Catalytic aromatic dehalogenation

It is interesting to remark that classical hydrogenation is a method used to prepare deuterium-labeled compounds by aromatic dehalogenation, but the usual reaction conditions suffer from several limitations. A microwave-enhanced catalytic dehalogenation procedure for rapid and specific deuterium labeling of N-4-picolyl-4-halogenobenzamide 68, by use of deuterated formate, [41] was recently reported (Scheme 8.25). 

The ease of dehalogenation of C H X by Ni(ll)/ IMes HCl 1/NaO Pr decreased in the order 1 > Br > Cl F. Subsequent work showed that a 1 1 combination of Ni and NHC in the presence of NaOCHEt resulted in enhanced reactivity towards aryl fluorides [6], Again, the A-mesityl substituted ligand IMes HCl 1 imparted the highest level of catalytic activity. Table 8.2 illustrates that hydrodefluorination is sensitive to both the nature of the substituents on the aromatic ring and the specific regioisomer. Thus, 2- or 4-fluorotoluene (Table 8.2, entry 2) proceeded to only 30% conversion after 15 h, whereas quantitative conversion of 2-fluoroanisole (Table 8.2, entry 3) and high conversion of 3-fluoropyridine (Table 8.2, entry 5) was achieved in only 2-3.5 h. The reactivity of 2-fluoropyridine was compromised by more efficient nucleophilic aromatic substitution. 

Dehalogenation of aromatic amines using catalytic hydrogenolysis takes place easily, usually without the reduction of C=C, COO, C=N, and N02 groups.454,455 The hydrogenolysis of 5-bromo-3-(l-methyl-2-pyrrolidinyl)pyri-dine (38) on Pd/C in EtOH produces nicotine hydrobromide (39) (Scheme... 

An interesting new homogeneous catalytic process was developed by Buijs [184] for the reductive dehalogenation of polychlorinated and -brominated aromatic hydrocarbons and ethers. Cu(I) benzoate catalyzed the reaction under Dow-Phenol conditions in the absence of air at 235 °C (Eq. (8)) ... 

The C-C coupling reaction between RMgX and R X is considered to proceed though an Ni(R)(R )Lm intermediate, and acceleration of the reductive elimination of R-R by coordination with olefinic or aromatic R X to Ni(R)(R )Lm is necessitated for a smooth catalytic reaction [15,16]. On these bases Ni-pro-moted dehalogenative polycondensation of dihalo organic compounds is suited to the preparation of 7i-conjugated aromatic and olefinic polymers. 

It is also known, that in addition to the catalytic reaction route the dehalogenation of aromatic halides can be carried out in stoichiometric reaction [ 1] in the presence of metal hydrides (e.g. LiAlH or NaBH ). Further characteristic feature of hydrodehalogenation reactions is the requirement for addition of free bases into the reaction mixture to fix the formed hydrochloric acid [2,4]. 

By heating above 200 C catalytic decarbonylation is possible using [RhCl(PPh3)3], and is particularly suitable for aromatic aldehydes since aliphatic aldehydes tend to dehalogenate under these conditions to form alkenes (equation 72). Cationic rhodium complexes, for example [Rh(Ph2P(CH2)2PPh2)2], are much more active catalysts and hence reactions can be carried out at below 100 Because of the milder conditions aliphatic aldehydes can be decarbonylated to the alkane using this catalyst system. Rhodium catalysts can also be used to decarbonylate a- and -diketones and keto esters (equations 73 and 74). ... 

Y. Zhu, C. Ching, K. Carpenter, R. Xu, S. Selvaratnam, N. S. Hosmane, J. A. Maguire, Synthesis of the novel ionic liquid [N-pentylp)ridinimn]-r [closo-CB,H j] and its usage as a reaction medium in catalytic dehalogenation of aromatic hahdes. App/. Organometal. Chem. 2003,17, 346. 

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