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2.7.1.1.1.1 Selective Catalytic Hydrogenation with Heterogeneous Catalysts

DOI: 10.1055/sos-SD-227-00139

Puylaert, P.; Savini, A.; Hinze, S.Science of Synthesis: Catalytic Reduction in Organic Synthesis, (20182333.

Halogen-containing nitroaromatics open the pathway to further functionalization, but tend to undergo hydrodehalogenation under hydrogenation conditions. A number of parameters influence the selectivity, one of them being the type of halogen substituent. The hydrogenation rate of nitroarenes typically follows the trend I < Br < Cl, which means that iodo-substituted nitroarenes are most prone to the accumulation of hydroxylamine intermediates. Hydrodehalogenation mainly takes place with the haloaniline products rather than their nitro precursors, owing to the electron-donating effect of the amine group weakening the carbonhalogen bond. The hydrodehalogenation reaction itself proceeds faster for chloroanilines than for bromo- and iodoanilines. In practice, however, the hydrogenation rates dominate, as the lower rate for iodo-substituted nitroarenes allows increased competition from hydrodehalogenation.[‌44‌] These issues may be circumvented by means of microfluidic processing.[‌49‌] In terms of straightforward applicability, Kasparian and co-workers have reported a rather general method utilizing a commercially available heterogeneous sulfided platinum catalyst [Pt(S)/C, B109 032–3] under mild conditions.[‌50‌] The catalyst is selective for nitro groups in the presence of halides on the hetarene core, showing only minimal hydrodehalogenation, even in the case of iodo substituents (Scheme 2).

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Me8 Meee (e) Meeee (%) Mee
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References