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A Theoretical Study of the Limits of the Acidity of Carbon Acids in Phase Transfer Catalysis in Water and in Liquid Ammonia

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A Theoretical Study of the Limits of the Acidity of Carbon Acids in Phase Transfer Catalysis in Water and in Liquid Ammonia Cent. Eur. J. Chem. • 11(11) • 2013 • 1711-1722 DOI: 10.2478/s11532-013-0311-7 Central European Journal of Chemistry A theoretical study of the limits of the acidity of carbon acids in phase transfer catalysis in water and in liquid ammonia# Invited Paper Ibon Alkorta*, José Elguero, Roger Gallo† Institute of Medical Chemistry, CSIC, E-28006 Madrid, Spain Received 9 May 2013; Accepted 6 July 2013 Abstract: The acidities of a large number of carbon acids have been theoretically calculated for the gas-phase and for DMSO solution. The gas-phase values, both DH and DG, are very well correlated with the available experimental data. From the calculated DG values in DMSO and the pKas in the same solvent, a homogeneous set of pKa (DMSO) values was devised that was used to generate pKa (water). These last pKas were used to establish the limits of the acidity of carbon acids for reactions under PTC conditions both alkylations and H/D exchange. A step further led to the pKas in liquid ammonia and from them to the virtual use of PTC using liquid ammonia instead of water. Keywords: PTC • Liquid ammonia • Carbon acids • pKa • DFT calculations © Versita Sp. z o.o. by their pK s. We have selected the C-alkylation 1. Introduction a reactions but we will also discuss another class of Few synthetic procedures have had a greater impact in reactions (deuteration, isomerization and oxidation) that preparative chemistry than[ Phase Transfer Catalysis occur at higher pKa values (less acid) than the reactions (PTC) [1-4]. Let us remind that the most used anionic of C-substitution. For this purpose, we have used nucleophiles in PTC conditions are those generated by extensively the discussion reported in Chapter 8 of the deprotonation of organic compounds having an acidic book of Starks, Liotta and Halpern [1]. C-H bond (carbon acids) because the reaction with The paper is organized in five sections: electrophilic carbon derivatives is used to build up the 1. DFT calculations of the gas-phase acidity (ΔG) will carbon skeleton of many organic molecules. be carried out on a set of 18 molecules whose pKas in Today, PTC together with ultra sounds and water and in DMSO are known (thanks to the work of microwaves activation, constitute three of the most useful Bordwell about 1200 compounds were measured in this approaches in the chemistry arsenal. PTC owes much to solvent while values in water are much rare) [20]. Mąkosza contributions that cover from 1965 [5-7] till very 2. From these comparisons, we will estimate the recently [8,9]. Although many modifications have been pKas in water of all the molecules needed in the following introduced to PTC, except for solvent-free PTC (with or section. without MW irradiation) [10,11], water is used in all other 3. From literature results we will establish the limits variants, triphasic [12-14], supercritical CO2 [15-18], of PTC in water in function of the pKas of the substrates fluorocarbons [19], etc. (carbon acids). The present work deals with a computational study 4. Discuss the pKas in liquid ammonia based on the of the possibility to replace water by liquid ammonia. We works of Lagowski [21] and Page et al. [22]. will examine the limits of generating carbanions in water 5. Examine the possibility to carry PTC in liquid and its related topic of carbon acids acidity as measured ammonia. * E-mail: [email protected] #This paper is dedicated to Prof. Mieczysław Mąkosza the discoverer of PTC 1711 dress: 736 La Salle, Impasse Jean de la Fontaine, 13320 Bouc Bel Air, France A theoretical study of the limits of the acidity of carbon acids in phase transfer catalysis in water and in liquid ammonia Figure 1. The 27 molecules discussed in this part of the manuscript. 2. Computational details The agreement between experimental and calculated gas-phase data is good enough to identify the site of The calculated values of ΔH and ΔG correspond deprotonation of 1: the CH2 not the CH3. The experimental to B3LYP/6-311++G(d,p) [23-26] and PCM [27-29] values are all from the NIST data-base [31]. (DMSO)/B3LYP/6-311++G(d,p) including DIS (Solute- From Table 1 the following equations are obtained solvent dispersion interaction energy), REP (Solute- (the RMS residuals are also given): solvent dispersion repulsion energy) and CAV (Solute cavitation energy) calculations, respectively. All the ΔG calc. = –(3.4±8.9) + (1.003±0.006) calculations were done using the Gaussian 09 package ΔH calc., n = 37, R2 = 0.999, RMS = 2.9 (1) [30]. ΔH calc. = –(133±54) + (1.08±0.04) 2 3. Results and discussion ΔH exp., n = 23, R = 0.977, RMS = 12.8 (2) ΔG calc. = –(123±43) + (1.09±0.34) 2 3.1. Comparison