Arenes direct release

Protiodesilylation has been exploited by many researchers as a mechanism for releasing arenes directly from the linker by cleavage of a C-Si bond (Figure 14.13) [94-99],... 

Electron-donating substituents direct the incoming nucleophile predominantly to the meta-position and electron-withdrawing substituents to the ortho-position. Oxidative demetallation (DDQ, iodine) is applied to reoxidize the cyclohexadienyl ligand, releasing a substituted arene. Addition of nucleophiles to halobenzene-FeCp complexes leads to nucleophilic substitution of the halo substituent (Scheme 1.34). Demetallation of the product complexes is achieved by irradiation with sunlight or UV light in acetone or acetonitrile. 

Under similar oxidative conditions, with activation of the aromatic C-H bond, some arenes could be used directly as aryl sources [41]. Unfortunately, by analogy with the Friedel-Crafts acylation, this reaction is regioselective for very few substrates only. High regioselectivity was, however, obtained if coordinating substituents on the arenes facilitate an orthopalladation reaction by a Pd(II) species [42]. After carbometallation and reductive elimination, Pd(0) is released, which has to be converted into the initial Pd(II) species in an extra oxidation step. Usually, quinines are used for this purpose, but in combination with certain heteropolyacids as cocatalysts even molecular oxygen can be employed as the oxidant. 

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

Substituents on an arene tend to direct addition in the way one might expect. Electron-releasing substituents, Q, usually direct attack meta, and electron-attracting ones ortho rather than para, perhaps because that puts Q at the terminus of the conjugated system of the resulting open polyenyl. 

When a halogen atom is employed to direct orr/i( -metallation, the Cr(CO)3 unit then allows substitution to follow, provided that an appropriate electrophile is employed (Scheme 10.58). In the case of butyrolactone, ring opening by the lithium reagent 10.225 releases an alkoxide that counterattacks onto the arene, leading to formation of lactone 10.227. 

This result represents the first example of a synthetic cycle for arene borylation facihtated by an f-element. The authors investigated the reaction mechanism and proposed a concerted direct B—H attack on an aromatic C—H bond, which resulted in the formation of the B—C bond and the release of one molecule of H2. This type of reactivity resembles a-bond metathesis if boron is considered a metal center. Albeit uranium was not directly involved in the borylation of the arene C—H bond, the formation of the diuranium arene inverse-sandwich complex plays an important role. The arene, i.e., benzene or naphthalene, is partially reduced upon coordination to uranium,which makes it more susceptible to attack by an electrophile such as borane. Therefore, although uranium is not direcdy involved in the C—H bond activation step, this example illustrates that f-elements can render arenes reactive in nonmetal-mediated transformations by forming activated arene metal complexes. 

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