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

1458 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

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 thiametallacycles 148509 (equation 156). Compound 146 was generated in situ and, after reaction with alkynes, gave rise to metallacycles 147 which, by treatment with SbCl3, afforded 148.

(156)

Finally, several complexes of chiral ˇ-thioxoketones have been prepared510 and the cobalt complex 149 has found an interesting synthetic application as catalyst in asymmetric cyclopropanation reactions (equation 157)511.

(157)

Chemical yields were acceptable (50 60%) but, notably, enantioselectivity was very good (75%). The ee increased to 97% when 1-octene was used as olefin.

I. Quantitative Aspects of the Basicity of Thiocarbonyl and Carbonyl

Compounds

The adiabatic ionization potentials of 5 and 6 are respectively 9.376 š 0.003 and 10.874 š 0.002 eV75. This helps explain why, in processes involving donor acceptor interactions such as CT512 516 and alkylation (formally similar to Menshutkin

reactions517)518,519, thiocarbonyl compounds are more reactive than their carbonyl homologues. Other factors come into play, notably the difference in the stabilities of the reaction products (in the case of the formation of covalent bonds), the size of the electronic ‘lone pairs’ in the case of hydrogen bonding and the polarizability of the heteroatoms in the case of electrostatic complexes with alkali metal ions116. These factors might lead to an inversion of the ranking of reactivities.

1. Proton and methyl cation basicities

The intrinsic basicity (i.e. the standard Gibbs energy change for reaction 158 in the gas phase) for a variety of compounds XC(DS)Y have been determined by means of Fourier

Transform lon Cyclotron Resonance Spectroscopy (FT ICR) by the groups of Abboud39 and of Gal520.

Table 16 summarizes these results together with data from other sources for the homologous carbonyl compounds. Other values are given elsewhere39. Figure 2 is a plot of GB (XCSY) vs GB (XCOY) obtained from the data given in Table 16.

As shown in this plot, the quality of the correlation is extremely good. The breadth of structural effects involved (72.1 and 59.1 kcal mol 1 for carbonyl and thiocarbonyl compounds, respectively) is possibly the largest ever reported for any linear free-energy relationship (LFER)521. It is known88 that, in the gas phase, carbonyl compounds protonate on the carbonyl oxygen. The LFER shown above very strongly suggests39 that the homologous thiocarbonyl compounds also have a constant basic center, namely the sulfur atom of the CS group. Theoretical calculations39 confirm this contention.

The slope of the correlation equation (ca 0.80) reflects the fact that differential substituent effects are 20% smaller in the thiocarbonyl series. This notwithstanding, thiocarbonyl compounds are consistently more basic than their carbonyl homologs over the entire range of reactivity examined in this work.

TABLE 16. Gas-phase basicities (GB) for thiocarbonyl and carbonyl compounds

aGB NH3 D 195.3.

bValues from Reference 39. cValues from Reference 520. dValues from Reference 536.

1460 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

FIGURE 2. Relationship between the experimental gas-phase basicities of thiocarbonyl and carbonyl compounds

Experimental evidence indicates that most ketones, esters, amides and ureas also protonate on the carbonyl oxygen when in acidic solution522,523 and the same holds for the homologous thiono compounds524,525. At variance with the gas-phase results, however, in the few instances in which a direct comparison of the pKa values of the corresponding conjugated acids can be carried out522,524 as in the case of the couples CH3CONH2/CH3CSNH2, C6H5CONH2/C6H5CSNH2 and ε-caprolactam/ε- thiocaprolactam, one finds that the tiocarbonyl compound is more basic by 1.5 2.0 pK units. This is likely a consequence of two facts: (i) the strong attenuation of polarizability effects in aqueous solution526 (pKa values are referred to a standard state of pure

water) and (ii) the poorer solvation of the protonated thiocarbonyl compounds in aqueous solution. This last point agrees with the hydrogen-bonding acidity of thiols being much smaller than that of alcohols527 as well as with the values of the experimental

solvation parameters, such as Olsen’s 528 or Marziano Cimino Passerini’s529,530 or Cox Yates531,532 mŁ . For the latter, it is found that mŁ (CS) > mŁ (CO). The reader is

referred to the important work by Bagno and Scorrano533 on the physical meaning and derivation of these parameters. These authors pointed out533 that, when the weaker base (as measured by the pKa of the conjugate base) has a larger mŁ value (as is the case here), the basicity gap narrows and eventually leads to a crossover as one moves from pure water to increasingly acidic solutions. This bridges smoothly the gap between the relative basicities of homologous carbonyl and thiocarbonyl compounds in the gas phase and in pure water. It is remarkable that, while it had been predicted39 that on the basis of the experimental data available in the gas phase and in solution for CH3CONH2 and CH3CSNH2 the crossover should take place in a 50% weight solution of H2SO4, Bagno and coworkers534 reported in an independent and parallel experimental study that the crossover for the cognate couple CH3CONMe2/CH3CSNMe2 takes place in a 48% solution of H2SO4. In Reference 39 the proton affinities PA (that is, the standard enthalpy changes for reaction 158) were computed theoretically at the MP2/6-31CG(d,p)//6-31G(d)CZPE level. The calculated and experimental values of the proton affinities displayed an excellent correlation. Furthermore, this allowed the estimation of the proton affinities for a large number of thiocarbonyl compounds HCSX for which experimental data were not available. A treatment of these results by means of the Taft Topsom88,535 method in terms of the ˛,F and RC parameters displayed excellent statistical quality and showed, as expected, that field and resonance effects oppose each other, as in the case of the carbonyl compounds88. As for the latter, polarizability contributions were found to be quite important.

