Eliminations Involving Cyclic Transition Structures

Amine oxide pyrolysis occurs at temperatures of 100°-150°C. The reaction can proceed at room temperature in DMSO.323 If more than one type of (3-hydrogen can attain the eclipsed conformation of the cyclic TS, a mixture of alkenes is formed. The product ratio parallels the relative stability of the competing TSs. Usually more of the /f-alkene is formed because of the larger steric interactions present in the TS leading to the Z-alkene, but the selectivity is generally not high. 

In cyclic systems, conformational effects and the requirement for a cyclic TS determine the product composition. This effect can be seen in the product ratios from pyrolysis of /V,/V-dimethyl-2-phcnylcyclohexylaminc-/V-oxide. 

Concerted Cycloadditions, Unimolecular Rearrangements, and Thermal Eliminations 

Sulfoxides also undergo thermal elimination reactions. The elimination tends to give (3, y-unsaturation from (3-hydroxysulfoxides and can be used to prepare allylic alcohols. 

Sulfoxide elimination in conjunction with [2,3]-sigmatropic rearrangement has been used to convert allylic alcohols to dienes. 

---

Alkylalkali compounds readily eliminate metal hydride to give the alkene this reaction is especially pronounced with the higher alkali metals. The microscopic reverse process has been modeled by the addition of LiH to ethylene. As shown in Figure 6, the reaction first involves a r-complex between ethylene and lithium hydride in which the structures of ethylene and of LiH are changed but little from the separated reactants. In the cyclic transition structure the C-Li bond has significantly shortened towards its value in the product ethyllithium. The C-H bond is still quite long in the transition structure but is on its way to a normal C-H bond. The same type of... 

The third possibility for radical rearrangement (path d. Scheme 9) involves protonation of the iminopropyl radical. In contrast to the addition-elimination mechanism for the neutral system, the protonated cyclic structure (TS 14-H+ 14 -H+) is found to be a transition structure, rather than a stable intermediate. This transition structure lies 19.0 kJ mol above the reactant radical (14-H ). Protonation of the reactant radical thus leads to a significant reduction in the barrier height (by 33.4 kJ mok ). 

Mechanisms for decarbonylation of ( )-2-butenal and ( )-2-methyl-3-phenyl-2-propenal have been studied with different levels of ab initio and DFT methods. Reactants, products, and transition structures were optimized for two kinds of reaction channels a one-step reaction involving a three-membered cyclic TS, and a two-step reaction involving an initial four-membered cyclic TS. The elimination of ( )-2-methyl-3-phenyl-2-propenal yields different products depending on the channel followed. Only one of the three possible one-step mechanisms leads directly to (E)-P-methylstyrene, the corresponding TS rising to the lowest activation Gibbs free energy. 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

---