Organic Letters
Letter
charge in going from diazonium cation to the free radical and
should display a LFER with a large positive slope, ρ. Figure 3
shows the log (KR/KH) versus σp Hammet substituent
constants with good linear correlation (R2 = 0.94), indicating
the para EWG substituents are generating partial positive
character on the ipso aryldiazonium carbon with a combination
of both inductive and resonance effects. The slope in Figure 3 is
greater than that of Figure S1, ρ = 8.8 and 6.3, respectively,
indicating greater substituent sensitivity to both inductive and
resonance effects (contained in the σp constants) as opposed to
just resonance effects.
We next evaluated the bond lengths in order to establish
whether a correlation is present between aryl-substituent and
aryldiazonium bond distance since increased bond lengths favor
homolytic bond cleavage. Figure S2 shows the change in bond
lengths versus σp for the substituted diazonium cations which
revealed good correlation, R2 = 0.91. Moreover, when we
plotted the change in C−N aryldiazonium bond frequency
versus σp (Figure S3), the electron-rich aromatic rings have
higher frequency C−N bond vibrations relative to electron
deficit aromatic rings, supporting a mechanism where the RLS
involves homolytic bond cleavage.
The picture that emerges from the DFT calculations is one in
which para EWGs weaken the ipso carbon−diazonium bond,
prompting homolytic bond cleavage to produce an aryl radical
which then abstracts a halogen atom. In cases such as 3d and
3n, we hypothesize that iodide generated in situ catalyzes the
formation of aryl radicals. Overall, the results from Figures 1, 3,
S1, and S2 demonstrate that the yield of aryl halide product is
tuned in response to the electron-withdrawing and electron-
donating ability of the aryl-ring substituents.
In conclusion, we have developed a Sandmeyer-type
synthesis of aryl halides from anilines. This metal-free
transformation is operationally simple, tolerates a variety of
functional groups, and can be conducted under mild acidic
conditions without the exclusion of air.
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ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
Experimental procedures; spectra for all new com-
pounds; computational details of for the production of
AUTHOR INFORMATION
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Corresponding Author
ORCID
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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We acknowledge the National Institutes of Health (AI097802-
02 and AI116723-01) for financial support.
D
Org. Lett. XXXX, XXX, XXX−XXX