Ullmann side-reactions

As an alternative to the Ullmann reaction, haloarenes are coupled to form the biaryls using palladium acetate in the presence of abase and tetra-n-butylammonium bromide [24], Yields are generally high (>70%) but dehalogenation of the haloarene may also occur as a side reaction. 

Chloro- or bromoarenes undergo an Ullmann-like reductive homocoupling to the respective biphenyls in the presence of PEG-400 (PT catalyst) and a trimetallic catalyst consisting of 4% Pd, 1% Pt, and 5% Bi on carbon with either hydrogen, formate and water, or zinc and water. Simple dehalogenation, a side reaction, can be minimized (Eq. 5). 

Various synthetic approaches have been demonstrated for the synthesis of PAEs since early days [35 0], PAEs were synthesized by Ullmann condensation between bisphenols and aryl fcis-halide monomers using Cu(I) salt/pyridine as catalyst [36], General Electric developed the first commercially successful PAE poly(2,6-dimethyl phenylene oxide) (PPO) [38], It was prepared by oxidative coupling of 2,6-dimethyI phenol. However, this process has its own restrictions, because it does not allow much structural variation or inclusion of any electron-withdrawing group into the polymer main chain. First attempts to synthesize polysulfones (PSF) were successfully done by Friedel-Crafts sulfonylation reaction of arylenedisulfonyl chlorides, for example, diphenyl ether-4,4 -disulfonyl chloride with diaryl ethers, for example, diphenyl ether, or by self-condensation of 4-phenoxy benzene sulfonyl chloride in the presence of FeCls [41], Whereas the former reaction involves side reactions (sulfonylation not only in the para- but also in the ort/io-position), the latter produces only the desired linear all-para products. 

Halophenols without 2,6-disubstitution do not pol5unerize imder oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or interrupt the propagation step of the chain process. To prepare polyCphenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmann ether synthesis. A cuprous chloride-pyridine complex in 1,4-dimethoxybenzene at 200° C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Ullmann condensation of the sodium salt of p-chlorothiophe-nol (31) with 2-iodobenzoic (32) acid gives 33. Cyclization by means of sulfuric acid affords the thioxanthone, 34. Reaction with the Grignard reagent from 3-dimethylaminopropyl chloride affords the tertiary carbinol (35). Dehydration by means of acetic anhydride affords chlorprothixene as a mixture of geometric isomers, 36. (Subsequent work showed the Z isomer-chlorine and amine on the same side—to be the more potent compound.) Chlorprothixene is said to cause less sedation than the phenothiazines. ... 

Fig. 16. Three dimension STM images showing the basic steps of the tip induced Ullmann reaction. Two adsorbed iodobenzene molecules at the lower terrace of the Cu(lll) step-edge (a). Iodine is abstracted from the molecules by injecting tunneling electrons (b). Iodine atoms (small) and phenyl molecules (large) are further separated by lateral manipulation (c). The iodine atom located between the two phenyls is removed onto the lower terrace to clear the path between the two phenyls (d). The phenyl molecule at the left side is moved close to the right phenyl and then 500 meV voltage pulses are supplied by the STM-tip to form a biphenyl molecule (e). [Image parameters +100 mV, 0.53 nA 70 x 30 A2] [3],...

The name Ullmann is not only associated with the biaryl synthesis (Figure 16.4, 16.5), but is also known from the synthesis of diaryl ethers (Ullmann synthesis of diaryl ethers). An example is given in the topmost reaction of Figure 16.6. Remember Side Note 5.6, where we asked the following question How can diphenyl ethers be prepared Now you are ready to give a correct answer, which is By way of Ullmann synthesis. ... 

---