Trapping of thioaldehydes

Several accounts of transient thioaldehydes generated from high-energy processes and detected spectroscopically or trapped as Diels-Alder adducts have appeared. Dice and Steer obtained thioformaldehyde from retro [2 + 2]-fragmentation by photolysis, and subsequent trapping afforded (11) (74CJC3518). This is the only report, before the work of Vedejs, that describes intermolecular Diels-Alder trapping of thioaldehydes. However the method is not preparatively useful. 

Vedejs Trapping of thioaldehyde by both Diels-Alder and ene reactions [88JOC2220]... 

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... 

Full details98 have been published on the use of hexamethyldisilathiane in the generation of thioaldehydes and thioketones, which can be trapped in situ (equation 23). The... 

Analogous chemistry was reported [215] with thioaldehyde intermediates generated from a non-classical thermal reaction of a sulfoxide via a sulfe-nate ester (Table 3, entry 2). The reaction of cyclic tetra- or pentasulfides with stabilised phosphoranes is also a good source of thioaldehydes [216], trapped by dienes (Table 3, entry 2). 

Vedejs et al. have used a versatile photochemically initiated fragmentation of a-thioacetophenone derivatives to generate a variety of thioaldehydes which have been trapped in situ by substituted 1,3-dienes [Eq. (3)].2,17... 

Kirby et al. have recently described some new fragmentation methods for generation of thioaldehydes and their subsequent trapping by conjugated dienes.18-21 One approach involves base-promoted elimination of HC1 from a sulfenyl chloride [Eq. (4)].18... 

An interesting method for preparation and trapping of alkyl and aryl thioaldehydes was recently disclosed.23 Fluoride-induced fragmentation of a-silyldisulfides such as 8 gave thioaldehydes which were trapped in situ by cyclopentadiene in high yields (Scheme 5-V). In all these systems, endo adducts were the predominant stereoisomers. 

Figure 9.5. Trapping of nucleophiles, here amines, with photogenerated thioaldehydes and selected examples of ToFSIMS images evidencing the successful spatially resolved surface grafting. The underpinning phenacyl sulfide ligation reaction can be found in Figure 9.1 b) (bottom reaction pathway). Reproduced with kind permission from the Royal Society of Chemistry from [PAU 13a]. For a color version of this figure, see WWW. iste. CO. uk/lalevee/dye.zip...

PAU 13a] Pauloehrl T., Welle A., Oehlenschlaeger K.K. et al, Spatially controlled smface immobilization of nucleophiles via trapping of photogenerated thioaldehydes , Chemical Science, vol. 4, pp. 3503-3507, 2013. 

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. ... 

Recyclization of (29) to the thiazole (30) is accountable in terms of intramolecular trapping of the intermediate thioaldehyde, itself a possible biogenetic precursor of penicillins, by an amide side-chain. 

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. 

Reaction of aliphatic aldehydes with H2S and TCS 14 at room temperature in the presence of pyridine leads to the adducts 596, which can be converted by NaH in DMF, via the adducts 597 and the intermediate thioaldehydes 598, into their sodium salts 599. These sodium salts 599 can be trapped by alkyl- or aUyl hahdes in up to 91% yield to give the vinyl sulfides 600 [147] (Scheme 5.47). 

Free intermediate thioaldehydes 598 or 602 and the selenoaldehydes 605 and HMDSO 7 are obtained in THF at 0°C on treatment of aliphatic and aromatic aldehydes with bis(trimethylsilyl)thiane 601 or bis(trimethylsilyl)selenide 604 in the presence of traces of butyllithium, while trapping the sensitive intermediate thio- or selenoaldehydes 602 and 605 with cyclopentadiene or cyclohexadiene to furnish mixtures of endo and exo Diels-Alder adducts such as 603 a and 606 a and 603 b and 603 b [148-150], the exo/endo ratio of which can be controlled [150] (Scheme 5.48). Analogous reaction of ketones such as 2-adamantanone or acetylene ketones with MesSiXSiMes 608 (a. X=S (601) b. X=Se (604)) in the presence of... 

The high reactivity of the C=S bond toward diazo dipoles was also helpful in the characterization of thiopivaldehyde, a monomeric thioaldehyde persistent in inert organic solvents for 16 h at 20 °C. This thioaldehyde underwent ready cycloaddition with ethyl diazoacetate (210). Similarly, the diazocumulene 9-(diazomethylene)fluorene could be generated by diazotization of 9-(aminomethy-lene) fluorene and was trapped by the cycloaddition with thiobenzophenone (211). 

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