Ring-closing reactions, activation energy

In the study of the potential energy surface [B3LYP/6-311G+-h(d,p)] for the reaction of the ethynyl radical HC2 with nitric oxides NO and N02, Peeters and co-workers reported that a cyclic 1,2-oxazete intermediate such as 17 may be involved. In the reaction of HC2 and NO, the four-membered cyclic structure 17, precursor of HCN and CO, could reasonably be formed from 16 by means of a ring-closing reaction having an activation barrier of 18 kcal mol-1 (Scheme 2) <1998CPL91>. 

Many examples of intramolecular ring-closing reactions have been presented in Chapter 4 and elsewhere, which makes it clear that this is an important concept for heterocyclic synthesis. However, certain types of proposed intramolecular reactions fail. These observations led J. E. Baldwin to develop a set of empirical mles that allow prediction of which structural types will be favored or disfavored for cyclization. For an intramolecular cyclization to occur, the functional group at one end of a chain must be able to reach the site where a new bond is to be formed, so that efficient orbital interaction can take place. The ease of the interaction is reflected in the activation energy for the formation of the transition state thus, cyclization is under kinetic control. Baldwin s rules are based on the following stmctural features ... 

The kinetics of hydrocracking reactions has been studied with real feedstocks and apparent kinetic equations have been proposed. First-order kinetics with activation energy close to 50 kcal/gmol was derived for VGO. The reactions declines as metal removal > olefin saturation > sulfur removal > nitrogen removal > saturation of rings > cracking of naphthenes > cracking of paraffins [102],... 

The two systems differ insofar as for stilbene the thermodynamic stability of the open form is greater, while for dihydropyrene the ring-closed form is more stable. The thermal back reaction 97 96, which requires an activation energy of 22 kcal/mol, is apparently a symmetry-forbidden concerted reaction. An estimate of the ground-state barrier based on EHT calculations yields values that are in good agreement with experimental activation energies (Schmidt, 1971). 

Which electrophile is lost from the amino acid residue is, of course, controlled by the enzyme. One way this may occur is by the enzyme binding the PLP imine so that the electrophile is in close proximity to a suitable or base to aid abstraction and also so that the a orbital of the bond to be broken is periplanar with the p r acceptor system, i.e. orthogonal to the plane of the pyridine ring (XXXI). Maximal orbital overlap, stereoelectronic control, will lower the activation energy for the reaction. Aldol-type reactions can also occur with PLP as in the laboratory the key to making carbon-carbon bonds is the formation of a stabilised carbanion. Proton abstraction from the initially formed imine gives a masked carbanion which can nucleophili-... 

Although the C—C—C bond angles in the C5 ring are close to that for tetrahedral carbon, the fission of a C—C bond by hydrogenolysis occurs more readily than that of linear alkanes. Reaction takes place at lower temperatures and with lower activation energies, and the order of reaction in hydrogen (for methylcyclopentane) is positive, where for an acyclic alkane it would be negative. This is... 

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