Sigma-complex

Back donation into the X-H a is essential for binding because pure Lewis acids such as AlMes or BF3 do not form isolable H2 or HX a complexes. On the other hand, very strong back donation breaks the X-H bond in an oxidative addition to give a classical dihydride (3.36). 

Coordinated H2 can deprotonate with base, even so mild a base as NEt3, for 3.37. Several H2 complexes can both exchange with free H2 or D2 and exchange with solvent protons and thus can catalyze isotope exchange between gas-phase D2 and solvent protons. 

Cp FeH(dppe) shows faster protonation at the Fe-H bond, so that the nonclassical [Cp Fe (H2)(dppe)] is obtained at —80°C on warming above —40°C, the complex irreversibly converts to the classical form [Cp Fe (H)2(dppe)]+. The Fe-H is the better kinetic base (faster protonation), but the Fe itself is the better thermodynamic base (dihydride more stable). 

Dihydrogen complexes have been characterized by X-ray, or, much better, by neutron diffraction. An IR absorption at 2300-2900 cm although not always seen, is assigned to the H-H stretch. The H2 resonance appears in the range 0 to -106 in the NMR spectrum and is often broad. Partial deuteration is useful because the H-D analog shows a /hd of 15-34 Hz in the NMR. This compares with 43 Hz for free HD and 1 Hz for classical L M(H)(D). The empirical Morris equation reliably relates /hd to the H. . . H(D) distance, 

Stretched H2 complexes with H-H distances 1A are rare and difficult to distinguish from classical hydrides other than by neutron diffraction or /hd- For example, hh in [Re(H2)Hs P(o-tolyl)3 2] is 1.36A by neutron diffraction. 

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Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

Sigma-bonded transition metal complexes are able to polymerize a range of vinyl monomers, the only limitation being that the monomer should not have groups that react chemically with the transition metal compound. An important observation is that styrene and its derivatives are polymerized by the sigma complexes. In this respect they differ from the jr-allyl compounds that show no reactivity at all toward these monomers. A reasonable explanation for this is that the mechanism of the initiation is different... 

The polymerization of vinyl monomers by transition metal sigma complexes has been shown by Ballard and van Lienden (25,28) to be catalyzed by white light which has been filtered through pyrex glass. The effect is best illustrated by the following experiment ... 

The Chemistry and Anionic Sigma Complexes by E. Buncel, M.R. Crampton,... 

F. Terrier, Eiectron Deficient Aromatic and Heteroaromatic Base Interactions. The Chemistry of Anionic Sigma Complexes, pp. 78-85. Elsevier, Amsterdam, 1984. 

Long NJ, Williams CK Angew Chem Int Ed 2003,42 2586-2617 Metal alkynyl sigma complexes synthesis and materials... 

Structure III in Figure 7-1 represents an arenium ion, more commonly called a sigma complex. Figure 7-2 shows the energy changes occurring during the reaction. 

Once formed, the electrophile behaves like any other electrophile, so the mechanism of the attack is the same as that for the previous situation where a nucleophile attacked the electrophile (described in the earlier section Basics of Electrophilic Substitution Reactions ). The attack leads to the formation of the resonance-stabilized sigma complex, followed by the loss of a hydrogen ion to a base. 

In order to understand any chemical process, you need to remember that stability is the key. In the case of aromatic systems, resonance is important to stability. To find out more about stability and resonance, we begin by examining the resonance in each of the different sigma complexes that may form, listed below and shown in Figure 7-15. The entering group can attack in one of three relative positions ... 

Sigma complexes always have a positive charge. 

The sigma complexes arising from ortho, meta, and para attack. 

How can the substituent influence the resonance shown in Figure 7-15 The answer is that if the substituent can create another resonance structure, the sigma complex is further stabilized. This additional stabilization leads to a preference for a certain attack. 

What happens if the substituent is electron withdrawing Figure 7-17 shows what happens in this case. When G attempts to pull electron density from an electron-poor atom (+), the result is a destabilization of the structure for both ortho and para sigma complexes, greatly reducing the probability that they will form. 

Unlike the positively charged sigma complex, the Meisenheimer complex has a negative charge. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

The authors speculated that Pd(ii) was reduced by reaction with the IL, followed by formation of sigma complex between the olefin and copper triflate. This polarized complex then reacts with the Pd(0)-7r-complex with the substrate to form the final product as shown by the scheme below. Scheme 7. 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

Figure 13.69. Sigma complex detected with diphenylnitrenium ion.

Step (1) is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation (a benzenonium ion) loses a cation (most often to give the substitution product, rather than adding a nucleophile to give the addition product. The benzenonium ion is a specific example of an arenonium ion, formed by electrophilic attack on an arene (Section 11.4). It is also called a sigma complex, because it arises by formation of a o-bond between E and the ring. See Fig. 11-1 for a typical enthalpy-reaction curve for the nitration of an arene. 

Because of the high electron density in the aromatic ring, toluene behaves as a base both in the formation of charge transfer r complexes and in the formation of sigma complexes. When only n-electrons are involved, toluene behaves much like benzene and xylene. When o-bonds and complexes arc involved, toluene reacts much faster than benzene and much slower than xylenes. 

Sigma complexes have also been observed in the nmr. For example, when m-xylene is dissolved in HF + SbF5 at — 35°G, the proton magnetic resonance spectrum shown in Figure 7.7 is obtained. The peak at 4.7 ppp downfield from TMS is due to two parafinnic protons. The structure that best fits the spectrum... 

Another class of gitonic superelectrophiles (based on the 1,3-carbodica-tion structure) are the Wheland intermediates or sigma complexes derived from electrophilic aromatic substitution of carbocationic systems (eq 8). 

Although several different rate laws have been drawn for different substrates, the kinetic equations are generally consistent with a mechanism involving an electrophilic attack by I2 on an anionic imidazole ring system, followed by proton abstraction from the sigma complex as the rate-determining step. 

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