Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of double-bonded functional groups / 1355 149

714 and 658 cm 1, respectively. The corresponding 5-amino derivative, 5-amino-1,3,4- thiadiazole-2-thiol (32b), present a very weak band at the typical S H stretching region (2526 cm 1), which is indicative that only a very small amount of the thiol tautomer is present. This is consistent with the observation of a band at 2923 cm 1, which was assigned to the NH stretching mode. The strong Raman and IR band at 765 cm 1 has been assigned to the C S stretching mode. Similarly, a Raman spectrum of 2,5-methyl-1,3,4- thiadiazole-5-thiol shows an extremely weak band at 2529 cm 1 corresponding again to the SH stretching, which indicates the predominance of the thione form. The weak bands in the 600 500 cm 1 region of the Raman and IR spectra were assigned to the C SH stretching mode136.

A complete analysis of the FT-Raman spectra of 1,3-dithiole-2-thione (37) and a series of related compounds were reported by Dyer and collaborators168. The bands at 3092 and 3075 cm 1 have strong sharp features corresponding to the CH stretch. The absorption at 1512 cm 1 was attributed to the CDC stretching. Although this is a rather low value for a typical CDC stretching vibration, the downshift in frequency was interpreted as being due to aromaticity in the ring168. The bands at 1077 and 1038 cm 1 were associated with the CDS stretching motion and its coupling to S C S motions. Depolarization ratio measurements show that the band observed at 512 cm 1 is highly polarized. This strongly suggests it to be associated with the ring-breathing motion.

H S

S

H S

(37)

Somogyi and coworkers169 have reported a FT-IR gas-phase spectrum of 4H-pyran-4- thione (38), which was interpreted using a general valence force field from ab initio calculations at the HF/4-21G level. The CDS stretching is assigned to a band at 1168 cm 1, although it seems clear that this mode contributes to different bands in the spectrum, namely 5, 6 and 9. The in-plane CDS bending is observed at quite a low frequency (300 cm 1), while the out-of-plane bending contributes to two b2 bands found at 685 and 415 cm 1.

S

O

(38)

In a paper by Grupce and collaborators170, the infrared spectra of protiated and partially deuterated thiosaccharin is reported in the N H, NDD and CDS stretching regions. Although, as is common for these kinds of compounds, the CDS stretching mixes in more or less proportion with other vibrational modes, at least the three bands observed at 1380, 1220 and 1040 cm 1 present significant contributions from the CDS stretch.

1396 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

The IR spectra of 2- and 4-thiouracils together with their N1and N3-methylated derivatives (27 29) and 2,4-dithiouracil in low-temperature inert matrices were obtained by Rostkowska’s group133 and the strongest and most characteristic bands related to CDO, CDS and CDC stretching modes discussed. For 2- and 4-thiouracil the CDO stretch appears at 1732 and 1752 cm 1, respectively. Methylation at N1 does not change the frequency of the CDO stretch for 2-thiouracil strongly. However, the CDO stretching modes of the methylated derivatives of 4-thiouracil are red-shifted with respect to the unsubstituted parent compound. It was also found that the CDC stretch, around 1650 cm 1, is very sensitive to sulfur substitution. For 2-thiouracil and all its derivatives the absorption in this region is weak, while for 4-thiouracil and all its derivatives it is as strong as the CDO stretch. This is explained133 in terms of the couplings between different modes. In 2-thiouracil the CDC stretch is coupled (out-of-phase) with the CDO stretch. However, when the S atom is substituted at the C4 position, as in 4-thiouracil, the CDC no longer couples with the CDS stretch.

There are also significant differences between 2- and 4-thiouracil in the 1500 1600 cm 1 spectral region, where the former presents relatively strong absorptions while for the latter only very weak bands are found. For all the thiouracils investigated the CDS stretch is located in the region 1100 1150 cm 1, except for the S-methylated derivatives. Strong absorptions which were assigned to out-of-plane displacements of the CDO groups were observed in the 750 800 cm 1.

