Transfer hydrogenation calculations

Figure 5-3. Active site and calculated PES properties for the reactions studied, with the transferring hydrogen labelled as Hp (a) hydride transfer in LADH, (b) proton transfer in MADH and (c) hydrogen atom transfer in SLO-1. (i) potential energy, (ii) vibrationally adiabatic potential energy, (iii) RTE at 300K and (iv) total reaction path curvature. Reproduced with permission from reference [81]. Copyright Elsevier 2002...

The results of these calculations have implications on the applicability of the rule of the geometric mean, which indicates that the KIE for a doubly labelled species should be the product of the KIEs for the corresponding singly labelled substrates. For instance, the KIE for the doubly labelled [17] should be the product of the secondary deuterium KIE, ]/ ]> associated with the nontransferring hydrogen and the primary deuterium KIE, / , produced by the transferring hydrogen (equation 58)). 

The overall conclusion drawn by Huskey and Schowen was that a combination of coupled motion and tunneling through a relatively sharp barrier was required to explain the exaltation of secondary isotope effects. They also noted that this combination predicts that a reduction of exaltation in the secondary effect will occur if the transferring hydrogen is changed from protium to deuterium for point A in Fig. 4, the secondary effect is reduced by a factor of 1.09. Experimentally, reduction factors of 1.03 to 1.14 had been reported. For points B, C, and D on the diagram, all of which lack a combination of coupled motion and tunneling, no such reductions in the secondary isotope effect were calculated. 

Racemic benzoin was reduced with (S,S)-28 in a formic acid-triethylamine mixture to give the R,R diol (dl meso=98.2 1.8) quantitatively in >99% ee via dynamic resolution, revealing that racemization at the benzylic carbon atom occurs rapidly under transfer hydrogenation conditions (Scheme 37) [108]. The reduction rate of (R)-benzoin was calculated to be 55 times faster than the S isomer. 

Reductive amination of ketones using p-anisidine and the Hantzsch ester for transfer hydrogenation is a low-yielding reaction in toluene at room temperature, but thiourea is an efficient catalyst, and yields of up to 94% are reported at 50 °C.334 A mechanism involving thiourea hydrogen bonding to the intermediate imine is supported by ab initio calculations. 

A mild, acid- and metal-free direct reductive amination of ketones has been achieved that relies on selective imine activation by hydrogen bond formation and utilizes the Hantzsch ester for transfer hydrogenation and catalytic amounts of thiourea as hydrogen bond donor. The mechanism in Scheme 18, supported by ab initio calculations, has been suggested.358... 

The connection of the 36 hydrogen atoms to the C60 cage lowers the molecular symmetry and activates Raman scattering from a variety of initially forbidden phonon modes (Bini et al. 1998). In addition, the appearance of the C-H stretching and bending modes and those related to various isomers of C6)0n%, results in a very rich Raman spectrum. The comparison of the phonon frequencies for live principal isomers of C60I f 6, obtained by molecular dynamics calculations, with experimentally observed phonon frequencies has led to the conclusion that the material prepared by the transfer hydrogenation method contains mainly two isomers, those with symmetries DM and S6 (Bini et al. 1998). 

The comparison of our experimental data with those of Ref. (Bini et al. 1998 Bensasson et al. 1997) shows that the Raman spectrum of the high-pressure hydrogenated C60H36 is richer more than five times than that of the transfer hydrogenated C60H36. The majority of the experimentally observed Raman peaks (86 peaks from a total number of 126) are very close, with an accuracy of 5 cm-1, to the calculated frequencies and cross-sections of the Raman active modes (their total number is 400) (Bini et al. 1998). The peaks, which are close to the calculated frequencies, are assigned to all principal isomers, but the majority of them belong to the isomers with the symmetry S6, T and D3d. 

Abstract Our publications dealing with problems related to aromatic heterocycles are discussed with the appropriate references from the literature. The three main topics are theoretical calculations, tautomerism, and NMR spectroscopy but other aspects are also discussed, such as crystal structures, proton transfer, hydrogen... 

Figure 3.33 PE profiles of the electronic ground state (circles), the lowest 1 tttt state (squares) and the lowest 17rcr state (triangles) of (a) the phenol-water cluster and (b) the phenol-ammonia cluster as a function of the hydrogen transfer coordinate, calculated with the CASPT2 method [32].

A key configuration in these water clusters, in ice and in clathrate hydrates is the pentamer (Fig. 4) in which one water molecule at the centre of a tetrahedron is hydrogen-bonded to four other water molecules at the vertices. Here two hydrogen bonds are formed by H-transfer and two by electron transfer . The calculated average bond energy is similar to that for the dimer (Kollmann and Allen, 1970 Hoyland and Kier, 1969). More sophisticated calculations on... 

In a very recent computational study, Diggle et al. have calculated the activation barriers for C(aryl)-X activation (X = H, F, OH, NH, CH3) as 0 (H), 9 (F), 12 (OH), 20 (NH ) and 21.3 kcal mol (CH3), respectively [155]. In comparison, the activation barrier for C(sp3)-H is 6.6 kcal moF [156]. C-X activation occurs under reaction conditions relevant for homogenous catalysis [157], but does not always result in decomposition as C-H activation is often reversible and can be exploited in catalytic transfer hydrogenations involving alcohols [156]. 

Even within the limits of maintaining chemically similar atoms, care is needed when one is invoking transferability. IMPT calculations on amide-amide and amide-water systems indicated that the change in the intermolecular interaction energy associated with the hydrogen bond exchange process... 

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