Thioketones aliphatic

In contrast with the normal behaviour of aliphatic thioketones, 3-exo,3 -exo-(lR,l R)-bithiocamphor cannot exist as enethiol, since the latter is obtained by reduction of its 1,2-dithiine, immediately stabilized by 1,5-prototropic rearrangement.6... 

Aliphatic unstable thioketones can be generated by thermolysis of their thiosulfinates in toluene.91... 

Many cycloadditions of nitrones86,172 174 and thiones give cycloaddition-cycloreversion equilibria. A-Methyl-C,C-diphenyl- and A-methyl-C-phenyl-nitrones, react with aliphatic thiones forming 1,4,2-oxathiazolidines, while 4 does not afford a cycloadduct.172 A-Methyl-C,C-2,2,4,4-tetramethyl-3-cyclo-butanonenitrone reacts with alicyclic thioketones to give 1,4,2-oxathiazolidines,174 while with 4 it enters into a metathesis reaction. 

In some cases involving aliphatic thiocarbonyl ylides, a 1,4-H shift occurs to afford vinyl thioethers. As an example, thiocarbonyl ylides (41), generated by the addition of isopropylidene carbene to aliphatic thioketones, are converted to divinylthioethers (42) (84) (Scheme 5.16). 

Aromatic thioketone (5)-methylides react with aromatic and nonenolizable aliphatic thioketones to regiospecifically give 1,3-dithiolanes of type 114 and 115... 

Finally, ylides 52 and 69 were found to react regioselectively with aliphatic thioketones to give the sterically less crowded products of type 117 (36,162,163). 

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). 

Sulfur dioxide reacts with aliphatic thioketone (5)-methylides in a sealed tube at 100 °C and l,2,4-oxadithiolane-2-oxides (138) are obtained. None of the regioiso-meric cycloadduct was formed (177). 

Another route to thioaldehyde and thioketone complexes uses the tungstate [W(CO)5SH] as the precursor. When [W(CO)5SH] was treated with ketones in the presence of trifluoroacetic acid, thioketone complexes [W(CO)5 S = C(R1)R2 ] were formed. Analogously, thiobenzaldehyde complexes were obtained from [W(CO)sSH]- and benzaldehydes bearing electron-releasing para substituents. Benzaldehyde, benzaldehydes with electron-withdrawing substituents, and aliphatic aldehydes did not react. Presumably, the binuclear dianion [p,-S W(CO)5 2]2 is an intermediate in the reaction, as on acidification of a mixture of [ju,-S W(CO)5 2]2 and p-N, A -dimethylaminobenzaldehyde the thioaldehyde complex [W(CO)5 S = C(C6H4NMe2-p)H ] is formed.143... 

The H2S/HC1 method of thionation of ketones was efficiently applied in the syntheses of aromatic thioketones, such as thiobenzophenone [129] and hindered aliphatic thiones, e.g. thiocamphor [128, 130] and thiofenchone [128, 131]. The following convenient procedure uses trimethyl orthoformate as a co-reactant [128]. 

For simpler, less hindered aliphatic thioketones, the method of Mayer and Berthold [132] of thionation of acetals by H2S was shown [133] to lead to tautomerically pure thioketones when the transformation was carried out under sufficiently mild conditions. Thus the reaction at 0°C in ethanol or THF in the presence of 0.05-0.01 eq of zinc chloride yielded reasonable yields of thioketones (2) devoid of the isomeric enethiols (2 ). 

The photodissociation of aromatic molecules does not always take place at the weakest bond. It has been reported that in a chlorobenzene, substituted with an aliphatic chain which holds a far-away Br atom, dissociation occurs at the aromatic C-Cl bond rather than at the much weaker aliphatic C-Br bond (Figure 4.30). This is not easily understood on the basis of a simple picture of the crossing to a dissociative state, and it is probable that the reaction takes place in the tt-tt Si excited state which is localized on the aromatic system. There are indeed cases in which the dissociation is so fast (< 10-12 s) that it competes efficiently with internal conversion. 1-Chloromethyl-Np provides a clear example of this behaviour, its fluorescence quantum yield being much smaller when excitation populates S2 than when it reaches Figure 4.31 shows a comparison of the fluorescence excitation spectrum and the absorption spectrum of this compound. This is one of the few well-documented examples of an upper excited state reaction of an organic molecule which has a normal pattern of energy levels (e.g. unlike azulene or thioketones). This unusual behaviour is related of course to the extremely fast dissociation, within a single vibration very probably. We must now... 

An interesting reaction is that of the mononuclear [W(CO)5(SH)] and the SH- -bridged dinu-clear /i-HS W(CO)s complexes with acetic anhydride to give the thioacetate complex [(MeCOS)W(CO)5]. The SH complexes react with aliphatic ketones and aromatic aldehydes to yield the complexes [(R2C=S)W(CO)5] and [(RCHS)W(CO)s] of these otherwise unstable thioketones and thioaldehydes.142... 

Aliphatic thioketones result tautomerically pure from the reaction of H2S with ketals in the presence of ZnCl2. Deprotonation with LDA gives the enethiolate which can be silylated to form the silyl vinyl sulphide. These are fairly stable to traces of water but react with MeOH to give the enethiol (equation 3)7. 

For simple aliphatic thioketone-enethiol tautomerism it has been assumed that the tautomers are similar in energy, and therefore one can anticipate only small differences, if any, in basicities of the sulphur and carbon sites191,192. 

Dimethyl acetals have been treated with hydrogen sulfide in the presence of zinc chloride to afford tautomerically pure aliphatic thioketones 60 (equation 46)305. 

For aliphatic thioketones, when an appreciable amount of enethiol is present, oxidation leads to divinyl disulfides397. However, Metzner409,410 has carried out the selective oxidation of symmetric and unsymmetrical aliphatic thioketones to afford quantitatively the corresponding sulfines (equation 105). 