Bigotto171 reported the IR and Raman spectrum of benzimidazol-2-thione (39), in the polycrystalline state and in the region from 4000 to 180 cm 1. The strong band observed at 3150 cm 1 was assigned to the NH stretching mode, while the C H stretching modes are observed at 3050 3070 cm 1. The skeletal displacements appear grouped as A1 and B2 fundamentals. The former are observed at 1624, 1501, 1462, 1279, 1234, 1199, 1020, 971, 819, 603 and 419 cm 1, while the latter are found at 1579, 1375, 1258, 1213, 614, 481 and 264 253 cm 1. The A1 band at 419 cm 1 is found to have a strong CDS stretching character. The bands at 970, 919, 850 and 741 cm 1 were assigned to the C H out-of-plane bending fundamentals, while the out-of-plane N H binding is observed at 705 cm 1.

H

N

S

N

H

(39)

The Raman and IR spectra of imidazoline-2-thione (40) were reported by Sathyanarayana and coworkers172. As for other thiocarbonyl derivatives the CDS stretching is coupled to other vibrational modes. In general, when, as in this case, the CDS group is attached to a strongly conjugated nitrogen atom, a large contribution is found for a band near 500 cm 1, due to extensive coupling of the vibrations and the contribution of the S CDNC mesomeric form. Thus for imidazoline-2-thione, similarly to imidazolidine-2- thione173 and thiazoline-2-thione174, the CDS stretching contributes to a medium intensity band at 520 cm 1 and to another medium intensity band at 910 cm 1. The CDS bending mode was assigned to a band at 340 cm 1, while the CDS out-of-plane bending was attributed to a weak band at 250 cm 1.

HN NH

S

(40)

Shurvell175 and collaborators have published the Raman and IR spectra of tetramethylcyclobutane-1-one-3-thione and the fully deuterated derivative. For this compound, the CDO stretching was observed as a very strong Fermi doublet at 1811 1782 cm 1. For the deuterated species this doublet is red-shifted and was found at 1808 1775 cm 1. The CDS stretching mode was assigned to a band of medium intensity at 1303 cm 1 and 1302 cm 1 in the IR and Raman spectra, respectively, and the same for the deuterated species are found at 1306 cm 1 and 1309 cm 1, respectively.

Ethylene dithione. This interesting molecule has been generated by means of Neutralization Reionization Mass Spectrometry (NRMS)176 by Sulzle¨ and Schwarz by flash pyrolysis, with subsequent isolation of the products in an argon matrix by the groups of Maier177 and of Wentrup178 as well as by Tesla coil discharges in Ar/CS2 mixtures (including various isotopomers of CS2) by Andrews and coworkers179. Although it had initially been suggested that in the ground state C2S2 is a singlet177, there seems to be increasing evidence that it is a triplet. The latter works177 179 provide IR data for this compound. The D1h symmetry of this molecule leaves one of three stretching vibrations (out-of-phase CS stretching) and one of the two degenerate bending vibrations as IR-

active. They have been reported at 1179.7 and 1904 cm 1177 . Further details on the structure and properties of this compound as well as on cognate cumulene-dithiones have been given by Sulzle¨180. Various papers176 179 report ab initio calculations on the stability, structure and vibrational spectrum of C2S2. They were extremely valuable for the purpose of establishing the structure and confirming the formation of this molecule.

1,2,3,4- pentatetraene-1,5-dithione (C5S2). This compound was obtained by Maier and coworkers181 by photolysis or flash pyrolysis of appropriate precursors. These authors succeeded in obtaining the IR spectrum of the argon matrix-isolated material. They detected the absorptions at 2105.0, 1687.9 and 783.5 cm 1, which were attributed, on the basis of PM3 calculations, to the three IR-active stretching modes 4 to 6 (assuming D1h symmetry).

Thioacetaldehyde. Maier and coworkers obtained this compound by flash pyrolysis of allyl ethyl sulfide in an argon matrix182. Its IR spectrum does not show a characteristic CDS stretching band. A comparison of the observed spectrum with the calculated one [the MP2/6-31G(d) level] allowed the identification of the molecule and further showed the extensive coupling between the normal CDS stretching mode and other normal modes.

