Synthesis of enol sulphonates from carbonyl compounds

Frydman, Norbert; Bixon, Ruth; Sprecher, Milon; Mazur, Yehuda

doi:10.1039/c2969001044b

Cited by 20 publications

(4 citation statements)

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“…A conceivable mechanism for this 1,2‐migration involves an initial β‐hydride elimination of the palladium(II) species after the oxidative‐addition step to give an intermediate alkyne‐coordinated palladium hydride species 17. To gain further insight into the plausibility of this reaction mechanism, a DFT study was conducted for the tert ‐butyl vinyl substrates (Figure 1).…”

Section: Methodsmentioning

confidence: 99%

Heck Coupling with Nonactivated Alkenyl Tosylates and Phosphates: Examples of Effective 1,2‐Migrations of the Alkenyl Palladium(II) Intermediates

et al. 2006

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Considerable efforts have been undertaken by numerous groups in academia and industry over the past decade to expand the repertoire of coupling reagents in palladium(0)-catalyzed cross-coupling reactions.[1] In particular, alkenyl phosphates and tosylates have proven their worth in various cross-coupling reactions, such as the Stille, [2] Suzuki, [2e,f, 3] Negishi, [2b] Kumada, [2b, 3e, 4] Sonogashira, [2-b,e,f, 5] Buchwald-Hartwig, [4a, 6] carbonyl enolate, [3d] and Heck couplings, [7] as effective alternatives to the less stable and typically more expensive alkenyl triflates and nonaflates. [8] However, the majority of this work has focused on the use of activated vinyl phosphates and tosylates, such as a,b-unsaturated systems or a-heteroatom-substituted alkenes, for which the oxidative-addition step is nonproblematic with palladium(0) catalysts bearing aryl phosphine ligands. Less attention has been devoted to nonactivated counterparts, most likely because of the greater difficulty in carrying out the first step of the catalytic cycle, namely the oxidative addition. [3d, 4, 5, 9] We now report on catalyst systems composed of a palladium complex with a basic, hindered alkyl phosphine that can promote the Heck coupling of nonactivated vinyl tosylates and phosphates with electron-deficient alkenes in good yields, thereby increasing the scope of this important cross-coupling reaction. Furthermore, during these studies we observed an interesting 1,2-isomerization with certain alkenyl tosylates and phosphates under reaction conditions that provide coupling yields as high as 95 %. A mechanistic proposal supported by experimental results and DFT calculations is included.To identify suitable reaction conditions for Heck couplings with nonactivated alkenyl tosylates and phosphates, we examined the use of palladium complexes with the ever increasingly popular bulky electron-rich phosphines.[10] All tosylates and phosphates were prepared from starting ketones by base-promoted proton extraction and reaction with tosyl anhydride (Ts 2 O) or (PhO) 2 POCl, as earlier reported. [4b, 11, 12] Initial coupling attempts were performed between the tosylate 1 and ethyl acrylate (Scheme 1). After examining a variety of reaction conditions, we noted that a combination of [PdCl 2 (cod)] (cod = 1,5-cyclooctadiene; 5 mol %) and the P(tBu) 3 HBF 4 salt [13] (10 mol %) in the presence of dicyclohexylmethylamine (2 equiv) in dimethylformamide (DMF) at 85 8C could furnish the diene 2, albeit in low yield (approximately 5 %). The addition of one equivalent of LiCl, however, had a dramatic effect on the reaction outcome, thus providing a 50 % yield of the diene 2 after 24 hours. [14] Increasing the reaction temperature to 100 8C improved the coupling yield to 66 %. Other reactions conditions, including change of solvent or examination of alternative catalyst combinations, were incapable of promoting the cross-coupling.To probe the scope of these reaction conditions, we examined the Heck coupling of a variety of alkenyl tosylates ...

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Section: Methodsmentioning

confidence: 99%

Heck Coupling with Nonactivated Alkenyl Tosylates and Phosphates: Examples of Effective 1,2‐Migrations of the Alkenyl Palladium(II) Intermediates

et al. 2006

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“…The tosylate anion and the brosylate anion have long been the most commonly used leaving groups for the study of solvolysis reactions, but only recently have they been employed in the generation of vinyl cations. 28 Solvolyses of the simple vinyl sulfonates, e.g., 1-cyclohexenyl and cfs-2-buten-2-yl sulfonates, were first investigated by a, X = OTs b, X = OBs c, X = OTf mation of the corresponding ketones, cyclohexanone and 2-butanone, respectively. However, as was shown by kinetic measurements, the products are not formed by a mechanism involving a vinyl cation but by protonation of the double bond and further reaction according to eq 5.…”

Section: Solvolysis Reactions Of Vinyl Sulfonatesmentioning

confidence: 99%

Vinyl cations in solvolysis reactions

Hanack¹

1970

Acc. Chem. Res.

106

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“…S−O bond cleavage in nucleophilic reactions of vinyl sulfonates were previously observed for nucleophilic vinyl sulfonates . Whereas most cycloalkenyl triflates solvolyze via S N 1,4b cyclopentenyl triflate gives [ 16 O]cyclopentanone in 50% ethanol−H 2 18 O, excluding an S N 1 route and suggesting reaction via an S−O bond cleavage 13b…”

Section: Discussionmentioning

confidence: 72%

“…Whereas most cycloalkenyl triflates solvolyze via S N 1,4b cyclopentenyl triflate gives [ 16 O]cyclopentanone in 50% ethanol−H 2 18 O, excluding an S N 1 route and suggesting reaction via an S−O bond cleavage 13b13c. The k (cyclohexenyl-OTf)/ k (( E )-MeC(OTf)CHMe) ratios in 50% dioxane are 10 6 and ca.…”

Section: Discussionmentioning

confidence: 99%

Triflate/Mesylate Ratios and Competing C−O and S−O Bond Cleavages in Nucleophilic Vinylic Substitution

Schottland

Rappoport

1996

J. Org. Chem.

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In an attempt to develop the k OTf/k OMs ratio as a mechanistic tool for the “addition−elimination” route in nucleophilic vinylic substitution, several pairs of vinyl mesylates and triflates were prepared. Whereas reactions of ArC(LG)C(CO2Et)2 (LG = OTf, OMs) with piperidine and morpholine in MeCN or THF gave the normal substitution product with k OTf/k OMs ratios of 4.3−10.6, the reaction of the mesylates, Ar = p-O2NC6H4, and of PhC(OMs)C(Me)CN with MeS- gave a ketone via an S−O bond cleavage. A related mesityl-substituted tosylate also reacted with p-MeC6H4X- (X = O, S) via an S−O bond cleavage. Hence, k OTf/k OMs ratios cannot be used as a general mechanistic tool. Several reactivity ratios in vinylic substitution are briefly discussed.

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Synthesis of enol sulphonates from carbonyl compounds

Cited by 20 publications

References 0 publications

Heck Coupling with Nonactivated Alkenyl Tosylates and Phosphates: Examples of Effective 1,2‐Migrations of the Alkenyl Palladium(II) Intermediates

Heck Coupling with Nonactivated Alkenyl Tosylates and Phosphates: Examples of Effective 1,2‐Migrations of the Alkenyl Palladium(II) Intermediates

Vinyl cations in solvolysis reactions

Triflate/Mesylate Ratios and Competing C−O and S−O Bond Cleavages in Nucleophilic Vinylic Substitution

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