Mechanism of the Diphenyldisulfone-Catalyzed Isomerization of Alkenes. Origin of the Chemoselectivity:  Experimental and Quantum Chemistry Studies

Abstract

Polysulfone- and diphenyldisulfone-catalyzed alkene isomerizations are much faster for 2-alkyl-1-alkenes than for linear, terminal alkenes. The mechanism of these reactions has been investigated experimentally for the isomerization of methylidenecyclopentane into 1-methylcyclopentene, and theoretically [CCSD(T)/6-311++G(d,p)//B3LYP/6-311++G(d,p) calculations] for the reactions of propene and 2-methylpropene with a methanesulfonyl radical, MeSO2*. On heating, polysulfones and (PhSO2)2 equilibrate with sulfonyl radicals, RSO2*. The latter abstract allylic hydrogen atoms in one-step processes giving allylic radical/RSO2H pairs that recombine within the solvent cage producing the corresponding isomerized alkene and RSO2*. The sulfinic acid, RSO2H, can diffuse out from the solvent cage (H/D exchange with MeOD,D2O) and reduce an allyl radical. Calculations did not support other possible mechanisms such as hydrogen exchange between alkenes, electron transfer, or addition/elimination process. Kinetic deuterium isotopic effects measured for the (PhSO2)2-catalyzed isomerization of methylidenecyclopentane and deuterated analogues and calculated for the H abstraction from 2-methylpropene and deuterated analogues by CH3SO2* are consistent also with the one-step hydrogen transfer mechanism. The high chemoselectivity for this reaction is not governed by an exothermicity difference but by a difference in ionization energies of the alkenes. Calculations for CH3SO2* + propene and CH3SO2* + 2-methylpropene show a charge transfer of 0.34 and 0.38 electron, respectively, from the alkenes to the sulfonyl radical in the transition states of these hydrogen abstractions.

Canonical Variational Transition-State Theory Study of the CF3CH2CH3 + OH Reaction

“…At higher temperatures, the two pathways of R2 denominated as TSa1 and TSb2 are preferred with respect to pathway TSb1 (at higher temperatures, entropic effects dominate over classical barrier heights), whereas the opposite trend occurs at lower temperatures. 49 The theoretically estimated global rate constant agrees quite well at all temperatures with the experimental values reported by Ravishankara and co-workers. 7 The worst agreement (within 41%) was at 238 K, while the best agreement (within 4%) appears at 348 K. Except for 373 K, the calculated values are slightly lower than the measured ones.…”

“…Those facts lead us to consider a second factor favoring hydrogen abstraction through CH 3 group versus CH 2 group: charge-transfer. 48,49 In Figure 3 QTAIM atomic net charges and charge-transfers to the hydroxyl radical for the five TSs are shown. For the two TSs corresponding to reaction R1, the charge-transfers are 0.23 (TS1) and 0.23 (TS2) electrons.…”

Use of sultines in the asymmetric synthesis of polypropionate antibiotics

“…The organic chemistry of SO 2 has been limited to the formation of arenesulfinic acids (Friedel-Crafts sulfinylation [1,2]), the copolymerization of SO 2 with alkenes or alkynes (polysulfone synthesis [3][4][5]), the synthesis of sulfinates by reaction with organometallic compounds [6][7][8], the ring-opening of oxiranes and oxetanes [8,9] leading to polysulfites [8,[10][11][12], the synthesis of sulfones [8,[13][14][15], the isomerization of alkenes via ene reaction/sigmatropic shift/retro-ene elimination sequence [16][17][18][19][20][21][22], and the cheletropic additions of conjugated polyenes [23][24][25] forming cyclic sulfones, as the [ω2s+π4s] ad-dition of SO 2 to isoprene producing 2,5-dihydro-3-methylthiophene-1,1-dioxide (sulfolene), a reaction known since 1914 [26,27].…”

Chemistry Unprotected

“…Baran proposed that total syntheses be analyzed with respect to the number of steps versus the oxidation state ( Figure 1) as well as their complexity. [6] Vogel also presented total syntheses of the natural products baconipyrone A/B, rifamycin, apoptolidine A, and (À)-dolabriferol. [5] It was also interesting that methallyl, prenyl, and methylprenyl ethers can be cleaved in the presence of allyl ethers by heating with diphenyldisulfone or solid poly- sulfone (10), generated by copolymerization of SO 2 with methylidenecyclopentane (Scheme 3).…”