Synchronous addition

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

Abb. 8a u. b. Diagramme fur rechtwinklige" und parallele" synchrone Addition zweier Doppelbindungen und einer Dreiiachbindung... 

The same MO methods can be applied to four- and five-center additions and eliminations, subject to certain restraints. Thus, the synchronous addition of X—X to a double bond in any unsaturate must be syn. The termolecular addition of X and Y to an unsaturate can be syn or anti. Provided that X and Y are nonbonded, YB theory, the Walsh-type correlations (Fig. 4), and extended HMO theory (Hoffmann, 1963) suggest an anti preference. In the next section, the powerful application of symmetry restrictions will confirm these predelictions. 

Mention should also be made of the addition of tervalent phosphorus compounds to alkynylphosphonic and to (alka-l,2-dienyl)phosphonic derivatives [in which addition occurs across the C(i)—C(2) bond]. The main product is of type 521 (X = Ph, OR or NR2), but is normally accompanied by smaller amounts of the isomers 522. Although this process is formally of a [3 + 2] format, a P NMR study of the kinetics suggests that the addition is not a synchronous addition but rather occurs in a stepwise manner . On the other hand, the phosphorylated Schiff base 523 undergoes regioselective and stereospecific reaction with a,j -unsaturated esters which, a kinetic study has shown, appears to be concerted. ... 

Naphthyl substituent shows a different character in the excited state in comparison with phenyl substituent. The photohydration of 1-phenyl-5,5-dim-ethyl-l,3-hexadiyne gives three photohydration products (58-60) (two acetylenyl ketone and one allenyl ketone) through both Si and Tj excited states (Scheme 20). An allenyl ketone product is obtained as a minor product. A proposed mechanism involves the protonation step as the rate limiting step in the formation of acetylenyl ketone products. On the other hand, the allenyl ketone product is formed by the synchronous addition mechanism of H3O+. 

Early data on the mechanism of the reaction of ozone with double bonds of organic compounds have been reviewed [35]. On the basis ofkinetics analysis, itwas suggested that this reaction does not occur by a synchronous addition mechanism but through a reversible stage involving the formation of an intermediate complex of ozone with the double bond. It was suggested that the subsequent stages of the reaction involve the... 

Field current is an important control element. It controls not only the power factor but also the pullout torque (the load at which the motor pulls out of synchronism). For example, field forcing can prevent pullout on anticipated high transient loads or voltage dips. Loads with known high transient torques are driven freqiiently with 80 percent power-factor synchronous motors. The needed additional field supplies both additional pullout torque and power-factor correc tion for the power system. When high pullout torque is required, the leading power-factor machine is often less expensive than a unity-power-factor motor with the same torque capabihty. 

Consider a process plant having a connected load of 15 000 kW and a running load of 12 500 h.p. at almost 0.65 p.f. lagging. Let a few large induction motors aggregating 2000 h.p. be replaced by as many oversized synchronous machines, with the purpose of improving the system p.f. in addition to performing the motor s duties. 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

The typical three-body TPG string is shown in Figure 4-71. With this arrangement the generated electricity is limited by the expander output. Additionally, both startup and synchronization must be accomplished by the expander instead of the steam turbine. 

Are there any circuits that are particularly noise-sensitive These include analog-to-digital and digital-to-analog converters, video monitors, etc. This may dictate that the supply has additional filtering or may need to be synchronized to the sensitive circuit. 

In addition to power factor considerations, synchronous motor efficiency is higher than similar induction motors. Efficiencies are shown in Table 7-1 for typical induction and unity power factor synchronous motors. Leading power factor synchronous motors have efficiencies approximately 0.5-1.0% lower. 

It is convenient to calibrate the burets so that the liquid is divided into 20 equal portions. Then, in the addition of the reagents, these calibrations aid in synchronizing the rates. 

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample. In addition, the pump and probe pulses are synchronized so that the lime interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

For reactions of A-acyliminium ions with alkenes and alkynes one has to distinguish between A-acyliminium ions locked in an s-trans conformation and those which (can) adopt an s-cis conformation. The former type reacts as a (nitrogen stabilized) carbocation with a C —C multiple bond. Although there are some exceptions, the intramolecular reaction of this type is regarded as an anti addition to the 7t-nucleophile, with (nearly) synchronous bond formation, the conformation of the transition state determining the product configuration. 

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