7t-complexation

Studies of the reaction mechanism of the catalytic oxidation suggest that a tit-hydroxyethylene—palladium 7t-complex is formed initially, followed by an intramolecular exchange of hydrogen and palladium to give a i yW-hydtoxyethylpalladium species that leads to acetaldehyde and metallic palladium (88-90). 

All lation of Phenols. The approach used to synthesize commercially available alkylphenols is Friedel-Crafts alkylation. The specific procedure typically uses an alkene as the alkylating agent and an acid catalyst, generally a sulfonic acid. Alkene and catalyst interact to form a carbocation and counter ion (5) which interacts with phenol to form a 7T complex (6). This complex is held together by the overlap of the filled TT-orbital of the aromatic... 

In this mechanism, a complexation of the electrophile with the 7t-electron system of the aromatic ring is the first step. This species, called the 7t-complex, m or ms not be involved directly in the substitution mechanism. 7t-Complex formation is, in general, rapidly reversible, and in many cases the equilibrium constant is small. The 7t-complex is a donor-acceptor type complex, with the n electrons of the aromatic ring donating electron density to the electrophile. No position selectivity is associated with the 7t-complex. 

Antimony(III) chloride forms colored 7t-complexes with double bond systems (e.g. vitamin A). 

Obviously the structures and yields of Birch reduction products are determined at the two protonation stages. The ring positions at which both protonations occur are determined kinetically the first protonation or 7t-complex collapse is rate determining and irreversible, and the second protonation normally is irreversible under the reaction conditions. In theory, the radical-anion could protonate at any one of the six carbon atoms of the ring and each of the possible cyclohexadienyl carbanions formed subsequently could protonate at any one of three positions. Undoubtedly the steric and electronic factors discussed above determine the kinetically favored positions of protonation, but at present it is difficult to evaluate the importance of each factor in specific cases. A brief summary of some empirical and theoretical data regarding the favored positions of protonation follows. 

When the iron sandwich complex bears an arene substituent with at least one benzylic hydrogen, the acidity of the latter is enhanced by the 7t-complexation to the 12e fragment FeCp+. The pKa of the conjugate acid of superoxide radical... 

First we will discuss reaction systems of the types shown here in which adducts were detected analytically and characterized as 7T-complexes. The idea of 7T-com-plexes as intermediates on the path to products of an electrophilic aromatic substitution was originally suggested by Dewar (1949). He did not, however, follow his hypothesis further. It was taken up again by Olah, particularly in relation to nitration (reviews Olah, 1971 Olah et al., 1987, 1989). 

In the reaction of the strongly electrophilic 4-nitrobenzenediazonium ion with 2-naphthol-6,8-disulfonic acid, which yields a sterically hindered o-complex, Roller and Zollinger (1970) actually observed the rapid formation of a 7T-complex spec-trophotometrically at low pH. The concentration of the 7T-complex decreases slowly and at the same rate as that of the formation of the azo product. H NMR data indicate that the 7t-complex is not localized. All 7T-electrons of the benzene and the naphthalene system are involved in the complex formation to a similar degree, in... 

On the basis of these correlations, Gold and Satchell463 argued that the A-l mechanism must apply (see p. 4). However, a difficulty arises for the hydrogen exchange reaction because of the symmetrical reaction path which would mean that the slow step of the forward reaction [equilibrium (2) with E and X = H] would have to be a fast step [equivalent to equilibrium (1) with E and X = H] for the reverse reaction, and hence an impossible contradiction. Consequently, additional steps in the mechanism were proposed such that the initial fast equilibrium formed a 7t-complex, and that the hydrogen and deuterium atoms exchange positions in this jr-complex in two slow steps via the formation of a a-complex finally, in another fast equilibrium the deuterium atom is lost, viz. 

These early results have since been confirmed and extended by a vast and still growing body of research. All of the contemporary spectroscopic techniques (ir, uv, visible, nmr, esr) have been brought to bear on the problem, and further confirmation has come from cryoscopic and conductometric studies. The early confusion that resulted from the coexistence of both donor-acceptor or non-covalently-bonded complexes) has been clarified. This research has been extensively reviewed10,13-15 and will not be detailed here. 

In Chapter 3, it was mentioned that positive ions can form addition complexes with 7T systems. Since the initial step of electrophilic substitution involves attack by a positive ion on an aromatic ring, it has been suggested that such a complex, called a % complex (represented as 10), is formed first and then is converted to the arenium ion 11. Stable solutions of arenium ions or 7t complexes (e.g., with Br2, l2> picric... 

Carbon-Rich Molecular Objects from Multiply Ethynylated 7T-Complexes... 

The methanolic cupric bromide oxidation of propargyl alcohol to trans-BrCH-CBrCH20H (30%) and Br2C=CBrCH20H (18%) and, under other reaction conditions, Br2C-CBr-CH20H (93 %) follows simple second-order kinetics with a rate coefficient of 1.5 x 10 l.mole . sec at 64 °C. A mechanism of ligand-transfer in a 7t-complex is proposed. ... 

Fig. 29. Possible mechanism for C—C activation involving formation of a 7r-complex followed by direct sp2-sp3 C-C insertion. This mechanism is ruled out based on the much larger potential energy barrier for C—C insertion relative to C—H insertion from the 7T-complex.

As mentioned in the introduction, 7T-complexes of conjugated dienes with palladium(II) are not stable enough to be isolated. However, reaction of a conjugated diene with PdCl2 in alcoholic solvents or acetic acid gives a (TT-allyl)palladium complex 27 in which the... 

Reagents which selectively retard certain chemical species can be incorporated into a thin-layer plate. Thus, silver nitrate, which forms weak 7t-complexes with unsaturated compounds, aids their separation from saturated compounds. 

The [5 + 2]-cycloadditions of air-stable 7]3-pyranyl and 7]3-pyridinyl molybdenum 7T-complexes (4758 and 48,59 respectively) with alkenes reported by Liebeskind and co-workers provide a novel method for the construction of oxabicyclo[3.2.1]octenes and highly functionalized tropanes (Scheme 20). This process involves the formation of a TpMo(CO)2 complex which in the presence of EtAlCl2 reacts with an alkene in a stereoselective [5 + 21-cycloaddition to give metal-complexed cycloadducts 50 and 51 (Tp = hydridotrispyrazolylborato). Metal decomplexation via protiodemetalation or oxidation affords the products in good to excellent yields (Scheme 21). 

In their resonance forms, the complexes are considered as acetylenic 7t-complexes (+2 oxidation state) or as metallacyclopropenes (+4 oxidation state), which lead to coupling reactions of the alkyne (insertion into the metallacyclopropene) or to a substitution of the alkyne by the substrate. 

The question as to whether a transition metal complex of type 4 is best described as an alkene 7T-complex 4A or as a metallacyclopropane 4B, which is of practical importance, has been addressed in several computational studies on the relationship between alkene 7T-complexes and three-membered rings [48—52]. It has been concluded that the titanium complexes of type 4 are best represented as titanacydopropanes, i.e. by resonance structure 4B, if one is willing to accept the notion that 4A and 4B are limiting resonance forms [52],... 

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