Overlap control

The predominance of cleavage at bond a over cleavage at bond b probably results from overlap control over the reaction pathway. <79) As shown below, there is greater overlap between the carbonyl orbitals and the cyclopropane a bond orbitals than between the carbonyl and b bond orbitals ... 

Orbital overlap controls angle of successful attack... 

Overlap control by steric factors of phenyl substituents in cyclopropanes additionally containing a carbonyl group have been described by Zimmerman and coworkers in the liquid ammonia reduction of the isomeric diphenyl cyclopropyl ketones cis- and trans-137. 

We proceeded, through overlapping control experiments [29, 30a, 30b], to exclude each individual function of the product molecule as a source of simple chemical catalysis. The control additives are shown in Figure 15 and the results are summarized in Table 1. In CHCI3 at 2.2 mM concentrations of (4) and (5), a 50% increase in initial rate was observed when the reaction was seeded with half an equivalent of product (6) (Table 1, entry 2). Our control experiments, detailed below, show that this 50% increase is the result of replication, i.e. a template effect as shown in Figure 3, complex (7). 

The geometric deformation of molecules can also induce orbital distortions. Seebach and coworkers [IS] have shown that dioxanones 1 are distorted. In the ground state, carbon-4 is pyramidalized through the mean plane of the ring toward carbon-2. This distortion is enhanced in the transition state for a nucleophilic attack, so such reactions are stereoselective because of better overlap control (Figure L7). 

The epimerization at both the ring stereogenic centers causes the diastereomeric interconversion 22< >30. Since the rates of [l,5]-hydride shifts in these two diastereomers are comparable, some of the product formed from the pyrolysis of 22 must actually arise from 30 and vice versa. After making corrections for the concurrent double epimerization by following an established procedure [9, 10], it was shown that the mechanistically significant ratio of the products /i,Z-31 and Z,Z-31 formed directiy from 22 was >220 1. Very clearly, the orbital overlap controlled the reaction of the stereochemically well defined reactant 22 and caused one symmetry-allowed pathway to take prominence over the other symmetry-allowed pathway. 

For another example of overlap control, in the Darzens condensation, see Vol. 2, Chap. 1.13. 

Figure 1 Overlap control as an explanation for the stereochemical control of the Darzens condensation...

A procedure has been developed which extends the Darzens synthesis to the preparation of aziridine esters and amides (equation 29). Reaction of benzalaniline (80) and ethyl chloroacetate (in BuKDK/DME) results in the formation of the trans-aziridine (82a) in 29% yield analysis of the crude reaction mixture showed no more than 10% of the isomeric cis adduct (83a). Similarly, condensation of 2-chloro-N,N-diethylacetamide and (80) affords aziridines (82b) and (83b) in 65% yield. However, in this case the cis isomer predominates ( 90 10, cis trans). The formation of (83a R = OEt) as the major product is consistent with the overlap control model suggested by Zimmerman. Apparently, the decreased stability of the amide (81b) enolate relative to the ester (81a) enolate causes ki (cyclization) to become greater than k (addition Scheme 19) it follows that the stereochemical outcome is determined in the initial aldol step, and steric arguments are advanced to explain this outcome. 

VDR requires RXR as a heterodimer partner to be fully active. Second, RXR-RXR homodimers could now regulate transcription from RXR target genes (response elements with a single nucleotide spacer). This dependence on RXR by RAR, TR, and VDR, coupled with the independent activity of RXR homodimers, allows for overlapping control of multiple gene networks in response to the relative ratios of the appropriate nuclear receptor ligands and 9-c/s-retinoic acid. 

Another important aspect is the compatibility of the organic concept with another powerful rural developement and marketing tool, widely used in Italy, i.e. the geographical indications (PGI/PDO) instruments. Compatibility of production mles is not always guaranteed, and the issue of overlapping controls is also of extreme importance. 

Coordination among Multiple Controllers As mentioned earlier, coordination problems are pervasive in this accident due to overlapping control responsibilities and confusion about responsibilities in the boundary areas of the controlled process. Most notably, the helicopters usually operated close to the boundary of the TAOR, resulting in confusion over who was or should be controlling them. 

Some constraints on the two system components (ATC and TCAS) are closely related, such as the requirement to provide advisories that maintain safe separation between aircraft. This example of overlapping control raises important concerns about potential conflicts and coordination problems that need to be resolved. As noted in section 4.5, accidents often occur in the boundary areas between controllers and when multiple controllers control the same process. The inadequate resolution of the conflict between multiple controller responsibilities for aircraft separation contributed to the collision of two aircraft over the town of Uberlingen (Germany)... 

In 1998, the BOOS health unit did receive a report on an MOE inspection of the Walkerton water system that showed some serious problems did exist. When the local Walkerton public health inspector read the report, he filed it, assuming that the MOE would ensure that the problems identified were properly addressed. Note the coordination problems here in an area of overlapping control. Both the MOE and the local public health inspector should have followed up on the 1998 inspection report, but there was no written protocol instructing the public health inspector on how to respond to adverse water quality or water system inspection reports. The MOE also lacked such protocols. Once again, the local public health authorities received no feedback that indicated water system operations were problematic. 

Jemmis, E.D. (1982) Overlap control and stability of polyhedral molecules. Closo-Carboranes. J. Am. Chem. Soc.,... 

Zimmerman, H. E., Singer, L., and Thyagarajan, B. S. "Overlap Control of Carbanionoid Reactions. 

Since we deal here with a cyclic system, orbital overlap control could, in principle, dominate the whole process. 

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