Insertion reactions thermodynamics

The thermodynamic properties of to-potactic insertion reaction materials with selective equilibrium are quite different from those of materials in which complete equilibrium can be assumed, and reconstitution reactions take place. Instead of flat plateaus related to polyphase equilibria, the composition-dependence of the potential generally has a flat S-type form. 

Reactions a and b in Scheme 8 represent different ways of coordination of butadiene on the nickel atom to form the transoid complex 27a or the cisoid complex 27b. The hydride addition reaction resulted in the formation of either the syn-7r-crotyl intermediate (28a), which eventually forms the trans isomer, or the anti-7r-crotyl intermediate (28b), which will lead to the formation of the cis isomer. Because 28a is thermodynamically more favorable than 28b according to Tolman (40) (equilibrium anti/syn ratio = 1 19), isomerization of the latter to the former can take place (reaction c). Thus, the trans/cis ratio of 1,4-hexadiene formed is determined by (i) the ratio of 28a to 28b and (ii) the extent of isomerization c before addition of ethylene to 28b, i.e., reaction d. The isomerization reaction can affect the trans/cis ratio only when the insertion reaction d is slower than the isomerization reaction c. 

The hydrogens within the octahedral olefin-dihydride intermediate are transferred consecutively with overall cis addition, and the rate-determining step (k9) is olefin insertion to give the alkyl- hydride. Kinetic and thermodynamic parameters for nearly all the steps of Fig. 1 have been estimated for the cyclohexene system. Because the insertion reaction is generally believed to require a cis disposition of the hydride and olefin... 

The overall enthalpy change of the insertion process contains contributions from four bonds (M-CO, M-COR, M-R and CO-R). As there is no significant difference between (Mn-R) and Zs(Mn-COR) then, at least in the case of manganese and hydrocarbon groups, R, the dominant factor will be the difference between T (Mn-CO) and E R-COX) [for R = CH3, E = 339 kJ mop1 (X = H), 370 kJ mol"1 (X = Cl) (Ref.23 )] which suggests that the insertion reaction is thermodynamically favoured with respect to decarbonylation. Kinetic studies of the carbonyl insertion reaction in solution have shown87) that the enthalpy of activation is 62 kj mol-1 for inser-... 

Unlike the reactions described in the previous two sections, competition between insertion and j5-hydrogen transfer is usually not an issue here. Ketone polymerization is nearly thermoneutral and disfavoured by entropy. However, aldehyde insertion is thermodynamically more favourable, and the Tishchenko reaction mentioned in the previous section can plausibly be written as a sequence of insertions and j -hydrogen transfer reactions (Scheme 4). 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

Chisholm and Clark have compared the reactivities of [PtHLg] and [PtMeL2] (83), where L is PMcaPh. The hydro cations [PtHLg] are more reactive toward insertion reactions. This result has been partly attributed to the thermodynamic properties of the Pt—H bond compared with those of the Pt—C bond either in the [PtMeL2] cation or in the products. This difference in reactivity may also be related to the availability of alternative reaction mechanisms. Thus reactions involving... 

The insertion reaction of NiH[P(OEt)3]4 with a series of dienes has shown that the anti isomer is commonly the kinetically preferred one, and an interpretation has been advanced that the diene rotates into the cisoid configuration as the Ni—H bond is added across 284) [Eqs. (84), (85)]. This addition to the cisoid diene appeared to be the preferred mechanism however, this did not occur when there was a cis substituent on one of the double bonds since cisoid m-pentadiene-1,3 did not give anti-anti-dimethyl-l,3-7T-allyl. The syn isomer appeared to be thermodynamically favored to a small extent for highly substituted zr-allyls. The addition of the Ni—H bond across a double bond to form the zr-allyl moiety was... 

With respect to carbonylation chemistry Sakakura and Tanaka have shown that irridiation of rhodium complex RhCl(CO)(PMe3)2 in pentane as solvent under a CO atmosphere (1 bar) at ambient temperature gives rise to carbonylated products. Selectivity for linear aldehyde is > 98 % (eq. (11)) [45]. This insertion reaction is photochemically driven, since it is known that aldehyde decarbonylation is a thermodynamically favorable process. Other photochemial C-H activation reactions have been investigated [46]. When toluene is reacted in the presence of CO and the same Rh complex, phenyl acetaldehyde is obtained as the major product (eq. (12)) [47]. 