of pKas in water and DMSO ΔG exp., n = 23, R = 0.986, RMS = 10.3 (3) with calculated acidities in DMSO (ΔG) The 27 molecules chosen for discussion of the Note that our values are the differences in energy boundaries of PTC in water are represented in Fig. 1. between the neutral and the protonated forms without Seventeen supplementary molecules used to establish any correction for the H+ difference. The purpose was linear relationships are reported in Fig. 2. to verify that our computed values represent adequately The numerical data are reported in Table 1 (gas the experimental values, i.e., that they are proportional. phase) and Table 2 (water and DMSO solutions). The Many authors have described that the relationships calculated values (see Computational details) of ΔH between experimental and calculated thermodynamic and ΔG correspond to B3LYP/6-311++G(d,p) and quantities related to the present question, for instance, PCM(DMSO)/B3LYP/6-311++G(d,p), respectively. carbanions, are generally excellent [32-35]. 1712 I. Alkorta, J. Elguero, R. Gallo Figure 2. The 17 supplementary molecules used for establishing linear relationships. Combining the data of Tables 2 and 3 leads to the following equations pKa DMSO = (–172±8) + (0.154±0.007) ΔG DMSO, n = 36, R2 = 0.950, RMS = 2.4 (5) Fitted values of Eq. 5 (fourth column) have been used to calculate Eq. 6: pK water = (–4.2±1.3) + (1.01±0.06) a 2 pKa DMSO, n = 25, R = 0.929, RMS = 3.0 (6) The last column of Table 3 contains the fitted values of Eq. 6 and the last seven values of column three (no calculations in the case of 4, 5, 7, 9, 10, 14 and 24). These fitted values are not very different from the pKa values measured in water (see Fig. 3), but they are more robust having been calculated using the very Figure 3. Plot of the experimental values of pKa in water against the reliable gas-phase values. fitted values of the last column of Table 3. Deviations larger than 4 pKa units correspond to There is a Table in reference [47] with several pKas compounds 32 (malonodinitrile, +5.7), 27 (toluene, in DMSO that, when common, are very similar to those +5.2), 16 (indene, +4.7) and 31 (2-nitropropane, –6.0). of Table 2. There is a lot of confusion in the secondary Compound 33 (methylmalono-dinitrile) also deviates like literature about the solvent used to determine the pKas, 32 but in this case the experimental value corresponds often the values are given without indicating the solvent. to benzyl-malonodinitrile. Thus, for instance, Starks, Liotta and Halpern [1] do not The two malonodinitriles 32 and 33 can be related explicitly write that their pKa values are in DMSO, but to the following comment by Pagani et al. [48]: “We are they are identical to those of Bordwell. Eq. 4 is obtained also convinced that cyanocarbanions are reluctant to from the data of Table 2: hydrogen bond. Proof for this is provided by the almost identical pKa values of malonodinitrile in water (pKa = pKa water = (–3.2±1.1) + (0.96±0.05) 11.41) [49] and in DMS0 (pKa= 11.0) [20], a behavior 2 pKa DMSO, n = 25, R = 0.941, RMS = 2.7 (4) which is dramatically different from that of comparably 1713 A theoretical study of the limits of the acidity of carbon acids in phase transfer catalysis in water and in liquid ammonia Table 1. Calculated and experimental values in the gas phase (all in kJ mol–1). No Compound ΔH calc. ΔG calc ΔH exp. ΔG exp. 1 Methylbenzylketone (CH2) 1444.2 1449.8 1465.0 1441.0 2 Acetylacetone 1407.6 1406.0 1438.0 1409.0 3 Acetophenone 1507.4 1508.4 1512.0 1487.0 8 Diethyl malonate 1440.1 1441.7 1442.0 1432.0 11 Phenylacetonitrile 1442.4 1447.7 1467.0 1443.0 12 Acetonitrile 1546.9 1544.4 1560.0 1536.0 13 Dimethylsulfoxide 1529.6 1528.7 1563.0 1536.0 15 Cyclopentadiene 1473.7 1472.5 1481.0 1455.0 16 Indene 1464.2 1464.6 1472.0 1451.0 17 Fluorene 1463.0 1461.8 1472.0 1439.0 18 Diphenylmethane 1504.9 1511.1 1512.0 1499.0 19 Dihydroanthracene 1496.7 1495.6 • • 20 Xanthene 1493.4 1491.8 • • 21 Allylbenzene 1503.4 1506.6 • • 22 Thiazole 1497.8 1493.6 • • 23 Thiophene 1600.4 1598.8 1595.0 1561.0 25 Triphenylmethane 1477.6 1482.8 1501.0 1476.0 26 Benzofurane 1585.1 1584.9 • • 27 Toluene 1587.7 1592.0 1577.0 1564.0 28 3-Nitroprop-1-ene 1403.5 1407.7 • • 29 Nitromethane 1472.8 1477.2 1491.0 1467.0 30 Nitroethane 1471.4 1474.5 1489.0 1469.0 31 2-Nitropropane 1473.2 1470.4 1490.0 1466.0 32 Malonodinitrile 1373.3 1372.5 1405.0 1376.0 33 2-Methylpropanedinitrile 1386.2 1383.6 • • 34 Bis(ethylsulfonyl)methane 1401.4 1400.7 • • 35 Bis(ethylsulfonyl)ethane 1419.9 1419.7 • • 36 Dimedone 1369.6 1371.4 1418.0 1385.0 37 Bis(methylsulfonyl)methane 1398.5 1397.2 • • 38 Ethyl acetate 1549.0 1550.0 1543.0 1527.0 39 Acetone 1534.2 1536.3 1544.0 1516.0 40 3-Ethanoylpentane-2,4-dione 1384.0 1386.5 • • 41 Meldrum’s acid 1369.1 1371.3 1389.0 1359.0 42 1,3-Cyclohexanedione 1371.0 1373.9 • • 43 Propanediamide 1446.0 1450.1 • • 44 Methane 1737.2 1741.3 1749.0 1715.0 1714 I.
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