The best-known exception to Kasha s rule is the anomalous fluorescence displayed by azulene and its derivatives (nonaltemant hydrocarbons) and some aliphatic and aromatic thioketones. 

The absence of a convenient method for the preparation of enethiolisab-le thioketones prompted us to fulfil this need. The development of aliphatic thioketones has so far been mostly restricted to examples with steric protection, such as thiocamphor and adamantanethione. Among the difficulties for a general synthesis of aliphatic and acyclic thioketones were easy enethiolisation (enethiols are stable tautomers with thermodynamic stabilities close to those of the thione form), oligomerisation, and susceptibility to air oxidation. They were solved by the adaptation of a former German reac-... 

This study was complemented by a selective synthesis of the tautomeric enethiols [77]. Aliphatic thioketones were deprotonated by LDA, silylated, and the resulting silyl vinyl sulfides were smoothly converted to enethiols by simple addition of methanol. These are stable compounds which do not equilibrate with thioketones, this behaviour probably related to the extremely mild conditions of the (easy) cleavage of the silicon-sulfur bond. 

A third method of access to sulfines is the oxidation of thiocarbonyl compounds. When the starting material is available it is an attractive route. There has been some dispute in the past whether enethiolisable thiocarbonyl derivatives would lead to the corresponding sulfines or to divinyl disulfides [101, 102]. It is now clear from our research that, even if the C=S molecules bear highly acidic a-protons, oxidation occurs on C=S and does not touch the a-protons. There are many examples of this behaviour. The most easily enethiolised compounds are thioketones. We have shown that their reaction with a peroxycarboxylic acid, mCPBA, is very fast at 0°C and quantitatively provides the corresponding sulfines [103]. In many examples the aliphatic sulfines are not very stable and have to be used in subsequent reactions that will be faster than their decomposition (t1/2 from some hours to days). 

The increasing utilisation of thiocarbonyl compounds brings about the necessity for efficient procedures for their transformation into carbonyl compounds. A straightforward method [172] involves the treatment of a thioketone with 4-nitrobenzaldehyde in the presence of TMSOTf. Thus thio-benzophenones and thiocamphor are converted into the corresponding ketones, but aliphatic thioketones or thioamides are resistant under these conditions. In order to convert an easily accessible bis(thiocamphor), de-... 

This reaction was intensely investigated by Mloston et al. In a classical, but not general reaction, they observed that heating aromatic or aliphatic thioketones [251, 252], thionoesters [253] and dithioesters [252, 253] with aryl-or benzylazides led to the formation of a carbon-nitrogen double bond. A [3+2] cycloaddition was assumed, with successive elimination of nitrogen and sulfur. 

The isomers of the now very popular dioxiranes, carbonyl oxides, are much less well-known species. However, the reaction of both isomers with two aliphatic thioketones was investigated [258]. A clear cut difference in reactivity was observed, with dioxiranes leading expectedly to the sulfines, while 5-membered ring thio-ozonides were produced from the reactions with carbonyl oxides, thus proving the possibility of a dipolar cycloaddition. 

It is interesting to note that nonsilylated enethiols have only been prepared as mixtures with their isomeric thioketones.4a,c The first selective synthesis of aliphatic enethiols was performed by Metzner52 by deprotonation of enethiolizable thioke-tones 42 with LDA and reaction of the enethiolates with trimethylsilylchloride. The subsequent methanolysis of silyl vinyl sulfides 43 afforded enethiols devoid of isomeric thioketones. Treatment of the enethiolates with various proton sources afforded instead mixtures of thioketones and enethiols (Scheme 31). 

The main drawback in the use of thiocarbonyl compounds as spin traps was represented by the fact that in most cases the resulting spin adducts either were as transient as the attacking radicals (aliphatic thioketones and dithioesters) or were characterized by very complex ESR spectra (thiobenzophenone and its derivatives). It was only after the introduction of thiobenzoyltriphenylsilane la that the use of thiocarbonyl compounds in spin trapping experiments acquired some practical value. 

Simple thioketones (6) are unstable compounds, and until comparatively recently they were known almost exclusively as the cyclic trimers (1,3,5-trithians), (7). However, a number of simple aliphatic and alicyclic thioketones have now been isolated as red or violet liquids they are easily oxidised and polymerised. In contrast, polycyclic thioketones, e.g. thiocamphor (8), are relatively stable, red crystalline solids (Figure 1) thiobenzophenone (6) (R = R = Ph) is also a stable solid. In general, thioketones are more sensitive to the... 

Thioketones (6) can be obtained by the acid-catalysed reaction of ketones with hydrogen sulfide (Scheme 2). The course of the reaction is dependent on the reaction temperature, the nature of the solvent, the concentration of the ketone and the stability of the thioketone (6), especially in relation to enolisation. This appears to be the most generally useful preparative route to thioketones, and many simple aliphatic derivatives are obtained by performing the reaction in ethanol at low temperature (-80°C to -55°C). The gem-dithiol (9) may also be converted into the thioketone (6) by heating it at approximately 200°C in the presence of a basic catalyst (Scheme 2). Reasonably stable thioketones, e.g. aromatic and heterocyclic derivatives like (10) and (11), can be prepared by heating the corresponding ketones with phosphorus pentasulfide in boiling toluene, pyridine or xylene (see Chapter 2, p. 21) (Scheme 3). 

At low temperatures, the monomeric thioketones are stable for some time, especially in the presence of a minute quantity of an antioxidant (hydroquinone). In a homologous series of aliphatic thioketones, the stability increases with the number of carbon atoms. Aliphatic thioketones often yield stable dimers, but do not polymerise. When thioketones are reacted with nucleophiles they generally... 

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