Thermodynamic stability, adducts

A major goal was to investigate the solid state structures of such compounds by single crystal X-ray diffraction. It was found that Lewis acid-base adducts R3M—ER3 show general structural trends, which allow estimations on the relative stability of the adducts. The experimental results were confirmed by computational calculations, giving even deeper insights into the structural parameters and the thermodynamic stability of simple Lewis acid-base adducts. In addition, their thermodynamic stability in solution was investigated by temperature-dependent NMR spectroscopy. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

Comparison of calculated and experimental data. Since the homologous series of adducts of the type Et3Al—E(Tms)3 (E = P 28, As 29, Sb 15, Bi 14) has been structurally characterized by single crystal X-ray diffraction, their structures and thermodynamic stabilities were calculated to allow a comparison between experimental and theoretical data (Table 7). 

These results suggest that the transition states leading to the formation of the cyclo-adducts (33) and (34) are product-like and that the greater than statistical formation of adducts (34) is due to the increased thermodynamic stability of a trisubstituted double bond. In agreement with this explanation is the fact that in reactions with for example p-xylene and durene (1,2,4,5-tetramethylbenzene) only the adducts (35) and (36) were obtained 54-59). Also as expected, two adducts were obtained with tetralin but only the compound (37) was obtained using 5,8-dimethyltetralin, which we may regard as a 1,2,3,4-tetra-alkylben-zene 54>. 

Cycloadditions selectively afford the adducts on the 6,6-ring junctions [65], and the products occasionally undergo a facile retro-Diels-Alder reaction as a consequence of the low thermodynamic stability of the adduct. Very stable Diels-Alder cycloadducts have, however, been prepared by use of different substituted o-quinodimethanes, probably because of stabilization by aromatization of the resulting adducts [66],... 

The partitioning of simple tertiary carbocations, ring-substituted 1-phenylethyl carbocations, and cumyl carbocations between deprotonation and nucleophilic addition of solvent strongly favors formation of the solvent adduct. The more favorable partitioning of these carbocations to form the solvent adduct is due, in part, to the greater thermodynamic stability of the solvent... 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

Theoretical calculations and experimental studies provided a detailed understanding of the parameters that determine the thermodynamic stabilities of R3M <— ER3 adducts.16 It was demonstrated that the strength of... 

Lewis acids and bases, which play the key roles for the thermodynamic stability of the resulting adducts,17 generally agree to the following trends, as can also be seen in Table I ... 

BC13, BBr3 and BI3. B-N distances obtained from single crystal X-ray diffraction studies as well as gas phase measurements yielded only small variations (X-ray 158.5-161.Opm gas phase 165.2-167.4pm),38 again demonstrating that the thermodynamic stability of group 13/15 adducts not necessarily correlates with their M-E bond length. 

These experimentally observed structural trends were confirmed by computational calculations on H3A1 and Me3Al adduct families. In each adduct family, the amount of increase of the M-X bond lengths and decrease of the X M-X bond angles (X = H, Me) compared to uncomplexed MX3 diminishes with the atomic number of the group 15 element. The structural trends very well reflect the trends observed for the thermodynamic stability of such adducts, as is illustrated in Tables XI and XII. 

These unusual products as well as adducts 109 and 106 may be attributed to the persistence of alkoxyamidyls they are isoelectronic with nitroxyl radicals and have significant delocalisation onto oxygen and hence, thermodynamic stability. The ESR... 

The [4 + 2]-cycloaddition reaction of dienylallene 204 with TCNE took place at 70 °C to give the adduct 205 in good yield [173], The dienylallene behaved not as butadiene but vinylallene, partly owing to the thermodynamic stability of the adduct. 

At the higher temperature, the thermodynamic stability of the product is the important consideration, with the 1,4-adduct, a disubstituted alkene, being more stable than the 1,2-adduct, which is a monosub-stituted alkene. An essential part of the reasoning is... 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

The observed reaction pattern for 4-nitro-7-methoxybenzofuroxan shows that the attack of the reagent on position 5 occurs under kinetic control, whereas the conversion of the ge/w-dimethoxy adduct 177 is favored by its greater thermodynamic stability. 

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

Calculations have shown that the HOMO in the [3,4-c]-annelated A,B-diheteropen-talenes is closely related to the nonbonding MO, thereby allowing ready reaction with electron-deficient dipolarophiles. In the case of the systems with two discrete ylide moieties as in thieno[3,4-c]pyrrole derivatives, it is not only the nature of the HOMO which directs the mode of addition, but also the thermodynamic stability of the adduct, leading to addition across the thiocarbonyl ylide at elevated temperatures and to the azomethine ylide at low temperatures (Section 3.18.4.2.1) (77T3203). 

Again, remarkably high At) values are found for C-6 and C-8 of tr-adduct 816 (Table III). This indicates once more the importance of para-para and, to a lesser extent, ortho-para quinoid resonance structures.3 On heating the solution of 8 and 9 in KNH2/NH3 from -40 to 10 C the mixture irreversibly converts to 9. The important azaallylic resonance stabilization present in 9 and not in 8 accounts for the higher thermodynamic stability of species 9. 

The same theoretical approach was used to explain the photochemical behavior of 2,3-dihydrofuran. The HOMO of 2,3-dihydrofuran showed at — 0.32 eV is reported in Figure 3.11. The atomic coefficients of the HOMO are in agreement with an attack of the LUMO (the carbonyl oxygen) on the (5-carbon. On the basis of this result, the thermodynamic stability of the possible adducts was tested. The structures of these compounds are reported in Figure 3.12. The endo isomer is more stable for 2 kcal moT 1 than the other isomer, in agreement with experimental results. 

The isomerization reactions of Pt-nucleobase adducts are expected to be difficult owing to the inertness and thermodynamic stability of the Pt-N bond [30]. For example, a half-life of about 23 years has been estimated for the direct NH3 exchange in [Pt(NH3)4]2+ in aqueous NH3 solution at 25 °C [31], Unfortunately, data on thermodynamic stability constants for Pt-N complexes is very limited because of their inertness (vide infra). Nevertheless, a few studies have reported Pt11 isomerization reactions in nucleobase complexes. 

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