Free activation enthalpy

The reversal of the thermal decomposition of 6 to ethylene and vinylacetylene cannot be utilized to generate 6, since, according to a quantum-chemical analysis, the reaction is slightly endergonic and requires a large free activation enthalpy (0.9 and 42 kcal mol-1, respectively) [59]. The intramolecular variant of this process as well as the addition of typical dienophiles of the normal Diels-Alder reaction to divinylace-tylenes will be discussed at the end of Section 6.3.3. 

In addition to the Hopf cydization of 176, there is a second pericydic reaction leading to 162, that is, the dehydro Diels-Alder reaction of butenyne with acetylene (Scheme 6.47). The theoretical treatment of this process by Johnson et al. [59] predicted a free reaction enthalpy and a free activation enthalpy, both at 25 °C, of -13.4and 42.0kcalmol-1, respectively. Ananikov [116] arrived at a similar result for the intramolecular case of non-l-en-3,8-diyne (202) and calculated the same quantities to be -15.3 and 30.9 kcal mol-1 for the formation of the isoindane 203. As already discussed regarding Scheme 6.40, the conversion of 162 into benzene and likewise that of 203 into indane have to be considered as a sequence of two [1,2]-H shifts 116, 117], whose highest transition state has a significantly lower energy than that for the formation of 162 and 203 by the dehydro Diels-Alder reaction. 

Table 16. Dynamic and static cis effects of the porphyrin ligand (P) on the axial ligands in carbonylruthenium porphyrins Ru(P)CO(t-BuPy) [36a-36h. Free activation enthalpies (AG298) for the displacement of (t-BuPy) according to Eq. (9) and CO-stretching frequencies (v< o) taken from Ref. (131). For abbreviations, see Table 2...

The X-ray data for 71 confirm its structure. The low-temperature 13C NMR suggests that 71 exists in two boat configurations (72 being the other). Estimate for the free activation enthalpies for the major confirmer of 71 is of c. 13-14 kcal mol-1 <2005HCA1421>. 

As has been emphasized at the beginning of this overview of asymmetric den-drimer catalysis, the kinetically controlled stereoselection depends on very small increments of free activation enthalpy. It is therefore an excellent sensitive probe for dendrimer effects and will continue to be studied in this fundamental context. As mono dispersed macromolecules, chiral dendrimer catalysts provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers. 

The regioselectivity and reactivity of Ar-SE reactions of naphthalene are explained correctly by comparing the free activation enthalpies for the formation of the sigma complexes 1- E— C10H10+ and 2-E— E— C10H10+ from the electrophile and naphthalene and for the formation of the sigma complex E— C6H6+ from the electrophile and benzene, respectively. 

We note that in most cases the free activation enthalpy, AGJ (estimated from the Eyring equation), is given as a measure for the rotational activation barrier. However, we also came across one example where the Arrhenius activation energy, E, had been calculated (193). Yet another manuscript listed both AG and E, and the activation enthalpy, AH, and activation entropy, ASt [see Mann et al. (792)]. 

The free activation enthalpy of rotation (AG ) is calculated to be 46 kJ/mol at the coalescence temperature this is significantly higher than the rotation barrier for a normal single bond, which is —10 kJ/mol (26). The increased barrier in triazenes is consistent with resonance (23a 23b)... 

The violet fluoro derivative WO(F)Fc3 is formed from WO(X)Fc3 (X = Cl, OFc, OnBu) in the presence of complex fluoride salts such as Mc30 BFT in CH2CI2 it is almost insoluble in organic solvents [176]. The H and NMR spectra of the diamagnetic oxotungsten(vi) complexes are temperature-dependent, suggesting a hindered rotation of the ferrocenyl ligands around the W-C(ferrocenyl) bond [175,176] in the case of WO(OFc)Fc3 the free activation enthalpy, AG (7 ), was determined to be 62.5 + 0.5 kJ/mol [175]. 

Fig. 5-15. Free activation enthalpy, AG (298 K), of the bridge-inversion rearrangement of 1,2,3-trichalcogena-[3]ferrocenophanes as a function of the bridge length [256, 258].

Contrary to the Z-forms of protonated azobenzenes, the Z-forms of donor/acceptor di-substituted azobenzenes isomerize very quickly at room temperature. To investigate this reaction, the Z-form is produced by flash excitation. The isomerization of these pseudo-stilbenes is strongly dependent on the polarity of the solvent. " For instance, 4-diethyIamino-4 -nitroazobenzene isomerizes with k g = 0.007 s in hexane, but with kz, g = 600 s in N-methylformamide. 4-Anilino-4 -nitroazobenzene in cyclohexane obeys the relation In kz, E = 22( 3) - 72( 9)T0V(8.314 T). Schanze et al. " have established two linear relationships between the free-activation enthalpy of isomerization and the Kosower Z values of aprotic and protic solvents. Sanchez and de Rossi report a strong pH sensitivity of... 

The high activity of the nickel catalysts frequently enables the hydrovinylation reaction to be carried out at low temperatures, thereby allowing full implementation of the small differences in the free activation enthalpy for the formation of the diastereomeric intermediates. The increase in the diastereomeric excess with decreasing reaction temperature for the hydrovinylation of p-divinylbenzene to... 

The roots for the activity in the field of preparation of enantiopure amino acids in the Leibniz-Institut fur Organische Katalyse an der Universitat Rostock e.V. (formerly known as Bereich Komplexkatalyse which was a part of the Zentral-institut fur Organische Chemie der Akademie der Wissenschaften der DDR ) were planted at the end of the 1960s by Horst Pracejus, who was its director at that time (Fig. 1). In the 1950s Pracejus had previously worked on asymmetric catalysis and published outstanding results on the reaction of nucleophiles with ke-tenes catalyzed by chiral bases and developed a fundamental understanding of the mechanism of such enantioselective processes controlled by opposed entropic and enthalpic parts of the free activation enthalpy [1],... 

Since the Eyring relation can be used to link a free activation enthalpy to a kinetic constant, the single-events method can be described in two different ways ... 

Analysis of exchange-broadened NMR spectra (that is, the simulation of temperature-dependent spectra as a function of the exchange rate) has become the method of choice for the determination of the free activation enthalpy for intramolecular conformational equilibria and for intermolecular exchange processes. Studying, in addition, the pressure dependence of the... 

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