Thiocarbonyl compounds unsaturated—

One of the great surprises of fluorothiocarbonyl chemistry is the ease with which these compounds undergo free-radical polymerization. This behavior is unique among thiocarbonyl compounds. Though thioacetone polymerizes in free-radical systems, it does not do so with anything like the avidity of fluorothiocarbonyl compounds. Thioacetone does not copolymerize with compounds containing carbon-carbon unsaturation, which is a most important property of fluorothiocarbonyl compounds. 

Some applications of the thio-Claisen rearrangement to the syntheses of thiocarbonyl compounds, particularly unsaturated dithiocstcrs and thionesters, and of its stereocontrol aspects are reviewed in [120]. A particularly high stereoselectivity was observed with S-allyl derivatives of P-hydroxydithioesters syn products were obtained exclusively, according to the accompanying scheme [495,496]. 

A facile route to isothiochromans involves the base-catalyzed reaction of a benzyl dibromide with an a-thiocarbonyl compound. The method has been extended to the synthesis of an anthraceno[2,3-f]thiopyran, a heterocyclic analogue of an anthracyclinone (Equation 169) <1994S363>. In like manner, alkylation of the 2-aryl-1-bromoethane 491 by thiourea and subsequent liberation of the thiol function creates an isothiochroman precursor. An intramolecular Michael addition to the cr, )-unsaturated ester side chain yields the 1-substituted heterocycle (Scheme 179) <1992JOC1727>. 

In search for control of absolute stereochemistry, the reaction of thio-chalcones was investigated with unsaturated amides bearing an Evans chiral oxazolidinone [223] and dimenthyl fumarate [224, 225]. For the first time with thiocarbonyl compounds, the efficiency of Lewis acid addition was demonstrated, and reactions could be conducted at room temperature. With EtAlCl2 (Table 4, entry 2) or A1C13 (entry 3), levels of induction up to 92% were attained for the endo isomer. Yb(OTf)3 in DMSO also caused the acceleration of the reaction with chiral acrylamides with p-facial selectivity [226]. This group has also reported [227] an intramolecular hetero Diels-Alder reaction with divinyl thioketones and the double bond of an allyloxy group (Table 4, entry 4). 

Table 4. Hetero Diels-Alder reactions of unsaturated thiocarbonyl compounds as dienes...

Whereas the reactions of aromatic and unsaturated organomagnesium compounds with thiocarbonyl compounds are analogous to their reactions with carbonyl compounds (see Chapter 7), saturated alkylmagnesium compounds (apart from methyl) tend to react by thiophilic addition [1] ... 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

New routes to heterocycles from unsaturated carbanions, heterocumulenes and thiocarbonyl compounds 01EJO4569. 

The Diels- Alder reaction has been used to prepare unsaturated ring systems in this series. A common example is the dimerization of a) -unsaturated thiocarbonyl compounds (Equation (49)) <86JA7045>. These thiocarbonyl compounds are commonly unstable at room temperature, but can be generated in situ in a number of ways . 

Enaminothioketones and /3-enaminothioaldehydes (a,/3-unsaturated thiocarbonyl compounds bearing a C-4 electron-donating substituent have been the most thoroughly investigated thiabutadiene system capable of... 

Thiocarbonyl compounds have been employed in Diels-Alder reactions to prepare thiopyranyl systems,1 and generally the carbon-sulfur double bond serves as the 2it dienophile component of the [4 + 2] cycloaddition (Chapter 5). However, there are several reported examples of a,j8-unsaturated thioaldehydes, thioketones, and dithioesters participating as the diene partners of Diels-Alder reactions in which the thiocarbonyl group comprises a component of the 4tt diene system. 

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