Diaryl sulfoxides

The main primary fragment ions of diaryl sulfoxides 13 and 14 have the structures 16a or 16b(C8H7S02 + m/z 167) and the ion m/z 152 (17) can be obtained from both by the loss of CH3 (equation 5)13. Ions 16a and 16b are formed from the sulfenate ester structure of the molecular ions of 13 and 14 through a cyclization process and a simultaneous loss of the other O Ar part. A similar ortho effect is not possible in 15 and hence its most intense ion is M+ (23% of the total ionization in comparison with 2.7 and 0.6% for 13 and 14, respectively) and its primary fragments are typical for a normal diaryl sulfoxide. 

In order to account for the unusually facile thermal racemization of optically active allyl p-tolyl sulfoxide (15 R = p-Tol) whose rate of racemization is orders of magnitude faster than that of alkyl aryl or diaryl sulfoxides as a result of a comparably drastically reduced AH (22kcalmol- ), Mislow and coworkers44 suggested a cyclic (intramolecular) mechanism in which the chiral sulfoxide is in mobile equilibrium with the corresponding achiral sulfenate (equation 10). 

In the case of diaryl sulfoxides the formation of both the aryl radical and the hydroxycyclohexadienyl radical was observed optically. Veltwisch and coworkers45 studied also the reaction of OH radicals from radiolysis of aqueous solutions of mixed (alkyl phenyl) sulfoxides (PhSOR). They found the formation of both alkylsulfinic and phenylsulfinic acids. 

Kinetic studies of the oxidation of diaryl sulfoxides by Crvi led to the proposition of a single electron transfer67 as in the case of aryl methyl sulfoxides68. These reactions were performed in aqueous acetic acid/perchloric acid medium, HCr03+ being the active... 

Bei der Reduktion von Dialkyl- und Benzyl-sulfoxiden mit Trichlorsilan in Ather wird ein Gemisch aus Thioacetal und Sulfan erhalten3, in der Gegenwart von a. -Wasserstoff-Atomen lauft also die Reduktion unter Spaltung und Rekombination des Molekiils ab. Dementsprechend werden Diaryl-sulfoxide in Ather praktisch quantitativ zu Sulfanen re-duziert (z.B. Diphenyl-sulfoxid zu Diphenyl-sulfid mit 98% d.Th.). 

When thionyl chloride is used, diaryl sulfoxides are usually the main products. Unsymmetrical diaryl sulfides can be obtained by treatment of an aromatic compound with an aryl sulfenyl chloride (ArSCl) in the presence of a trace amount of iron powder.Aromatic amines and phenols can be alkylthiolated (giving mostly ortho product) by treatment with an alkyl disulfide and a Lewis acid catalyst. With certain substrates (primary amines with a chloro group, or a group not replaceable by chloro, in the para position), treatment with S2CI2 and NaOH gives thiophenolate salts ... 

The other fragmentation pathways are typical for diaryl sulfoxides "" . A corresponding ortho effect was found in chlorodiphenyl ethers and sulfides but not in sulfones (12) were the sulfinate ester rearrangements " and the consequent formation of the m/z 125 and m/z 159 ions suppress the other possible fragmentations of the molecular ions (equation 4). It is also noteworthy that the ratio [m/z 125] [m/z 159] increases with increasing distance between the chlorine and the sulfur (equation 4). 

Treatment of arylmagnesium bromides with AT -sulfinyldiimidazole yields diaryl sulfoxides [102] [103]... 

The structural determination of diphenyl sulfoxide has also been performed (4), ahd shows that this diaryl sulfoxide also has an approximately pyramidal structure. The S—O bond length in this molecule is reported as 1.47 A, again suggesting some double-bond character. 

To summarize, free sulfoxides carry a net positive charge on the sulfur atom which (1) is independent of chain length in dialkyl sulfoxides, (2) is greater for dialkyl sulfoxides than diaryl sulfoxides, (3) is virtually unaltered by O-bonding, and (4) is increased by S-bonding. 

The second approach to chiral aminosulfonium salts consists of the conversion of chiral sulfoxides into aminosulfonium salts by means of MA -diethylaminosulfinyl tetrafluoroborate (165). The reaction occurs with predominant retention of configuration at sulfur with dialkyl and alkyl aryl sulfoxides. However, its stereospecificity is strongly dependent on the nature of substituents in the starting sulfoxide. In the case of diaryl sulfoxides this method failed to give chiral aminodiarylsulfonium salts. 

Andersen (75,76), as well as Mislow (221), discovered that the ORD curves of alkyl aryl sulfoxides show a strong Cotton effect in the region below 250 nm. An extensive study by Mislow and his coworkers (47) led to the following empirical rules, correlating the sign of the Cotton effect with the absolute configurations of chiral dialkyl, alkyl aryl, and diaryl sulfoxides, as well as menthyl esters of aromatic sulfinic acids ... 

