Desulfonylation via Radical Process: Recent Developments in Organic Synthesis

As the "chemical chameleon", sulfonyl-containing compounds and their variants have been merged with various types of reactions for the efficient construction of diverse molecular architectures by taking advantage of their incredible reactive flexibility. Currently, their involvement in radical transformations, in which the sulfonyl group typically acts as a leaving group via selective C-S, N-S, O-S, S-S, and Se-S bond cleavage/functionalization, has facilitated new bond formation strategies which are complementary to classical two-electron cross-couplings via organometallic or ionic intermediates. Considering the great influence and synthetic potential of these novel avenues, we summarize recent advances in this rapidly expanding area by discussing the reaction designs, substrate scopes, mechanistic studies, and their limitations, outlining the state-of-the-art processes involved in radical-mediated desulfonylation and related transformations. With a specific emphasis on their synthetic applications, we believe this review will be useful for medicinal and synthetic organic chemists who are interested in radical chemistry and radical-mediated desulfonylation in particular.

C-H Sulfonylation via 1,3-Rearrangement of Sulfonyl Group in N-Protected 3-Bis-sulfonimidoindole Derivatives Using Fluorine Reagent

A C-H sulfonylation of N-protected 3-bis-sulfonimidoindole derivatives via a 1,3-rearrangement of a sulfonyl group on a bis-sulfonimide moiety using tetra-n-butylammonium fluoride (TBAF) was developed to provide 2-sulfonyl-3-sulfonamidoindole derivatives in high yields as a novel C_sp2-H functionalization of heterocycles. The rearrangement of the sulfonyl group proceeded through an intermolecular addition of the desorbed sulfinyl ion from the bis-sulfonimide moiety in the substrate.

Vicinal difunctionalization of alkenes by four-component radical cascade reaction of xanthogenates, alkenes, CO, and sulfonyl oxime ethers

Four-component coupling reactions between xanthogenates, alkenes, CO, and sulfonyl oxime ethers were studied. In the presence of hexabutylditin, working as a propagating radical reagent, the chain reaction proceeds, as expected, taking into account reagents polarities, affording the corresponding functionalized α-keto oximes. Although yields are modest, this rare one-pot four-component process is easy to carry out and the resulting compounds, bearing multiple functionalities, have the potential for further elaboration.

Oxidant-directed chemoselective sulfonylation and sulfonyloximation of alkenes via cleaving the C–S bond in TosMIC

Oxidant-directed chemoselective sulfonylation and sulfonyloximation reactions for the divergent synthesis of valuable vinyl sulfones and α-sulfonylethanone oximes are developed.

Electron transfer-induced reduction of organic halides with amines

Reduction of a variety of organo halides was examined by using amines as a sacrificial hydrogen source. UV light-induced reduction of vinyl and aryl halides with triethylamine proceeded smoothly to give the corresponding reduced products. High temperature heating also caused the reduction and DABCO (1,4-diazabicyclo[2.2.2]octane) also served as a good reducing reagent.

Palladium/Light Induced Radical Alkenylation and Allylation of Alkyl Iodides Using Alkenyl and Allylic Sulfones

Alkenylation and allylation of alkyl iodides with alkenyl and allyl sulfones, respectively, took place under Pd/photoirradiation system. The initial alkyl radical, derived from a single electron transfer between Pd(0) and RI, underwent the title transformations. Pd(0) was regenerated through a reductive elimination of PhSO₂PdI, which is formed by the combination of the sulfonyl radical and the palladium radical. The addition of water was effective, presumably by pushing the equilibrium through hydrolysis of PhSO₂I.

Polycyclic heteroaromatic ring construction driven by silver/cobalt co-catalyzed desulfonylative and defluorinative fragment-recombination of enol nonaflates with amidines

A novel Ag(i)/Co(ii) co-catalyzed tandem fragmentation and recombination reaction commencing from detachable nonaflates and amidines has been developed.

