Thioaldehyde compounds

Selenoaldehydes 104, like thioaldehydes, have also been generated in situ from acetals and then directly trapped with dienes, thus offering a useful one-pot procedure for preparing cyclic seleno-compounds [103,104], The construction of a carbon-selenium double bond was achieved by reacting acetal derivatives with dimethylaluminum selenide (Equation 2.30). Cycloadditions of seleno aldehydes occur even at 0 °C. In these reactions, however, the carbon-selenium bond formed by the nucleophilic attack of the electronegative selenium atom in 105 to the aluminum-coordinated acetal carbon, may require a high reaction temperature [103], The cycloaddition with cyclopentadiene preferentially gave the kinetically favorable endo isomer. 

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... 

On transforming the cyclopentenone 1431 into the trimethylsilyloxy diene 1432 the ensuing Diels-Alder cyclization gives rise to 69% tricychc compound 1433 [10] (Scheme 9.8). For Diels-Alder-reactions of thioaldehydes, selenoaldehydes, or unsaturated nitroso compounds with cyclopentadiene, see the reactions of 602 to 603 and of 605 to 606 in Scheme 5.48 and of 1092 to 1093 in Scheme 7.43. For Diels-Alder-reactions of silyloxyazadienes such as 510 with maleic anhydride to give 511, see Scheme 5.29. 

A nice example of a degenerate rearrangement has been reported for the complex pentacarbonyltungsten(0)-thioaldehyde-l,2-dithiol 177 (R = H) or the thioketone-1,2-dithiol complex 177 (R = CH3). These compounds (obtained by the reaction of silver nitrate with tetraethylammonium pentacar-bonyliodotungstate(O) in the presence of l,6,6 z -trithiapentalenes) appeared to exist as an equilibrium mixture of the two isomers 177/178 (Scheme IV.70) [83JCS(CC)289]. The symmetrically substituted 1,2-dithiol (R = CH3, R = H) displays fluxional behavior as appears by NMR spectroscopy... 

Higher thioaldehydes seem to have been overlooked. Thioketones, however, have received much attention. Work in this area has involved principally thio-acetone. This compound polymerizes spontaneously and rapidly, though it is not as active as thioformaldehyde. Some work has also appeared on higher... 

Thiocarbonyl compounds can be generated thermally in the gas-phase from a variety of precursors. Bock et al. [77JCS(CC)287 82CB492] have shown that pyrolysis of 1,2,4-trithiolane derivatives (81) is especially advantageous for this purpose. The fragmentation reactions were monitored by PE spectroscopy. By this method thioaldehydes including thioformaldehyde and thioketones that polymerize readily were obtained. 1,2,4-Trithiolane 4-oxide (82) yields a mixture of thioformaldehyde and thioformaldehyde oxide. 

Usually these heteroaldehydes and heteroketones are generated in solution in the presence of suitable substrates and immediately trapped. The synthesis, properties, and reactivity of thioaldehydes and thioketones have been the subject of several reviews.1-5 The chemistry of selenocarbonyl compounds has likewise been reviewed in recent years.6-9... 

The thioaldehyde complexes 81a,b were formed from the thiolato complexes 80a,b on treatment with PMe3 (Scheme 21). The thiolato compounds were obtained from 79 and MeCH2SH or PhCH2SH. Although the complexes 81a,b were stable even in refluxing benzene for several hours, they... 

Complexes with a bridging vinylidene ligand were also used as the starting compound for the synthesis of thioaldehyde complexes. Sequential treatment of 104 with cyclohexene sulfide, HBF4, and Li[BHEt3] gave the thioaldehyde complexes 105 [Eq. (24)].209... 

However, the reaction of the binuclear rj2-thioaldehyde titanocene complexes 112 (see Scheme 26) with valeronitrile, benzonitrile, and methyl thiocyanate proceeded in a similar fashion (Scheme 39).69 Related metalla-cyclic compounds were also obtained from the reactions of 112 with benzo-phenone (Scheme 39), various imines, phenyl isothiocyanate, or dicyclohexyl carbodiimide via insertion of the C = 0 and C = N bond, respectively, into the Ti-C bond.69,70... 

