This side-reaction appears to be avoided when the synthesis is carried out with o-bromophenyl (pinacolato)boronic ester instead of ortho-dibromobenzene. In an interesting example of this type of side-reaction, hexabenzotriphenylene was produced from 9-bromophenanthrene ( Scheme 11). 92 One side-reaction encountered frequently in this approach is the single or double insertion of in situ generated arynes into the initial Ni–aryne species.

91 Another synthetic method involves the dehydrohalogenation of haloarenes in the presence of Ni(0) sources bearing labile ligands such as ethylene the reaction is carried out at low temperatures to avoid the competing oxidative addition to give the Ni–aryl species ( Scheme 11). 90 The 2,3-naphthalyne complexes have also been prepared similarly by reacting Ni IIBr(3-bromonaphthyl)L 2 with Na/Hg. The synthesis of Ni–benzyne complexes is typified by the reaction of ortho - dibromobenzene with Ni(cod) 2/PEt 3, followed by reaction of the resulting ortho-halophenyl moiety with Li, as shown in Scheme 11. Unsymmetrical phosphines result from the full sequence of reactions in Scheme 6, and p-anisyldiphenylphosphine (isolated as its oxide) was produced in 55% yield, based on the phosphorus used, when chlorobenzene (ArX) and p-methoxyanisole (Ar′X) were used.īy far, the most commonly studied species among Ni–cycloalkynes are the complexes of benzynes or arynes. 146 As an example of the synthesis of a symmetrical triarylphosphine, triphenylphosphine (isolated as its oxide) was obtained in 75% yield, with iodobenzene as the aryl halide (ArX in Scheme 6, steps i–iii only). Rossi and coworkers have developed a procedure for ‘one-pot’ preparation of triarylphosphines starting from elemental phosphorus ( Scheme 6). Operation of the radical ion mechanism in DMSO, or liquid ammonia, in which marginally improved yields are obtained, was confirmed by Swartz and Bunnett, 48 but no extension to the scope of the reaction was made. 145 The absence of cine substitution products is a synthetic advantage and would have been taken as a prima facie indication that the displacements are examples of the S rn1 reaction, had the mechanism been recognized at the time. Triarylphosphines were prepared by the reaction between lithium diphenylphosphide in THF and m-and p-iodotoluene (or the corresponding bromo compounds), 4-bromobiphenyl and p - dibromobenzene in yields of 70–80% (isolated after oxidation, as the phosphine oxides).

Bromination mechanism chem draw