Elimination reaction transition state

Since the -elimination mechanism requires formation of a six-membered cyclic transition state, this reaction is not possible for five- or six-membered lactones, but may be applied to higher homologs. 

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. 

Ab initio and density functional calculations of the thermal syn elimination transition states for ) reaction of organic amine oxide, sulfoxide, and phosphoxide have confirmed the expected planar geometry and known order of reactivity.71... 

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. ... 

Experimental evidence and computational analysis point to a mechanism in which the alkene (or alkyne) carbons and the M-H bond must be nearly coplanar to react. Once the metal alkene complex has achieved such geometry, 1,2-insertion can occur. During insertion, the reactant proceeds through a four-center transition state. 14The reaction involves simultaneous breakage of the M-H and C-C n bonds, as well as the formation of an M-C a bond and a C-H bond at the 2-position of the alkene (or alkyne). The result is a linear compound, L M(CH2CH3), in the case of ethene insertion. The reverse reaction, (3-elimination, follows the same pathway starting from a metal-alkyl complex with an open coordination site. 

Elimination of selenoxides takes place through an intramolecular, syn elimination pathway. The carbon—hydrogen and carbon—selenium bonds are coplanar in the transition state. The reaction is highly traws-selective when acyclic a-phenylseleno carbonyl compounds are employed. The formation of conjugated double bonds is favored. Endocyclic double bonds tend to predominate over exocyclic ones, unless there is no syn hydrogen available in the ring. Some examples of selenoxide-mediated syn eUmination reaction are given in Scheme 6.23. 

Figure 11 shows the fully optimized geometry of the transition state of reaction (17) as well as the calculated energies. The results indicate that the transition state is quite similar to that of reaction (16), with a long transitional C-C bond (2.256 A) and productlike structure. The calculated Ea is 34.96 kcal/mol at the PMP2/6-31G level, and 33.24 kcal/mol at the B3LYP/6-31G level. Both of them are very close to that of reaction (16). It can be concluded that without other reactions (H-transfer reaction, termination reaction, and addition reaction), the radical decomposition will proceed in a chain reaction fashion until the P position is eliminated. 

Manufacture. Cinnamaldehyde is routinely produced by the base-cataly2ed aldol addition of ben2aldehyde /7(9(9-with acetaldehyde [75-07-0], a procedure which was first estabUshed in the nineteenth century (31). Formation of the (H)-isomer is favored by the transition-state geometry associated with the elimination of water from the intermediate. The commercial process is carried out in the presence of a dilute sodium hydroxide solution (ca 0.5—2.0%) with at least two equivalents of ben2aldehyde and slow addition of the acetaldehyde over the reaction period (32). 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

When the addition and elimination reactions are mechanically reversible, they proceed by identical mechanistic paths but in opposite directions. In these circumstances, mechanistic conclusions about the addition reaction are applicable to the elimination reaction and vice versa. The principle of microscopic reversibility states that the mechanism (pathway) traversed in a reversible reaction is the same in the reverse as in the forward direction. Thus, if an addition-elimination system proceeds by a reversible mechanism, the intermediates and transition states involved in the addition process are the same as... 

Fig. 6.3. Variable transition state theoiy of elimination reactions. J. F. Bunnett, Angew. Chem. Int. Ed. Engl. 1, 225 (1962) J. F. Bunnett, Surv. Prog. Chem. 5, 53 (1969) W. H. Saunders, Jr., and A. F. Cockerill, Mechanisms of Elimination Reactions, Wiley, New York, 1973, pp. 48—55 D. J. McLennan, Tetrahedron 31, 2999 (1975) W. H. Saunders, Jr., Acc. Chem. Res. 9, 19 (1976).

There is another useiiil way of depicting the ideas embodied in the variable transition state theory of elimination reactions. This is to construct a three-dimensional potential energy diagram. Suppose that we consider the case of an ethyl halide. The two stepwise reaction paths both require the formation of high-energy intermediates. The El mechanism requires formation of a carbocation whereas the Elcb mechanism proceeds via a caibanion intermediate. 

The nature of the transition state in elimination reactions is of great importance, since it controls the regiochemistry of p elimination in compounds in which the double bond can be introduced in one of several positions. These effects are discussed in the next section. 

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

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