Thioaldehydes, stability

Thioaldehydes stabilized by mesomeric effects include I,11 2,12 and 3.13 Corresponding selenoaldehydes, e.g., 4,11 have also been described. 

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). 

Ammo-5-thioformyluracils, e.g. (108a), have their thioaldehyde function stabilized by the amino group this may involve tautomerization to an imino-mercaptomethylene structure (108b). The reactivity of (108a) with enamines has been investigated.189... 

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... 

In the sulfur literature only a few examples of thioaldehyde S-oxides 99 (mono-substituted sulfines) have been reported due to the low stability of these compounds, and they were not prepared by oxidative methods since the starting thioaldehydes are also very unstable. These problems were circumvented by using silyl thioketones249,250 as precursors which, by controlled oxidation with mCPBA, afforded the corresponding thioacylsilane S-oxides 98. These sulfines were subsequently desilylated with BU4NF (equation 104) and the stereochemistry of this process has been studied in detail408. This indirect route has been applied to the preparation of aromatic and aliphatic, not enethi-olizable, thioaldehyde S-oxides. 

Partially and perfluorinated thioketones and thioaldehyde were stabilized as anthracene adducts (70). The adducts (70) were prepared in moderate yield from the corresponding carbonyl compounds with P4S10 or Lawesson s reagent in the presence of anthracene under toluene reflux. The generated thiocarbonyl compounds are not accessible in bulk due to their tendency towards polymerization. By thermolysis of the anthracene adducts (70) in the presence of C,N-bis(triisopropylsilyl)nitrilimine (NI), 1,3,4-thiadiazole derivatives (71) were obtained. Also, 1,3-dipolar cycloaddition with bis(trimethylstannyl)diazomethane (BTSD) to give consecutive products (72) from a 1,2-metallotropic migration of primary adducts was discussed. [95LA95]... 

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... 

Simple thioaldehydes and thioketones are too unstable to exist and attempts at their preparation lead to appalling smells (Chapter 1). The problem is the poor overlap between the 2sp2 orbital on carbon and the 3sp2 orbital on sulfur as well as the more or less equal electronegativities of the two elements. Stable thiocarbonyl compounds include dithioesters and thioamides where the extra conjugation of the oxygen or nitrogen atom helps to stabilize the weak C-S bond. 

In 1960, Woodward et al. obtained the first heterocyclic thioaldehyde (3) by reacting the hydrobromide of the corresponding /V-(ethylformimino) compound with H2S in the presence of base (60JA3800). This thioaldehyde (3) is stabilized by conjugation to N, and it was a famous precursor in the total synthesis of chlorophyll. 

Ab initio quantum chemical calculations concluded that HN=S is more stable than the isomeric structure HS=N and that thermal isomerization between the two species is unlikely (86IC4221 81PS325). It is now clear, both from experiment and from theory, that election-donating substituents attached to nitrogen stabilize the RN=S system, while electron-withdrawing substituents detabilize these compounds. This is the same effect observed for substituents attached to the carbon of thioaldehydes and selenoaldehydes. 

The thioketene dimetallic complex 493 reacts with H to give an anionic thioacyl complex (494). Protonation of494, by cleavage of a metal - carbon bond, gives a thioaldehyde complex (495) which is stabilized by PPhj... 

The size difference between carbon and sulfur atoms leads to relatively inefficient overlap of -tr-orbi-tals in the C=S bond. Consequently, thiocarbonyl compounds are in general highly reactive and have a tendency to di-, oligo- or poly-merize. This is particularly true for thioaldehydes, thioketones, and thio-ketenes. In contrast, thioamides (1) are usually perfectly stable and can be handled without problems. This stability can be understood in terms of a pronounced resonance interaction between the C =S TT-bond and the nonbonding electron pair on nitrogen. The analogous electron delocalization prevails in thiolactams. ... 

Electrophilic substitution or reaction via electron transfer at the methylene carbon gives a number of products with electrophiles. Note that the fulvenealdehyde (114) and -thioaldehyde (115) are stabilized as push-pull systems (Scheme 25) <88JOC3529,87TL5635,94POL675>. 

Reactions.—Owing to the low thermal stability of thioaldehydes obtained by flash thermolysis, their reactivity has not yet been investigated. However, one paper reports that thioformaldehyde can be trapped with cyclopentadiene to give Diels-Alder cycloadducts. ... 

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