(E,Z) Equilibria, 12. Differential NMR Shielding by Phenyl, Assigned from Chemical Labelling and (Z,E) Equilibration

Abstract

Unequivocal NMR assignments of the (E,Z) isomeric pairs 3 and 8 are possible by (Z,E) equilibration. Deprotonation of 2-methyl-l-phenylpropene (1) with n-BuLi under any conditions gives only a single allyl-metal derivative 2. Methylation of the latter gives the isomer (2)-3, while deuteriolysis leads to the isotopically labelled derivative 6 of the olefin 1; this proves the NMR assignments for 1 as well as the endo configuration of 2. The phenyl group gives rise to a differential shielding effect on the y-carbon nuclei in methyl and rnethylene groups along a CC double bond (ca. 7 ppm), and this effect is comparable to that along a CN double bond as in 8. The nitrile function in 5c is much less effective.

Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1′-Lithio-1′-(2,6-dimethylphenyl)propenes

“…6.5 Hz, C-1′) ppm, assigned through comparison with ( Z )- 1 in Et 2 O and confirmed at 25 °C through gated decoupling and selective { 1 H} decoupling as follows: {2′-CH 3 } → C-1′ as a d with 2 J = 16 Hz, 2′- C H 3 as a d with 2 J = 8.5 Hz; {2-/6–C H 3 } → C-1 as a dt with 3 J = 13.5 and 5.5 Hz, C-1′as a d, C-2/-6 as a d with 3 J = 6.8 Hz, and 2-/6- C H 3 as two narrow d with 2 J = 4.9 Hz for ( Z )- 1 and ( E )- 1 ; {2′-H} → 2′- C H 3 as a sharp q. 13 C NMR of THF-solvated (4 equiv), monomeric ( Z )- 1 ([D 8 ]­toluene, –85 °C, 100.6 MHz) δ 18.5 (2′-CH 3 cis to aryl), 22.3 (2-/6-CH 3 ), 114.9 (C-4), 116.0 (C-2′), (C-2/-6 obscured), 126.5 (C-3/-5), 158.5 (C-1), 197.4 (C-1′) ppm, assigned through comparison with monomeric ( Z )- 1 in THF; 13 C NMR of monomeric ( Z )- 1 in a Z / E = 32:68 mixture (THF, 0.24 M, –88 °C, 100.6 MHz) δ 18.0 (2′-CH 3 cis to aryl), 22.0 (2-/6-CH 3 ), 114.2 (C-2′), 114.9 (C-4), 125.0 (C-2/-6), 126.0 (C-3/-5), 158.4 (C-1), 197.6 (triplet 1:1:1, 1 J CLi = 11.4 Hz, C-1′) ppm, assigned through the γ-aryl effect, temperature dependent Z / E ratios and comparison with ( Z )- 1 in Et 2 O and t -BuOMe.…”

“…4 Hz, 2′-CH 3 trans), 119.6 (dt, 1 J = 159 Hz, 3 J = 7.5 Hz, C-4), 124.6 (dt, 1 J = 154 Hz, 3 J = 6.5 Hz, C-2/-6), 128.2 (dd, 1 J = 156 Hz, 3 J = 7.5 Hz, C-3/-5), 134.4 (quasi-septet, apparent J = ca. 5 Hz, C-2′), 158.0 (t, 3 J = 6.5 Hz, C-1), 183.0 (qi 1:2:3:2:1, 1 J ( 13 C, 6 Li) = 7.5 Hz, C-1′) ppm (Table S12), assigned through the γ-aryl effect 33 and comparison with 1 in Et 2 O, the qi 1 J ( 13 C, 6 Li) = 7.5 Hz of C-1′ was observed also at −71 (Figure S6), –79, –91 (prepared through Hg/Li interchange), –95 (with 6 LiBr), and −106 °C; 13 C NMR (THF, –81 °C, 25.1 MHz) δ 20.3 (q, 2′-CH 3 cis), 30.6 (q, 2′-CH 3 trans), 115.0 (d, C-4), 121.0 (quat, C-2′), 122.2 (d, C-2/-6), 127.2 (d, C-3/-5), 164.0 (quat, C-1), 194.0 (unresolved, C-1′) ppm, assigned through { 1 H} off-resonance decoupling, the γ-aryl effect, and comparison with 1 in THF; the triplet (1:1:1) with 1 J ( 13 C, 6 Li) = 11.5(1) Hz of C-1′ was observed at −106, –117, –124, and −126 °C (Table S13).…”

