Addition reactions transition-state structures

A deeper understanding of carbenic philicity requires a more detailed representation of the addition reaction transition state than that afforded by structure 4. Early MO calculations furnished structure 6 as representative of the transition state for addition of a singlet carbene to an alkene (Fig. 7.6). " ... 

Both experimental [7] and theoretical [8] investigations have shown that the anti complexes of acrolein and boranes are the most stable and the transition states were located only for these four anti complexes. The most stable transition-state structure was calculated (RHF/3-21G) to be NC, while XT is the least stable of the four located. The activation energy has been calculated to be 21.6 kcal mol for the catalyzed reaction, which is substantially above the experimental value of 10.4 1.9 kcal mol for the AlCl3-catalyzed addition of methyl acrylate to butadiene [4a]. The transition-state structure NC is shown in Fig. 8.5. 

Problem 6.18 What about the second step in the electrophilic addition of HCl to an alkene—the reaction of chloride ion with the carbocation intermediate Is this step exergonic or endergontc Does the transition state for this second step resemble the reactant (carbocation) or product (alkyl chloride) Make a rough drawing of what the transition-state structure might look like. 

Stanton and Merz studied the reaction of carbon dioxide addition to zinc hydroxide, as a model for zinc metallo-enzyme human carbonic anhydrase IIJ 36. It was shown that the LDA calculations (DFT(SVWN)) were not reliable for locating transition state structures whereas the post-LDA ones (DFT(B88/P86)) led to the transition state structures and ener-... 

Ab initio calculations and density functional theory studies of the gas-phase addition of HF to CH2=CH2 have revealed the possibility of forming trimolecular (two HF and one ethylene) and dimolecular (one FIF and one ethylene) complexes and transition-state structures and of the catalytic effect of the second molecule of the reagent. An energetically favourable pathway was selected on the basis of the computed potential-energy surface for these two reactions. ... 

As in intermolecular reactions, enantioselectivity can be enforced in intramolecular Diels-Alder additions by use of chiral structures. For example, the dioxolane rings in 2 and 3 result in transition-state structures that lead to enantioselective reactions.72... 

In this section we generated an -value of by taking the family of identity exchange reactions of benzyl derivatives (109) (Pross, 1983). That particular -value is sufficiently anomalous to deter any possible application to questions of transition state structure. Yet there are cases that are, in principle, quite similar to the identity exchange reaction in which a-values are utilized as a measure of transition state structure. In such cases we believe the conclusions may not be valid. A typical example (Bell and Sorenson, 1976) is the addition of hydroxide ion to substituted benzaldehydes (110). For this system the sensitivity of rates and equilibria to substituent effects was similar. 

A double ir-face selective aptotic Michael addition of the lithium dienolate, derived from the chiral senecioate (119), to cyclopentenone served for the total synthesis of (-)-kushimone (120 Scheme 45).137-138 The selectivity of the key reaction was rationalized by the assumption of a transition state as shown in (121a). A related approach involves the Michael addition of enolates, derived from chiral propionates, to methyl ( )-crotonate (Scheme 46).139 The formation of the threo isomer (122) as the major product indicates a transition state structure as shown in (121b). This method was utilized in the synthesis of the marine natural product 7,20-diisocyanoadociane (123 Scheme 47).140... 

Mikami has also reported a related ene-like process involving glyoxylates and ketone-derived enolsilanes 42 (Eq. 8B2.11) [17]. The enol ether adducts 43 yield the corresponding P-hydroxy ketone upon treatment with mild acid. On the basis of an analysis of the stereo- and regiochemical outcome of the addition reaction Mikami has invoked a monodentate complex between aldehyde and metal, in contrast to the typical transition-state structures involving glyoxylates that are suggested to involve metal/aldehyde chelates. 

Mikami has carried out a number of investigations aimed at elucidating mechanistic aspects of this Si-atom transfer process. In particular, when the aldol addition reaction was conducted with a 1 1 mixture of enoxysilanes 60 and 62, differentiated by the nature of the 0-alkyl and 0-silyl moieties, only the adducts of intramolecular silyl-group transfer 63 and 64 are obtained (Scheme 8B2.6). This observation in addition to results obtained with substituted enol silanes have led Mikami to postulate a silatropic ene-like mechanism involving a cyclic, closed transition-state structure organized around the silyl group (Scheme 8B2.6). 

Despite the short lifetimes of most silylenes, improvements in flash photolysis techniques for their generation and time-resolved spectroscopic detection methods in the past decade have made possible direct kinetic measurements on the reactions of silylenes. The purpose of these kinetic studies has been to elucidate the mechanisms of silylene reactions. While considerable work remains to be done, transition state structures and activation barriers are emerging from these experiments, and aspects of silylene insertion and addition mechanisms have been revealed that were not uncovered by product studies and were, indeed, unexpected. 

The reaction was found to be first order with respect to amines and acrylamides and no base catalysis was observed. The reaction is believed to occur in a single step in which the addition of amine to Cp of acrylamide and proton transfer from amine to Ca of acrylamide take place concurrently with a four-membered cyclic transition-state structure. The Hammett (px) and Brpnsted (/3X) coefficients are rather small, suggesting an early transition state (TS). The sign and magnitude of the cross-interaction constant, pxy(= —0.26), is comparable to those found in the bond formation processes in the. S n2 and addition reactions. The normal kinetic isotope effect ( h/ d > 1.0) and relatively low A and large negative Avalues are also consistent with the mechanism proposed.192... 

The key idea of the Zimmerman-Traxler model is that aldol additions proceed via six-membered ring transition state structures. In these transition states, the metal (a magnesium cation in the case of the Ivanov reaction) coordinates both to the enolate oxygen and to the O atom of the carbonyl compound. By way of this coordination, the metal ion guides the approach of the electrophilic carbonyl carbon to the nucleophilic enolate carbon. The approach of the carbonyl and enolate carbons occurs in a transition state structure with chair conformation. C—C bond formation is fastest in the transition state with the maximum number of quasi-equatorially oriented and therefore sterically unhindered substituents. 

Leffler (1953, 1963a) has proposed a more general relationship (hence referred to as the Leffler Hammond postulate) which represents an extension of the Hammond postulate since it treats the whole spectrum of reaction types. Thus the transition state is viewed as changing gradually from reactant-like in highly exothermic reactions, to intermediate in character for thermoneutral reactions, to product-like for endothermic reactions. In addition to this proposal, which relates the transition state structure to that of the products and reactants, a free energy relationship (1) which relates changes in... 

There are additional factors which may invalidate the use of a as a measure of transition state structure. Murdoch (1972) has demonstrated that for multi-step reactions, even when proton transfer is rate-determining, the value of a obtained may be greatly influenced... 

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