Thioketones, reactions with

In addition to carbenes and nitrenes, organometallic species such as germylenes 107 have been utilized in [2 + 1] cycloadditions and, upon reaction with thioketones, afforded the corresponding thiagermiranes which are very stable and do not decompose even when heated to their melting point438 (equation 124). 

Reaction with thioketones. The reaction of this reagent with thiocamphor (1) in THF at —50° followed by addition of an acid chloride results in imsaturated thioesters (2). Evidently thiocamphor is deprotonated to a thioenolate anion (a), which then reacts with the acid chloride to form 2. 

The reactivity of alkyllithiums towards ketones and thioketones has been investigated. " Whereas alkyllithiums add to the carbonyl carbon of ketones to give tertiary alcohols, the reaction with thioketones proceeds to give secondary thiols by reduction of the C=S group. Transition states for such addition and reduction reactions of acetone and thioacetone in ethereal solution have been located and the computed activation free energies are in agreement with experimental results. 

Across C=S bonds Allenes undergo a thermal and a photochemical [2+2] cycloaddition reaction with thioketones to give four-membered ring heterocycles. For example, irradiation of tetramethylallene and thiobenzophenone in benzene affords the [2+2] cycloadduct 246 in 64 % yield. As the result of the thermal reaction the linear adduct 247 was formed in 36 % yield... 

This example is a carbotriene DTHDA reaction with thioketone and carbon dienophiles and may be included in Section 2.3. 

Reaction with Thioketones. The reaction of thiobenzophenone derivatives (48) and similar compounds with Fe2(CO)9 has been studied by Alper. It has been shown to result in the formation of orf/jo-metalated complexes (49) in reasonable to high yields (eq 30). Treatment of the complexes (49) with Mercury(II) Acetate effects orf/io-mercuration. Thioketene complexes have also been examined. Complexes of type (49) offer a new route to lactones. ... 

Dihydrothiopyrans have also been prepared by cycloaddition between a,jS-unsaturated thioketones and carbonyl-activated dienophiles under Lewis-acid catalysis [78]. A marked dependence of the reaction yield on the catalyst was observed. The results of the cycloaddition reaction of thioketone 77 with methyl metacrylate, catalyzed by different catalysts, are illustrated in Equation 3.24. 

Azadienes undergo Diels-Alder reactions to form pyridine, dihydro- and tetrahydropyridine derivatives. N-Vinyl lactim ethers undergo Diels-Alder reactions with a limited set of dienophiles. " Thioketones react with dienes to give Diels-Alder cycloadducts. The carbonyl group of lactams have also been shown to be a dienophile. Certain heterocyclic aromatic rings (among them furans) can also behave as dienes in the Diels-Alder reaction. Some hetero dienes that give the reaction are -C=C-C=0, 0=C-C=0, and N=C-C=N. ... 

Raasch, and Schaumann and co-workers, have studied the reactions of bis (trifluoromethyl)thioketene (83 R1 = CF3) with alkenes, with thioketones, and with carbodiimides or Schiff-base imines. The products were, respectively, thietans, 1,3-dithietans (84 R1 = CF3), and 1,3-thiazetidines (85 R1 = CF3) (from the carbodiimide).114,115 With phenyl azide the 1,2,3,4-thiatriazoline (86 R = CF3) is formed subsequent pyrolysis yields2,l-benzisothiazole.114... 

Diphenylcyclopropene thione (156) was prepared11S-12°) from 3,3-dichloro-1,2-diphenyl cyclopropene (154) by reaction with thioacetic acid, since transformation of the carbonyl function of diphenyl cyclopropenone with P4S10121 was complicated by ring expansion to the trithione 155122 In a useful recent thioketone synthesis123) 156 was obtained directly from diphenyl cyclopropenone in a quantitative yield by simultaneous treatment with HC1 and H2S. 

Both saturated or unsaturated thioimides and dithioimides behave like thioketones in their photocycloaddition reaction with alkenes (equation 129) and alkynes (equation 130)185. 

A very similar reaction to that of Pechmann and Nold but which probably does not proceed through a dipolar cycloaddition manifold is the formation of 1,2,3-thiadiazole (6) via a thionoester and lithium trimethylsilyldiazomethane (Equation (17)) <86H(24)589>. Lithium trimethylsilyl-diazomethane also reacts with thioketones to produce 1,2,3-thiadiazoles <87H(26)1467>. 

Some thioketones are reported to give 1,3,2-dithiastannolanes in moderate to high yields in reactions with a stannylene Dis2Sn [Dis = CH(SiMc3)2] <930M4>. The key difference with the process discussed above is inversed polarization of intermediate ylide (380) which can exist in an equilibrium with a thiastannirane (381). 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

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. 

No isolated derivatives of 1,2,3-dioxathietanes have been reported. It is reasonable to assume their intermediacy in the reaction of singlet oxygen with thioketones and sulfines to give ketones (71CC118, 70TL4683), and energy profiles have been calculated (CNDO/B) on the supposed 1,2,3-dioxathietane intermediate (80JCS(P2)188). 

As mentioned before, it was with thioketones that the first cases of thiophilic addition were evidenced, but many other modes of reaction have been observed concurrently carbophilic addition, reduction, coupling, double addition, and enethiolization [357]. We will not discuss these reactions here, which have not been used much for synthetic purposes. 

Applied to dialkenyl disulfides, the dihetero sigmatropic shift gives rise to 1,4-dithiocarbonyl derivatives. The precursors are easily obtained by oxidation of the enethiolates, and the reaction has been observed with thioketones [500]. thioamides [501], dithioesters [356, 502] and thionesters [503],... 

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