William A. Smit

Lewis Acid Catalyst Free Electrophilic Alkylation of Silicon‐Capped π Donors in 1,1,1,3,3,3‐Hexafluoro‐2‐propanol

The electrophilic alkylation of p donors most often implies the use of Lewis acid catalysts for the generation of reactive electrophilic species from covalent precursors. A promising alternative to this procedure has recently been suggested in studies by Mayr and co-workers. 3] In particular, it was disclosed that cationic intermediates generated by solvolysis of SN1-active substrates in polar solvents, such as acetone/ water, acetonitrile/water, 2,2,2-trifluoroethanol (TFE), or 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), can be trapped by p nucleophiles (activated arenes, heteroarenes, or enol ethers) provided the nucleophilicity of the latter is higher than that of the solvent system. Herein , we show that a similar approach can be extended well beyond the scope of purely solvolytic reactions, and might also be applicable for elaboration of the Lewis acid free protocols for a diverse set of reactions, such as barylthioalkylation of p donors, Hosomi–Sakurai acetal allylation, the Mukaiyama aldol reaction, and Sakurai– Mukaiyama conjugate additions. Since the use of HFIP as a reaction medium was found to be a factor of crucial importance for performing these transformations (see below), it is appropriate to briefly discuss the specific physical properties of HFIP and some aspects of its preparative utilization relevant to this subject. HFIP is well-known as a polar solvent of high ionizing power and low nucleophilicity. Notably, in sharp contrast to the majority of other polar solvents, HFIP is capable of strongly solvating anions but not cations, as was deduced from both conductometric studies and gas-phase determination of the anion-binding energy. HFIP also serves as a powerful hydrogen-bond donor, as was demonstrated not only by the spectral studies and calorimetric measurements, but also by the isolation of extremely stable complexes with a number of nucleophilic species. The above properties of HFIP make it an optimal solvent for the generation of persistent carbocations and cationic radicals from various precursors and under a fairly diverse set of reaction conditions. A considerable amount of experimental data also attests to the usefulness of HFIP as a medium for synthetically useful reactions, such as C O cleavage of oxiranes with O, N, or S nucleophiles under neutral conditions, or in certain C C bond-forming transformations such as arene p-methoxybenzylation, Diels– Alder cycloaddition, or cationic cyclization. As was demonstrated in previous studies by our group, the interaction of b-chloroalkyl arylthioethers with Lewis acids leads to the formation of electrophilic species capable of reacting with a number of silicon-capped p donors, to give the respective products of b-arylthioalkylation (Scheme 1, [Eq. (1)]).

Discrete introduction of the electrophile and nucleophile in AdE -reactions of dicobalt hexacarbonyl complexes of conjugated enynes.

Abstract The dicobalt hexacarbonyl complex of the enyne isopropenylacetylene ( 1 ) is effective in permitting stepwise Ad E reactions to the double bond of 1 via stabilization of the intermediate carbenium ion.

A new approach to the synthesis of linearly fused triquinanes

Abstract A short pathway for the stereocontrolled synthesis of the title compounds from readily available precursors is elaborated which involves as a key step a stereoselective Pauson-Khand cyclization of the substrates of trans-2-allyl-1-ethynylcyclopentanol type.

Carrol rearrangement on the surface of chromatographic grade alumina

Abstract Thermolysis on the surface of Al2O3 is proposed as a mild and convenient preparative method for /3,3/-sigmatropic Carrol rearrangement of acetoacetic esters of tertiary allyl alcohols into respective γ-δ-unsaturated ketones.

Reaction of vinyl ethers with ArSCl adducts of d-glucal

Abstract Lewis acid mediated reactions of vinyl ethers with ArSCl adducts of benzyl protected D-glucal followed by quenching of the five-membered sulfonium salt intermediates with external nucleophiles, H2O, MeOH, and n-Bu4NBH4, lead to a highly stereoselective formation of β-C-glucopyranosides with different functional groups in the lateral chain.

The role of electronic factors in reagents in controlling thestereochemistry of arylsulfenyl chlorides addition

Abstract For a series of substituted arylsulfenyl chlorides the stereochemistry of AdE-reactions with a series of substituted Z- and E-β-deuterostyrenes has been elucidated. Non-stereospecific reaction was observed only for the pair 2,4-dinitrobenzenesulfenyl chloride-p-methoxystyrene; for all the other reactants the reaction proceeded in a strictly stereospecific manner.

Case of an acid-free cationic polymerization of alkenes. An efficient preparation of poly-β-pinene in hexafluoroisopropanol medium

An unprecedented capacity of hexafluoroiso-propanol to serve both as a sole proton source and polar medium in the cationic polymerization of β-pinene is disclosed. The elaborated procedure is simple-to-operate; it furnishes the respective polymer in a nearly quantitative yield.

Structure and reactivity of thiophanium salts, the elusive intermediates in ArS+-mediated one-pot sequence of three AdE reactions

Abstract The structure of the previously postulated cationoid intermediate, presumably formed in ArS + -mediated coupling of two vinyl ether units, was firmly established as (1R * ,2R * ,4S * ) -2,4-dimethoxy-3,3-dimethyl-1- p -tolylthiophanium salt and its reactivity was elucidated.

ArSCl adducts of glucal derivatives in the preparation of C-glucosides

Reaction of the ArSCl adducts formed in situ from the protected glucals with carbon nucleophiles in the presence of SnCl4 is proposed as a novel route for a stereoselective synthesis of various C-β-D-glucosides.

Inversion of the substituent effects on the rates of ArSC1 AdE-reactions with styrenes

Abstract The inversion of the reactivity order of R 2 C 6 H 4 SC1 in Ad E -reactions with R 1 C 6 H 4 CHCH 2 owing to the variations in R 1 and R 2 has been found and interpreted in terms of two kinetically distinguishable Ad E -2 mechanisms.