1.2- Dithiols cyclic

Enemark et al,98 reported solution redox potentials and heterogeneous electron transfer rate constants for [W(E)(Tp )(SS)] (E = O, NO SS = dithiolate). Cyclic voltammograms reveal a one-electron quasi-reversible oxidation process for [WO(Tp )(tdt)], which has the most negative electrochemical potentials among a large series of Mo and W investigated.98... 

EWG-Activated 2-methylene dithioles (= cyclic ketene dithioacetals with electron-withdrawing substituents) 38 undergo a ring-opening reaction mediated by primary aliphatic amines to give polyfunctionalized 2-(alkylamino) thiophenes (40) after hydrolysis in a one-pot procedure [82] ... 

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

Many acyclic enamines form thioamides when they are allowed to react with elemental sulfur at room temperature in dimethylformamide solvent 138). They also produce cyclic 1,3-dithiole by-products, which become main products at higher temperatures 135). For example, the reaction of l-(N-morpholino)-l-phenylethene (106) and sulfur in dimethylformamide solvent yields 1,3-dithiole 107. 

From the reactions of sulfur and carbon disulfide with cyclic ketone-derived enamines (570-573) 3H-l,2-dithiole-3-thiones were obtained, whereas the addition of carbon disulfide to other enamines gave a-dithio-pyrones (574), through initial dimerization of the enamine. 

The photochemical behavior of a number of substituted derivatives of thiochroman-4-one 1-oxides has been examined by Still and coworkers192-194. These authors also report that rearrangement to cyclic sulfenates, with subsequent reaction by homolysis of the S—O bond, appears to be a particularly favorable process. For example, ultraviolet irradiation of a solution of 8-methylthiochroman-4-one 1-oxide (133) in benzene for 24h afforded a single crystalline product which was assigned the disulfide structure 134 (equation 54). More recently, Kobayashi and Mutai195 have also suggested a sulfoxide-sulfenate rearrangement for the photochemical conversion of 2,5-diphenyl-l,4-dithiin 1-oxide (135) to the 1,3-dithiole derivatives 136 and 137 (equation 55). 

The cycloaddition of alkynes with the tributylphosphine-carbondisulfide adduct 131 results in the in situ formation of the ylides 132 which react with aldehydes to give the novel 2-arylidene or 2-alkylidene-l,3-dithioles 133 (Scheme 36) [132]. Concerning ylides C-substituted by sulfur we can also mention a publication on the behavior of various keto-stabilized ylides towards acyclic and cyclic a s-disulfides allowing the synthesis of substituted thiazoles, thiols, and dithiols [133]. 

The coordination of redox-active ligands such as 1,2-bis-dithiolates, to the M03Q7 cluster unit, results in oxidation-active complexes in sharp contrast with the electrochemical behavior found for the [Mo3S7Br6] di-anion for which no oxidation process is observed by cyclic voltammetry in acetonitrile within the allowed solvent window [38]. The oxidation potentials are easily accessible and this property can be used to obtain a new family of single-component molecular conductors as will be presented in the next section. Upon reduction, [M03S7 (dithiolate)3] type-11 complexes transform into [Mo3S4(dithiolate)3] type-I dianions, as represented in Eq. (7). 

Dithiolate ligands form stable anionic square pyramidal complexes with the Re03+ core [10]. This has been exploited in the dimer-captosuccinic acid complexes of 186Re and 188Re (vide infra), and in the use of the cyclic anhydride of dimercaptosuccinic acid as a bifunctional chelator for bioconjugate formation. The anhydride, in protected form such as 38, is reacted with antibody or other protein to form an amide or... 

Synthesis. These macrocycles are prepared from seven-membered ring dinitrile complexes, 84a-84c (Scheme 17), which contain either methylene, sulfur or oxygen in the five position (129). These cyclic dinitriles are synthesized by alkylating maleonitrile dithiolate or derivatives thereof with the corresponding dihalide. The dinitriles 84a-84c can be cyclized in magnesium propoxide to form porphyrazines 85a (33%), 85b (19%), and 85c (27%) (Scheme 17), which can be demetalated with trifluoroacetic to form 86a-86c. Additionally, 86a has been remetalated with nickel (87a, 92%), copper (88a, 95%), and zinc (89a, 94%). The sulfur and oxygen derivatives 85b, 85c, 86b, and 86c are of low solubility and are not suitable for further manipulation. 

Second, the oxidation of a [l,n]-dithiol by Ti(III)-Fl202 at pH 7 produces the cyclic disulfide as shown in Scheme 3.26. The hnal product of the dithiol oxidation forms a rather stable anion-radical. 

Otto, Sanders, and coworkers have utilized disulfide exchange to generate dynamic libraries of diastereomeric receptors [3]. DCLs made from a racenfic dithiol led to numerous cyclic structures including four cyclic tetramers, with the RR,RR,RR,RR diastereomer being the most stable (along with its all-5 enantiomer). Upon addition of N(CH3)J, the meso-diastereomer shown below was amplified 400-fold (Fig. 5.3). The structure of the diastereomer was confirmed by NMR and re-equilibration... 

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

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