TRANS-PATH

These rules show that the G<- G transition, in contrast with the others, is purely rotational. In the coordinate system shown in Figure 8.20, the transition states for the cis and trans paths of interconversion have symmetry axes and C2y and relate to the symmetry groups and C2h, respectively. The different symmetries of the transition states results from the fact that the same permutation relates to different symmetry operations in C2v and C2h. For example, (ab)(14)(28)(36) is equivalent to inversion in C2h, while in it corresponds to the reflection in the axy plane. The symmetry of the reaction path does not affect the symmetry of states with even Ka (and Ka = 0). However, the selection rules for transitions Ka = 1 0 are different for cis and trans paths. The classifica-... 

It is possible to account for the stereoselectivity in the double bond isomerization of 1-butene over silica-alumina and other acid catalysts by assuming that the chemisorption leads to a butenyl carbonium ion (Fig. 51). According to this scheme, 1-butene can be chemisorbed to form either a cis (path A) or a trans (path B) butenyl carbonium ion depending upon the conformation of the butene molecule as it approaches the catalyst surface. 

Recent stereochemical studies have established two principal modes of addition of nucleophiles to 7r-allyl ligands coordinated to Pd(II), as shown in Scheme Dimethylmalonate and amines add trans (path A), whereas... 

As a multidimensional PES for the reaction from quantum chemical calculations is not available at present, one does not know the reason for the surprismg barrier effect in excited tran.s-stilbene. One could suspect diat tran.s-stilbene possesses already a significant amount of zwitterionic character in the confomiation at the barrier top, implying a fairly Tate barrier along the reaction path towards the twisted perpendicular structure. On the other hand, it could also be possible that die effective barrier changes with viscosity as a result of a multidimensional barrier crossing process along a curved reaction path. 

Schlegel H B 1994 Some thoughts on reaction-path following J. Chem. Soc. Faraday Trans. 90 1569... 

Important synthetic paths to azirines and aziridines involve bond reorganization, or internal addition, of vinylnitrenes. Indeed, the vinylnitrene-azirine equilibrium has been demonstrated in the case of trans-2-methyl-3-phenyl-l-azirine, which at 110 °C racemizes 2000 times faster than it rearranges to 2-methylindole (80CC1252). Created in the Neber rearrangement or by decomposition of vinyl azides, the nitrene can cyclize to the p -carbon to give azirines (Scheme 4 Section 5.04.4.1). 

For gas-phase diffusion in small pores at lowpressure, the molecular mean free path may be larger than the pore diameter, giving rise to Knudsen diffusion. Satterfield (Ma.s.s Tran.sfer in Heterogeneous Catalysis, MIT, Cambridge, MA, 1970, p. 43), gives the following expression for the pore dimisivity ... 

Rechenburg, I. (1965) Cybernetic solution path of an experimental problem. Royal Aircraft Establishment Translation No. 1122, B.F. Toms (trans.), Farnborough, Hants, Ministry of Aviation, Royal Aircraft Establishment. 

Figure 13.37 shows the UV spectrum of the conjugated diene cis,trans-, 3-cyc o-octadiene, measured in ethanol as the solvent. As is typical of most UV spectra, the absorption is rather broad and is often spoken of as a band rather than a peak. The wavelength at an absorption maximum is refened to as the X ax of the band. There is only one band in the UV spectrum of 1,3-cyclooctadiene its X ax is 230 ran. In addition to UV-VIS bands are characterized by their- absorbance (A), which is a measure of how much of the radiation that passes through the sfflnple is absorbed. To correct for concentration and path length effects, absorbance is converted to molar absorptivity (e) by dividing it by the concentration c in moles per liter and the path length I in centimeters. 

We have already considered two reactions on the H2CO potential energy surface. In doing so, we studied five stationary points three minima—formaldehyde, trans hydroxycarbene, and carbon monoxide plus hydrogen molecule—and the two transition structures connecting formaldehyde with the two sets of products. One obvious remaining step is to find a path between the two sets of products. 

Determine the reaction path connecting trans hydroxycarbene and H2 + CO. Predict the activation energy, referring to the values for the SCF and zero-point energies for the products and reactants summarized at the conclusion of this problem. This reaction occurs via a two step process ... 

We can easily identify both structures by the value of the dihedral angle. In the one on the left, the dihedral angle has increased to 118.3°, indicating that this side of the path is leading to the trans form. Indeed, if we look at ail of the points in the reaction path, we see that the dihedral angle steadily increases on this side of the transition structure, and steadily decreases on the opposite side. From the latter, we can conclude that the right structure is tending toward the cis form. Thus, we have confirmed that this transition structure does in fact connect the cis and trans isomers of hydroxycarbene. 

The red line follows the progress of the reaction path. First, a butadiene compound b excited into its first excited state (either the cis or trans form may be used—we will be considering the cis conformation). What we have illustrated as the lower excited state is a singlet state, resulting from a single excitation from the HOMO to the LUMO of the n system. The second excited state is a Ag state, corresponding to a double excitation from HOMO to LUMO. The ordering of these two excited states is not completely known, but internal conversion from the By state to the Ag state i.s known to occur almost immediately (within femtoseconds). 

In an investigation by Yamabe et al. [9] of the fine tuning of the [4-1-2] and [2-1-4] cycloaddition reaction of acrolein with butadiene catalyzed by BF3 and AICI3 using a larger basis set and more sophisticated calculations, the different reaction paths were also studied. The activation energy for the uncatalyzed reaction were calculated to be 17.52 and 16.80 kcal mol for the exo and endo transition states, respectively, and is close to the experimental values for s-trans-acrolein. For the BF3-catalyzed reaction the transition-state energies were calculated to be 10.87 and 6.09 kcal mol , for the exo- and endo-reaction paths, respectively [9]. The calculated transition-state structures for this reaction are very asynchronous and similar to those obtained by Houk et al. The endo-reaction path for the BF3-catalyzed reaction indicates that an inverse electron-demand C3-0 bond formation (2.635 A... 

Figure 30.6 Thermal ring-openings of cis- and trans-dimethylcyclobutene occur by conrotatory paths.

Ethyl l//-azepine-l-carboxylate and chlorotrimethylsilane, in hexamethylphosphoric triamide in the presence of magnesium, followed by quenching into water, yields the 1 2 trans-adduct 3.279 The reaction follows a similar path with dichlorodimethylsilane, whereas with dichloro-diphenylsilane a mixture of the 1 2 adduct 4 and the disiloxane 5 is obtained. 

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

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