5. Electron paramagnetic resonance spectroscopy (EPR)

Davies and Neville183 have reported the first EPR spectra of several radical anions of simple aliphatic thioaldehydes and thioketones in fluid solutions. They were generated (in the absence of tert-butyl peroxide) by photolyzing solutions of the appropriate potassium thiolates in tert-butyl alcohol. Coupling constants, a[n(˛-H)] and a[n(ˇ-H)], and g values were reported for RHCDS ž species (R D Me, Et, C7H15 and C6H5) as well as for (CH3)2CDS ž . These experiments suggest a significant spin delocalization from carbon onto sulfur, the effect of this on the a(C-˛) coupling constant being offset by some degree

1398 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

of pyramidalization at the carbon center. The authors further showed the great similarity existing between these EPR spectra and those for the homologous carbonyl radical anions. They also determined the spectrum of cyclohexanethione radical anion and analyzed it in connection with the time scale of the ring inversion. Placucci and coworkers184 have reported the EPR spectrum of the thioaldehyde radical anion H2CDCHCHDS ž 184 obtained under conditions similar to those used by Davies and Neville183, that is, the photolysis of the allyl thiolate, but in the presence of tert-butyl peroxide. This allowed these workers to obtain the spectra of both E and Z rotational isomers of this species. The origin of this isomerism can be traced back to the restricted rotation around the C CS bond. Neville and Davies could only detect one of the rotamers under their working conditions. Placucci and colleagues had already reported the EPR spectrum of the radical anion of thiobenzaldehyde, C6H5CHS ž (41), and 2,4,6-tri-tert-butylthiobenzaldehyde and deuterated derivatives185. This important study showed, inter alia, that: (i) Similarly to those of the radical anion of benzaldehyde, C6H5CHO ž (42), the a[ ˛ H ] values of 41 indicate that these corresponding to the pair of ortho hydrogens (positions 2 and 6) as well as to the pair of meta hydrogens (positions 3 and 5) are not equivalent. This suggested that the radical adopts a planar (or quasi-planar) conformation with a substantial barrier to the Ar CHS rotation. (ii) A comparison of the data for 41 and 42 shows that the aH values corresponding to the aromatic hydrogens in 41 are smaller than those of 42, in agreement with the observation that the CDS moiety is less able than the CDO one to delocalize the unpaired electron into the aromatic ring.

Alberti and coworkers have reported the EPR spectra of several adducts of group VI radicals with thioketones186.

(ii) series of metal chelates derived from 1,2-dithiol-3-thion-4,5-dithiolate (dmt)188. The equilibria between 2-ethoxycarbonylthiolane-3-thiones and their (Z)-enethiol tautomers were also studied by these techniques189. It was found that the equilibrium is largely shifted in favor of the enethiols. 1H NMR was also used to study the syn anti conformational equilibria of seven N-(1-methoxycarbonylethyl)- 4-thiazoline-2-thiones with S conformation of the chiral rotor (43)190. A variety of substituents were used.

(43)

To our knowledge, very few systematic studies of the NMR spectra of thiocarbonyl compounds have been carried out that allow a good comparison with the homologous

carbonyl derivatives. An important study was carried out by Barbarella, Bongini and coworkers191,192. They determined the 13C chemical shifts, υ13C , of the CDX carbon in series of compounds [(CH3)3C]2CDX (X D NH, O, S, Se)191. The values (in ppm relative to TMS) are, respectively, 193.5, 218.0, 278.0 and 292.5. These results agree with the fact that the C atom in CDS is deshielded by several tens of ppm with respect to that in the homologous carbonyls193,194. These authors further examined the problem using Pople’s expression for the local paramagnetic term p, considered to

be the dominant contributor to υ(13C)195. This treatment links this magnitude to E, the mean electronic excitation energy, that in this particular case is essentially determined by the energy of the n ! Ł transition. It was further calculated that this term is preponderant. E is then linked to max for the longest-wavelength absorption maximum in the UV-vis region. The experimental values for the same compounds (in nm) are as follows: 195, 298, 540 and 706. There is an excellent correlation between υ13C and max for these compounds193. This led the authors to conclude that for this family, the chemical shift variation is dominated by the energy factor. De Marco and Doddrell194 examined the effect of replacing a t-Bu by a (CH3)3Si group for both ketones and thioketones. This allowed one to quantify the influence of the electronegativity of the substituent and to confirm the existence of linear relationships between the υ13C values for carbonyl and thiocarbonyl compounds (see References 1 and 196). Sliwka and Liaanen-Jensen have synthesized and examined the spectroscopic properties of a set of carotenoid thiones197. They determined υ13C for the CDS groups of these molecules and their carbonyl homologues and found again adherence to a linear relationship.

