Exchange arenes

Thermal arene exchange of tetramethylthiophene with [(/ -cymene)RuCl2]2 affords 130 (89JA8828), which on reaction with AgBE4 and excess tetramethylthiophene yields 131. The Ru—S thiophenic cluster, 132, was synthesized by reaction of 130 with (Mc3Si)2S followed by anionic metathesis and formation of the PFg salt. The coordination geometry around each ruthenium atom is pseudooctahedral. 

Scheme 6.1-6 Arene exchange reactions of ferrocene in [BMIM]CI/AICl3.

The general process is shown in equation (3). The L unit in the [Cr(CO)3L3] can be CO (most common),15"20 MeCN,21 22 2-alkylpyridine,23 ammonia24-27 and other donor ligands. The rate (reaction temperature) is related to the nature of L the most reactive readily available source of Cr(CO)3 is [(q6-naphthalene)Cr(CO)3], which undergoes favorable arene exchange under mild conditions with many... 

The requirement for AlCh adds some obvious limitations, but a modest number of substituted arenes have been successfully coordinated to the [FeCp]+ unit. Photoinduced arene exchange has been described43 and is particularly effective with the -r -chlorobenzene-r -cyclopentadienyliron cation without the need for Lewis acids.42 More basic (electron rich) arenes readily replace chlorobenzene, leading to FeCp cation complexes of O-phenylethyl-p-toluenesulfonate and variously substituted thiophenes.42 The charged complexes are not purified by conventional organic techniques, such as chromatography, but re-crystallization is possible. The complexes are very air and heat stable again, methods of removal of the arene from the Fe are few. The simplest is pyrolysis at >200 C.46... 

In addition to these exchange reactions, a number of alkane/alkane and al-kane/arene exchange reactions could be studied as equilibria (benzene, toluene, cyclopropane, methane, ethane, neopentane, cyclohexane). Determination of equilibrium constants allowed calculation of AG° values and estimation of relative metal-carbon bond energies. Wolczanski concluded that the differences between metal-carbon bond energies and the corresponding carbon-hydrogen bond energies were essentially the same [82]. 

Synthetic applications of the photochemistry of transition-metal arene complexes are rare, except for the (h -arene)M(CO)j (M = Cr, Mo, W) complexes where photo-substitution of ligands for CO occurs. These complexes also undergo arene exchange upon photolysis ... 

Table 1. Results of Arene Exchange Induced by UV Irradiation OF (h -ARENElRuCLjfPRj) ...

Arene exchange can be induced by irradiation, but give few results of preparative significance, Irradiation of -(p-xylene)-) -cyclopentadienyl-Fe(I) in the presence of hexamethylbenzene gives the corresponding hexamethylbenzene complex in high yield ... 

Arenes show higher reactivity than alkanes, but this difference is not so pronounced as in many other cases benzene, which is the least reactive arene, exchanges its hydrogen atoms only twice as fast as the most reactive of the alkanes, cyclohexane. In the case of substituted aromatic molecules the steric effect is again very pronounced. 

Thermo chemical studies show the arene-Mo bond (68kcal mol in [(q -CgHg)Mo(CO)3] (40)) to be stronger than the arene-Cr bond (53 kcal mol" in [(q -CgHg)Cr(CO)3] (1)) [63,64]. Kinetically, however, the situation is reversed. The metal arene bond in the Mo complex 40 is far more labile than that in the Cr complex 1. In the absence of a Lewis base catalyst, arene exchange in (arene)Mo(CO)3 complexes is measurable at temperatures as low as 60 °C (com-... 

The most recent addition to the synthetic arsenal are arene exchange reactions in complexes containing condensed aromatics. Like in the Cr(CO)3 chemistry detailed above, the most reactive complexes are those of naphthalene and its substituted analogues. They are accessible in high yield via the Ag(I) route but care needs to be taken to use rigorously dry solvents and avoid contact with donor solvents [76]. Examples are given in Scheme 8. 

Scheme 8 Arene exchange reactions with (naphthalene)Mn(CO)3. Arene exchange reactions are also successful with complexes 73-76 [76]...

The metal arene bond in (arene)FeCp complexes is photolabile [93], and this forms the basis of the use of these complexes as photoinitiators for cationic polymerizations of epoxides [94]. Arene exchange via this route has not been used extensively. On photolysis of (p-xylene)FeCp in CH2CI2 or acetone by a medium pressure Hg-arc or by bright sunlight , the complex undergoes arene exchange with hexamethylbenzene, paracyclophane and thiophene under mild conditions... 

Scheme 10 Synthesis of (Arene) FeCpcomplexes by arene exchange...

While investigating the nature of bonding in these complexes, Evans et al. reported [106] an arene exchange reaction in which a bridged xylene complex was formed by the reaction of (C5Mes)3U, KCg and p-xylene as shown in Eq. 21 ... 

Dean et al. (65) have prepared organometallic mercury (2+) compounds of the type Hg2(AsF6)a Arene. The NMR spectra of these complexes exhibit averaged arene carbon signals due to rapid free-complexed arene exchange at NMR probe temperatures. On the basis of the variation of the aryl carbon chemical shift with the arene/Hg22 ratio for the system Hg2(AsFe)2-hexa-methylbenzene (HMB) the existence of the following two equilibria was determined ... 

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