Aryl halides soft acids

General Characteristics. In accordance with the high tendency for formation of Co(CO)4 (2 ) and the acidity of (3), the former componnd behaves in many respects as a psendohalide see Pseudohalide). Relative rates of electrophilic snbstitntion with Mel and MeOTs classify (2 ) as a hard and relatively weak nncleophile see Hard Soft Acids and Bases). Compounds of the type RCo(CO)4 are prepared by two different methods reaction of an electrophihc halide, snlfate, or tosylate (RX) with (2 ), or reaction of HCo(CO)4 with alkenes. Simple alkyl derivatives have low stability nevertheless, many studies have been conducted in which species have been identified as RCo(CO)4 intermediates. Apart from alkyl and aryl derivatives, for example MeCo(CO)4 and PhCo(CO)4, (2 ) forms derivatives with main group residues R3E and R2E and with the main group metals Zn, Cd, and Hg as well as with transition metals, as shown by examples in Scheme 4. Co E bonding in these compounds appears to be predominantly covalent. The bond distances can be long, and consequently, the coordination number of Co can be high. 

Isopropyl alcohol under UV irradiation converts bromobenzene to benzene in 72% yield (Table 4). Similar replacement of bromine by hydrogen is accomplished by treatment of aryl bromides dissolved in dichloromethane with a mixture of ethanethiol and anhydrous aluminum chloride. This hard acid-soft base combination reacts with polycyclic aromatic halides and halogenated phenols by an addition-elimination mechanism, leading to an aryl ethyl sulfide through a radical anion intermediate. This is converted by another molecule of ethanethiol to the debrominated arene and diethyl disulfide. 1-Bro-monaphthalene is thus transformed into naphthalene (equation 59), 2,4,6-tribromophenol into phenol (equation 60), and bromochlorophenols into chlorophenols in 61-91% yields. ... 

One of the most general approaches to the synthesis of this type of compounds involves the Lewis acid promoted rearrangement of acetals of a-substituted propiophenones [4,5]. Besides silver salts, a large variety of soft and borderline Lewis acids have been found to be convenient catalysts for the 1,2-aryl shift, noticeably zinc halides in substoichiometric amounts [6]. As final products need to be free of metal traces for human consume, the use of Lewis acids supported on microporous solids can be advantageous since a better recovering of the catalyst can be anticipated. 

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