Fluoroarene-Cr

The reactivity of complexed haloarenes toward thiolates has been studied, and it has been reported that o-, m-, and p-dichlorobenzenetricarbonylchromium complexes 18a-c react with thiolates (RS R = Me, nBu, tBu Scheme 17, path i) under phase-transfer conditions or in DMSO to give 39 and 40a-c. The ortho- and para-complexes 18a and 18c undergo stepwise substitution of the two Cl atoms in a reaction sequence that can be easily controlled by the amount of added thiolate. The meta complex 18b shows a lower selectivity and gives a mixture of mono- and disubstituted products even in the presence of substoichiometric amounts of thiolate (Scheme 17) [22]. Similarly, LiCH(C02Et)CN and BuSH react with the o-dichlorobenzene complex 18a to give complex 39d and then disubstituted arene 40d, showing that this substitution can be performed with two different nucleophiles (Scheme 17) [23]. Phase-transfer catalysis has also been applied to fluoroarene-Cr(CO)3 complexes, which are more reactive toward thiolates than are the corresponding chloro derivatives [22]. 

Fluoroarene-Cr(CO)2L complexes 33p [L = CO, PPh3, P(OPh)3, P(pyrrolyl)3, P(pyrolyl)2 (NMeBn)], where L is a potential linker ligand for solid-phase synthesis, have been evaluated with regard to the rates of nucleophilic substitution by amines [35]. The preparative and kinetic results indicate that SNAr reactions on tris(pyrrolyl)phosphine-modified fluoroar-enechromium complexes proceed rapidly and with high efficiency, and are thus appropriate for the development of solid-phase versions for use in combinatorial synthesis (Scheme 18). 

Hydroxylamines deprotonated by KOH substitute for chloride in (chloro-arene)Cr(C0)3 in high yields [24]. Reaction of a (fluoroarene)Cr(CO)3 complex with dialkyl phosphite 12 resulted in formation of a phosphate 13 [25]. The ar-ylation of oximes under phase transfer conditions leads to a simple process for benzofuran formation [26]. 

A comparison of the yields of para-substituted [18F]fluoroarenes indicate that (i) acetyl hypofluorite is an inferior fluorination agent for the fluorodegermylation reaction and (ii) the aromatic substituents have considerable influence over the reactivity of the fluorination. A decrease in the fluorodegermylation yield was observed with electron-withdrawing aromatic substituents. The electrophilic aromatic degermylation reaction is thought to proceed via a cr-complex intermediate (Scheme 3). It has been hypothesized that the yield of aryl fluoride is influenced to some extent by the aromatic substituents ability to stabilize the cr-complex intermediate. 

A series of o-arylhydroxylamines, which are interesting intermediates for the preparation of benzofurans, hydroxybiphenyls, and catechols, have been synthesized starting from Cr(CO)3-activated fluoroarenes. Oxime 21 reacts with m-fluorotoluenetricarbonylchromium... 

The addition of two nucleophiles to a coordinated //6-arene is a synthetically important goal. A one-pot synthesis of 1,3-disubstituted cyclohexadienes involving an initial ipso addition to fluoroarene complexes is possible. Indeed, p-fluorotoluenetricarbonylchromium complex 22c reacts with isobutyronitrile carbanion (2 equiv.) in THF to give, after 5 days at —30 °C and acidic treatment under CO atmosphere, the cyclohexadiene 66 in 47 % yield (Scheme 31) [53] the yield can reach 75 % after several weeks Carbon monoxide was used in order to decoordinate the // -cyclohexadiene intermediate and to recover Cr(CO)6 needed for the preparation of the starting material [54]. 

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