Photochemical Transformation of Telomers III. Photolysis of Dimethyl (2R*,4R*)-2-Bromo-2,4-dimethyl-4-(2,2,2-trichloroethyl)glutarate

Abstract

ABSTRACT:The photolysis of dimethyl (2R*,4R*)-2-bromo-2,4-dimethyl-4-(2,2,2-trichloroethyl)glutarate was examined and reaction paths for ten products are proposed. A homolytic cleavage of the carbon-bromine bond of the substrate occurred first forming a radical intermediate and subsequent reactions yielded the products via decomposition of the intermediate and recombination of secondly formed radical species. Ionic lactonization of the substrate and diastereoisomerization proceeded simultaneously.KEY WORDS Photolysis / Telomer / Methyl Methacrylate Dimer Homolytic Cleavage / Carbon-Bromine Bond / Recombination Diastereoisomerization / Lactonization / The photochemistry of organic halides is characterized by cleavage of the carbon-halogen bond. 1 · 2 In recent studies on the solution photoreaction of aliphatic halides, 3 -5 workers have concentrated their attention on understanding whether the products are formed from a radical species or from an ionic one.We previously reported the photolysis of methyl 2-bromo-4,4,4-trichloro-2-methylbutyrate (1) 6 obtained from the telomerization of methyl methacrylate (MMA) using bromotrichloromethane (BTCM) as a telogen. 7 In the preceding paper,8 we described the effects of copper powder on the photolysis of 1. The results indicated that the conversion of the substrate and amount of products via a homolytic cleavage of the carbon-bromine bond increased with the amount of copper powder added. Since there have been a few studies on the photochemistry of aliphatic * To whom all correspondence should be addressed.halides containing different halogen atoms, 9 • 10 we had interest in the photochemistry of telomers such as 1.meso-and di-Dimers of MMA with a headto-head linkage, formed by recombination of the tertiary carbon radicals produced by a homolytic cleavage of the carbon-bromine bond, are interesting. Rae et al. reported the synthesis and characterization of the mesoand di-isomers of tetramethyl 2,4,5,7-tetramethyloctane-2,4,5, 7-tetracarboxylate and thermal degradation of its meso-isomer as a model to study the effects of head-to-head linkage in thermal degradation of poly-(MMA). 11,12

Conformational Structure of Methacrylate Radicals as Studied by Electron Spin Resonance Spectroscopy: From Small Molecule Radicals to Polymer Radicals

“…The preferential formation of the r -dimer rather than the m -dimer and of the r,r -trimer rather than the other unisolated trimers is very consistent with the predominant syndiotactic propagation of the methacrylates, where r and m represent racemo and meso, respectively. These results agree with the results by Kimura et al 15c).…”

“…Preparation of Precursors. The precursors for the radicals were prepared by the reaction of the corresponding alkenes with bromotrichloromethane in the presence of 2,2‘-azobis(isobutyronitrile) (AIBN) at 90 °C (Scheme ). The 1:1 adduct (unimer) ( 1aBr and 1bBr ), dimers ( 5aBr-I , 5aBr-II , 5bBr-I , and 5bBr-II ), and one of the trimers ( 6Br ) in Chart were isolated by flash chromatography from a mixture of the products in the reaction of MMA or MMA- d 3 .…”

“…Reaction with MMA. A mixture of MMA (4 mL, 37 mmol), bromotrichloromethane (30 mL, 300 mmol), and AIBN (1.5 g, 9.1 mmol) was flushed with argon and heated at 90 °C for 6 h …”

“…Methyl ( 2 SR, 4 SR) -2-bromo-4-carbomethoxy-2,4-dimethyl-6,6,6-trichlorohexanoate (5aBr- II): needles, mp 79.1−79.8 °C (pentane) (lit. mp 79−80 °C);15c yield 0.33 g (2%); R f 0.50 (pentane/CH 2 Cl 2 /ether = 20/4/1); 1 H NMR (CDCl 3 , 300 MHz) δ 3.80 (s, 3H, OCH 3 ), 3.69 (s, 3H, OCH 3 ), 3.57 (d, J = 15.3 Hz, 1H, CH 2 ), 2.95 (d, J = 15.3 Hz, 1H, CH 2 ), 2.85 (d, J = 14.9 Hz, 1H, CH 2 ), 2.50 (d, J = 14.9 Hz, 1H, CH 2 ), 1.99 (s, 3H, CH 3 ), 1.57 (s, 3H, CH 3 ); 13 C NMR (CDCl 3 , 75.5 MHz) δ 174.72 (CO), 171.38 (CO), 96.34 (CCl 3 ), 61.95 (CH 2 ), 58.43 (CBr), 53.39 (OCH 3 ), 52.49 (OCH 3 ), 51.43 (CH 2 ), 47.28 (C q ), 29.96 (CH 3 ), 19.44 (CH 3 ); MS (EI, 70 eV) m/z 339 (5), 317 (20), 285 (19), 257 (13), 221 (30), 181 (13), 153 (16), 141 (6), 127 (13), 115 (15), 101 (47), 87 (8), 69 (56), 59 (57), 51 (11), 41 (100); MS (CI, NH 3 ) m/z 416 (M + + 17, 100), 336 (5), 317 (42), 204 (6). Anal.…”

“…Methyl ( 2 SR, 4 SR, 6 SR) -2-bromo-4,6-dicarbomethoxy-8,8,8-trichloro-2,4,6- trimethyloctanoate (6Br): prism, mp 111.0−112.2 °C (methanol) (lit. mp 111−112 °C);15c yield 1.14 g (6%); R f 0.30 (pentane/CH 2 Cl 2 /ether = 20/4/1); 1 H NMR (CDCl 3 , 300 MHz) δ 3.77 (s, 3H, OCH 3 ), 3.694 (s, 3H, OCH 3 ), 3.686 (s, 3H, OCH 3 ), 3.44 (d, J = 15.3 Hz, 1H, CH 2 ), 2.795 (d, J = 15.3 Hz, 1H, CH 2 ), 2.793 (d, J = 14.2 Hz, 1H, CH 2 ), 2.70 (d, J = 14.2 Hz, 1H, CH 2 ), 2.12 (d, J = 14.2 Hz, 1H, CH 2 ), 2.04 (d, J = 14.2 Hz, 1H, CH 2 ), 1.83 (s, 3H, CH 3 ), 1.35 (s, 3H, CH 3 ), 0.94 (s, 3H, CH 3 ); 13 C NMR (CDCl 3 , 75.5 MHz) δ 176.89 (CO), 175.82 (CO), 171.73 (CO), 96.30 (CCl 3 ), 63.66 (CH 2 ), 58.32 (CBr), 53.28 (OCH 3 ), 53.12 (OCH 3 ), 52.38 (OCH 3 ), 52.20 (CH 2 ), 51.87 (CH 2 ), 46.36 (C q ), 45.75 (C q ), 26.03 (CH 3 ), 17.58 (CH 3 ), 16.88 (CH 3 ); MS (EI, 70 eV) m/z 439 (11), 417 (5), 385 (11), 317 (6), 297 (11), 279 (8), 265 (5), 247 (10), 218 (12), 201 (6), 181 (12), 153 (13), 141 (15), 127 (20), 101 (84), 94 (10), 81 (11), 69 (75), 59 (56), 51 (6), 41 (100); MS (CI, NH 3 ) m/z 516 (M + + 17, 64), 467 (8), 420 (100). Anal.…”