Thioaldehydes from aldehydes

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... 

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... 

Segi et al. reacted some aldehydes with bis(trimethylsilyl) sulfide in THF with n-BuLi as catalyst. The thioaldehydes (41) thus formed were trapped with cyclopentadiene to afford adducts 42 as a mixture of endo and exo isomers (88JA1976). French workers have recently reported a gas-phase synthesis of thioxoethanal (44) from precursor (43) 44 was detected as an intermediate by photoelectron spectroscopy and by the formation of adduct 45 (90JOC2596). 

Essentially all of the work on [4 + 2] cycloadditions of selenocarbonyl compounds has been reported within the past five years.Krafft and coworkers have developed a novel method for producing seleno-aldehydes and -ketones, and have investigated in some detail the Diels-Alder chemistry of these species.Alkyl- and aryl-substituted selenocarbonyl compounds could be formed from silyl seleno-cyanates (198) (equation 105). As is the case with thioaldehydes, selenoaldehydes react with cy-clopentadiene to afford predominantly endo cycloadducts. This stereochemical preference has also been observed by Segi et using a different method for generating selenoaldehydes. 

A plausible intermediate for the enantio- and diastereoselective addition of diethylzinc to a-thioaldehyde catalyzed by 20o is shown in Fig. 3-4. When 20o ((—)-DFPE) is used in the ethylation of the racemic aldehyde 41, the (li)-enantiomer reacts faster than the (5)-enantiomer and the newly produced stereogenic center from (P)-41 has the S-configuration. This stereochemical property can be reasonably explained by considering the 7/6-fused bicyclic intermediate depicted. Compound (P)-41 has lower steric hindrance than (S)-41 in the reaction complex and ethylzinc preferentially attacks from the Si face of (7 )-41 to afford the S-configuration. Therefore, (i )-41 reacts faster than (S)-41 to afford (3S,4R)-42. 

It has long been known that 1,3-azoles can be assembled from a component providing the two heteroatoms, for example a thioamide or an amidine, and an a-haloketone, A more recent route employs the interaction between the anion of an isonitrile and an aldehyde, thioaldehyde or imine. 

Considering all of these observations, a possible mechanism for this interesting biochemical reaction is shown in Figure 26. Central to this proposed mechanism is the involvement of a persulfide (R-S-SH), a thioaldehyde and a protein-bound disulfide in the proposed reaction mechanism. Persulfides are known to be widely involved in the biosynthesis of many important sulfur containing metabolites. Here they function as a protein-bound nucleophilic sulfur that can add to an aldehyde and be reductively released from the protein to form the thioaldehyde (reactions 2-3, Figure 26). The persulfide could be formed by the transfer of hydrogen sulfide to a disulfide (reaction 1, Figure 26). 

Thionation of Aldehydes. Thioaldehydes are extremely labile compounds. One cannot, therefore, expect that LR wiU smoothly transform aldehydes into the corresponding thioanalogs. If, for instance, diphenylacetaldehyde reacts with LR the divinylsulfide (14) and the l,3,5-trithia-2-phosphacyclohexane (15) are formed instead of the thioaldehyde (eq 24). However, the corresponding enethiols, although terminal (aldo-) enethiols (16), can be obtained by use of LR via an indirect synthetic route (eq 25). Also the transient pentafluorothiobenzaldehyde (17) can be prepared with LR and trapped as the anthracene cycloadduct, from which it can be regenerated by pyrolysis (eq 26). ... 

The thioaldehydes (18) are obtained from the corresponding aldehydes with, LR. They dimerize spontaneously to yield 1,3-dithietanes, which extrude sulfur to give the interesting poly-methinetetrathiafulvalenes (19) (eq 27). ... 

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