State secondary formation

These structures were then used to generate the force fields and calculate the secondary /3-deuterium-d6 equilibrium isotope effects (EIEs) for the formation of the isopropyl carbocation (Table 30). Because the transition states for formation of the carbocation will be close to the structure of the carbocation, these KIEs should be excellent approximations of the maximum secondary /3-deuterium KIEs expected for the limiting SN1 solvolytic reaction. 

There would appear to be no difference in the reactions expected photo-chemically as the primary products are H atoms and OH radicals in both the electric discharge and on irradiation. Chen and Taylor (22) state that there is no evidence for oxygen atoms either in the photolysis or in the decomposition of water vapor in an electric discharge. However, the secondary formation of O atoms (2) and the formation of ozone (31, 50) in an electric discharge through water vapor have been demonstrated. It might be expected that under the proper experimental conditions similar results could be obtained photochemically. 

The reaction was carried in boiling benzene with 5,5-dimethylcyclopentadiene 12 selectively deuterated (X = D, Y = H or X = H, Y = D). The secondary deuterium isotope effects were measured by means of 1H and 2H NMR.65 The large and inverse (0.84) isotope effect for C5-C6 bond formation and no effect (1.00) for C1-C7 bond formation corresponds to extensive sp2->sp3 rehybridization66 of Cl atom and no rehybridization at C2 atom. At the rate-determining transition state only formation of one carbon-carbon bond is advanced in formation of [2+2] cycloadduct. 

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropylmethyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 65 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost, iv. The rate enhancement in the solvolysis of secondary cyclobutyl substrates is probably caused by participation by a bond leading directly to 65, which accounts for the fact that solvolysis of cyclobutyl and of cyclopropylmethyl... 

With silyl-substituted oxiranes, dibal-H favors the primary alcohol and Bu 3A1H favors the secondary alcohol. These observations have been interpreted in terms of the timing of the hydride transfer to one of the oxirane carbons. dibal-H, which exists as a Lewis complex in donor media (R3N-A1H(Bu )2, or R20-A1H(Bu )2) acts as a nucleophilic hydride source, which preferentially attacks the least-hindered carbon. With Bu 3A1, complexation with the oxirane oxygen precedes isobutene elimination and the generation of the Al—H bond. A considerable carbocation character is acquired in the transition state, hence formation of the primary alcohol is favored. It is worthy of note that trialkylstannyl-substituted oxiranes are reduced with Red-Al invariably at the oxirane... 

However, when the reaction of 1,3-pentadiene + DCl is carried out under kinetic control, essentially the same relative amounts of 1,2- and 1,4-addition products are obtained as are obtained from the kinetically controlled reaction of 1,3-butadiene + HBr. The transition states for formation of the 1,2- and 1,4-addition products from 1,3-pentadiene should both have the same stability because both resemble a contributing resonance stmcture in which the positive charge is on a secondary allylic carbon. Why, then, is the... 

Subsequently, the bond dissociation energy of a secondary C-H was determined to be 98 kcal/mol, ° which places the heat of formation of the diyl species roughly 42 kcal/mol above 1,5-hexadiene and 8 kcal/mol above the heat of formation of the transition state for the 3,3-shift. This would appear to be a mininum estimate given that a similar calculation of butane 1,4-diyl is only 61 kcal/mol above cyclobutane while the transition state for formation and cleavage of this species is 4 kcal/mol higher in energy (see Chapter 5, Section 3). 

FIGURE 8,31 The more substituted cation is at lower energy than the less substituted cation. Note also that the transition state for formation of the tertiary cation is lower than the transition state for formation of the secondary cation. Why ... 

The factors that operate to stabilize a full positive charge also operate to stabilize a partial positive charge. We therefore expect the transition state for the formation of a tertiary carbocation to be lower in energy than the transition state for formation of a secondary carbocation, although the difference will not be as great as that for the fully developed cations (Fig. 8.31). 

FIGURE 9.23 The transition states for formation of the high-energy carbocations will contain partial positive charges (5 ). The same order of stabibty that holds for full positive charges applies to partial positive charges. A 5 on a tertiary carbon is more stable than a 5 on a secondary carbon, and so on. 

The next question we need to answer is why is the 1,2-addition product formed faster For many years, chemists thought it was because the transition state for formation of the 1,2-addition product resembles the resonance contributor in which the positive charge is on a secondary allylic carbon, whereas the transition state for formation of the 1,4-addition product resembles the resonance contributor in which the positive charge is on a less stable primary allylic carbon. 

Theoretical work by the groups directed by Sustmann and, very recently, Mattay attributes the preference for the formation of endo cycloadduct in solution to the polarity of the solvent Their calculations indicate that in the gas phase the exo transition state has a lower energy than the endo counterpart and it is only upon introduction of the solvent that this situation reverses, due to the difference in polarity of both transition states (Figure 1.2). Mattay" stresses the importance of the dienophile transoid-dsoid conformational equilibrium in determining the endo-exo selectivity. The transoid conformation is favoured in solution and is shown to lead to endo product, whereas the cisoid conformation, that is favoured in the gas phase, produces the exo adduct This view is in conflict with ab initio calculations by Houk, indicating an enhanced secondary orbital interaction in the cisoid endo transition state . 

Pyridoxal Derivatives. Various aldehydes of pyridoxal (Table 3) react with hemoglobin at sites that can be somewhat controlled by the state of oxygenation (36,59). It is thereby possible to achieve derivatives having a wide range of functional properties. The reaction, shown for PLP in Figure 3, involves first the formation of a Schiff s base between the amino groups of hemoglobin and the aldehyde(s) of the pyridoxal compound, followed by reduction of the Schiff s base with sodium borohydride, to yield a covalendy-linked pyridoxyl derivative in the form of a secondary amine. 

In spite of low copper contents, massive horizontal development renders porphyry deposits amenable to large-scale production methods. Porphyry deposits are associated with igneous activity and intmsion of molten rocks into cooler parts of the earth s cmst, often in connection with the formation of mountains. Erosion of mountainous areas exposes these deposits to weathering, and, under the right conditions, enables the formation of oxidized or secondary copper deposits. Copper mines in the United States are Usted in Table 2. 

In the earliest applications of numerical methods for the computation of blast waves, the burst of a pressurized sphere was computed. As the sphere s diameter is reduced and its initial pressure increased, the problem more closely approaches a point-source explosion problem. Brode (1955,1959) used the Lagrangean artificial-viscosity approach, which was the state of the art of that time. He analyzed blasts produced by both aforementioned sources. The decaying blast wave was simulated, and blast wave properties were registered as a function of distance. The code reproduced experimentally observed phenomena, such as overexpansion, subsequent recompression, and the formation of a secondary wave. It was found that the shape of the blast wave at some distance was independent of source properties. 

Enamines formed in this way may be distilled or used in situ. The ease of formation of the enamine depends on the structure of the secondary amine as well as the structure of the ketone. Thus pyrrolidine reacts faster than morpholine or piperidine, as expected from a rate-controlling transition state with imonium character. Six-membered ring ketones without a substituents form pyrrolidine enamines even at room temperature in methanol (20), and morpholine enamines are generated in cold acetic acid (21), but a-alkylcyclohexanones, cycloheptanone, and linear ketones react less readily. In such examples acid catalysis with p-toluenesulfonic acid or... 

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