Mechanistic Evidence for a Novel Rearrangement Sequence in the Synthesis of 4-Aryl-5,6-dihydro-1,2-oxathiine-2,2-dioxides from Homopropargyl
Benzosulfonates

Abstract

Under similar literature conditions, tributyltin hydride-promoted free radical reaction of homopropargyl benzosulfonates 1 initiated a novel rearrangement sequence and led to formation of two kinds of cyclic sultones: 4-aryl-5,6-dihydro-1,2-oxathiin-2,2-dioxides 2 and 4-aryl-3-tributyltin-1,2-oxathiane-2,2-dioxides 7. Isolation and X-ray structure characterization of previously unreported cyclic α-tributyltin sultones 7 provided evidence for a cyclization-fragmentation-cyclization mechanism proposed by Motherwell.

4-aryl-5,6-dihydro-1,2-oxathiin-2,2-dioxides - 4-aryl-3-tributyltin-1,2-oxathiane-2,2-dioxides - α,β-unsaturated sultones - ipso substitutions - free radicals

References

1

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3 Studer A. Bossart M. In Radicals in Organic Synthesis   Vol. 2:  Renaud P. Sibi MP. Wiley-VCH; Weinheim: 2001.  p.62 

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10

Procedure for tinhydride reaction of homopropargyl benzosulfonate 1: A solution of Bu₃SnH (4.6 mmole) and AIBN (4.6 mmole) in 50 mL of dry benzene was added to a refluxing solution of 2 (4.6 mmole) in 90 mL of dry benzene over a period of 10 h via a syringe pump. After an additional 4-6 h, the reaction mixture was concentrated in vacuo. Purification of the residue by flash column chromatography on silica gel (gradient elution; 10% EtOAc-hexanes then 100% EtOAc) furnished in order of elution, the cyclic α-tributyltin substituted sultone 7 and α,β-unsaturated cyclic sultone 2. ¹H NMR, IR and MS spectra of 2a and 2b are identical with those provided in the literature (ref. [8] ) Analytical data for 7a: ¹H NMR (300 MHz, CDCl₃) δ 0.73 (t, 3 CH₃), 0.40-1.30 (3 × 3 CH₂), 2.17 (br dd, 1 H), 2.41 (qd, 1 H), 4.57 (d, 1 H), 4.66 (m, 1 H), 4.80 (td, 1 H), 5.21 (dt, 1 H), 7.43 (dd, 1 H), 7. 53 (t, 1 H), 7.68 (d, 1 H), 7.79 (d, 1 H), 8.18 (d, 1 H), 8.99 (d, 1 H). ¹³C NMR (75 MHz, CDCl₃) δ 9.9 (t, 3 CH₂), 12.0 (q, 3 CH₃), 25.6 (t, 3 CH₂), 27.0 (t, 3 CH₂), 27.3 (t), 37.1 (d), 1.4 (d), 69.6 (t), 119.9 (d), 124.6 (d), 124.9 (d), 126.2 (d), 127.0 (s), 134.9 (d), 137.6 (s), 144.6 (s), 148.3 (d). IR(neat)1334 (SO₂), 1150 (SO₂) cm^-1 _. MS m/e (rel. intensity) 552 (M⁺-1, 10), 405(40), 361(97), 294(70), 262 (M⁺-SnBu₃, 100). Analytical data for 7b: ¹H NMR (300 MHz, CDCl₃) δ 0.74 (t, 3 CH), 0.50-1.30 (3 × 3 CH₂), 2.11 (br d, 1 H), 2.51 (qd, 1 H), 2.88 (s, 2 CH₃), 3.78 (dd, 1 H), 4.63 (dd, 1 H), 4. 81 (m, 2 H), 7.15 (d, 1 H), 7.35 (d, 1 H), 7. 43 (t, 1 H), 7.51 (t, 1 H), 7.77 (d, 1 H), 8.24 (d, 1 H). ¹³C NMR (75 MHz, CDCl₃) δ 10.2 (t, 3 CH₂), 12.0 (q, 3 CH₃), 25.6 (t, 3 CH₂), 27.0 (t, 3 CH₂), 27.6 (t), 43.8 (d), 43.8 (q, 2 CH₃), 52.4 (d), 70.1 (t), 112.9 (d), 115.9 (d), 121.3 (d), 122.8 (d), 123.1 (d), 125.5 (d), 128.1 (s), 130.5 (s), 135.1 (s), 150.5 (s). IR(neat)1340 (SO₂), 1153 (SO₂) cm^-1 _. MS m/e (rel. intensity) 594 (M⁺ + 1, 25), 538(25), 332(24), 240(100).