B. Low-lying and Excited States

1. UV-visible spectroscopy

Over the last few years, several excellent reviews have been published on this subject. That of Clouthier and Moule100 is a comprehensive study of the information available on the optical spectroscopy of small carbonyls, thiocarbonyls and selenocarbonyls. Electronic transitions, their vibronic structures and the geometrical structures of various excited states were treated in detail. The case of thiones in solution was discussed by Steer and Ramamurthy11. Two major reviews were also published later on the photoreactivity14 and photophysics and photochemistry12 of thiocarbonyls. These topics are obviously related to the relative energies of the ground and excited states of these compounds. Schaumann1 provides a coverage of earlier work.

Both experimental and theoretical studies14,39 reveal that the sulfur p-type orbitals (ns) and the CDS orbitals are higher in energy when compared to those of the corresponding carbonyl compounds, whereas the CŁ DS orbital is lower in energy than that of the corresponding carbonyl group.

It is known1 that the long-wavelength band in the visible region (400 700 nm) can be attributed to a dipole forbidden n ! Ł transition. This band is appreciably redshifted with respect to the same transition in carbonyls. As indicated elsewhere14, owing to the strong overlap between the 1n ! Ł and 3n ! Ł transitions, it is often difficult to differentiate these transitions in thiocarbonyl compounds. High-intensity absorption bands at short wavelengths (UV) are observed that are attributed to the allowed ! Ł transition (S0 ! S2). In the case of thioketones, this transition generally occurs in the near-UV (see Table 14). The electron promotion is largely localized on the CDS moiety. These transitions are broad and exhibit poorly resolved vibrational structure, even

dAr matrix at 12 K, from Reference 177. eAr matrix, from Reference 179.

fIn MeCN, from Reference 177. gIn c-C6H12, from Reference 201.

The S1 energy surface is essentially built up from the 1 n0, Ł state. Very recently, multireference configuration interaction studies were performed on the singlet199 and triplet200 states of 5. The first study indicates that the S2 surface is composed of , Ł , , Ł , (n, 4s), and n0, Ł2 . The former three states have very similar equilibrium energies and are separated by low barriers. The diabatic 1, Ł state was shown to be planar, whereas 1, Ł and 1 n0, Ł2 are nonplanar. The 1, Ł state is considered to be the photoreactive state in S2 since it lies slightly lower in energy than , Ł , can be easily populated by internal conversion and has a low oscillator strength with the ground state. The latter study indicates that the 3, Ł state is found in the same energy region as S2. This is potentially relevant for the understanding of the photochemistry associated with the S2 state.

Janoschek has studied by ab initio techniques the UV spectra of various carbon suboxides and subsulfides201,202.

Steer and coworkers203 have performed electrochromism studies on pyranthione (4H- pyran-4-thione) (38) and xanthione (44). They found that the magnitude of the change in the dipole moment upon excitation, m D jmS2 mS0 j is close to 2 D. The transition dipole moment for the S0 ! S2 transition is parallel to the direction of the ground state dipole moment, i.e. along the C2 axes which contains the CDS bond.

1402 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

S

O

(44)

2. Photoelectron spectroscopy (PES)

This is the choice technique for probing the ground and low-lying electronic levels of molecules. The PES of sulfur-containing compounds was reviewed in 199118.

Some uses of this technique, in combination with mass spectroscopy, have been discussed earlier.

The combination of FVT with PES has allowed the study of several, highly reactive species. Thus, Pfister-Guillouzo and colleagues89 have succeeded in obtaining the various ionization potentials (IP) and assigning the various electronic levels of thioformyl cyanide. Their results (IP in eV and MOs) are as follows: 10.1 (ns), 11.8 CS CN , 12.8 CN0 ,

13.7 CN CS , 14.1 CS C CN and 15.2 CN C CS C nN . They mean inter alia that there is a sizable transfer of electron density from the CS to the CN groups, as indicated

by the 0.8 eV lowering of the ns orbital with respect to that of 5. Also, the CS C CN orbital is stabilized with respect to the CS and CN orbitals of 5 and HCN, while the out-of-phase combinations are barely affected. This situation can be compared to that prevailing in thioxoethanal, H C(DS)C(DO) H (45), also studied by the same group204.

