Transition state addition reactions

Table 10.4 lists the rate parameters for the elementary steps of the CO + NO reaction in the limit of zero coverage. Parameters such as those listed in Tab. 10.4 form the highly desirable input for modeling overall reaction mechanisms. In addition, elementary rate parameters can be compared to calculations on the basis of the theories outlined in Chapters 3 and 6. In this way the kinetic parameters of elementary reaction steps provide, through spectroscopy and computational chemistry, a link between the intramolecular properties of adsorbed reactants and their reactivity Statistical thermodynamics furnishes the theoretical framework to describe how equilibrium constants and reaction rate constants depend on the partition functions of vibration and rotation. Thus, spectroscopy studies of adsorbed reactants and intermediates provide the input for computing equilibrium constants, while calculations on the transition states of reaction pathways, starting from structurally, electronically and vibrationally well-characterized ground states, enable the prediction of kinetic parameters. 

The pre-exponential factor Ac H for the reaction R02 + RH per attacked C—H bond differs for aliphatic hydrocarbons and for hydrocarbons, where the attacked C—H bond is in the a-position to tt-C—C bond. This difference is the result of additional loss of the activation entropy due to retardation of group rotation, resulting from the interaction of tt-electrons with electrons of reaction center. When the peroxyl radical attacks the C—H bond in neighborhood with the n-C—C bond, the retardation of free rotation around the C—C bond in the transition state additionally lowers the entropy of the transition state. The values of E0 and AC H are given here [119] ... 

The analogy between electron-transfer via addition/elimination (Eq. 2b,c) or abstraction/elimination (Eq. 2a, c) and classical solvolysis involving closed-shell molecules (nonradicals) is seen by comparing Scheme 1 with Scheme 3, in which XY, the precursor of the ions X and Y , is formally derived from the two radicals X and Y". Analogous to Scheme 1, on the way to the ionic products that result from the interaction between X and Y there are two possibilities if XY denotes a transition state, the reaction (Eq. 3a, a ) is a case of outer-sphere electron transfer. If, however, a covalent bond is formed between X and Y, the path (Eq. 3b, b ) is an example of inner- sphere electron transfer. Obviously, part b of the scheme describes the classical area of S l solvolysis reactions (assuming either X or Y to be equal to C) [9, 10]. If a second reaction partner for C (other than the solvent) is allowed for (the (partial) ions then represent transition states), then Eq. 3b also covers Sn2 reactions. If looked upon from the point of view of radical-radical reactivity, Eqs. 3a and b show well-known reactions radical disproportionation in Eq. 3a,a and combination in Eq. 3b. 

The 1,2-addition of diethylzinc to aldehydes is a powerful method for C-C bond formation. As there is a wide variety of possible transition states, the reaction is very sensitive to changes in the ligand structure. For this reason the diethylzinc addition in Scheme 2.1.3.2 is a suitable test reaction for developing and establish-... 

Some organic reactions can be accomplished by using two-layer systems in which phase-transfer catalysts play an important role (34). The phase-transfer reaction proceeds via ion pairs, and asymmetric induction is expected to emerge when chiral quaternary ammonium salts are used. The ion-pair interaction, however, is usually not strong enough to control the absolute stereochemistry of the reaction (35). Numerous trials have resulted in low or only moderate stereoselectivity, probably because of the loose orientation of the ion-paired intermediates or transition states. These reactions include, but are not limited to, carbene addition to alkenes, reaction of sulfur ylides and aldehydes, nucleophilic substitution of secondary alkyl halides, Darzens reaction, chlorination... 

The Diels-Alder reaction is highly stereospecific. The diene reacts in an unfavorable conformation in which its double bonds lie in a plane on the same side (cis) of the single bond connecting them. This s-cis (or cisoid) conformation is required to give a stable product with a cis double bond. Addition of ethene to the alternate and more stable (transoid) conformation would give an impossibly strained Pimv-cyclohexene ring. Possible transition states for reaction in each conformation follow, and it will be seen that enormous mo-... 

As would be expected for reactions with polar transition states, additions of per-fluoroalkyl radicals to alkenes are faster in CH3CN than in Freon 113 with the observed solvent effects being greater for additions to alkenes which are more electron-rich [70,117]. Table 10 provides comparisons of rates in the two solvents. 

