Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization

Abstract

Alkyne cross-metathesis of molybdenum carbyne complex [TolC≡Mo(OCCH(CF))]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (>98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e][8]annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followed by ring-opening polymerization of strained ε-caprolactone.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…[172] Am ethodf or controlled chain terminationo fl ivingR OAMP through ynamines and ynamides such as 66 was introduced also by the group of Fischer,w hiche nabled selective functional group transfer to the end of polymer chains (Scheme 24). [173] The functionalized end-groups were used to initiate ring-opening polymerization of e-caprolactone to access amphiphilic block copolymers. In stoichiometric reactions, molybdenum benzylidyne complexes reactedw ith ynaminest of orm stable, paramagnetic metallacyclobutadiene complexes, whereas ynamides led to diamagnetic analogues.…”

“…In stoichiometric reactions, molybdenum benzylidyne complexes reactedw ith ynaminest of orm stable, paramagnetic metallacyclobutadiene complexes, whereas ynamides led to diamagnetic analogues. [69,173] Thus, ynamides 66 Scheme22. Synthesis of fully conjugated polymers 62 and 63 with different topologies.…”

“…were employed as termination reagents, which reactedw ith the propagating metal complex 65 to form stable metallacyclobutadiene complexes of the type 67,a nd simultaneously transferred end-groups selectively to the polymerc hain 68. [173]…”

Chemistry of Metallacyclobutadienes

“…[39] Molybdenacyclobutadienes 60 a [40] and 60 b [41] were obtainedf rom the reactions of the carbyne complex [Mo( CMes){OC(CF 3 ) 3 } 3 ]( Mes = 2,4,6-trimethylphenyl) with an excess amount of EtCCEt and PhCCMe, respectively.C omplexes 61 and 62 were generated from the reactions of the carbyne complex [Mo( Ctolyl){OCMe(CF 3 ) 2 ) 3 }(dme)] with two equivalents of the corresponding alkynes PhCCNR 2 . [42] Complexes 63 werep roduced from the reactions of the carbyne complex[ Mo( CMes){OCMe(CF 3 ) 2 } 3 ]w ith diaminoacetylenes R 2 NCCNR 2 (NR 2 = 4-methylpiperidinyl, NEt 2 ). [43] In contrast to typical five-coordinate, highvalent molybdenacyclobutadienes, complexes 63 are paramagnetic and adopt ag eometry intermediate between idealized square-pyramidal and trigonal-bipyramidal geometries.…”

“…The dihedral angles are 122.4, 147.0,a nd 145.98 for 140 a, 63 a,a nd 82 a,r espectively.I na ddition, the MÀC(b)d istances in complexes 140 a (2.339(5) )a nd 82 a (2.462 )a re appreciably longert han those in typical highvalentd 0 metallacyclobutadienes,a lthough they are also shorter than that expected forn on-bonded metal-carbon centers. In the complexes 63 a, [43] 63 b, [43] 61, [42] and 62, [42] each of which has two NR 2 substituents, the MÀC(b)d istances are also longer than 2.6 (see Table 1).…”