Bromoallylation of Alkenes Leading to 4-Alkenyl Bromides Based on Trapping of β-Bromoalkyl Radicals

A radical-chain addition of allyl bromides to aryl alkenes, vinyl ester, and vinyl phthalimide was studied in which elusive β-bromoalkyl radicals were trapped efficiently to give 5-bromo-1-pentenes in good to high yields (16 examples). A subsequent carbonylative radical cyclization with AIBN/Bu₃SnH/CO was successful in giving the corresponding 3,5-disubstituted cyclohexanone derivatives in moderate yields. Synthesis of a piperidine ring was also successful by subsequent reaction with primary amine.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Radical azidation reactions and their application in the synthesis of alkaloids

Recent advances in radical azidation using sulfonyl azides are presented. For instance, radical carboazidation using α-iodoketones, desulfitative carboazidation, and anti-Markovnikov hydroazidation of alkenes are described. These novel methods tolerate a large number of functional groups and allow the synthesis of organic azides that would be difficult to synthesize otherwise. The transformation of the azides using reductive processes as well as a Schmidt reaction under nonacidic conditions were used to synthesize alkaloids including indolizidine 167B, monomorine I, cylindricine C, and lepadiformine C.

Aminomethylation of Michael acceptors: complementary radical and polar approaches mediated by dialkylzincs

Phtalimidomethyl iodide and substituted maleimidomethyl iodide were used as radical precursors in dialkylzinc-mediated radical addition to diethyl fumarate. The reactions led stereoselectively to functionalized pyrrolizidines. The radical mechanism was supported by spin-trapping experiments and rationalized by theoretical calculations. Radical additions, on the one hand, and carbozincation followed by transmetalation with copper(I), on the other, were shown to be complementary methods to achieve the formal aminomethylation of activated unsaturated compounds.

Silylboranes as new sources of silyl radicals for chain-transfer reactions

Various silylboranes, which were outfitted with a catecholborane moiety at one end and a (Me(3)Si)(3)Si moiety at the other end of a carbon chain, were prepared through the hydroboration of the corresponding unsaturated silanes. The C-centered radical species generated from these silylboranes efficiently cyclized to provide, through a 5-exo intramolecular homolytic substitution at the silicon center, the corresponding silacycle and a Me(3)Si radical that was subsequently trapped by sulfonyl acceptors. These cyclizations proceeded at unprecedented rates, due, in part, to a strong gem-dialkyl effect that was attributable to the presence of bulky substituents on a quaternary center located on the chain. In parallel, we designed arylsilylboranes that produced silyl radicals through a 1,5-hydrogen transfer. Such silyl radicals may be valuable radical chain carriers, for instance, in oximation reactions of alkyl halides. Finally, computational studies allowed calculation of activation barriers of the homolytic substitution step and additionally illustrated that the overall reaction mechanism involved a transition state in which the attacking carbon center, the central silicon atom, and the Me(3)Si leaving group were collinear.

Efficient carboazidation of alkenes using a radical desulfonylative azide transfer process

The radical-mediated carboazidation of terminal alkenes using electrophilic alkanesulfonyl azides is reported. A single reagent delivers the necessary electrophilic alkyl radical as well as the azido group, and good yields are obtained by using a moderate excess of the carboazidating reagent (1.5-2 equiv). Interestingly, in addition to the starting sulfonyl azide, this method requires only the use of a radical initiator, di-tert-butyldiazene. In terms of atom economy, this azide transfer reaction is close to ideal, as SO2 (1 equiv) is the only side product. The synthetic potential of this process has been demonstrated by a formal synthesis of the alkaloid lepadiformine C.

Cobalt catalyzed functionalization of unactivated alkenes: regioselective reductive C-C bond forming reactions

The cobalt catalyzed hydroaldoximation and hydrocyanooximation of unactivated alkenes is reported. Secondary and tertiary aldoximes and oximonitriles are synthesized with excellent regioselectivity under mild conditions, and conversion of the products to valuable intermediates is documented. The reactions expand the arsenal of reductive carbon-carbon bond forming reactions as well as regioselective functionalizations of unactivated double bonds.

Tin-free radical carbonylation of alkylsulfonyl derivatives into alkylcarbonyl derivatives

A simple radical approach based on tin-free radical carbonylation is devised for the direct conversion of alkylthiosulfonates, alkylsulfonyl cyanides, and alkylsulfonyl oxime ethers into the corresponding alkyl thiol esters, acyl cyanides, and acylated oxime ethers in a single step. The present approach is very simple, highly efficient, and minimizes the formation of byproducts.

A novel approach for the direct conversion of alkylsulfonyl derivatives into alkylcarbonyl derivatives via tin-free radical carbonylation

A novel radical approach for the direct conversion of RSO(2)X into RCOX in a single step is devised; the present approach is very simple, highly efficient, and minimizes formation of by-product.