Os complex

We have seen ( 6.2.3) hat there is a close relationship between the rates of electrophilic substitutions and the stabilities of tr-complexes, and facts already quoted above suggest that no such relationship exists between those rates and the stabilities of the 7r-complexes of the kind discussed here. These two contrasting situations are further illustrated by the data given in table 6.2. As noted earlier, the parallelism of rate data for substitutions with stability data for o"-complexes is not limited to chlorination ( 6.2.4). Clearly, rr-complexes have no general mechanistic or kinetic significance in electrophilic substitutions. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Introduction of substituents on the carbocyclic ring relies primarily on electrophilic substitution and on organometallic reactions. The former reactions are not under strong regiochcmical control. The nitrogen atom can stabilize any of the C-nng o-complexes and both pyrrole and benzo ring substituents can influence the substitution pattern, so that the position of substitution tends to be dependent on the specific substitution pattern (Scheme 14.1). 

Od-fumace blacks used by the mbber iadustry contain over 97% elemental carbon. Thermal and acetylene black consist of over 99% carbon. The ultimate analysis of mbber-grade blacks is shown ia Table 2. The elements other than carbon ia furnace black are hydrogen, oxygen, and sulfur, and there are mineral oxides and salts and traces of adsorbed hydrocarbons. The oxygen content is located on the surface of the aggregates as C O complexes. The... 

The case for the generality of the o-complex mechanism is further strengthened by numerous studies showing that benzenium ions (an alternative name for the o-complex) can exist as stable entities under suitable conditions. Substituted benzenium ions can be observed by NMR techniques under stable-ion conditions. They are formed by protonation of the aromatic substrate ... 

C) rate-controlling o-complex formation (selective electrophile)... 

There is, however, no direct evidence for the formation of Cl", and it is much more likely that the complex is the active electrophile. The substrate selectivity under catalyzed conditions ( t j = 160fcbenz) is lower than in uncatalyzed chlorinations, as would be expected for a more reactive electrophile. The effect of the Lewis acid is to weaken the Cl—Cl bond, which lowers the activation energy for o-complex formation. 

At the next organizational level are factors directly causing error 1) job characteristics such o Complexity, time stress, noise, lighting, environment, or mental requirements, and 2) individual factors such as personality, and team performance. These, collectively, are called performance-influencing factors, or PIFs. 

However, treatment of 4-chloro-3-nitrocoumarin (81) with 2-mercaptophenol (254) provided the product of displacement of the chlorine atom 263. Treatment of compound 263 with triethylamine gave a mixture from which low yields of 266 and 267 were isolated (92ZOR1489). This fact can be explained by the formation of the o-complex 264. This complex is stabilized by carbonyl group participation and therefore an equilibrium of 263 and 265 can be expected. This is in accordance with the formed products (Scheme 41). A similar situation was described earlier for the reaction of 4,5-dichloropyridazin-6(17/)-one with the disodium salt of 2-mercaptophenol (82JHC1447). 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

Reductive elimination—to yield the coupling product 3 and regeneration of the catalytically active palladium-(O) complex 5. 

The electrophilic character of the palladium atom in the complexes formed by oxidative addition of aryl halides and alkenyl halides to palladium(o) complexes can be exploited in useful ways. 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

A mixture of ethyl 1//-azepine-l-carboxylate (24, R = Et 2.74 g, 17.3 mmol) and pentacarbonyliron(O) (4.16 g, 22 mmol) inTHF (25 mL) was irradiated with UV light for 30 h, during which time CO (g)(710mL) was evolved. (Caution reaction must be carried out in an efficient fume hood). The solution was reduced in volume and the residue chromatographed (neutral alumina, hexane). After removal of the solvent the product was purified by sublimation under high vacuum at 60 C yield 4.04g (77%) mp 87°C. The tricarbonyliron(O) complex of ethyl 2,4,7-trimethyl-1//-azepine-1-carboxylate (11.6% mp 54 C) was prepared similarly. 

Fig. 7-2. Potential energy E as a function of the reaction coordinate for reactions of the P-nitrogen of arenediazonium ions with nucleophiles yielding (Z)- and (is)-azo compounds, a) Reactant-like transition states (e. g., reaction with OH) b) product-like transition states (e. g., diazo coupling reaction with phenoxide ions product = cyclohexadienone-type o-complex (see Sec. 12.8).

Replacement of electrofugic leaving groups other than protons is called //750-substitution. For a discussion of o-complexes see Sec. 12.8. 

On the other hand, the rate constant k does not depend on the changing steric influence of substitutents in the 8-position, but correlates surprisingly well with the Hammett-Brown constant cr. This result indicates that the formation of an sp3-hybridized carbon atom (at the 1-position of the o-complex) leads to a compound without significant steric interaction of the electrophile with substituents in the 8-position. The o-complex cannot be planar and is asymmetric. The preferred conformation of a o-complex of this type is illustrated in Figure 12-6. The pseudoax-ial position of the electrophile E reduces the steric interaction between this group and the peri substituent R. 

As the o-complexes in these azo coupling reactions are steady-state intermediates (Wheland intermediates, named after Wheland s suggestion in 1942), their stereochemistry cannot be determined directly. Bent structures like that in Figure 12-6 can, however, be isolated in electrophilic substitutions of 1,3,5-triaminobenzene... 

Only one group of compounds is known for which o-complexes containing azo groups have been observed. Geidysh et al. (1969) added an equimolar amount of... 

On the other hand, the catalytic effect of water as a base is stronger at the 2-position. This result can be explained if one assumes that the proton is transferred by a water molecule which solvates the O- group in the reagent 3-sulfo-l-naphth-oxide dianion. As can be seen in 12.148, the base is already in the optimum position when the stage of the o-complex is reached. This explanation is supported by a comparison of the entropies of activation for reaction at the 2- and 4-positions. 

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