Carbonylative thiocarbonylation, alkyne

Enophiles may include carbonyls, thiocarbonyls, imines, alkenes and alkynes. When the carbonyl is an enophile, the reaction is called a carbonyl ene reaction. The enol form of an unsaturated ketone may serve as an ene in an intermolecular ene reaction known as the Conia-ene reaction. Ene reactions proceed best when the enophilic double bond is electron deficient. Hydrogen is the most common atom transferred in an ene reaction. Other atoms or groups may, however, participate in ene-like transformations. 

The process of transfer of one hydrogen atom from an allylic alkene (an ene) to an electrophilic alkene (an enophile) followed by formation of a new cr-bond and migration of allylic double bond is known as ene reaction or Alder ene reaction [1]. The ene may be an alkene, alkyne, allene, cyclopropane ring or arene, whereas an enophile may be an alkene, carbonyl, thiocarbonyl, imino or diazo compound having an electron withdrawing substituent. Oxygen may also serve as an enophile. The reaction usually takes place from left to right, since a new [Pg.161]

Further use of metal-catalyzed thiocarbonylation has been demonstrated in the intramolecular carbonylative strategy that led to five- and six-membered lactones 54 and 56 (Scheme 46.9). ° The use of conjugate enynes 57 in the Pd-catalyzed carbonylative thiocarbonylation also incorporates carbon monoxide at the internal carbon of terminal alkynes. ... 

The versatility of these [4+2] heterocyclization reactions is a consequence of the wide range of ene and diene components which can be used. In addition to alkenes and alkynes functioning as ene components, a variety of heterodienophiles is available such as electron-deficient imines (e.g. equation 89), nitriles e.g. equation 90), electrophilic carbonyl compounds (e.g. equation 91), thiocarbonyl compounds (e.g. equation 92), singlet oxygen (e.g. equation 93), nitroso compounds (e.g. equation 94), sulfenylsulfonamides (e.g. equation 95) and azo compounds (e.g. equation 96). Many of these reactions proceed with excellent regioselectivity and stereoselectivity, probably because in many instances they involve... 

Besides alkoxy-, amino-, and amidocarbonylation reactions, a carbonylation process using a thiol as nucleophile, that is, thiocarbonylation, has been extensively studied.It has been shown that the thiocarbonylation takes place with 1-alkynes, " " " propargyl alcohols,allenes, " 1,3-dienes, propargylic mesylates, " and bicyclopropylidene . 

The ene and Prins reactions are not mechanistically distinct. Coverage will therefore be organized by the nature of the carbonyl compound, with intermolecular reactions presented first, followed by intramolecular reactions. The emphasis will be on material published since the field has been reviewed " and on examples demonstrating the stereo-, regio- and chemo-selectivity of these reactions. Coverage is restricted to the addition of carbonyl and thiocarbonyl compounds to simple alkenes. Addition of carbonyl compounds to vinylsilanes, allylsilanes and enol ethers is covered in the following chapters. Addition of imines and iminium compounds to alkenes is presented in Part 4 of this volume. Ene reactions with alkenes and alkynes as enophiles are covered in Volume 5, Chapter 1.1. Use of aldehydes and acetals as initiators for polyene cyclizations is covered in Volume 3, Chapter 1.6. 

As well as undergoing addition to enones, arenes, carbonyl groups and thiocarbonyl groups, alkynes also undergo additions to alkenes. Thus tolan affords cyclobutenes on photoaddition to the tetrahydronaphthalene 126. The triplet-sensitized irradiation of vinylacetylene in the presence of 1,1-dichloroethene yields the (2 + 2)-cycloadduct 127 as well as the dimers 128 and 129. Similar behaviour is observed when dienes are the reaction... 

Thermal reactivity of a-carbonyl- and a-thiocarbonyl-stabilized methylenet-riphenylphosphoranes has been investigated. The kinetic data provided valuable support and insight into the mechanism of pyrolysis and a quantitative basis for structure/molecular reactivity correlations. The pyrolysis reactions of triphenylphos-phonium ylides stabilized by sulfone and C=X (X=0, S) groups represent alternative and environmentally friendly routes to the synthesis of novel alkene and alkyne compounds. 

The latter carbonylation involves the formation of PtH(SR)(PPh3)2 by the oxidative addition of RSH to the zero-valent platinum complex. A possible pathway may include the CO insertion into the S-Pt bond of PtH(SR)(PPh3)2. Then, acylplatination of alkynes generates p-thiocarbonyl-substituted vinylplatinum intermediate, which undergoes reductive elimination to give the a,p-unsaturated thioesters with regeneration of the catalyst. 

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