Vasily V. Tumanov

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)]).

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.

Cationic polymerization of styrenes under essentially neutral conditions

An acid-free protocol for the cationic polymerization of styrene and its derivatives is described based upon utilization of hexafluoroisopropanol (HFIP) both as the sole proton source and a polar medium. For the electron-enriched styrenes no other activator is required while the polymerization of unsubstituted styrene proceeds readily only in the presence of the catalytic amounts of β-pinene as a co-initiator. Polymers thus prepared reveal MWD characteristics comparable to those for the products of the conventional non-living cationic polymerization.

Cationic polymerization of styrene and indene initiated by aryl acetals in hexafluoroisopropanol medium

Benzaldehyde acetal and its para–susbstituted derivatives are capable of initiating the cationic polymerization of styrene and indene in hexafluoroisopropanol medium under essentially neutral conditions. As a result, polystyrenes carrying diverse functionalities at the α-terminus and having MWD characteristics comparable to those for the products of the conventional non-living cationic polymerization are produced.

Novel approach toward the preparation of 1,6-enynes: substrates for the intramolecular Pauson-Khand cyclization

A novel approach to the synthesis of polysubstituted 1,6-enynes is elaborated based on the protocol of the three-component coupling of arenesulfenyl chlorides, dicobalthexacarbonyl complexes of conjugated enynes, and nucleophiles of the π-donor type and a possibility to employ the 1,6-enyne derivatives thus prepared as the substrates for the intramolecular Pauson—Khand cyclization is demonstrated.