“…6 Hz, C-2/-6), 154.2 (unresolved m, C-1), 190.5 (dm. 2 J = 16 Hz, C-1′); assigned through the γ-aryl effect and comparison with ( Z )- 1 in Et 2 O; 13 C NMR of dimeric ( Z )- 1 in a Z / E = 94:6 mixture (Et 2 O, 0.35 M, 25 °C, 100.6 MHz) δ 17.2 (qd, 1 J = 124 Hz, 2 J = 8.5 Hz, 2′-CH 3 cis to aryl), 21.5 (qd, 1 J = 125 Hz, 3 J = 4.9 Hz, 2-/6-CH 3 ), 120.2 (sharp d, 1 J = 155 Hz, C-4), 122.7 (dq, 1 J = 144 Hz, 3 J = ca. 5 Hz, C-2′), 127.4 (dm, 1 J = 155 Hz, C-3/-5), 127.9 (m, apparent J = ca.…”

“…Without warming, the needles were dissolved for NMR spectroscopy or derivatization. 1 H NMR of monomeric ( E )- 1 in a Z / E = 94:6 mixture (Et 2 O, 0.35 M, 25 °C, 400 MHz) δ 1.91 (d, 3 J = 6.0 Hz, 3H, 2′-CH 3 trans to aryl), 2.07 (s, 6H, 2-/6-CH 3 ), 6.14 (q, 3 J = 6.0 Hz, 1H, 2′-H), 6.46 (t, 3 J = 7.3 Hz, 1H, 4-H), 6.72 (d, 3 J = 7.3 Hz, 2H, 3-/5-H) ppm; 1 H NMR of THF-solvated (4 equiv), monomeric ( E )- 1 ([D 8 ]­toluene, –85 °C, 400 MHz) δ 2.38 (s, 6H, 2-/6-CH 3 ), (2′-CH 3 obscured), 6.44 (q, 3 J = 6 Hz, 1H, 2′-H), 6.83 (t, 3 J = 7.4 Hz, 1H, 4-H), 7.14 (d, 3 J = 7.4 Hz, 2H, 3-/5-H) ppm, assigned through comparison with ( E )- 1 in THF; 1 H NMR of monomeric ( E )- 1 in a Z / E = 47:53 mixture (THF, 0.68 M, –10 °C, 400 MHz) δ 1.94 (s, 6H, 2-/6-CH 3 ), (2′-CH 3 obscured), 5.72 (q, 3 J = 5.8 Hz, 1H, 2′-H), 6.20 (t, 3 J = 7.4 Hz, 1H, 4-H), 6.57 (d, 3 J = 7.4 Hz, 2H, 3-/5-H) ppm; 13 C NMR of THF-solvated (4 equiv), monomeric ( E )- 1 ([D 8 ]­toluene, –85 °C, 100.6 MHz) δ 23.0 (2-/6-CH 3 ), 25.2 (2′-CH 3 trans to aryl), 115.6 (C-4), 116.6 (C-2′), (C-2/-6 obscured), 127.2 (C-3/-5), 161.2 (C-1), 197.8 (C-1′) ppm, assigned through comparison with ( E )- 1 in THF; 13 C NMR of monomeric ( E )- 1 in a Z / E = 32:68 mixture (THF, 0.24 M, –88 °C, 100.6 MHz) δ 22.7 (2-/6-CH 3 ), 24.8 (2′-CH 3 trans to aryl), 115.4 (C-2′), 116.0 (C-4), 126.1 (C-3/-5), 126.6 (C-2/-6), 161.2 (C-1), 198.1 (triplet 1:1:1, 1 J CLi = 11.4 Hz, C-1′) ppm, assigned through the γ-aryl effect, the temperature dependent Z / E ratios, and comparison with ( Z )- 1 in Et 2 O and t -BuOMe.…”

“…n -Bu 6 Li (0.16 mmol) in cyclopentane (0.11 mL) was added to the tube, which was warmed up to rt after 15 min and showed that the I/Li interchange reaction was complete. 1 H NMR (saturated hydrocarbon solvents, −45 °C, 400 MHz) δ 1.73 (s, 6H, 2-/6-CH 3 ), (2′-CH 3 obscured), 5.74 (q, 3 J = 5.6 Hz, 2′-H), 6.71 (t, 3 J = 7.3 Hz, 1H, 4-H), 6.78 (d, 3 J = 7.3 Hz, 2H, 3-/5-H) ppm; 13 C NMR (saturated hydrocarbon solvents, –45 °C, 100.6 MHz) δ 17.3 (2′-CH 3 cis to aryl), 21.1 (qd, 2-/6-CH 3 ), 123.0 (d, C-4), 128.8 (narrow d, C-2/-6), 129.0 (dd, C-3/-5), 131.4 (d, C-2′), 150.3 (narrow m, C-1), 179.5 (unresolved narrowed m, C-1′) ppm, assigned through the γ-aryl effect and selective { 1 H} decouplimg. After the long-term measurements, the clear solution was poured onto dry CO 2 , and the warmed-up material was dissolved in aqueous NaOH (3 M) and Et 2 O. Acidification of the alkaline aqueous layer afforded the diastereomerically pure carboxylic acid ( E )- 8 (12 mg, 42%).…”