Arenes oxygenation

The increased reactivity of the twix methyl gronp for the photooxygenation of dimethylstyrenes, in solntion, has been rationalized in terms of attractive arene-oxygen interactions, in the transition state for the formation of the /wix-oriented intermediate. 

As mentioned earlier, Pd-catalyzed arene oxygenations have been known for over four decades. However, the vast majority of the early reports are plagued by low TONs and competing biphenyl formation, which often leads to catalyst decomposition [42]. Furthermore, oxygenations of substituted arenes generally afford the desired products as mixtures of olmlp isomers, which decrease the synthetic... 

While this method represents an important advance in the field of arene oxygenations, the use of PhI(OAc)j as a terminal oxidant is a significant limitation. Phl(OAc)j is expensive and leads to the formation of nndesired iodobenzene as a stoichiometric by-product. In order to address this drawback, the use of K S Og in place of Phl(OAc)j has been explored [45]. Potassium persulfate (KjSjOg) is more than an order of magnitude less expensive than Phl(OAc)j and leads to easily separable water soluble by-products. Based on previous results using Phl(OAc)j (Scheme 24.50), the use of 1 1 Pd/pyr catalytic system was investigated for the acetoxylation of benzene with as... 

The solid state structure of a Cs clOa complex evidenced a folded tub-shaped calix[8]arene conformation composed by four 3/4-COTie clefts (Fig. 7.28) [43]. The Cs" cation was electrostatically coordinated by the eight caUx[8]arene oxygens [43]. A similar solid state stmcture was reported for a dicesium complex of 1,5-bridged derivative lOi [36]. 

There are a total of eighteen different hydrocarbon series, of which the most common constituents of crude oil have been presented - the alkanes, cycloalkanes, and the arenes. The more recent classifications of hydrocarbons are based on a division of the hydrocarbons in three main groups alkanes, naphthanes and aromatics, along with the organic compounds containing the non-hydrocarbon atoms of sulphur, nitrogen and oxygen. 

Phenolic compounds are commonplace natural products Figure 24 2 presents a sampling of some naturally occurring phenols Phenolic natural products can arise by a number of different biosynthetic pathways In animals aromatic rings are hydroxylated by way of arene oxide intermediates formed by the enzyme catalyzed reaction between an aromatic ring and molecular oxygen... 

Numerous reactions have been described in which the oxygen of the oxepin system is removed to give benzene derivatives. The formation of the aromatic products can be rationalized by an arene oxide as intermediate. A suitable reagent for the elimination of an oxygen atom from this heterocycle is triphenylphosphane, e.g. formation of l,24 2a,12 and 2b.1,9... 

Transition-metal complexes such as [Rh(CO)2Cl]2,204 Rh(butadiene)2Cl,205 or Cr(CO)3(NH3)312 have also been used for the deoxygenation of oxepins to give 312,204 205 and benzoxepins to give 4.12,204 Occasionally, substantial amounts of phenolic compounds have been isolated due to the competing NIH shift of the arene oxide.204 1-Benzoxepin and 3-benzoxepin resist oxygen extrusion under these conditions probably due to their inability to form arene oxi-des.133,204... 

Chromium carbene complexes having electron-rich arenes tethered to the car-bene oxygen or carbon underwent photodriven intramolecular Friedel-Crafts acylation in the presence of zinc chloride (Eqs. 32 and 33) [118]. The process was highly regioselective, undergoing acylation exclusively para to the activating group. 

More recently, Curran and Keller found that the (TMSlsSiH-mediated addition of aryl iodides to arenes are facilitated by oxidative rearomatization with oxygen (Reaction 69). Here, AIBN is not necessary for good performance of the reaction. The reaction proceeds well in both inter- and intra-molecular (see above) versions. 

Arene hydrocarbon dioxygenases are capable of carrying out a number of reactions other than the introduction of both atoms of oxygen into the substrate. Illustrative examples of monooxygenation carried out by dioxygenases include the following ... 

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

A set of oxygen donor atoms, providing both a and tt donation to a metal center, is not appropriate to stabilize any low oxidation state of a metal.19 This is, however, a synthetic advantage since very reactive, unstable, low-valent metalla-calix[4]arenes can be generated in situ and intercepted by an appropriate substrate. In the absence of a suitable substrate, the reactive fragment, however, can collapse to form metal-metal bonded dimers. The formation of metal-metal bonds has been, however, so far observed in the case of Group V and VI metals only. The most complete sequence so far reported has been for tungsten, molybdenum, and niobium. 

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