During a very short initiation period the cation [Ni(Ci2H 9)] , which is present mainly in the thermodynamically more stable syn form b, reacts via the less stable but more reactive anti form a with insertion of butadiene into the anti--polybutadienyl complex c. As a result of the very rapid anti-syn isomerization this complex also exists in equilibrium (cf. Kf) with the more stable syn complex d, which must be regarded as the stable storage complex under the conditions of polymerization. With butadiene the polybutadienyl-butadiene complexes e and f are formed as the actual catalysts. By the much higher reactivity of the less stable anti complex e, the formation of cis units is catalyzed. Since all the equilibria can be assumed to be rapid, the insertion reaction of butadiene (/ 2c) has to be taken as the rate-determining step in the catalytic cycle. Thus, the catalytic activity is dertermined thermodynamically by the concentration of the -c/5-butadiene complex in the anti form e and kinetically by its reactivity k2c-... 

Insertion into element-hydrogen bonds tend to be less favored thermodynamically than insertions into other bonds (e.g., element-carbon). This is often attributed to the high element-hydride bond strength, which is lost upon insertion. Since the insertion reaction is also entropically disfavored, the reverse deinsertion of the unsaturated moiety to generate an element-hydride bond can be thermodynamically favored. When the hydride exists in the P position of the inserted product, this process is commonly referred to as /S-hydride elimination. Nevertheless, there are many examples of insertions into element-hydride bonds that generate stable compounds, and when this insertion reaction is an uphill process, chelation to the element or subsequent chemistry (i.e., catalytic cycles) can be employed to facilitate the initial insertion step. 

The actinide compounds [Cp2ThH(/i-H)]2 (Cp = sterically encumbered Cp) react reversibly with CO to yield formyl complexes. The product of the reaction of Cpf(NpO)ThH (Np = neopentyl) was characterized spectroscopically, and the thermodynamics and kinetics of the insertion reaction were analyzed ... 

Thermodynamic aspects of the insertion reactions have been considered, especially with regard to reactions (b), Y = O. The enthalpy change of the reaction is related to the disruption of the metal-carbon bond, the formation of the new metal-carbon bond, the formation of the new carbon-carbon bond, and the reduction of the CO bond order. From the estimated metal-carbon bond dissociation enthalpies (BDE) of reagent and product and from the energetics of the other quantities involved in the process, the AC of reaction (g) has been calculated to be negative for R = Me or Ph and positive for R = CF3. This is in agreement with the observation that the trifluoroacetyl derivative... 

Thermodynamic control of many insertion reactions is further demonstrated by (1) their reversibility and (2) the occurrence of various displacement reactions which are directed to the most stable products. Point (2) is neatly indicated by the contrasting behavior of the hard Si" or the soft Sn" reactions ... 

We have studied the energetics of the ethylene insertion reactions in a previous paper [3]. The energy change from the 7t-complex 2a to the direct product of insertion 2b was found to be 10 7 kcal/mol, indicating the chain growing reactions are thermodynamically favorable. However, the kinetic feature of the insertion, i.e. the transition state structure and the reaction energy barrier have not been discussed. 

The naked metal ion insertion reaction seems to indicate that high M-C bond strengths allow easy C-C bond cleavage for the bare ions. Apparently this is not the case for coordinated metals. Although C-C bond breaking appears to be kinetically facile as the initial step for the unhindered metal complexes, in the case of usual metal complexes, steric congestion at the metal center seems to retard such a process. " C-H activation is generally both thermodynamically and kinetically favored over C-C activation nevertheless, appropriate selection of reaction conditions and catalyst systems may allow C-C activations. " ... 

Substitution of CO by phosphines 145 The Dotz reaction 149 Rearrangement reactions with loss of CO 151 Photochemical reactions 153 Reactions at the carbene carbon 158 General features 158 Amine nucleophiles 159 Phospine and phosphite nucleophiles 167 Alcohols and alkoxide ion nucleophiles 171 Thiol and thiolate ion nucleophiles 179 Intramolecular nucleophilic reactions 191 Hydroxide ion and water as nucleophiles 194 Insertion reactions initiated by nucleophilic attack Acid-base reactions at the a-carbon 207 General features and methods 207 Kinetic and thermodynamic acidities 209 Effect of structure on pKa values 210 Intrinsic rate constants for proton transfer 219 Thermodynamic acidities in organic solvents 223 Hydrolysis of ionizable carbene complexes 228 Acknowledgments 232 References 233... 

In addition to the kinetic enhancement, the formation of an acyl adduct stabilizes the insertion product Thus by means of the introduction of Lewis acids it is possible to carry out insertion reactions which are thermodynamically unfavorable in the absence of the acid. A good example is given in Eq. (42) where the presence of AlBr3 leads to the formation of the stable... 

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