Org. Chem. XXXX, XXX, XXX-XXX