Coordination benzene

This work was extended to consider the analogous toluene (179+) and p-xylene (180+) systems, and [K2-(HTp )Pt(Ph)(r 2-C6H6)]+ (181+, Scheme 14). Each of these were obtained by protonation of the respective aryl hydride complex Tp Pt(R)H(Ar) (R = H, Ar = Tol 182, 2,5-Xyl 183 R = Ar = Ph 153).72 As with 178+, no evidence for arene dissociation is observed for either 179+ or 180+ below 0 °C, though both lose the arene above this temperature to afford the same dihydride-bridged dimer. In contrast, 181+ slowly exchanges coordinated benzene with the solvent at -100 °C. 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

A peculiar complex is formed by if coordination of Os(II) ammine complex to one of the double bonds of benzene rings, rather than rf coordination, and the coordinated benzene rings show interesting reactivity [82]. For example, Os(II) coordinates regioselectively to the 2,3-double bond of anisole to form the complex 333, and hence localization of the remaining 7r-electrons occurs. As a result, at 20 °C an electrophile attacks easily at C(4) due to electron-donation of the methoxy group. The 4H-cationic intermediate 334 is stabilized by backdonation from the metal, and the monosubstitution product 334 is formed without deprotonation. The / ara-substituted anisole 335 is... 

The use of photochemical dinitrogen loss has been selectively employed in similar reactions with some notably enlightening results [124,125]. An early study shows that Cp Re(CO)(L)(N2) (L is P(OEt)3, P(OMe)3, PMe2Ph) photochemically produces frans-Cp Re(CO)(L)(Ph)Cl under UV irradiation in chlorobenzene, similar to the reports above [124]. However, the analogous reaction of Cp Re(CO)2(N2) with 1,4-difluorobenzene, produces both the C-H oxidative addition product Cp/Re(CO)2(Ar)H (Ar is 2,5-C6F2H3) and the coordinated benzene product, Cp Re(CO)2( 2-l,4-C6F2H4) [125]. The two isomers interconvert around 213 K. 

As we saw with metal complexed vinyl carbons, or-ganometallic complexes of aromatic compounds exhibit l3C chemical shifts upfield of the uncomplexed compound. For example, metal-coordinated benzene carbons are found in the range 8 74-111, depending on the exact structure of the complex. Similarly, metal-coordinated cyclopentadienide (C5H5-, abbreviated Cp ) appears in the range 8 75-123. 

It seems likely that this complex is three-coordinate with weakly coordinated benzene occupying the fourth coordination position. 

Such structures do not seem to be of great general relevance as HDS models, since they produce activation predominantly of the coordinated benzene ring towards nucleophilic attack [107, 108], For instance, [Mn(CO)3(T -BT)]BF4 reacts with nucleophiles (H , Me, Ef, Ph, CHjCN ) by addition to either the 4- or 7-position of the carbocyclic ring, without any indication of attack to the heterocycle [111]. 

The distances between the metal atoms and the plane of the coordinated benzene ring are slightly increased compared to open chain analogs (chromium [116, 117], iron [118. 121]). 

The internal coordinates were selected so as to make it physically meaningful to compare force constants of different molecules. For the free and the coordinated benzene ring, the internal coordinates selected were based on suggestions by Pulay et al. [10b, 13c]. The C-Cr-C bending coordinates of Cr(CO)g were chosen in a way that allowed analogous definitions in the case of... 

In Equation (19.8), a similar mechanism occurs for the 18-electron [(> 6 6H6) Mo(CO)3] molecule, with the coordinated benzene ring slipping to an i/ -linkage to allow room for the initial PR3, where the intermediate [(>/4-C6H6)Mo(CO)3(PR3)] has been isolated and characterized in the solid state. Sometimes an associative mechanism will also involve a second term in its kinetics rate law involving a competition between addition of Y and addition of solvent, as was observed in Chapter 17 for square planar substitution reactions. In this case, the solvent term will follow pseudo-first-order kinetics. 

Rate constants for the oxidative addition to the 14-electron intermediate were obtained directly from flash photolysis experiments. Flash photolysis of RhCl(CO)(PPh3)j causes selective dissociation of CO to form the 14-electron intermediate RhCl(PPh3)j or a derivative of it containing coordinated benzene (Scheme 6.2). Oxidative addition of dihydrogen to this intermediate was observed directly by timeresolved UV-vis spectroscopy and was shown to be fast k = 1.0 x 10 M" s and AG = 10 kcal/mol). A small kinetic isotope effect was measured, k /k = 1.4, and this value is consistent with an early, three-centered transition state in which little H-H bond breaking has occurred. 

The chemical shift of coordinated benzene is lower (the shift of the signal due to coordination toward higher fields) compared to the free ligand. This is in agreement with the lowering of the multiplicity of the C —C bond due to coordination of the arene molecule (Table 10.5). 

PhMe2SiLi cleaves silyl enol ethers to give the lithium eno-late more rapidly than does methyllithium, allowing f-butyldi-methylsilyl enol ethers to be purifed, and then converted into the lithium enolate at low temperature. It reacts with the N-sulfonamides of pyrroles, indoles, and secondary amines, but not primary amines, to remove the sulfonyl group (eq 25), but it opens A -sulfonylaziridines by attack on carbon. It adds to iron-coordinated cyclohexadiene (eq 26) and to chromium-coordinated benzene rings (eq 27). ... 

The overall reaction consists of addition of two hydrides followed by two protons to coordinated benzene. In this last step, the two protons reoxidize the metal and liberate free cyclohexene [28]. The reaction is thus catalytic in the platinum metal even though it is stoicheiometric in hydride and acid. 

The C-H bond activation of coordinated benzene is the rate-determining step and is favoured by electron-donating ligands L. By craitrast the withdrawing ligand L=CO provides a more efficient catalyst for hydroarylation of ethylene, by decreasing the energy barrier of the olefin insertion [120]. 

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