Thioketenes, cycloaddition with

Cycloaddition with thioketenes 3-Substituted imidazole 1-oxides 228 react with thioketenes to give a mixture of l,3-dihydro-2H-imidazol-... 

Dimerization and analogous cycloadditions of thiocarbonyl compounds also lead to 1,3-dithietans. Adamantanethione on irradiation or on treatment with methanesulphonic acid gave the dimer, containing a 1,3-dithietan ring, and 2-isopropylidene-5,5-dimethyl-6-oxo-l,3-oxathian-4-thione also dimerized on irradiation. Bis(trifluoromethyl)thioketen reacted with a variety of thiocarbonyl compounds, including isothiocyanates, to give 2-(hexafluoroisopropylidene)-l, 3-dithietan derivatives. ... 

Reactions.—Thioketen (83) reacts with azomethines to give /ff-thiolactams. It has been suggested that the dipolar species (84) is formed, instead of the classical dipolar species (85), in the rate-determining step of the cycloaddition. Bis-(trifluoromethyl)thioketen (86) adds to azomethines, isothiocyanates, and azides to form 1,3,5-dithiazines and thiazetidines, 1,3-dithietans, and A -1,2,3,4-thia-triazolines, respectively. The thioketen (86) also undergoes [4 + 2]cycloadditions with dienes. ... 

Also, a [3+2] cycloaddition reaction of thioketene -oxides with 2-azaallyl anions 76 affords ionic cycloadducts, which after acidification give the heterocycle 77 . 

Cycloaddition may be varied by the use of fluorinated thioketenes in place of ketenes. Olefins and Schiff bases serve as the other component. Thus thioketene 56 cycloadds to a SchifT base to give product 57 in a 79% yield. In a further variation, reaction of dithiobenzoate esters with diphenylketene yields 61% of product 58. ... 

The desUylation strategy has been used for the cycloaddition of the parent thiocarbonyl yhde la with aldehydes and reactive ketones. The product obtained using A-methyl-3-oxoindolinone as the trapping agent corresponds to the spiro-cyclic compound 125 (168). Thioketene (5)-methylide (127) was reported to react with aromatic aldehydes and some ketones to furnish 2-methylene-substituted 1,3-oxathiolanes (128) (51) (Scheme 5.42). 

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

Cycloaddition of 2-ethoxy-2,3-dihydro-4//-1,3-benzoxazin-4-one with conjugated diene 167 gave tetrahydropyrido[2,l-b][l,3]oxazin-6-ones (168) (71JHC865). Diels-Alder reactions of 3,4-dihydroisoquinolines and thioketenes (169), formed in situ, yielded 4,6,7,116-tetrahydro[l,3]ox-azino[2,3-a]isoquinoline-4-thiones [83AG(E)55 88CB1165],... 

We have reported the first electroactivity of a thioketene dimer compound [116]. The CV measurement of 2,4-dibenzylidene-l,3-dithietane (31), which was prepared by a basic dimerization of phenylthioketene derived from ben-zyltriphenylphosphonium chloride, showed irreversible two-step oxidation peaks at 0.25 and 0.61 V vs Ag/Ag+, indicating that 31 acts as a stronger electron donor than 2,6-bisphenyl-l,4-dithiafulvene (30) and TTF (2). The dimer (31) can form a 1 1 CT complex with TCNQ in DMSO. Cycloaddition polymerization of bisthioketene derived from p-xylenebis(triphenylphosphoni-um chloride) gave a -conjugated polymer (32) with thioketene dimer unit in the main chain (Scheme 12). This polymer was the first polymer contain-... 

It has been shown that thioketenes, isothiocyanates, and carbon disulfide can react with hydrazoic acid to form 5-alkyl-, 5-amino-, and 5-thiosubstituted-l,2,3,4-thiatriazoles <1996CHEC-II(4)691>. Most probably these reactions proceed via [3+2] cycloaddition of azide anion to C=S bond. 

It is known that aryl azides undergo 1,3-dipolar cycloaddition reaction with bis(trifluoromethyl)thioketene 156 to form the yellow A3-l,2,3,4-thiatriazolines 159 in very low to fair yield supporting the mechanism of reaction of this thioketene with hydrazoic acid (Equation 14) <1978JOC2500>. 

SCHEME 2.22 Cycloaddition reaction of quadricyclane (12) with hexafluorothioacetone and bis(trifluoroniethyl)thioketene. 

Dithiocarboxylic acids (118) can be converted into 1,3-dithietans (119) by acid chlorides,iodine, HCl, DCCI, or upon standing for a long time, and they are formally thioketen dimers. The cycloaddition of two C=S groups yields thioketen dimers and (120) from methyl isothiocyanate. Derivatives (121) are prepared by the reaction of dithiocarboxylic acids (118) with phosgene. ... 

High-level quantum-chemical calculations on the 3 + 2-cycloadditions of thioformaldehyde 5 -imides, S -methylide, S-oxide, and 5-sulfide have been reviewed. Theoretical studies on the 1,3-dipolar cycloaddition between thioketene 5-oxide and methyleneimine show that this reaction is concerted but non-synchronous. Adamantanethione 5-methylide reacts with thiocarbonyl compounds to produce 1,3-dithiolanes. A density-functional-theory study of the cycloaddition of the sulfine H2CSO predicts the 2 + 3-mechanism having the lowest pathway, with an activation barrier of 12.3kcalmoP. R The thermal and photochemical reactions of fluorenethione 5-oxide (69) with cyclooctyne (70) involves an initial 1,3-dipolar cycloaddition to produce the adduct (71), followed by an efficient sulfur transfer to cyclooctyne to produce the enone (72) and the dithiin (73) (Scheme 26). ° ... 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

As a general rule ketenes undergo non-catalyzed [2+2] cycloaddition reactions across their C=C bonds, with the exception of ketene itself. In contrast, disubstituted thioketenes undergo cyclodimerization across their C=S bonds. Mono substituted thioketenes undergo dimerization via a [3+2] cycloaddition reaction, also involving the C=S bonds. 

Carbon cumulenes undergo [2+2] cycloaddition reaction with numerous double or triple bonded substrates to give four-membered ring cycloadducts. Examples of cycloaddition to C=C, C=C, C=0, C=N, C=S, N=0, N=N. N=S. S=0, P=C, P=0, P=N and P=S bonds are known. When the two adjacent double bonds in the cumulenes are different, cycloaddition across either one of the double bonds occurs, and sometimes addition across both bonds is observed. However, more often the cycloaddition reactions follow only one pathway. As a general rule, in ketenes the non-catalyzed cycloaddition occurs preferentially across the C=C bond, whereas catalyzed cycloaddition reactions proceed across the C=0 bond. In thioketenes, isothiocyanates and sulfenes addition mainly occurs across the C=S bond. In isocyanates addition across the C=N bond is preferred. 

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