Sulfoxides are capable of forming stable complexes with transition metals and there have been many comprehensive studies into their binding properties. However, the use of sulfoxides, particularly chiral sulfoxides, as ligands in transition metal catalysis is rather less well developed. This review aims to describe these catalytic studies and covers new developments that are showing very promising results and that have led to a renewed interest in this field.

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The emergence of sulfoxides as efficient ligands in transition metal catalysis.

The rapid generation of molecular complexity in a relatively easy manner has made the [5+2] cycloaddition approach a highly useful tool in the synthesis of a wide number of complex natural products and important biologically active products containing seven-membered rings. Besides the common and highly efficient [5+2] cycloadditions of oxidopyrylium and oxidopyridinium ions with π-systems which offer an easy access to a wide range of novel heterocyclic seven-membered ring molecules with a oxygen or nitrogen bridge, and the less employed [5+2] photocycloadditions, the metal-catalyzed [5+2] cycloadditions have attracted a lot of attention and have become one of the most popular ways of constructing seven-membered ring products. All these cycloadducts can be synthetically manipulated easily toward a number of interesting molecular structures with high potential applications. Abbreviations: Ac: acetyl; Ar: aryl; BarF: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; BINAP: 2,2′-bis(diphenylphosphanyl)-1,1′-binaphthyl; Bn: benzyl; Boc: tert-butoxycarbonyl; Bu: butyl; Bz: benzoyl; Cbz: benzyloxycarbonyl; cod: cyclooctadiene; Cp: cyclopentadienyl; DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene; DCE: 1,2-dichloroethane; de: diastereomeric excess; DFT: density functional theory; DMF: dimethylformamide; DMSO: dimethyl sulfoxide; dppb: 1,4-bis(diphenylphosphino)butane; dppe: 1,2-bis(diphenylphosphine)ethane; dppp: 1,3-bis(diphenylphosphino)propane; ee: enantiomeric excess; Et: ethyl; HFIP: hexafluoroisopropyl alcohol; L: ligand; M: metal; Me: methyl; MOM: methoxymethyl; Ms: mesyl; Ph: phenyl; PMB: para-methoxybenzoyl; Pr: propyl; Segphos: 5,5′-bis(diphenylphosphino)-4,4′-bi-1,3-benzodioxole; TBDPS: tert-butyldiphenylsilyl; TBS: tert-butyldimethylsilyl; TEA: triethylamine; Tf: trifluoromethanesulfonyl; TFE: trifluoroethanol; THP: tetrahydropyran; TIPS: triisopropylsilyl; TMS: trimethylsilyl; Tol: tolyl; Tp: hydridotris(pyrazolyl)borate; Ts: 4-toluenesulfonyl (tosyl); VCP: vinylcyclopropane.

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Recent Developments in the [5+2] Cycloaddition

The chloroperoxidase (CPO) catalyzed oxidation of a series of rigid aromatic sulfides of well-defined geometry has been studied by the use of enantioselective gas chromatography for the determination of product composition and sulfoxide enantiomeric excess. The almost planar 1-thiaindane was found to be an excellent substrate, giving a quantitative yield of the (−)-(R)-1-oxide in 99% e.e. The sterically more demanding next higher homolog with a six-membered heterocyclic ring, 1-thiatetrahydronaphthalene (1-thiochroman), also gave a sulfoxide in high e.e (≥96%) but in a much lower yield, indicating a preserved stereorecognition ability but a lower turnover rate. A carbonyl group in the 4-position (1-thiochroman-4-one) had no further effect on the CPO-mediated sulfoxidation. Enantioselectivity was lost for the symmetric disulfide, 1,3-benzodithiol, indicating an equal accessibility of the FeO complex towards both sulfur atoms and an active site centred above the heme-iron atom. In all cases with pronounced enantioselectivity the reaction gave the (R)-sulfoxide as observed previously with alkyl aryl sulfides. A change of oxygen source from hydrogen peroxide to t-butyl hydroperoxide and a longer reaction time gave a higher chemical but a lower optical yield, most likely due to an increased competition by the uncatalyzed oxidation reaction in this case.

Chloroperoxidase-catalyzed asymmetric synthesis of series of aromatic cyclic sulfoxides

Cycloadditions in Synthesis.

Asymmetric [4,2] cycloadditions involving the use of enantiomerically pure sulfoxides as the main controller of stereoselectivity are reviewed. Diels-Alder reactions with sulfinyldienes and sulfinyldienophiles, 1,3-dipolar reactions with vinylsulfoxides, and hetero-Diels-Alder reactions are the main objectives of this review. The influence of the sulfinyl group on the reactivity, regioselectivity, and especially stereoselectivity of these reactions has been mainly considered in order to understand the synthetic scope and limitations of the sulfinyl group acting as a chiral inductor in these reactions. Apparent deficiencies in proposed stereochemical models have been also highlighted.

Asymmetric [4+2] Cycloadditions Mediated by Sulfoxides

(1R,3R)-2-Methylene-1,3-dithiolane 1,3-dioxide: A highly reactive and selective chiral ketene equivalent.

trans-Dioxides of Cyclic Ketene Thioacetals: Highly Selective Chiral Ketene Equivalents

trans‐Dioxides of Cyclic Ketene Thioacetals: Highly Selective Chiral Ketene Equivalents.

Trans-1,3-dithiolane-1,3-dioxide is a more reactive and more selective chiral ketene equivalent than trans-1,3-dithiane-1,3-dioxide.

Chiral ketene equivalents for use in asymmetric synthesis

Chiral Ketene Equivalents for Use in Asymmetric Synthesis

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