Bromides from arenes

Skulski and Kraszkiewicz have also reported a new direct method for the preparation of various symmetrical diaryliodonium bromides (15-88% crude yields) from arenes by the reaction of ArH with NaI04 in sulfuric acid followed by the addition of KBr [23],... 

Butyllithium/butylmagnesium bromide oxygen Phenols from arenes... 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ X-. The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Another recent development in the field of palladium-catalyzed reactions with alkynes is a novel multicomponent approach devised by the Lee group. Starting from a-bromovinyl arenes and propargyl bromides, the assembly ofeight-membered car-bocycles can be realized via a cross-coupling/[4+4] cycloaddition reaction. The authors also presented the combination of a cross-coupling and homo [4+2], hetero [4+2], hetero [4+4] or [4+4+1] annulation leading to various cyclic products [147]. 

From Chapter 7 it is apparent that the trichloromethyl anion is formed under basic conditions from chloroform, as a precursor of the carbene. The anion can also react with Jt-deficient alkenes (see Section 7.3) and participate in nucleophilic substitution reactions, e.g. 1,1-diacyloxy compounds are converted into 1,1,1-trichloroalkan-2-ols [58] (Scheme 6.35). Similarly, benzyl bromides are converted into (2-bromoethynyl)arenes via an initial nucleophilic displacement followed by elimination of hydrogen bromide [59] (Scheme 6.35). 

Finally, polymer 594 has been used as an arene-catalyst to activate nickel from nickel(II) chloride and lithium, in order to perform hydrogenation of different organic substrates such as afkenes, afkynes, carbonyl compounds and their imines, alkyl and aryl halides (chlorides, bromides and iodides), aromatic and heteroaromatic compounds as well as nitrogen-containing systems such as hydrazines, azoxy compounds or Af-amino oxides, giving comparable results to those obtained in the corresponding reaction in solution . 

Figure 7.44 X-ray crystal structure of the host portion of p I bulylcalix 4 arene (A) guest-free crystal showing a, b, c, d... repeating layers and (B) after exposure to vinyl bromide showing an a, b, a, b... repeat pattern arising from a shift of ca. 6A between layers b and c (reprinted with permission from AAAS from [55]).

Multiple arylations of polybromobenzenes have been conducted to generate electron-rich arylamines. Tribromotriphenylamine and 1,3,5-tribromobenzene all react cleanly with A-aryl piperazines using either P(o-tolyl)3 or BINAP-ligated catalysts to form hexamine products [107]. Reactions of other polyhalogenated arenes have also been reported [108]. Competition between aryl bromides and iodides or aryl bromides and chlorides has been investigated for the formation of aryl ethers [109], and presumably similar selectivity is observed for the amination. In this case bro-mo, chloroarenes reacted preferentially at the aryl bromide position. This selectivity results from the faster oxidative addition of aryl bromides and is a common selectivity observed in cross-coupling. Sowa showed complete selectivity for amination of the aryl chloro, bromo, or iodo over aryl-fluoro linkages [110]. This chemistry produces fluoroanilines, whereas the uncatalyzed chemistry typically leads to substitution for fluoride. 

The coupling of aryl bromides with primary aHphatic amines often suffered from the formation of reduced arene by-products similar to the reactions with secondary amines. For example, the use of ( )-BINAP as a Hgand greatly improved the yield in the coupling of 5-bromo-raefa-xylene and -hexylamine, Eq. (64) [27]. 

The Tosoh group reported that a (f-Bu)3P/Pd-catalyst allowed the preparation of terf-butyl ethers from the corresponding aryl bromides and NaOf-Bu [147]. Electron-withdrawing as well as electron-donating substituents were tolerated on the aryl bromide, although increased electron density on the aryl bromide results in greater amounts of reduced arene and biaryl side products, Eq.(185). 

---