Thioketenes 4+2 cycloaddition reactions

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. 

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>. 

Cycloaddition reactions of thioketene and isocyanic acid forming four-membered ring products have been studied by means of the ab initio RHF/6-31G method. The two alternative reactions are concerted but nonsynchronous, taking place through twisted or planar four-membered cyclic transition states. The activation barriers were calculated to be... 

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

Raasch, M.S. Bis(trifluoromethyl)thioketene. I. S3mthesis and cycloaddition reactions. 

Calculations at HF/6-31G and B3LYP/6-31++G have been used to study the mechanism of the cycloaddition reaction between thioketene and thio-formaldehyde. In addition an AIM analyis was carried out to study the electronic structures of stationary points for the two reactions under investigation. 

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. 

In [2+2] cycloaddition reactions of carbon cumulenes, often only one four-membered ring compound is obtained. This reaction is of considerable importance in the synthesis of 8-lactams from ketenes and C=N double bond containing substrates. The j8-lactam structure is present in a variety of antibiotics. Also, j8-thiolactams are obtained from thioketenes and imines. 

Thioketene 5-oxides, which are sulfines with an extended cumulenic system, also participate in the cycloaddition reaction with 2-diazopropane. In this case, exclusive reaction across the C=C bond is observed to form the cycloadduct 29 in 70 % yield. ... 

Bis(trifluoromethyl)thioketene also undergoes the [2+2] cycloaddition reaction with isothiocyanates to give the four-membered ring cycloadducts 27, resulting from addition across the CHS bonds of both the thioketene and the isothiocyanate... 

The cycloaddition reactions of thio- and selenoketenes are similar to ketenes, but some exceptions are observed. For example, the dimerization of thioketenes occurs across the C=S double bonds, while in ketenes, dimers resulting from addition across the C=C bonds and unsymmetrical dimers, resulting from addition across both the C=C and the C=0 bonds are obtained. The [2- -2] cycloaddition reactions of thioketenes can involve the C=S or the C=C bonds, and additions across either one of these groups occurs. In their additions across C=N bonds both types of additions are also encountered. The progress of these reactions is monitored by the disappearance of their intensive color. Diarylthioketenes are blue, dialkyl derivatives are purple and monosilylthioketenes are red . 

The high reactivities of thioketenes prompted the synthesis of sterically hindered thioketenes which undergo [2-1-1], [2-1-2], [3-1-2] and [4-1-2] cycloaddition reactions similar to other heterocumulenes. Review articles on thioketenes were written by Schaumann in... 

Alkylidenethioketenes and thioketene 5-ylides (R2C=C=S=CH2) are also described in this chapter. The latter were trapped by an intramolecular cycloaddition reaction to give dithiolanes. Also, propadieneselon, CH2=C=C=Se, is described in this chapter. The unusual extended system, ArN=C=C=C=S, was recently generated by flash vacuum thermolysis ". Thiocarbonyl- -sulfides are treated as Thiosulfines in Section 4.2.2. 

In the attempted thioketene trapping experiments, sometimes dimerization of the generated thioketenes is observed. The dimerization occurs by a [2-1-2] cycloaddition reaction across the C=S bond to give the 1,3-dithietane derivatives 3. A similar dimer is isolated in the thermal generation of MeCOCH=C=S... 

The nucleophilic ynamines also undergo a [2+2] cycloaddition reaction with thioketenes. The initially produced cycloadducts 23 rearrange to give allene thiocarboxamides 24. ... 

The [2+2] cycloaddition reaction of thioketenes with reactive isocyanates, such as ni-trophenyl or sulfonyl isocyanates, proceeds similarly to give 4-thioxo-2-azetidinones 46". The reactivity of the isocyanates is as follows CISO2NCO > Ph0S02NC0 > 4-MePhS02 NCO > MeS02NC0 > 4-02NPhNC0. 

In the [2+2] cycloaddition reaction of thioketenes with chloro- and fluorosulfonyl isocyanates the [2+2] cycloadducts are not isolated, but j8-thiolactams resulting from the hydrolysis of the halosulfonyl groups are obtained. 

A [2+2] cycloaddition reaction of bis(trimethylsilyl)thioketene across the C=0 bond in acid amides is also observed. The initial cycloadduct 48 rearranges to an intermediate new thioketene 49, which undergoes further tfansformation to give the isolated products 50". ... 

The [2+2] cycloaddition reaction of thioketenes across C=S bonds is also observed. For example bis-trifluoromethylthioketene reacts with C=S bond-containing substrates to give the four-membered ring dithietane derivatives 51 . 

Bis(trifluoromethyl)thioketene and carbometoxycyanothioketene" undergo [3+2] cycloaddition reactions with nitrones 56 (X = O) and nitrilimines 56 (X = NR) to give the... 

Likewise, mono substituted thioketenes, generated in situ, undergo [3+2] cycloaddition reactions with nitrile oxides and nitrileimines" . 

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

The cumulenes discussed in this book are subdivided into carbon- and noncarbon cumulenes, and the 1-carbon cumulenes (sulfines, sulfenes, thiocarbonyl S -imides and thiocar-bonyl S -sulfides) are excellent dipolar species. The 2-carbon or the center-carbon cumulenes (carbon dioxide and carbon sulfides) are less reactive but their imides (isocyanates, isothiocyantes and carbodiimides) readily participate in many of the discussed reactions. The 1,2-dicarbon cumulenes (ketenes, thioketenes and ketenimenes) similarly participate in cycloaddition reactions, as well as the more exotic 1,2-dicarbon cumulenes (1-silaalene, 1-phosphaallene and other metal allenes). In contrast, 1,3-dicarbon cumulenes are only... 

Apparently, the reaction involves cycloaddition of nitronate at the C=S double bond of thioketene. This approach can be useful for deoxygenation of labile nitronates. 

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. ... 

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],... 

The thermal formation of a nitrene has been invoked to account for the result in the reaction between a sterically hindered thioketene and benzyl azide437 (equation 123). Although initially a 1,3-dipolar cycloaddition between the C=S bond and the azide was expected, the temperature of the reaction (138 °C) led to decomposition of benzyl azide into benzyl nitrene and the subsequent [2 + 1] cycloaddition. 

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