Thioaldehydes isolation

The addition of H2S to an aldehyde or ketone can result in a variety of products. The most usual product is the trithiane (8). ° ot-Hydroxy thiols (5) can be prepared from polychloro and polyfluoro aldehydes and ketones.Apparently, compounds 5 are stable only when prepared from these compounds, and not even for all of them. Thioketones (6) can be prepared from certain ketones, such as diaryl ketones, by treatment with H2S and an acid catalyst, usually HCl. They are often unstable and tend to trimerize (to 8) or to react with air. Thioaldehydes are even less stable and simple ones apparently have never been isolated, though t-BuCHS has been... 

TBA-F in THF (0.01 M, 200 ml) is added slowly with stirring over 30-60 min to the a-silyldisulphide RCH(SiR3)SSAr (5 mmol). The mixture is stirred until all of the disulphide is consumed and the reactive thioaldehyde is reacted in situ without isolation. 

Thioaldehyde oxides are reported to be one of the lachrymatory agents in onions and are speculated to dimerize via an initial [4+2] cycloaddition to a five-membered ring that rearranges to a 1,2-dithietane dioxide. The latter has also been isolated from extracts of onions (80JA2490) and is reported to be the only characterized derivative of a 1,2-dithietane (Scheme 121). 

Similarly, the thioaldehyde complexes 66a,b were formed via addition of thiols to the vinylidene complex 64 and elimination of ethene. The vinylidene complex 64 reacted with benzyl mercaptan to yield the isolable intermediate 65. On heating 65 lost ethene in a first-order reaction to give 66b.188 The complexes 66a-d could also be prepared from the benzyne-(hydrido) complex 67 and thiols RCH2SH (Scheme 18).188... 

The reactions of dinuclear thioaldehyde and -ketone complexes with PR3 take a different course. When the dimeric complexes 112 are treated with PMe3 a Ti-S bond in 112 is cleaved and the monomeric complexes 113 are reversibly formed (see Scheme 26), 69 The reaction of 112 with benzene-thiol led to a Ti-C cleavage and formation of 1,3-propanedithiolato-(phenylthiolato) complex.69 The reaction of binuclear p-191 rf-thioketone molybdenum complexes related to 115 (M = Mo, E = S, R = Aryl) with P(OEt)3 also gave mononuclear complexes [Mo(CO)2(Et) i72-S = C(Aryl)2 (i75-C5H5)] by a rather complicated reaction sequence. When other phosphites were employed additionally binuclear complexes derived from addition of the phosphite to one molybdenum atom were isolated.225... 

As for thioaldehydes, the stability of thioketenes is largely influenced by the nature of the substituents and bulky groups tend to stabilize this functional group. Electronic factors such as those originating in silicon, phosphorus or trifluoromethyl substituents lead to a similar result. In general, however, the synthesis of monomeric thioketenes is difficult and requires the use of special techniques such as FVT, matrix isolation at low temperatures or generation under conditions which allow trapping in situ of the transient species. The... 

Only one isolated example of formation of thioketene and thioaldehyde dimolybdenum complexes has been reported507, but in contrast, zirconocene thioacetaldehyde and thiobenzaldehyde complexes, introduced by Buchwald, have been the subject of several studies508, including their application in the synthesis of novel antimony thiametallacy-cles 148509 (equation 156). Compound 146 was generated in situ and, after reaction with alkynes, gave rise to metallacycles 147 which, by treatment with SbCR, afforded 148. 

A number of studies dealing with a-oxo sulfines has appeared. The stabilities of the attractive molecules usually do not allow isolation. The strategy used for the manipulation of unstable thioaldehydes was applied here. A precursor is submitted to an elimination reaction (or a cycloreversion) to generate the sulfine in the presence of a diene, and it is the product of... 

The reaction of alkoxides with thioaldehyde S-oxides, generated from sul-finates, was carefully examined by Baudin et al. [163]. A number of products were isolated, formation of which was explained by carbophilic addition of the alkoxide anions. 

Cycloaddition reactions of thioaldehydes and sulfines are most probably encountered in plants, as elegantly and soundly shown by the group of Eric Block during their investigation of sulfur products occurring in the Allium species (for a review see [91]). They were able [92, 93] to isolate bicyclic dithioacetal oxides, called zwiebelanes, and also to synthesise them from a thioxosulfine, already described in this review (Sect. 2.6, Scheme 18). An extremely rich stereochemical and analytical study has resulted. 

One of the first thioaldehydes stabilized by an aromatic system and by heteroatoms was 2-ethoxynaphthalene-l-thiocarboxaldehyde (2), which was isolated as a trimer by reacting the corresponding aldehyde with H2S in alcohol under acidic conditions. By heating the trimer, it was possible to distill a small amount of monomer, which, however, polymerized after a few hours at room temperature (37JA1721). 

Monomeric thioaldehydes have not been isolated, although several, e.g. thioformaldehyde, have been identified by photoelectron spectroscopy. The thione group is relatively unstable in the monomeric form (1), and thioaldehydes can only be isolated as dimers (2), trimers (3) polymers (4), or enol derivatives (5), (Scheme 1). 

Thioaldehydes have rarely been described in literature. If an alkyne is added to 5-unsubstituted l,2-dithiole-3-thiones, ethylenic thioaldehydes (e.g.. 135, R1 = H) can be isolated. They are described by McKinnon as amorphous, unstable compounds,194 but Davy has isolated them as crystalline, stable compounds197 with well-defined mass spectra.204... 

The thioaldehydes (308), mentioned under (i), can be obtained by photochemical methods. These are substances of high reactivity, but instead of the expected 3-lactam product, only polymers were isolable [299]. On the other hand, it was assumed [300] that (309)-like compounds could rearrange to fi-lactams via an ene mechanism, but the photochemically generated (309) led to thiazolines (310) [301 ]. 

Thioaldehydes are highly reactive compounds, which cannot be isolated under classical conditions. Therefore, any Claisen transposition, that would lead to a thioaldehyde, can be efficient only if the expected product is converted in situ in an isolable derivative. As an example, thermolysis of vinyl sulfides in the presence either of mercuric oxide [59] or dimethyl sulfoxide and water [60] led to y-unsatu-rated aldehydes (Scheme 9.10). 

As compared to thioaldehydes, these compounds are somewhat easier to isolate, their reactivity in solution or towards the oxygen in air being more moderate. The first evidence is due to Ojima and Kondo [55], who studied a simple allyl vinyl sulfide rearrangement (Scheme9.11) at ambient temperature. By NMR, they detected an unsaturated thioketone, which was stable under a nitrogen atmosphere, but further converted to the corresponding ketone just by standing in air. 

Although thioaldehydes belonging to certain classes of heterocyclic compounds have been isolated and characterized (Volume 1, p. 184), all attempts to prepare aliphatic thioaldehydes have so far proved unsuccessful. However, monomeric thioformaldehyde has recently been generated at low... 

The dimer of trifluoromethylsulfine is obtained in the thermal generation of the sulfine from the anthracene adduct ". Also, the cyclic dimers of y-acetylenic-(3,(3-disubstituted thioaldehyde S -oxides are isolated in their generation from allenic sulfinates with vinyl... 

The thioaldehyde (1), associated with penicillin synthesis, has been trapped chemically, but not isolated. It undergoes polymerization or thioenolization,... 

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