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I've been looking for ways to synthesize biphenyl - $\ce{(C6H5)2}$.

The Ullmann coupling is inaccessible, as bromobenzene is inaccessible in my country due to the manufacture of PCP.

I had the idea to use a version of the Sandmeyer coupling between 2 molecules of aniline - $\ce{C6H5NH2}$.

My assumptions that this is possible are because of the synthesis of diphenic acid, wherein anthranilic acid is diazonated and coupled using a copper(I) ammonia hydroxylamine complex, as per this paper: http://orgsyn.org/demo.aspx?prep=cv1p0222.

I believe it would involve dimerization of the phenyl radical, similar to how the Sandmeyer reaction involves radical addition to copper(II), where the phenyl radical comes from copper(I) induced expulsion of nitrogen from diazonium salts. Is this mechanism correct?

Is is possible to do something similar with anilin? Is there literature for this method?

Please be mindful that I'm not too experienced in the mechanisms of these types of reactions.

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  • $\begingroup$ Looks like you have a good idea. If the route via the diazonium salt fails, you can prepare bromobenzene from it via the Sandmeyer reaction. If you have access to benzene and bromine, you can make bromobenzene. $\endgroup$
    – user55119
    Commented Dec 25, 2021 at 15:30
  • $\begingroup$ I see...thanks for the feedback. Would you mind leaving an answer? $\endgroup$
    – user102173
    Commented Dec 25, 2021 at 17:49

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As OP commented that they don't want benzene in the synthesis due to its carcinogenicity, here is an alternative route involving halobenzene:

  1. Sandmeyer reaction to form halobenzene

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  1. Wurtz reaction or Ullmann reaction

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The coupling reaction can be greatly improve (in terms of selectivity and % yield) by using high-end catalyst and improved reaction conditions. Here are some methods:

  • Fluorobenzene and phenyllithium

enter image description here

  • Using palladium based catalyst

    • Palladium on carbon. Here is a paper describing the reaction enter image description here
    • Suzuki-Miyaura coupling enter image description here
  • Kharasch coupling

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Reaction sources:

  1. Continuous Processes for the Synthesis and Isolation of Functionalized Biphenyls via Suzuki-Miyaura Cross-Coupling Reactions by Georg Johannes Lichtenegger (pdf)
  2. Zenish J. Jain, Paraag S. Gide, Rani S. Kankate, Biphenyls and their derivatives as synthetically and pharmacologically important aromatic structural moieties, Arabian Journal of Chemistry, Volume 10, Supplement 2, 2017, DOI: 10.1016/j.arabjc.2013.07.035

OP's route through Diphenic acid can also work. Diphenic acid can be decomposed thermally as well as catalytically to form various PAH and its derivative, one of the product being biphenyl. [Beware: PAH are carcinogenic]

enter image description here

Ref.: Oxidative Ring-Opening of Aromatics: Decomposition of Biphenyl Carboxylic Acids and Zinc Biphenyl Carboxylates by Natalia Montoya Sanchez and Arno de Klerk. (pdf)

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  • $\begingroup$ Thanks, but I don't want to work with benzene. It's a carcinogen, and can't be stored in a nonvolatile state. $\endgroup$
    – user102173
    Commented Dec 27, 2021 at 19:15
  • $\begingroup$ @Jacob Tbh, biphenyl is also carcinogenic and it can cause kidney stone formation and liver tumors. That bein said, do you want an alternative route involving making a biphenyl derivative and then reduce it to biphenyl? $\endgroup$ Commented Dec 28, 2021 at 6:27
  • $\begingroup$ Sure. Benzene is more volatile than biphenyl, hence I don't want to use it. $\endgroup$
    – user102173
    Commented Dec 28, 2021 at 17:46
  • $\begingroup$ The thermolysis of diphenic acid works for me. $\endgroup$
    – user102173
    Commented Jan 1, 2022 at 19:46

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