The correlation portrayed in Figure 2 is deceiving, in that it suggests a great similarity between CDO and CDS compounds. Yet, O and S atoms widely differ in size and electronegativity1. This formal analogy originates in canonical structures such as 150b and 151b wherein the positive charge of the incoming proton is relayed to the sp2 carbon through O and S.

Alcami and coworkers116 have examined in detail the protonation of 5 and 6 and they found that the charge redistributions undergone upon protonation are very different. In the case of 6HC , the net atomic charge on the oxygen is practically the same as in 6. This is a consequence of the large electronegativity of oxygen: this atom overcomes the loss of 0.24 electrons with the gain of 0.24 electrons. In the case of 5HC , the more polarizable and less electronegative sulfur atom loses 0.7 electrons and recovers 0.18 electrons only. In the cases of substituted compounds, the protonated heteroatoms become

1462 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

again -electron deficient and polarize the p system(s) of the substituents, if available. This helps to understand the comparable (albeit somewhat reduced in the case of CS) stabilizing effect of potential electron-donor substituents in the two series of protonated compounds as well as the planarity of the amino groups in protonated amides/thioamides (a similar behavior has been reported for urea536). The -interactions mentioned in the case of the neutral species are enhanced in the case of the protonated forms. The possible CT from the substituent to the electron-deficient carbon is less favorable the greater is the electronegativity of the substituent. As a result, for monosubstituted, protonated CDO and CDS compounds, the ranking of substituent effects on basicity is NH2 > OH > F.

In the case of disubstitution, the ranking of substituent effects on the intrinsic basicities of CDS compounds, both measured or calculated39, is as follows:

Arbelot, Chanon and coworkers have reported518,519 kinetic data on the alkylation of

Representative values are given in Table 17.

A direct comparison of Gibbs energies of activation, G‡, and the corresponding gas-phase basicities is not yet possible, because their database is mostly built on cyclic compounds for which gas-phase data are not available. However, the trend of increasing

G‡ with substitution ˛ to the thiocarbonyl is exactly the same as decreasing intrinsic basicity given above. It has been shown537 that ‘extended’ Brønsted equations can be obtained that link the gas-phase basicities of N(sp2) and N(sp3) bases with their nucleophilicities towards MeI in MeCN solution at 25.0 °C. The above points at the likelihood of this being also the case here. These experimental facts and inferences derived therefrom are further supported by G2(MP2) calculations of the methyl cation affinities MeA (i.e.

These calculations show that: (i) the methyl affinities (MeAs) of CDS compounds are consistently much higher (for X D H, 15 to 25 kcal mol 1 over the range Y D F to Y D NH2) than those of the CDO homologs, and (ii) the sensitivity to substituent effects of the thiocarbonyls is ca 72% that of the carbonyls. Protonation and methylation therefore display the same pattern of structural effects (there is also a nearly perfect correlation between the PAs and MeAs for each family, although in all cases the PA exceeds the MeA by some 100 kcal mol 1).

Arbelot and Chanon518,519 have also reported a comprehensive series of semiempirical calculations that conclusively show the dependence of the kinetic reactivity in reaction 159 on the energy of the sulfur lone-pair orbitals.

1464 M. T. Molina, M. Ya´nez,˜ O. Mo,´ R. Notario and J.-L. M. Abboud

TABLE 18. Equilibrium constants Kc (in l mol 1) for the formation of 1 : 1 CT complexes between I2 and selected thiocarbonyl compounds in n-heptane at 25.0 °C539

good degree of precision. In this case, and at variance with protonation, the sensitivity of the stability constant to -electron donation by the substituents is ca 60% higher for CDS than for CDO. This should follow from the increased electron demand in the case of the CDS. . .IDI complexes in which the extent of actual CT is more important.

UV-visible spectroscopic data for these systems also display a number of remarkable features. Thus, in the case of bulky thioamides and tetrasubstituted ureas, there are indications of the existence of two different 1 : 1 complexes. Their possible structures are presently being studied by means of high level ab initio calculations540.

Freeman, Po and coworkers541 have determined the X-ray structures of the crystalline 1 : 1 CT complexes of I2 with imidazole-2-thione (152), 1-methylimidazole-2-thione and 1,3-dimethylimidazole-2-thione. Devillanova and coworkers542,543 have carried out similar studies on 1 : 1 complexes involving 5,5-dimethylimidazoline-2,4-dithione (153) and 5,5-dimethyl-2-oxoimidazolidin-4-one. In all cases, the S. . .I I arrangement is essentially linear. The angles CSI are in the range 90 100°. The stability of the complexes of 152 and its derivatives is much higher than that of 153 as shown by the increase and decrease of the C S and S. . .I bond lengths, respectively being more important in 152 and its derivatives. The thermodynamic stability of the complex of 153 substantially increases on going from CCl4 to CH2Cl2 solutions. This confirms the substantial degree of CT involved in these complexes.

3. Hydrogen-bonding (HB) interactions

The diffusiveness of the electronic charge in the ‘lone pairs’ of sulfur leads to CDS compounds being substantially weaker HB bases than the homologous CDO derivatives. A number of cases in which the CDS group acts as a HB basic site in intramolecular interactions have been described earlier. The following deals with 1 : 1 intermolecular associations.

The problem of the quantitative ranking of HB basicities was treated simultaneously and independently by groups of Taft544 and of Abboud545. Both took as a starting point the formation constants Kc pertaining to the 1 : 1 association between HB bases B and HB donors (‘acids’) H A in ‘inert’ solvents such as cyclohexane and CCl4 (reaction 161):

Taft used log10 Kc for H A D 4-fluorophenol to define the pKHB scale of basicities through pKHB D log10 Kc. Abboud and Bellon used their own experimental data to define parameters quantitatively describing both HB acidity and basicity, log10 Kc being given by a bilinear form of these descriptors. Later on, this approach was applied by Abraham,