On the Stereoselectivity of Ring-Opening Metathesis Polymerization (ROMP) of N-Arylpyrrolidine-Fused Cyclobutenes with Molybdenum– and Ruthenium–Alkylidene Catalyst

Abstract

Ring-opening metathesis polymerization (ROMP) of cyclobutenes fused with N-arylpyrrolidene with Schrock−Hoveyda catalyst containing a racemic biphenolate ligand [Mo(N-2,6-i-Pr 2 C 6 H 3 )(CHCMe 2 Ph)(biphenolate)] gives polycyclobutenes with homogeneous tacticity and predominantly double bonds in Zconfiguration. Reactions of the same substrates with the first-generation Grubbs catalyst [(Cy 3 P) 2 Cl 2 RuCHPh] or Schrock molybdenum carbene with monodendate alkoxy ligands [Mo(N-2,6-i-Pr 2 C 6 H 3 )(CHCMe 2 Ph)(OCMe(CF 3 ) 2 ] yield the corresponding polycyclobutene containing a mixture of Z-and E-double bonds. Upon diimide reduction, all these polycyclobutenes give the same tactic hydrogenated polymers, indicating that the stereochemistry at the asymmetric carbons remains the same in all these reactions. The stereoselectivities of ROMP with cyclobutenes and with norbornenes are compared, and the plausible mechanisms are proposed. ■ INTRODUCTIONMetal−carbene-catalyzed ring-opening metathesis polymerization (ROMP) 1 of strained cycloalkenes has been extensively used in synthesizing different kinds of polymers for 193 nm photoresists, 2 nonlinear optical properties, 3 brush polymers for optoelectronic applications, 4 assembly of double-stranded ladderphanes, 5 and membrane transporting carriers. 6 Bicyclo[2.2.1]heptene systems have been extensively studied, and the reaction can proceed stereoselectively depending on the nature of the substrates and catalysts. Thus, syndiotacticcis, 7 isotactic-trans-, 8 and isotactic-cis-polynorbornenes 9 are readily obtained. Relatively speaking, ROMPs of cyclobutene derivatives have received much less attention, and the reactions in general are nonselective, polymers with a mixture of Z-and E-double bonds being obtained, 10 unless specially designed substrates are employed. 11 We have previously shown that, upon treatment with the first-generation Grubbs catalyst 1a, 12 ROMPs of norbornenes fused with N-aryl-endo-pyrrolidine give the corresponding polynorbornenes having isotactic stereochemistry with trans double bonds. 5,8 It is believed that the endo-fused substituents in 2 provide a bulky environment so that the stereoselectivity of the reaction may be directed. More recently, treatment of 2 with a Schrock−Hoveyda molybdenum−carbene catalyst 13 containing a bidendate ligand 3a yields polynorbornenes in cis, isotactic selectivity. 9a The span for each of the monomeric species in polynorbornenes is about 5.5 Å, and interactions between adjacent pendants in polynorbornenes or linkers in ladderphanes have been shown to be prominent. 5,8 The spacing occupied by each of the monomeric units in polycyclobutenes would be expected to be somewhat shorter because the bridging methylene groups are missing in polycyclobutenes. The distance separating adjacent pendants in polycyclobutenes would therefore be shorter than that in polynorbornenes. Accordingly, interactions between pendants in polycyclobutenes would be different from those in polynorbornenes. In this paper, we describe a syst...

Excimer Formation in a Confined Space: Photophysics of Ladderphanes with Tetraarylethylene Linkers

“…The absorption spectrum of monomer 10 a is essentially the sum of the absorptions of tetraarylethylene 6 and aminobenzoate 8 c ( Figure 1). [25] Like other single-and double-stranded polynorbornenes, [7,[14][15][16]19] the absorption due to the aminobenzoate chromophores in ladderphane 2 a (l max = 314 nm) exhibited a hypsochromic shift in comparison with that of 10 a (l max = 320 nm, Figure 1), which may be due to release of strain from the norbornene moiety. [28] On excitation at 350 nm, no emission was observed from solutions of 10 a, 6, and 8 c in CH 2 Cl 2 (1.0 10 À5 to 1.0 10 À3 m).…”

“…Depending on the nature of the polymeric backbones in 1, the spacing between arene linkers would be 5.5 for double-stranded polynorbornenes [15] and around 4.5 for polycyclobutene-based ladderphanes. [16] Significant perturbation of photophysical properties has been observed in these ladderphanes in comparison with those of corresponding monomers, although these distances are somewhat longer than those in typical p-p interactions between aromatic layers, such as those in DNA molecules [17] and in graphite. [18] To illustrate this, the Soret band splitting with porphyrin linkers, excimer emission with diethynylene-terphenylene linkers, and fluorescence quenching with a range of oligoaryl linkers are frequently observed in 1.…”

Determination of the Orientation of Pendants on Rigid‐Rod Polymers

“…For the past two decades, we have extensively studied the chemistry of double stranded ladderphanes 1 and 2 (Scheme 1), [1][2][3][4][5] which are synthesized by ring opening metathesis polymerization (ROMP) of bisnorbornenes 3 [2] or biscyclobutenes 4 [3] respectively. Unsymmetrical double stranded ladderphanes 7 are obtained from a single stranded polynorbornene or polycyclobutene using replication procotol (5 to 7 via 6) [4] or sequential polymerization (8 to 7 via 6).…”

Stromaphane: A New Type of Two-dimensional Covalent Organic Framework with Subnanometric Slit-Shaped Pores

“…25 We have extensively applied ruthenium- 26 or molybdenum-based catalysts 27 for the stereoselective ring opening metathesis polymerisation (ROMP) 28 of N-arylpyrrolidenefused norbornene or cyclobutene derivatives (NB or CB) leading to the corresponding polymers, PNB and PCB, respectively. Double bonds in PNB are all in trans con guration, when the Grubbs G-I catalyst 26 is used, 28-30 and in cis form in both PNB 31 and PCB 32 upon treatment with a Schrock-Hoveyda catalyst SHMo (Figure 1b). 27 The presence of the N-aryl pyrrolidine moiety in NB and CB, is vital for the stereoselective formation of the corresponding polymers PNB and PCB.…”

“…The pendants in PNB and PCB thus obtained are all aligned towards a similar direction. [29][30][31][32][33] This kind of conformation offers a unique route to synthesise both symmetric and unsymmetrical double stranded ladderphanes (Figure 1a). [21][22][23][24] ROMPs of cyclopropene derivatives have been brie y explored.…”