Thiocarbonyl shift

A linear relation (eq. (4.11)) permits prediction of thiocarbonyl shifts from the C = O shift values of the parent carbonyl compounds [330]. 

If one assumes k is at unity, i.e. constant change in force constants upon adduct formation, the carbonyl frequency shift upon adduct formation should be 0.865 of the thiocarbonyl shift for the formation of the same adduct. If k is assumed to be as large as 1.47, that is, equal to the ratio of the frequencies, vco/vcs- then the ratio in equation 32 becomes 1.27. A comparison of changes in stretching frequencies upon adduct formation is given in Table 22. 

Steric overcrowding associated with the interaction betw een the thiocarbonyl group and a bulky alkyl group gives a bathochromic shift. This has been interpretated as evidence for a smaller thiocarbonyl group" in the first excited state (73). 

Methods for the A-acylation of similar heterocycles, such as simple thiazolidinethiones, have been reported since 1977, namely acyl chlorides in miscellaneous conditions,586 or carboxylic acids under DCC-activation.60,61 However the easiest and most effective method involves acyl chlorides or carboxylic anhydrides in the presence of an amine.47 Applying that procedure on carbohydrate scaffolds Rollin and co-workers62 reported the synthesis of diverse /V-acylated OZTs. The reactions were performed with good yields and the /V-selective acylation was ascertained by NMR— namely the thiocarbonyl 13C chemical shift (Scheme 41). Thanks to the dual nature of the carbanion drifting in the reaction,596 60 no competitive formation of the thioester, as mentioned by Plusquellec el al. in the case of benzothiazole, was observed. 

In some cases involving aliphatic thiocarbonyl ylides, a 1,4-H shift occurs to afford vinyl thioethers. As an example, thiocarbonyl ylides (41), generated by the addition of isopropylidene carbene to aliphatic thioketones, are converted to divinylthioethers (42) (84) (Scheme 5.16). 

In the absence of a dipolarophile, thiocarbonyl ylide 84 undergoes a 1,4-hydrogen shift to give the naphthoannelated thiophene derivative 85. Many examples of related syntheses have been reported (139-143). The photolysis of tetraarylthiiranes in the presence of tetracyanoethene represents an approach to tetrahydrothiophenes via a SET mechanism (75,76). 

In so far as the decrease in chemical reactivity is an indication of diminished transition metal basicity, it was proposed143 that the thiocarbonyl complex is less basic than its carbonyl analogue. This conclusion is substantiated by the spectral shifts in Table 22 and is also in agreement with molecular orbital calculations which predict the thiocarbonyl complex to be less basic than the carbonyl complex155,156. 

Whereas coordination of a CA to a lone electron pair of a heteroatom containing only single bonds typically results in a hypsochromic shift of the CA absorption maxima, upon complexation of germylenes with S atoms of thiocarbonyl compounds (and also... 

Thiocarbonyl oxides are a subject of active investigation. The natural occurrence of sulfines and related compounds in plants of the genus Allium (onion, garlic, etc.) is included in a superb and extensive review by Block [91]. Two detailed papers [92, 93] report the isolation of zwiebelanes from onions and their chemical synthesis involving intermediate sulfines produced by oxidation of di-l-propenyl disulfide, subsequent sulfoxide accelerated [3.3] sigmatropic shift and the [2+2] cycloaddition of the C=S and C=S=0 moieties. A further article [94] provides a great deal of information on the mechanism of formation of (Z)-propanethial S-oxide, the lachrymatory factor of the onion, as well as its chemical synthesis and reactions. Techniques of analysis of the volatiles of onions have been further improved [95]. 

A new class of nucleophiles have been introduced for sulfur addition. Degl Innocenti and his group [145, 146] have shown that allyl or benzylsi-lanes, in the presence of tetra-n-butylammonium fluoride, reacted in a thiophilic fashion and led to allyl sulfides or dithioacetals. It is remarkable that this selective reaction is general for a large variety of thiocarbonyl compounds thioketones [145], dithioesters [146], and even with the normally sluggish trithiocarbonates [145]. With substituted allyl silanes retention of configuration of the allyl chain is observed. It is noteworthy that the possible [2,3] sigmatropic shift of the intermediate anionic species was not observed. 

An outstanding comparison was made with the best known dipolarophi-le, dimethyl acetylenedicarboxylate (DMAD). The cycloaddition of N-methyl-C-phenylnitrone to adamantanethione is 1500 times faster than the addition to DMAD. As indicated in Scheme 74 the reaction is an equilibrium, leading to 56 44 ratio of product vs starting material. This equilibrium is shifted towards the left side for thiobenzophenones, as the cycloadduct formation breaks the conjugation, and thus these thiones do not appear to react with nitrones. Ab initio calculations were carried out to model the high reactivity of nitrones with thiocarbonyl compounds [257]. 

The alkylation of (21) with phenacyl or p-bromophenacyl bromide to yield the 1,3-dithiolylium salts (171) offers a more special case. The methylene group of these salts can be deprotonated to form the unstable thiocarbonyl ylides (172) which collapse to the non-isolatable episulfides (175). Further reaction of (175) by loss of sulfur produces (174) whereas episulfide ring-opening and hydrogen shift followed by air oxidation leads to the disulfide (173) (80BCJ2281). 

The kinetics of the xanthation of sucrose were studied in the same year by Cherkasskaya, Pakshver, and Kargin, who determined potentiometric-ally the concentrations of the dithiocarbonate derivative and also of inorganic sulfide and trithiocarbonate. The rate of formation of 0-(sodium thiol-thiocarbonyl)sucrose was found to pass through a maximum with increasing alkali concentration, presumably due to a shift of the equilibrium in favor of side reactions in strongly alkaline solution. This result appears to parallel the qualitative findings of Lieser and Hackl for polysaccharides. 

Transformation of a thione into a carbonyl group [Eq. (1)] is today a standard technique in sulfur chemistry and generally proceeds in very good yields with widely different classes of compounds. Efficient methods are based on hydrolysis, alcoholysis, or oxidation. Hydrolysis involves nucleophilic attack at the thiocarbonyl group, followed by elimination of H2S [Eq. (2)]. Factors tending to shift the equilibrium... 

The CD spectrum of the thiolactam (R)-5-methylpyrrolidine-2-thione reveals a pronounced solvent dependence [490], Its long-wavelength n k thiocarbonyl CD band at Imax = 326 nm (in cyclohexane) undergoes a large solvent- and concentration-dependent blue shift of Av = +4880 cm (A1 = —54 nm) on going from cyclohexane to water. This corresponds to the blue shifts usually observed for n n bands in UV absorption spectra (see Section 6.2.3) and is best explained in terms of a monomer dimer equilibrium of the thiolactam involving the formation of intermolecular solute/ solute hydrogen bonds in nonpolar solvents. In HBA solvents e.g. DMSO) and HBD solvents [e.g. EtOH), this equilibrium is disturbed by competitive solute/solvent H-... 

---