This monograph covers the period 1989-1995 and is partially an update of the work by Schaumann1. The scope is essentially the same, attention being focused primarily on thioaldehydes (thials) and thioketones (thiones). In keeping with the spirit of Reference 1, only some new developments in the field of thioketene, hetero cumulene and thioquinone chemistry are considered. Because of the fast evolution in the field, the spirit is somewhat different and physicochemical and theoretical topics are emphasized. Whenever possible, systematic comparisons with homologous carbonyl compounds, a natural yardstick, are carried out. 

The problem of the synthesis of thioaldehydes has been partly circumvented in recent years by using silyl thioketones as their synthetic equivalents. This clever methodology was introduced by Italian chemists and they have successfully exploited many different applications of these compounds, which have been reviewed by Bonini249. In this... 

Retro-Diels-Alder reactions giving thiocarbonyl compounds are favored when simultaneously a comparatively stable diene is formed1. This is the case with anthracene and cyclopentadiene Diels-Alder adducts 81 and 82 which, upon heating, afford a wide array of thioaldehydes and thioketones (equation 84). These adducts are stable at room temperature and have become a convenient way of storing very reactive thiocarbonyl compounds. Cyclopentadiene is the cheapest and most reactive diene for use in Diels-Alder reactions. Also, strain in the bridged cycloadducts facilitates retro-Diels-Alder cleavage224. 

Thermal cycloreversion of the adducts can be accomplished at a convenient rate when heated in toluene under reflux. If a new diene is present in the reaction mixture, the thioaldehyde thus generated in the retro-Diels-Alder reaction may give a new adduct. Therefore, adducts 81 and 82 act as thioaldehyde or thioketone transfer reagents. These adducts dissociate reversibly on heating, thus ensuring that the concentration of the labile species remains very low. For this reason, polymerization is not a serious problem especially in the case of thioaldehydes224. The transient thiocarbonyl compounds can be trapped not only by dienes but also by 1,3-dipolar cycloadditions332 (equation 85). 

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. 

Organosilicon compounds have also been reacted with thiocarbonyl ylides to afford a variety of heterocycles468,469 and in Section III.F.5 we pointed out the 1,3-dipolar cycloaddition between in situ generated thioaldehydes and a pyrazolidinium ylide to produce a nuclear analogue of pyrazolidinone antibacterial agents332. Moreover, Vedejs has applied the dipolar cycloaddition between a thione, generated from a Norrish-type II fragmenta-... 

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. 

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]... 

Among the compounds reported in this article, thioaldehydes are the least stable. They will not be reported in detail here as a recent series includes an up-to-date review by Okazaki [4]. 

A novel synthesis of a-unsaturated sulfines has been introduced by Bra-verman et al. [99]. Et3N or DABCO treatment of allylic and benzylic tri-chloromethyl sulfoxides triggered the elimination of chloroform and formation of the sulfines. It must be stressed that these sulfines are thermally relatively stable, and this stands in high contrast to the corresponding thio-carbonyl compounds unsaturated thioaldehydes cannot be monitored under the same experimental conditions and have to be used at very low temperature or trapped in situ. The first synthesis of thioacrolein S-oxide was achieved by flash vacuum thermolysis of an anthracene allyl sulfoxide [100], and both isomers in a (Z E) ratio of 78 22 were characterised by NMR spectroscopy at -60 °C. 

Exceptions to the carbophilic addition of heteronucleophiles to thiocarbonyl compounds were demonstrated with thiols and thioaldehydes [160] or dithioesters [36] bearing an a-electron withdrawing group, which undergo selective thiophilic addition. 

Heating these products (endo and exo mixtures) was also a source of thiocarbonyl compounds, by cycloreversion [213], and led, in the presence of a diene, to different adducts (exo rich equilibrium mixtures). These reactions were applied in the field of morpholine alkaloids and thioshikimic acid synthesis [5]. Another effective source of thioaldehydes are sulfenyl phthalimides [214], which are very prone to elimination, as described hereunder in the work of Capozzi. 

Thioaldehyde S-oxides behave as dienophiles with 1,3-dienes [241]. Cycloaddition involves the carbon-sulfur double bond of the heterocumulene to furnish dihydro-2H-thiapyran S-oxides (Table 6, entry 1). The cis trans product ratio was found to depend upon the initial diene/sulfine ratio. A large excess of diene selectively provided the cis cycloadducts, as a result of a stereospecific reaction of the (Z) sulfine. With a produced proportion of diene, mixtures of cis trans isomers are obtained, with a preference for the cis compound. The Italian authors have demonstrated that the slower reaction allows a (Z) to (E) isomerisation of the sulfines prior to addition. 

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