Sulfinyl cation

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

This reaction, important in the synthesis of cefaclor and other cephalosporins, has been studied in detail and is discussed in Volume 1, Chapter 2. The postulated mechanism involves formation of a sulfinyl cation (88) which undergoes an ene reaction resulting in the observed product. In an alternate explanation the sulfinyl cation reacts with the double bond to give the episulfoxonium ion (89). A six-electron sigma-tropic rearrangement would then result in the observed exomethylene isomers (Scheme 3) (see Volume 1, Chapter 2). The possible intermediacy of the sulfinyl cation suggested that derivatives other than the sulfinyl chloride could be potential sources of the requisite sulfinyl cation and hence could cyclize to the exomethylenecepham sulfoxide. This has in-... 

This review is concerned with the formation of cation radicals and anion radicals from sulfoxides and sulfones. First the clear-cut evidence for this formation is summarized (ESR spectroscopy, pulse radiolysis in particular) followed by a discussion of the mechanisms of reactions with chemical oxidants and reductants in which such intermediates are proposed. In this section, the reactions of a-sulfonyl and oc-sulfinyl carbanions in which the electron transfer process has been proposed are also dealt with. The last section describes photochemical reactions involving anion and cation radicals of sulfoxides and sulfones. The electrochemistry of this class of compounds is covered in the chapter written by Simonet1 and is not discussed here some electrochemical data will however be used during the discussion of mechanisms (some reduction potential values are given in Table 1). 

Application of electron impact ionization mass spectrometry (EI-MS) techniques for the analysis of 1,2-thiazines has waned since the publication of CHEC-II(1996). In one recent example of this technique, bicycle 44 was ionized at 70 eV and 180°C to afford radical cation 53, 54 via loss of CO2, and W-sulfinyl compound 55 and 1,3-cyclohexa-diene radical cation 56 via a retro-[44-2] reaction in the gas phase (Scheme 5) <2002TA2407>. 

Therefore, it is clear that protonation with retention will occur cis to oxygen but alkylation with inversion will place the new C-C bond trans to oxygen. Interestingly, when a second lithium cation is allowed to complex to the sulfinyl oxygen, the intermolecular Li—O bond has the dominant structural effect. In the presence of excess lithium ions, the stereochemical behavior of an a-lithio sulfoxide should, therefore, resemble that of an a-sulfinyl carbanion. 

Padwa and Kuethe have also used vinylogous Pummerer reactions of amido sulfoxides in the preparation of nitrogen-containing heterocycles. Vinyl amido sulfoxide (224) underwent an additive Pummerer reaction, on treatment with trifluoroacetic anhydride, to yield product 226 (Scheme 57).123 The a-thiocarbo-cation 225 generated from the Pummerer reaction of N-methyl-N-phenyl-2-[2-(toluene-4-sulfinyl)phenyl]acetamide (224) underwent a Friedel-Crafts reaction at the y-carbon with the tethered aromatic ring. Reductive removal of the... 

The structurally related optically active a-acyl vinyl p-tolyl sulfoxide 269 underwent asymmetric cyclopropanation. Michael addition of the carbanion of bromomalonate to 269 and the subsequent intramolecular alkylation yielded the corresponding optically active a-acyl-cyclopropane 271, with a high degree of diastereoselectivity (Scheme 70).142 It was proposed that the stereochemical outcome of the reaction can be rationalized by transition state 270, in which there is chelation of the oxygen atom of the carbonyl and sulfinyl groups to the metal cation. 

Schemes 88 and 89, in which the lithium cation is coordinated to the sulfinyl oxygen and, in the former, with the sulfur lone pair also. Methyl iodide, which is a nonpolar substrate, prefers to react at the more nucleophilic side which is anti to the sulfur lone pair. It has been suggested, in other words, that the si and the re faces of the benzyllithium derived from S(s -(1) are hard and soft reaction centers respectively, whereas the same si face has both hard and soft reaction centers therefore the soft methyl iodide is expected to react at the re face in the first case and at the si face in the second one, which in fact explains the observed products. This type of rationalization also explains the stereochemistry of the protonation (deuteration) of these organometallics (for a controversal discussion see ref. 34).

