Sulfoxides, elimination reaction

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Early investigations have indicated that sulfinyl radicals apparently do not add, at least in the usual way, to olefmic double bonds24. However, some recent results by lino and Matsuda25 obtained by studying the thermal decomposition of benzhydryl p-tolyl and benzhydryl methyl sulfoxides in the presence of cis-/2-deuteriostyrene lead one to believe that sulfinyl radicals add reversibly to CH2 =CHPh. The molar ratio of trans to cis /3-deuteriostyrene that they observed at nearly 50% conversion was explained by addition-elimination reaction of sulfinyl radicals. 

Sulfoxides also undergo thermal elimination reactions. The elimination tends to give (3, y-unsaturation from (3-hydroxysulfoxides and can be used to prepare allylic alcohols. 

Recently, Carreno, Urbano and coworkers were also able to synthesize almost enantiopure [7]helicene bisquinones 4-58 and 4-59 (96% ee) by reaction of the sulfoxide (S,S)-4-55 with the diene 4-56 in dichloromethane at -20 °C (Scheme 4.12) [19]. This six-step domino process includes a double Diels-Alder reaction, sulfoxide elimination, and aromatization of the rings B and F of the intermediate 4-57 to give 4-58, which could be oxidized to the fully aromatized 4-59. 

Such nucleophilic displacements are likely to be addition-elimination reactions, whether or not radical anions are also interposed as intermediates. The addition of methoxide ion to 2-nitrofuran in methanol or dimethyl sulfoxide affords a deep red salt of the anion 69 PMR shows the 5-proton has the greatest upfield shift, the 3- and 4-protons remaining vinylic in type.18 7 The similar additions in the thiophene series are less complete, presumably because oxygen is relatively electronegative and the furan aromaticity relatively low. Additional electronegative substituents increase the rate of addition and a second nitro group makes it necessary to use stopped flow techniques of rate measurement.141 In contrast, one acyl group (benzoyl or carboxy) does not stabilize an addition product and seldom promotes nucleophilic substitution by weaker nucleophiles such as ammonia. Whereas... 

An unusual one-pot intramolecular sulfoxide alkylation-elimination reaction was found by Gibson et al. <2001SL712>. These authors found that treatment of 459 with potassium bis-trimethylsilylamide resulted in a ring closure to 460 in acceptable yield. Furthermore, Batori and Messmer found an effective method for preparation of [l,2,3]triazolo[l,5- ]pyrimidinium salts <1994JHC1041> oxidative cyclization of hydrazones 461 by 2,4,4,6-tetrabromo-2,5-cyclohexadienone gave rise to the quaternary salts 462. Under certain reaction conditions, the formation of 6-bromo-salts 462 (R6 = Br) was also experienced. As neither the starting compound nor the quaternary triazolopyridinium salt underwent bromination in this position, the authors assumed that this bromination process occurred on one of the intermediates in the course of the above-mentioned cyclization reaction. 

Chloroallyl) thiocarbamate sulfoxides (. ., 5-2) un-.doubtedly rearrange in an analogous manner but in tbis case tbe sulfenate quickly undergoes an additional 1,2-elimination reaction (7 ). Tbe resulting products are tbe IJ, -dialkylcarbamoyl-sulfenyl chloride (M) and tbe carbonyl compound, aldehydes... 

This thermal rearrangement along with the 1,2-elimination reaction is analogous to a sequence previously reported for aryl 3-chloroallyl sulfoxides (15). 

N-Acylated Cys derivatives 32 on treatment with silver(I) oxide or silver(I) carbonate/1061 mercury(II) oxide/1071 iron salts/1081 or DCC1 091 undergo a (3-elimination reaction to yield AAla derivatives 33. The various (3-leaving groups that have been reported include sulfoxide/1101 sulfone/1111 5,5-dimethylsulfonium [Me2+S]/1121 2,4-dinitrophenylsulfanyl11131 and 5-dimethylphosphinothionyl11141 (Scheme 12). 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

Potassium /crt-butoxidc is a good base for bringing about elimination reactions of secondary alkyl halides suitable solvents include tert-butyl alcohol and dimethyl sulfoxide. 

Oxidation of alkyl iodides, bearing electron-withdrawing groups such as car-bomethoxy and sulfonyl at the a-carbon, with m-chloroperbenzoic acid results in clean elimination to give olefins [Eq. (27)]. This reaction involves reductive / -elimination of the intermediate iodosylalkanes, as observed in thermal peri-cyclic -elimination of sulfoxides and selenoxides. Exclusive syn stereochemistry in the reductive /1-elimination was established by deuterium labeling... 

Similarly, in vitro tests with CHO cells (with activation) caused an increased frequency of chromosome-damaged cells and SCEs and dose-related decreases in colony formations. In a large number of in vitro tests reviewed by Fishein (ref. 184a), diallate was the more active compound of the two. The ultimate mutagen postulated is 2-chlorocrolein formed via a series of sulfoxidation, rearrangement and 1, 2-elimination reactions ... 

Optically active vinyl sulfoxide was prepared by a combination of resolution and elimination reaction. Firstly, inclusion complexation of rac-2-chloroethyl m-tolyl- sulfoxide (118) and 14b in benzene gave, after two recrystallizations from benzene, a 1 1 complex of 14b and (+)-118 of 100% ee in 72% yield. Secondly, treatment of the complex with 10% NaOH gave optically pure (+)-m-tolyl vinyl sulfoxide (119) by HC1 elimination as colorless oil. Rapid polymerization of the (+)-119 proceeded by treatment with BuLi or BuMgBr at -78 °C to give optically active polymer (120). Oxidation of 120 with H2O2 gave optically active polysulfone (121).48... 

As a further stereoselective organic synthesis [40-47] using reactive sp2 carbon-centered radicals, eq. 10.23 shows the preparation of chiral 4-te/7-butylcyclohexene (49) from the optically pure o-bromophenyl sulfoxide (48) through 1,5-H shift by sp2 carbon-centered radical, followed by (3-elimination. This reaction looks like a thermal concerted intramolecular elimination reaction (Ei). 

The two metabolites identified in the rat, derived from the 3-lyase pathway, (2) and (3), have one and two sulfoxide groups respectively. These promote elimination reactions on hot surfaces in the GC/MS, as well as imparting higher polarity to the molecules. Reduction of the sulfoxide groups to sulfide with titanium trichloride (29) produces a single analyte... 

In this book, there have been many references to oxidation and reduction reactions. While these reactions are not within the scope of the discussions of this book, their mechanisms do involve the processes presented herein. In the case of the Swem oxidation, the first step is an addition-elimination reaction between dimethyl sulfoxide and oxallyl chloride. This process, illustrated below using arrow pushing, involves addition of the sulfoxide oxygen to a carbonyl with subsequent elimination of a chloride anion. 

The thermal elimination of sulfoxides (example below) is a first-order reaction with almost no rate dependence on substituent at sulfur (Ar) and a modest negative entropy of activation. It is accelerated if R is a carbonyl group (that is, R = COR )- The reaction is (slightly) faster in less polar solvents. Explain. 

E2 elimination reactions occur preferentially when the leaving groups are in an anti copla-nar arrangement in the transition state. However, there are a few thermal, unimolecular sy -eliminations that produce alkenes. For example, pyrolysis of several closely related amine oxides, sulfoxides, selenoxides, acetates, benzoates, carbonates, carbamates and thio-carbamates gives alkenes on heating (Scheme 4.10). The syn character of these eliminations is enforced by a five- or six-membered cyclic transition states by which they take place. 

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