Aryl halides groups

The diazonium salts 145 are another source of arylpalladium com-plexes[114]. They are the most reactive source of arylpalladium species and the reaction can be carried out at room temperature. In addition, they can be used for alkene insertion in the absence of a phosphine ligand using Pd2(dba)3 as a catalyst. This reaction consists of the indirect substitution reaction of an aromatic nitro group with an alkene. The use of diazonium salts is more convenient and synthetically useful than the use of aryl halides, because many aryl halides are prepared from diazonium salts. Diazotization of the aniline derivative 146 in aqueous solution and subsequent insertion of acrylate catalyzed by Pd(OAc)2 by the addition of MeOH are carried out as a one-pot reaction, affording the cinnamate 147 in good yield[115]. The A-nitroso-jV-arylacetamide 148 is prepared from acetanilides and used as another precursor of arylpalladium intermediate. It is more reactive than aryl iodides and bromides and reacts with alkenes at 40 °C without addition of a phosphine ligandfl 16]. 

When allene derivatives are treated with aryl halides in the presence of Pd(0), the aryl group is introduced to the central carbon by insertion of one of the allenic bonds to form the 7r-allylpalladium intermediate 271, which is attacked further by amine to give the allylic amine 272. A good ligand for the reaction is dppe[182]. Intramolecular reaction of the 7-aminoallene 273 affords the pyrrolidine derivative 274[183]. 

The terminal amino group of 2-hydrazino-4-phenylthiazole is also the reactive center in reactions with activated aryl halides such as 288. A solution of the product (289) obtained from this reaction when shaken with PbOj gives a deeply colored radical, whose structure has been studied by ESR (Scheme 173) (532. 533). 

The value of alkyl halides as starting materials for the preparation of a variety of organic functional groups has been stressed many times In our earlier discussions we noted that aryl halides are normally much less reactive than alkyl halides m reactions that involve carbon-halogen bond cleavage In the present chapter you will see that aryl halides can exhibit their own patterns of chemical reactivity and that these reac tions are novel useful and mechanistically interesting... 

One group of aryl halides that do undergo nucleophilic substitution readily consists of those that bear a nitro group ortho or para to the halogen... 

The generally accepted mechanism for nucleophilic aromatic substitution m nitro substituted aryl halides illustrated for the reaction of p fluoromtrobenzene with sodium methoxide is outlined m Figure 23 3 It is a two step addition-elimination mechanism, m which addition of the nucleophile to the aryl halide is followed by elimination of the halide leaving group Figure 23 4 shows the structure of the key intermediate The mech anism is consistent with the following experimental observations... 

The —OH group of phenols makes it possible for them to participate m hydrogen bonding This contributes to the higher boiling points and greater water solubility of phenolic compounds compared with arenes and aryl halides... 

Ullman type coupling occurs between aryl halides and trifluoromethyl copper species generated by the action of copper iodide on sodium tnfluoroacetate [168, 169] or on methyl fluorosulfonyldifluoroacetate [170] (equation 145) Similarly the pentafluoroethyl group can be introduced from potassium pentafluoropropion-ate [171] (equation 146)... 

Aryl halides with a halogen activated by electron-withdrawing groups react with pyrrolidine enamines of cyclic ketones (68) to give the a-arylated ketones after hydrolysis. The enamine (28) with 2,4-dinitrochlorobenzene gave an excellent yield of 2(2,4-dinitrophenyl)cyclohexanone (88). The... 

Alkylation reactions are subject to the same constraints that affect all Sn2 reactions (Section 11.3). Thus, the leaving group X in the alkylating agent R—X can be chloride, bromide, iodide, or tosylate. The alkyl group R should be primary or methyl, and preferably should be allylic or benzylic. Secondary halides react poorly, and tertiary halides don t react at all because a competing E2 elimination of HX occurs instead. Vinylic and aryl halides are also unreactive because backside approach is sterically prevented. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

The synthesis of (hetero)aryl cyanides from (hetero)aryl halides via transition-metal catalysis is a very valuable reaction since a nitrile can be easily transformed into several other functional groups. Not until 2000 were the first examples on microwave-assisted cyanation reported in the literature. Alter-man and Hallberg found that 3-bromopyridine and 3-bromothiophene were... 

Alkenylboranes (R2C=CHBZ2 Z — various groups) couple in high yields with vinylic, alkynyl, aryl, benzylic, and allylic halides in the presence of tetra-kis(triphenylphosphine)palladium, Pd(PPh3)4, and a base to give R C CHR. 9-Alkyl-9-BBN compounds (p. 1013) also couple with vinylic and aryl halides " as well as with a-halo ketones, nitriles, and esters.Aryl halides couple with ArB(IR2 ) species with a palladium catalyst. ... 

For aryl halides and sulfonates, even active ones, a unimolecular SnI mechanism (lUPAC Dn+An) is very rare it has only been observed for aryl triflates in which both ortho positions contain bulky groups (fe/T-butyl or SiRs). It is in reactions with diazonium salts that this mechanism is important ... 

---