This unique system allows one to examine directly the electronic interaction between a carbonyl and a thiocarbonyl group. The effect is appreciably smaller: the ns orbital is stabilized by some 0.34 eV with respect to 5 while the no orbital (of the CDO group) is practically unaffected (relative to 6). The cs orbital was slightly stabilized (0.21 eV) relative to 5 while the co orbital was moderately destabilized (0.4 eV) relative to 6. According to the authors, this is evidence of the opposing roles of field and resonance effects. Other species have also been examined by this combination of techniques205,206.

III. SYNTHESES

Sulfur plays a pivotal role in many transformations (chiral sulfoxide auxiliary, powerful electron-withdrawing sulfone substituent) and it is a very effective metal radical scavenger in the form of thiocarbonyl group. The radical chemistry associated with the thiocarbonyl group will not be discussed in this chapter but it has been reviewed recently207 213.

At the same time, thiocarbonyl-derived heterocycles are conveniently employed as key intermediates for the preparation of a plethora of both synthetic and natural products ranging from macrocyclic lactones to the powerful HIV-inhibitor TIBO214. Heterocyclic thiones are not included in this chapter. The interested reader is referred to a number of recent reviews215 222, while this chapter will focus on the synthesis and reactivity of thioaldehydes, thioketones, thioketenes and thioquinones. The material will be organized according to Schaumann’s review1 although a few new sections have been included. Only the major advances achieved since Schaumann’s publication shall be examined. The syntheses of thioaldehydes and thioketones have been the subject of recent reviews223 225, which include their use in C C bond-forming reactions226. Following Schaumann1 this section is organized according to the type of bond being formed and the reagent used. However, some new subsections (sulfines, reactions with organometallics, ene reactions) are arranged according to individual subjects.

As discussed in Section II, thiocarbonyl compounds differ from their carbonyl counterparts in at least two important characteristics. Because of the higher energy of the sulfur p orbitals, they are much more reactive as electron donors. On the other hand, the CDS bond is also much less polarized than the CDO bond, due to the smaller difference in electronegativities between carbon and sulfur. The latter fact leads to the reactions of the thiocarbonyl group being less selective than those of the carbonyl group. This happens, for instance, in the case of nucleophilic additions (see Section IV.C), and an enhanced reactivity against dipoles has also been observed (see Section IV.E.3).

In general, simple thioaldehydes such as thioacetaldehyde and thiobenzaldehyde are extremely reactive and immediately oligomerize. This considerably restricts their potential use in organic synthesis. However, thioaldehydes can be stabilized in several ways: by bulky substituents (But, Me3Si,. . .), -donor groups at the thiocarbonyl carbon (heterocyclic rings) or by coordination to a transition metal. The latter strategy has experienced a steady growth in recent years and we have added a new section on this particular subject (see Section III.K). In spite of their intrinsic instability, thioaldehydes can be prepared by many methods and used as transient intermediates, provided that the requisite reagent is present and compatible during the preparation. To achieve this purpose one of the most useful techniques has been Flash Vacuum Thermolysis (FVT)20.

Thioketones are more stable than thioaldehydes and Campaigne has given a personal account on the early development of their chemistry7. Although there are many methods to prepare thioketones, the simplest cyclic thione, cyclopropanethione (45), still remains unknown and it has been the subject of theoretical calculations91,227, Interestingly, the silicon analog dimethylsilathione (46) has been formed by thermolysis of silylthioketenes and characterized by analysis of the reaction products228. Very recently, kinetic stabilization with bulky substituents has allowed the isolation of pure silanethione (47) for the first time229.

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

1404 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

chemistry of thioketenes has been reviewed by Schaumann230. Based on the isolation of its dimerization product231, the phosphorus analog, phosphathioketene (48), has been claimed to be a reactive intermediate.

In spite of many synthetic efforts, thioquinones 49 still remain very elusive and only the anthraquinone member 51 has been isolated as a stable compound232. In this case, the reason for this is the higher stability of the cyclic forms, benzothietes, 50 (see Section II).

P C S

S

S

S

S

S

During the preparation of this manuscript two new reviews on thioaldehydes and thioketones233 and on thioketenes234 have appeared.

A. Formation of the a-Carbon Thiocarbonyl Bond

Conjugated ω-dimethylamino thioaldehydes have been formed in the reaction of aminals with thioacetoacetates (equation 15)235.

R = H, Me

Pentamethylcyclopentadienyllithium by treatment with thiophosgene affords a carbothioyl chloride which, in the presence of boron trifluoride-etherate, reacts with a stannane