All diaryl sulfoxides, some alkyl aryl sulfoxides (e.g., methyl p-tolyl sulfoxide, 41), and some dialkyl sulfoxides (e.g., methyl 1-adamantyl sulfoxide), were found to undergo racemization according to the pyramidal inversion mechanism. Mislow and co-workers (248) have found that for most of the compounds investigated, irrespective of the nature of the substituents attached to sulfur, the first-order rate constant for racemization in p-xylene at 210 C is about 3 X 10" sec", corresponding to a half-life of about 6 hr. Moreover, the activation parameters do not show significant differences and their values were contained in a narrow range 35 to 42 kcal/mol and... 

An extraordinarily easy thermal racemization was observed for aryl arenethiosulfinates (256). It occurs at a convenient rate at about 50°C. The following activation parameters were estimated for the racemization of p-tolyl p-toluenethiosulfinate 218 A// = 23 kcal/mol A5 = -4 e.u. Thus, the rate of racemization is about 10" times greater than that of diaryl sulfoxides. An internal displacement of sulfenyl sulfur rather than pyramidal inversion was proposed as the mechanism. Recent studies on the chemistry and stereochemistry... 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

Dialkyl and diaryl sulfoxides were reduced (deoxygenated) to sulfides by several methods. As a little surprise sulfides were obtained from sulfoxides by catalytic hydrogenation over 5% palladium on charcoal in ethanol at 80-90° and 65-90 atm in yields of 59-99% [684. In the case of p-tolyl -styryl sulfoxide deoxygenation was achieved even without reduction of the double bond giving 66% of p-tolyl -styryl sulfide and only 16% of p-tolyl -phenyl-ethyl sulfide [684],... 

Chemical deoxygenation of sulfoxides to sulfides was carried out by refluxing in aqueous-alcoholic solutions with stannous chloride (yields 62-93%) [186 Procedure 36, p. 214), with titanium trichloride (yields 68-91%) [203], by treatment at room temperature with molybdenum trichloride (prepared by reduction of molybdenyl chloride M0OCI3 with zinc dust in tetrahydrofuran) (yields 78-91%) [216], by heating with vanadium dichloride in aqueous tetrahydrofuran at 100° (yields 74-88%) [216], and by refluxing in aqueous methanol with chromium dichloride (yield 24%) [190], A very impressive method is the conversion of dialkyl and diaryl sulfoxides to sulfides by treatment in acetone solutions for a few minutes with 2.4 equivalents of sodium iodide and 1.2-2.6 equivalents of trifluoroacetic anhydride (isolated yields 90-98%) [655]. 

Various diaUsyl, alkyl aryl and diaryl sulfoxides were readily oxidized to the corresponding sulfone in excellent yields under mild conditions at —30°C by 51. Interestingly,... 

The benzothiadiazepine 1-oxides (550), formed by the reaction of the diaryl sulfoxides (549) with hydrazoic acid, rearrange on treatment with sodium hydroxide to give (551) (72CB757). The benzothiadiazine 1,1-dioxide (552) also undergoes base-induced ring expansion to give (553) (71JOC2968). 

Olah et al.466 observed immediate formation of protonated benzenesulfinic acid upon addition of SO2 to benzenium ion formed in H SO3F-Sb I 5-SO2C11 solution at —78°C. Based upon this observation, Laali and Nagvekar467 developed a method for the synthesis of aromatic sulfoxides [Eq. (5.169)]. Product formation was interpreted in terms of dehydration of protonated benzenesulfinic acid followed by nucleophilic attack by the aromatic to the formed arenesulfinyl cation. Mixed sulfoxides (4-fluorophenyl-4-methylphenyl and 4-fluorophenyl-3-trifluoromethyl sulfoxides) were also prepared by sequential addition of the two aromatics. The direct synthesis of symmetric diaryl sulfoxides in high yields (room temperature, 2—48 h, 50-95%) has been reported through the electrophilic activation of thionyl chloride with triflic acid.468... 

Scheme 5.44. ortho-Metalation of diaryl sulfoxides. All reactions proceed... 

Furukawa, N. Ogawa, S. Matsumura, K. Fuji-hara, H. Extremely facile ligand-exchange and disproportionation reactions of diaryl sulfoxides, selenoxides, and triarylphosphine oxides with organolithium and Grignard reagents. 

Sulfones and sulfonamides - the S+O class are similarly powerful directors,141 and do not suffer electrophilic attack at sulfur (though occasionally suffer nucleophilic attack on the aromatic ring)134143 but are less useful because of their more limited synthetic applications.144 143 Aryl J-butylsulfoxides are also powerful ortho-directors,145 but less hindered diaryl sulfoxides are susceptible to attack by organolithiums at sulfur146 (see section 3.3.3 for discussion of this sulfur-lithium exchange ). 

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