Asymmetric selective polymerization of racemic methacrylates with the cyclohexylmagnesium bromide‐(−)‐sparteine system

Abstract

Asymmetric selective (or stereoelective) polymerization of various racemic methacrylates with cyclohexylmagnesium bromide (c‐HexMgBr)‐(‐)sparteine (1/1.2) catalyst was studied in toluene at −78°C. The methacrylates of α‐ethylbenzyl (EBMA), α‐isopropylbenzyl (i‐PBMA), α‐tert‐butylbenzyl (t‐BBMA), sec‐butyl (s‐BuMA), 1‐methylallyl (1‐MAMA), 2,3‐epoxypropyl (2,3‐EPMA), 2‐phenylpropyl (2‐PPMA), and menthyl (MentMA) alcohols were used as racemic monomers. In the polymerization of EBMA and i‐PBMA (S) enantiomers were consumed preferentially and the optical purity of initially polymerized i‐PBMA was as high as 97%. Optically pure (R) monomers were recovered at about 60% conversion for i‐PBMA and 80% for EBMA. In t‐BBMA, however, the (R) monomer was consumed preferentially over the (S) isomer. In the polymerization of s‐BuMA and 1‐MAMA (S) monomers were consumed in excess and the optical purity of the polymers formed in the early stage was about 30%. In 2,3‐EPMA and 2‐PPMA, which have asymmetric centers at the β position from the ester oxygen, (R) antipodes were more reactive. MentMA did not polymerize at −78°C. Enantiomer selectivity ratios rS and rR were determined in the polymerization of EBMA, i‐PBMA, and 1‐MAMA. All polymers except poly(t‐BBMA) were highly isotactic, but the tacticity of poly(t‐BBMA) could not be estimated. Circular dichroism spectra of optically active polymers of α‐substituted benzyl esters were measured.

Enantiomer-Selective Radical Polymerization of rac-2,4-Pentanediyl Dimethacrylate by 2,2’-Azobisisobutyronitrile/Copper(II) Trifluoromethanesulfonate/Chiral Diamine as Asymmetric Reverse Atom Transfer Radical Polymerization Initiating System

“…value of the resulting polymer was 14.2% at a 22.8% monomer conversion and 2.1% at a 59.6% monomer conversion. 3 These results indicated that the chiral copper complex should affect the addition of rac-1 to the growing end, in which the SS-1 enantiomer was predominantly polymerized, i.e., the enantiomerselective radical polymerization based on the reverse ATRP. In summary, we achieved the enantiomer-selective radical polymerization by the cyclopolymerization of rac-2,4-pentanediyl dimethacrylate (rac-1) based on the reverse atom transfer radical polymerization (reverse ATRP) method using chiral initiating systems.…”

“…In general, ionic and coordination polymerization systems were easily modified into chiral ones, so that they were used for preparing various types of asymmetric polymerizations, e.g., high enantiomer-selectivity was achieved for the anionic polymerization of the racemic α-monosubustitued benzyl methacrylate using the (-)-sparteine/Grignard reagent [1][2][3] and the ringopening polymerization of propylene sulfide with (-)-binaphthol/ZnEt 2 . 4,5 Although the radical polymerization is also available for asymmetric polymerization, little is known about the enantiomer-selective radical polymerization.…”

Enantiomer‐selective radical cyclopolymerization of rac‐2,4‐pentanediyl dimethacrylate using a ruthenium‐mediated chiral atom transfer radical polymerization initiating system

“…The ee values of the recovered monomers increased from 13.2 to 37.5% with increasing monomer conversion, whereas the specific rotations of the resulting polymers decreased from +40.30 to +26.3° with increasing monomer conversion. In addition, the optical purity (op) of the resulting polymer could be calculated as follows:22 The op values were 48.2% for a 26.6% monomer conversion and 25.7% for a 59.3% monomer conversion.…”

(−)‐Sparteine: The compound that most significantly influenced my research

“…When the chain end is S-PEMA, it is 33.7 times more reactive to S-PEMA than to R-PEMA, but when it is R-PEMA, it is only about 4 times more reactive. 17 Other racemic methacrylates and an acrylate were also enantiomer-selectively polymerized with the SpGrignard reagent complex in toluene at Ϫ78°C (Table 1). 17,18 A higher enantiomer selectivity (r S ϭ 50 and r R ϭ 0.09) was observed for ␣-isopropyl benzyl methacrylate (run 4), which produced a polymer with ee ϭ 96% during the early stage of the polymerization.…”