Various types of electron-deficient sulfur diimides react as hetero-dienophiles. In general, these cycloadditions occur under conditions similar to those used for AT-sulfinyl compounds. However, fewer types of sulfur diimides have been utilized in this process relative to Af-sulfinyl compounds. Some examples of symmetrical sulfur diimide Diels-Alder reactions are listed in Table l-II. It should again be noted that the orientational selectivity in these cycloadditions is the same as that shown by N-sulfinyl systems (cf. Table l-I). Several examples of cycloadditions with unsymmetrical sulfur diimides are shown in Table l-III. In all cases, these reactions were totally regioselective, and as noted above, reactions occurred at the least electron-deficient nitrogen-sulfur bond. Frontier molecular orbital (FMO) theory has been used to rationalize the regio-selectivity of addition of the cationic sulfur diimide shown in entry... 

Recently, cationic iV-sulfinyl amines and sulfur diimides have been reported. These species are readily prepared by alkylation of a IV-sulfinyl compound or a sulfur diimide with a trialkyloxonium tetrafluoroborate [Eq. (8)].5... 

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. 

A number of other asymmetric enolate protonation reactions have been described using chiral proton sources in the synthesis of a-aryl cyclohexanones. These include the stoichiometric use of chiral diols [68] and a-sulfinyl alcohols [69]. Other catalytic approaches involve the use of a BlNAP-AgF complex with MeOH as the achiral proton source, [70] a chiral sulfonamide/achiral sulfonic acid system [71,72] and a cationic BINAP-Au complex which also was extended to acyclic tertiary a-aryl ketones [73]. Enantioenriched 2-aryl-cyclohexanones have also been accessed by oxidative kinetic resolution of secondary alcohols, kinetic resolution of racemic 2-arylcyclohexanones via an asymmetric Bayer-Villiger oxidation [74] and by arylation with diaryhodonium salts and desymmetrisation with a chiral Li-base [75]. 

These and other results are best explained, at least when the reaction is carried out in liquid SOj, by the mechanism shown in Figure 12.2. Initial 5 2 attack by the electrophilic sulfur of SOj inverts the configuration at carbon and forms an ion pair. Collapse of the ion pair to form an 0-bound sulfinate occurs reversibly, and the more stable S-bound sulfinate is eventually formed. The cation [CpM(L)(L )] is slow to invert, leading to retention of stereochemistry at the metal. Sulfur electrophiles that are similar to SOj, such as N-sulfinyl-sulfonamides (RS02)NS0 and sulfur bis(sulfonylimide)s (RSOjN)jS, also insert into transition metal-carbon a-bonds with inversion of stereochemistry at carbon, - probably by a mechanism analogous to that shown in Figure 12.2. 

Whether this can be rationalized directly by reaction (11) yielding sulfinyl radicals besides thiols, or involves a more complex reaction scheme still remains an open question. There seems little doubt about the production of the sulfinyl radicals, as they could unambiguously be identified in complementary ESR experiments in which the hydroxyl radicals were generated via Fenton chemistry in acid solutions [23]. One mechanistic suggestion put forward [22] discusses the intermediacy of an OH-adduct radical, R-S (OH)-S-R which in basic solution may suffer OH attack to yield RS together with the hydrated form of the sulfinyl radical, RS(OH)2 . In acid solution, on the other hand, a secondary oxidation of the sulfenic acid (formed in reaction (10)) by disulfide radical cations (also formed upon OH reaction with disulfides, vide infra), that is reaction (12) has been brought into the discussion. 

A sulfinium cation (80) was suggested as a probable intermediate in the ring closure of sulfinyl chlorides by Lewis acids in an intramolecular ene reaction (Kukolja, 1977). The process is visualized as a complexation of the Lewis acid with either the oxygen or the chlorine atom of the sulfinyl chloride followed by formation of the S—C-2 bond with olefinic carbon, concomitant with hydrogen abstraction from the methyl group of the isopropenyl functionality (Scheme 4). In support of this mecha-... 

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