Tr-Complexes

The solubility of hydrogen chloride in solutions of aromatic hydrocarbons in toluene and in w-heptane at —78-51 °C has been measured, and equilibrium constants for Tr-complex formation evaluated. Substituent effects follow the pattern outlined above (table 6.2). In contrast to (T-complexes, these 7r-complexes are colourless and non-conducting, and do not take part in hydrogen exchange. 

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. 

R" = H, R = H) gives similar butadienethiolate complexes. Tr-Complexation of thiophene in compounds of the type 123 activates the heterocycle relative toward other nucleophiles, such as OMe , SMe , SEt , S(i-Pr) , andCH(COOMe)2. The products 127 include the ring-opened butadiene-thiolate framework. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

The complex that is formed can dissociate to form a cation (n-tr-complex) and an iodide anion, with the iodide ion reacting with the excess iodine molecules that are present. In addition the decomposition of the n-cr-complex can lead to the formation of highly reactive iodine cations, which can initiate further reactions — e.g. oxidations or electrophilic substitutions of aromatic systems [11, 13]. 

Bunz UHF (1999) Carbon-Rich Molecular Objects from Multiply Ethynylated tr-Complexes. 201 131-161... 

As stated earlier, this reaction did not match perfectly with the Curtain-Hammett postulate. The chiral Mo complex can select the favored face (either A or B) from L-C. However, facial selection of B-C on formation of the tr-complex (A or B) should be dictated by the orientation of the carbonate itself not by the chirality of the Mo complex. At the same time, we would expect the chiral Mo complex to... 

The NR3 substituent will, of course, exert, overall, a powerful electron-withdrawing, i.e. destabilising, inductive (polar) effect on all three positively charged tr complexes (40, 41 and 42), compared with the o complex for similar attack on benzene itself (cf. p. 132). 

The transformation of the Tr-complex into the o-complex is supposed to be intramolecular. 

The existence of tr-complex intermediates in C-H activation chemistry has been suggested to explain inverse kinetic isotope effects in reductive elimination processes whereby alkanes are formed from alkyl metal hydrides (Scheme 3).9... 

Other clear-cut evidence that the dichotomy between primary and secondary amines cannot be due to differential steric compression in the tr-complexes formed in these reactions has been afforded by Nudelman and Cerdeira82 in their study of the reactions of o-and p- lluomn i Irobenzenes with two primary amines n- and iso-propylamine in... 

Metal migration about a tr-complexed polyene ligand... 

Figure 3.IO. A proposal rationalizing the observed fluorescence perturbation and selective deuteration of the Sd-Cd(II) complex (a) n-Ar-Cd(ll) complex (b) Ar-Cd(II) tr-complex (c) Ar-Cd(II) tr-complex. (Reproduced from Ref. 24. Copyright 1990 American Chemical Society.)...

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

Fig. 1. Decrease of activation energy in dissociative chemisorption by van der Waals adsorption (Bj), tr complex adsorption (E ), and resonance effects (Ei) Ej is activation energy of homogeneous reaction.

Specific stabilisation of canonical forms of the tr-complexes for 0- and p-attack can also be effected by a phenyl group, e.g. (47ft 47d),... 

This step is not favored by the protonation of the d-complex for polar reasons. A fast equilibrium between the protonated tr-complex and the nonprotonated quinoline in acetic acid cannot be excluded, however [Eq. (20)]. 

Bunce, N. J., K. U. Ingold, J. P. Landers, J. Lusztyk, and J. C. Scaiano, Kinetic Study of the Photochlorination of 2,3-Dimethyl-butane and Other Alkanes in Solution in the Presence of Benzene. First Measurements of the Absolute Rate Constants for Hydrogen Abstraction by the Free Chlorine Atom and the Chlorine Atom-Benzene tr -Complex. Identification of These Two Species as the Only Hydrogen Abstractors in These Systems, . /. Am. Chem. Soc., 107, 5464-5472 (1985). 

As intuitively deduced, coordinating Y and Z must be removed or displaced in order for alkenes to coordinate and form an alkene tr-complex (Scheme 2). Since these intermediates, Cp 2MR(alkene), have never been observed in any detectable amounts for early transition metal-mediated polymerization, several chelate model systems have been devised (Figure 3). 

Vinyl ethers and amines disclose little tendency to revert to type thus, the intermediate formed by reaction with an electrophilic reagent reacts further by adding a nucleophilic species to yield an addition compound cf the sequence (8) — (11). Thiophene and pyrrole have a high degree of aromatic character consequently the initial product formed by reaction of thiophene or pyrrole with an electrophilic species subsequently loses a proton to give a substituted compound cf the reaction sequence (12) — (15). Furan has less aromatic character and often reacts by overall addition as well as by substitution. In electrophilic addition, the first step is the same as for substitution, i.e. the formation of a tr-complex (e.g. 13), but instead of losing a proton this now adds a nucleophile. 

NMR spectra of 1,2,4-triazines are well documented, especially the XH and 13C spectra. The parent compound (1) shows three signals in the XH NMR spectrum, as expected, the shifts depending on the solvent as shown in Table 2. In liquid ammonia a tr-complex is formed. The signal for H-6 is a doublet of doublets, while the signals for H-3 and H-5 are simple doublets. Since to our knowledge coupling between H-3 and H-5 is never observed in simple 1,2,4-triazines, XH NMR spectroscopy provides a simple method to distinguish between isomeric derivatives. 

Since the reaction of Pd (II) species with acetate is slow, we might predict the reactions of olefins with the Pd (II) species to give the intermediate tr complexes which are also slow. However, this is only partially true. 

The Effects of Ionizing Radiation on Solid Catalysts Ellison H. Taylor Organic Catalysis over Crystalline Aluminosilicates P. B. Venuto and P. S. Landis On the Transition Metal-Catalyzed Reactions of Norbornadiene and the Concept of tr Complex Multicenter Processes G. N. SCHRAUZER... 

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