The effect of protic additives was rationalized by the formation of hydrogen-bonded adducts, such as that between benzoic acid and the carbonyl group of the coordinated benzoate, as observed by 1H NMR. Such an adduct would be expected to facilitate the formation of a polar transition state. The reaction rate also increases upon modification of the ligand with anionic groups and in polar solvents. Radical initiators have no effect. 

However, because the transition states for reactions of the two enantiomeric organolithiums with the chiral electrophile are diastereoisomeric, one product must be formed faster than the other. If the organolithium is configurationally stable on the timescale of addition to the aldehyde, this rate difference will be of no consequence eventually both enantiomers will... 

We will see many additional reactions whose products may be determined by kinetic control or by thermodynamic control, depending on the conditions. In general, reactions that do not reverse easily are kinetically controlled because no equilibrium is established. In kinetically controlled reactions, the product with the lowest-energy transition state predominates. Reactions that are easily reversible are thermodynamically controlled unless something happens to prevent equilibrium from being attained. In thermodynamically controlled reactions, the lowest-energy product predominates. 

When 1-methylcyclohexene is used as the starting material, there is additionally a question of regioselectivity. The alcohol attacks the more hindered end of the bromonium ion—the end where there can be greatest stabilization of the partial positive charge in the loose transition state. This reaction really does illustrate the way in which a mechanism can lie in between S l and S>j2. We see a configurational inversion, indicative of an S>j2 reaction, happening at a tertiary centre where you would usually expect S>jl. 

Silver BINAP complex was used as well by Yanagisawa and coworkers . The particular feature of this catalyst is the marked anti selectivity when nsing crotyltins, regardless of whether the donble bond is (E) or (Z). This selectivity is explained by a fast transmetaUation step between the chiral complex and the organotin reagent, followed by the addition on the carbonyl gronp via a cyclic transition state. This reaction has been extended to other organometallics snch as 2,4-pentadienylstannanes . 

There have been both experimental and theoretical studies to probe the degree of concertedness in gas-phase substitutions as shown in Scheme 1. Is (2) an intermediate with a finite lifetime, or are the addition and elimination steps concerted so that (2) is a transition state Experimental molecular beam studies on the femtosecond time-scale have been reported for the reaction of chloride ions with the iodobenzene cation to yield chlorobenzene and iodine. The results show an 880 fs reaction time for the elimination process, indicating a highly non-concerted process, so that here the (x-complex is an intermediate rather than a transition state. The reactions of halobenzene cations with ammonia have been interpreted in terms of the formation of an addition complex which may eliminate either halogen, X , or hydrogen halide, HX, depending on the nature of the halogen. ... 

The Next Important Elements to Life Occur in Period 3 P and S are the smallest elements capable of multiple covalent bonds to C, O and N, and which also have available d-shells. The d-shells allow additional transition states and reaction mechanisms. P and S are particularly important in the capture, storage, and distribution of chemical energy. 

The mercurinium ion is attacked by the nucleophilic solvent—water, in the present case— to yield the addition product. This attack is back-side (unless prevented by some structural feature) and the net result is anti addition, as in the addition of halogens (Sec. 7.12). Attack is thus of the Sn2 type yet the orientation of addition shows that the nucleophile becomes attached to the more highly substituted carbon— as though there were a free carbonium ion intermediate. As we shall see (Sec. 17.15), the transition state in reactions of such unstable three-membered rings has much SnI character. 

Any explanation of facial selectivity must account for the diastereoselection observed in reactions of acyclic aldehydes and ketones and high stereochemical preference for axial attack in the reduction of sterically unhindered cyclohexanones along with observed substituent effects. A consideration of each will follow. Many theories have been proposed [8, 9] to account for experimental observations, but only a few have survived detailed scrutiny. In recent years the application of computational methods has increased our understanding of selectivity and can often allow reasonable predictions to be made even in complex systems. Experimental studies of anionic nucleophilic addition to carbonyl groups in the gas phase [10], however, show that this proceeds without an activation barrier. In fact Dewar [11] suggested that all reactions of anions with neutral species will proceed without activation in the gas phase. The transition states for reactions such as hydride addition to carbonyl compounds cannot therefore be modelled by gas phase procedures. In solution, desolvation of the anion is considered to account for the experimentally observed barrier to reaction. 

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