Sulfinyl diene complexes

As we have shown, the sulfinyl group has been widely used as a chiral inductor in Diels-Alder reactions when bonded to the dienophilic double bond, due to its strong ability to control the 7r-facial selectivity. However, only a small number of papers on the use of sulfinyl dienes in asymmetric synthesis have been written, perhaps due to the poor reactivity of many of these substrates and the complex course of their reactions (mainly in the case of 1-sulfinyl dienes, see later). Additionally, the fact that synthetic methods to obtain enantiomerically pure sulfinyl dienes have been available only in the last six years would also explain the low number of papers concerning these asymmetric Diels-Alder reactions. During the preparation of this account, an excellent review on the synthesis and asymmetric Diels-Alder reactions of chiral 1,3-sulfinyl dienes has been published [11]. 

Scheme 1.54 Synthesis of sulfinyl iron diene complexes.

Scheme 5.36 Synthesis of sulfinyl diene Fe(0) tricarbonyl complexes.

The reaction of enantiomerically pure sulfinyl dienes with Fe(GO)s yields ( 7" -(lZ)-sulfinyl diene)iron(O) tricarbonyl complexes.Planar and axial chirality are introduced upon complexation of similar ligands with (dba)Fe(GO)3 (dba = dibenzylideneacetone). Gomplexation of sorbic acid with Fe(GO)s in acetone upon irradiation with UV light produced the ( 7" -2,4-hexadienoic acid)Fe(GO)3 complex in good yields.Stobart efal. reported the reaction of 1,4,5,8,9,10-hexahydroanthracene with Fe(GO)s under UV irradiation to form a complex in which two tricarbonyliron moieties are bound in an 77 -fashion to the ends of the isolated conjugated diene units (Equation (8)). 

A recent attempt to selectively control the dehydrative spirocyclization of a dihydroxyketone using a chiral-directing element has been reported by Paley et al. [51]. Incorporation of an iron(0)-tricarbonyl diene complex into the dihydroxyketone skeleton was used to direct spirocyclization (Scheme 6). Thus, the unsubstituted spiroacetals 18 and 19 were obtained selectively in high yield from the dehydrative spirocychzation of sulfinyl iron(O) diene complexes 16 and 17, respectively. 

In order to make these oxidative reactions of 1,3-dienes catalytic, several reoxidants are used. In general, a stoichiometric amount of benzoquinone is used. Furthermore, Fe-phthalocyanine complex or Co-salen complex is used to reoxidize hydroquinone to benzoquinone. Also, it was found that the reaction is faster and stereoselectivity is higher when (phenylsulflnyl)benzoquinone (383) is used owing to coordination of the sulfinyl group to Pd, Thus the reaction can be carried out using catalytic amounts of PdfOAcji and (arylsulfinyl)benzoquinone in the presence of the Fe or Co complex under an oxygen atmosphere[320]. Oxidative dicyanation of butadiene takes place to give l,4-dicyano-2-butene(384) (40%) and l,2-dicyano-3-butene (385)[32l]. 

The initial work on the asymmetric [4-1-2] cycloaddition reactions of A -sulfinyl compounds and dienes was performed with chiral titanium catalysts, but low ee s were observed <2002TA2407, 2001TA2937, 2000TL3743>. A great improvement in the enantioselectivity for the reaction of AT-sulfinyl dienophiles 249 or 250 and acyclic diene 251 or 1,3-cyclohexadiene 252 was observed in the processes involving catalysis with Cu(ll) and Zn(ii) complexes of Evans bis(oxazolidinone) (BOX) ligands 253 and 254 <2004JOC7198> (Scheme 34). While the preparation of enantio-merically enriched hetero-Diels-Alder adduct 255 requires a stoichometric amount of chiral Lewis acid complex, a catalytic asymmetric synthesis of 44 is achieved upon the addition of TMSOTf. 

Asymmetric [4 + 2] cycloaddition reactions of A-sulfinyl compounds and dienes have been developed <2002TA2407, 2000TL3743>. Excellent enantioselectivity is observed for the preparation of product 148 by the reaction of A-sulfinyl dienophiles 146 and acyclic diene 147 catalyzed with Cu(II) and Zn(II) complexes of Evans bis(oxazolidinone) ligands (Scheme 74) <2004JOC7198>. 

Recently, Sisko and Weinreb have developed a new, in situ procedure for effecting A -sulfonyl imine Diels-Alder reactions. The method consists of treating an aldehyde with an A -sulfinyl sulfon-amide/boron trifluoride etherate in the presence of a diene (Scheme 3). Under these conditions the IV-sul-finyl sulfonamide reacts with the aldehyde to produce an /V-sulfonyl imine. This reaction, developed by Kresze, has been used previously for preparing imines from simple, non-enolizable aldehydes (c/. 1,2), but had not been applied to more complex aldehydes. As can be seen from the scheme, the cycload-... 

Two groups have recently examined enantioselective A -sulfinyl dienophile Diels-Alder reactions. In one case, Whitesell et al. found that a phenylmenthol derived N-sulfinyl carbamate adds to ( , )-2,4-hexadiene under Lewis acid catalysis to afford a single diastereomer (equation 52). If the Lewis acid was omitted, a complex mixture of cycloaddition products was obtained. Attack of the diene on the N-sulBnyl dienophile as shown in the equation would rationalize the observed results. The site of Lewis acid complexation, however, is unknown. 

A useful synthesis of functionalized dienes from alkenes used the initial formation of a stable rr-allyl palladium complex, followed by reaction with a-sulfinyl ester enolate and pyrolytic sulfoxide elimination (Scheme 55). ... 

Kresze and Wagner offered a mechanistic model for the [4 + 2] cycloadditions of A-sulfinyl dienophiles to rationalize the kinetically formed regioisomeric products.10 They proposed a concerted mechanism for the reaction via a transition state which has dipolar character (Scheme 1-1). For 1-substituted dienes, transition states A and B can be considered. If R is an electron-donating group which stabilizes the cationic center, a 3-substituted product will result. If R is electron-withdrawing (e.g., C02Me), the 6-substituted isomer will be the kinetic product of the cycloaddition. A similar argument can be made for 2-substituted and more complex dienes. 

The hetero-Diels-Alder (HDA) reaction of 2,3-dimethylbuta-1,3-diene and cyclohexa-1,3-diene with ethyl glyoxylate catalysed by chiral Zn(II) complexes yields both the dihydropyran and the ene product, the former predominating. The enantiomeric excess, which can reach 87%, is dependent on the solvent as well as the catalyst <97JCS(P1)2345>. Use of (Rs,E)-3-[(15)-isoborneol-10-sulfinyl]-l-methoxybutadiene gives the 5,6-dihydropyran with good endo and facial diastereoselectivity <97TA2989>. Scandium(III) perfluoro-octanesulfonate is an efficient catalyst in the HDA reaction of aldehydes with non-activated dienes <97BCJ1421>. 

The imino function adjacent to (diene)iron complexes can be employed in a hetero-Diels-Alder reaction with l-methoxy-3-trimethylsilyloxy-1,3-butadiene. This leads to tetrahydropyridinones pendent to the (diene)iron system. The biologically active piperidine alkaloid SS20846 A has been synthesized using this procedure. Ketones adjacent to the tricarbonyl(diene)iron unit can be reacted with nucleophiles using the stereodirecting effect of the tricarbonyliron moiety. Spiroketals are formed in good diastereoselectivity from appropriately functionalized 1- and 2-sulfinyl l,3-dien-5-ones under the stereodirecting influence of the tricarbonyliron moiety (Scheme 4-139). ... 

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