Communication www.rsc.org/chemcomm CHEMCOMM Copolymerization of carbon monoxide and aziridine† Li Jia,* Errun Ding and William R. Anderson Department of Chemistry, Lehigh University, 6 E. Packer Avenue, Bethlehem, PA 18015, USA. E-mail: lij4@lehigh.edu; Fax: +1 610-758-6536; Tel: +1 610-758-5715 Received (in Cambridge, UK) 1st May 2001, Accepted 15th June 2001 First published as an Advance Article on the web 19th July 2001 Alternating copolymerization of carbon monoxide with aziridine was realized, which serves as a prototype of a novel route for synthesis of poly-b-peptides. Rational discovery of new catalytic reactions is an important scientific challenge for synthetic chemists. We are interested in designing new catalytic polymerization reactions which involve reaction and incorporation of heteroatoms in the polymer chain to produce functional polymers. In light of the successful examples of carbonylation of aziridines and epoxides and alternating copolymerization of CO and alkenes, 1–4 we set out to develop metal-catalyzed alternating copolymerization of CO with aziridines [eqn. (1)]. 5 This reaction in principle provides an (1) attractive route to poly-b-peptides, which have received considerable current attention as biomimetic materials. 7,8 Prior to this work, Sen and Arndtsen independently suggested the possibility of copolymerization of imines with CO to produce poly-a-peptides and demonstrated the first examples of imine insertion into Pd–acyl bonds. 9,10 We report here the initial identification of a catalyst for CO–aziridine copolymerization and that alternating polymerization can be achieved under carefully controlled experimental conditions. At the onset of this project, we conceived a catalytic cycle leading to the copolymerization. First of all, CO insertion and aziridine insertion into a metal–carbon bond are inevitable steps in any catalytic cycle that one might design for the copoly- merization of CO and aziridine. Examples of the latter reaction are absent in organometallic chemistry to our knowledge. However, an interesting reaction of aziridine insertion into acetyl chloride was reported [eqn. (2)]. 11 It is well established (2) that some metal–acyl species resemble organic acyl chlorides and undergo nucleophilic cleavage by alcohols and amines to afford esters and amides. By such analogy, insertion of aziridine into metal–acyl bonds might also occur. The aziridine insertion coupled with CO insertion into a metal–alkyl bond then forms a reasonable catalytic cycle leading to the alternating copoly- merization (Scheme 1). Under the above hypothesis, we screened a number of potential catalysts and arrived at Heck’s complex CH 3 COCo- (CO) 3 PPh 3 1. In the presence of 10 mol% of 1 under 1000 psi CO in THF solution, aziridine and CO was copolymerized to produce a crystalline, hot-water soluble polymer in good yield (entry 1, Table 1). The FT-IR and NMR spectra together confirm that the product is poly-b-alanine. In the IR spectrum, two prominent amide bands are present at 1645 and 1539 cm 21 .† The 1 H NMR spectrum reveals two triplet resonances (a and b) at d 3.25 and 2.26 ppm (J = 6.5 Hz) (Fig. 1a), in agreement with the chemical shifts reported for poly-b-alanine prepared from acrylamide. 12 Additional fine features (labeled with asterisks) are present overlapping with or in close vicinities of a and b. The small differences in chemical shifts between them and the main peaks a and b lead us to believe that they belong to b-alanine units located at or close to the end of the chain. These resonances are not due to amine microstructures, which would possibly be present if repetitive aziridine inser- tions occurred (see below), because they do not move downfield in acid solutions in contrast to what should be expected for amines upon protonation. Work is in progress in our laboratory to positively identify these resonances. A resonance (c) at d 1.80 ppm is observed due to the acetyl end group that originates from † Electronic supplementary information (ESI): NMR spectra, GPC sum- mary, elemental analyses, experimental procedure of polymerization, and a scheme rationalizing the imperfect alternating enchainment. See http:// www.rsc.org/suppdata/cc/b1/b103899k/ Table 1 Copolymerization of CO with aziridine using 1 as the catalyst a Entry [Cat]/mM Aziridine/g Aziridine–1 (molar ratio) CO pressure/psi Reaction time/h Yield/g Amine units/mol% b M w c /10 3 PDI c 1 5.8 0.25 10 1000 12 0.25 (60%) <2 14.1 4.85 2 5.8 0.25 10 500 12 0.25 (60%) <2 — — 3 5.8 0.25 10 250 12 0.17 (41%) <2 — — 4 5.8 0.75 30 1000 12 0.98 (79%) ~8 36.6 2.52 5 5.8 1.25 50 1000 12 1.84 (89%) ~ 12 57.8 11.56 6 d 5.8 0.25 3 3 30 1000 12 3 3 1.06 (86%) ~2 36.6 5.67 7 d 5.8 0.25 3 5 50 1000 12 3 5 1.92 (93%) ~2 63.3 9.32 a In 100 mL THF at 80 °C. b Amine defects estimated by 1 H NMR. c Determined by GPC in 1,1,1,3,3,3-hexafluoropropan-2-ol with 0.01 M sodium triflate, light scattering–viscometry–refractive index triple detector. d Aziridine was added in portions. Scheme 1 Working model for alternating copolymerization of CO with aziridine (L = PPh 3 ). This journal is © The Royal Society of Chemistry 2001 1436 Chem. Commun., 2001, 1436–1437 DOI: 10.1039/b103899k Published on 19 July 2001. Downloaded by University of Pittsburgh on 28/10/2014 22:55:42. View Article Online / Journal Homepage / Table of Contents for this issue