Transition state cyclic hydrocarbons

The Diels-Alder reaction is one of the most important methods used to form cyclic structures and is one of the earliest examples of carbon-carbon bond formation reactions in aqueous media.21 Diels-Alder reactions in aqueous media were in fact first carried out in the 1930s, when the reaction was discovered,22 but no particular attention was paid to this fact until 1980, when Breslow23 made the dramatic observation that the reaction of cyclopentadiene with butenone in water (Eq. 12.1) was more than 700 times faster than the same reaction in isooctane, whereas the reaction rate in methanol is comparable to that in a hydrocarbon solvent. Such an unusual acceleration of the Diels-Alder reaction by water was attributed to the hydrophobic effect, 24 in which the hydrophobic interactions brought together the two nonpolar groups in the transition state. 

Among the recently published works, the one which showed that the cyclic structures of water clusters open up to form a linear structure above a certain threshold electric field value a was a systematic ab initio study on the effect of electric field on structure, energetics, and transition states of trimer, tetramer, and pentamer water clusters (both cyclic and acyclic) [36], Considering c/.v-butadiene as a model system, the strength and the direction of a static electric field has been used to examine the delocalization energy, the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations [37]. Another recent work performed by Rai et al. focused on the studies using the DFT and its time-dependent counterpart of effects of uniform static electric field on aromatic and aliphatic hydrocarbons [38],... 

Another class of isomerization involves cyclization-decyclization reactions. For example, cyclic hydrocarbons may decompose with the formation of unsaturated products. Again these reactions may be viewed to go through biradical transition states, as seen, for example, in... 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

The fact that lithiation usually occurs in hydrocarbon solvents suggests that the enamine deprotonation will only occur if it undergoes a Lewis acid-base interaction forming a complex with the base used. This is consistent with the assertions of a number of authors that formation of a cyclic transition state is necessary to explain the regiochemistry of these reactions53. 

Existence of cyclobutadiene as the simplest cyclic hydrocarbon with conjugated double bonds was predicted by Hiickel in the 1930s. Until recently, however, all attempts to prepare this compound ended in failure. In 1956 Longuet-Higgins deduced 166) that coordination of cyclobutadiene with a transition metal should stabilize its unstable triplet state. 

Direct concerted insertion is characteristic of singlet carbene and the three-center cyclic transition state 139 suggested by Doering and Prinzbach" has been supported by kinetic data" and theoretical studies." Singlet methylene inserts into C-H bonds of hydrocarbons in a statistical fashion" [Eq. (6.128)] ... 

Table I lists a few representative examples of estimated A-factors for some hydrocarbon reactions proceeding through six-membered ring, cyclic transition states. We see that although the simple rotor rule is fairly effective in estimating A-factors, it generally errs on the high side. The more complex analysis (10) which takes into account all the changes in frequencies in going to the cyclic complex does a somewhat better job of estimating the A-factors.

Electrocyclic ring closures of acyclic trienes and tetraenes have substantial negative entropies of activation, as anticipated for reactions in which rotational freedom is lost on assembling the cyclic transition states. Conversions of the more rigid cyclic trienes and tetraenes to bicyclic hydrocarbons usually have less negative entropies of activation. 

Gimer V, KhuongKS, Leach AG, Lee PS, BartbergerMD, HoukKN A standard set of peri-cyclic reactions of hydrocarbons for the benchmarking of computational methods the performance of ab initio, density functional, CASSCF, CASPT2, and CBS-QB3 methods for the prediction of activation barriers, reaction energetics, and transition state geometries, / Phys Chem A 107(51) 11445—11459, 2003. 

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