PUBLISHED IN ITS FINAL FORM ON: Chem. Rev. 2019, 119, 2611-2680. DOI: 10.1021/acs.chemrev.8b00547 REDUCTION OF NITRO COMPOUNDS USING 3d- NON-NOBLE METAL CATALYSTS Dario Formenti, Francesco Ferretti, Florian Korbinian Scharnagl, and Matthias Beller* Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany Corresponding author e-mail: [email protected] Dedicated to Dr. Michelangelo Scalone, F. Hoffmann-La Roche, for his important contributions to catalysis. ABSTRACT. The reduction of nitro compounds to the corresponding amines is one of the most utilized catalytic processes in the fine and bulk chemical industry. The latest development of catalysts with cheap metals like Fe, Co, Ni and Cu has led to their tremendous achievements over the last years prompting to their greater application as “standard” catalysts. In this review we will comprehensively discuss the use of homogeneous and heterogeneous catalysts based on non-noble 3d-metals for the reduction of nitro compounds using various reductants. The different systems will be revised considering both the catalytic performances and synthetic aspects highlighting also their advantages and disadvantages. 1 Contents 1. Introduction: setting the scene .................................................................................................................. 4 2. Survey of reducing agents employed in the reduction of nitro compounds .............................................. 9 3. Heterogeneous iron-based catalysts ........................................................................................................ 13 3.1. Reductions with molecular H2 ........................................................................................................ 13 3.2. Transfer hydrogenations ................................................................................................................. 19 3.2.1. Reductions with N2H4 ............................................................................................................. 19 3.2.2. Reductions with alkali/alcohols .............................................................................................. 31 3.2.3. Reductions with NaBH4 .......................................................................................................... 32 3.2.4. Other reductants ...................................................................................................................... 35 4. Heterogeneous cobalt-based catalysts .................................................................................................... 37 4.1. Reductions with molecular H2 ........................................................................................................ 37 4.2. Transfer hydrogenations ................................................................................................................. 58 4.2.1. Formic acid and CO/H2O as reductants .................................................................................. 58 4.2.2. Reductions with NaBH4 .......................................................................................................... 61 4.2.3. Reductions with N2H4 and other reducing agents ................................................................... 66 5. Heterogeneous nickel-based catalysts .................................................................................................... 70 5.1. Reduction with molecular H2 .......................................................................................................... 70 5.2. Transfer hydrogenations ................................................................................................................. 88 5.2.1. Reductions with NaBH4 .......................................................................................................... 88 5.2.2. Reductions with alcohols/bases, N2H4, HCOOH and other reductants ................................... 98 6. Heterogeneous copper-based catalysts ................................................................................................. 102 6.1. Reduction with molecular H2 ........................................................................................................ 103 6.2. Transfer hydrogenations ............................................................................................................... 104 6.2.1. Reductions with NaBH4 ........................................................................................................ 104 6.2.2. Reductions with alcohols/bases, N2H4, HCOOH and other reductants ................................. 114 7. Multimetallic heterogeneous catalysts .................................................................................................. 116 7.1. Reduction with molecular H2 ........................................................................................................ 116 7.1.1. Nickel and cobalt based catalysts.......................................................................................... 116 7.1.2. Copper-nickel based catalysts ............................................................................................... 120 7.2. Transfer hydrogenations ............................................................................................................... 121 7.2.1. Reductions with alcohols, N2H4 and ammonia-boranes ........................................................ 121 7.2.2. Reductions with NaBH4 ........................................................................................................ 124 7.2.2.1. Bimetallic alloy nanoparticles ........................................................................................... 124 7.2.2.2. Ferrite-based nanoparticles ............................................................................................... 127 7.2.2.3. Manganese-containing composites ................................................................................... 129 8. Homogeneous catalysts ........................................................................................................................ 131 8.1. Iron-based catalysts ...................................................................................................................... 132 2 8.1.1. Reduction with gaseous reductants ....................................................................................... 132 8.1.2. Transfer hydrogenations ....................................................................................................... 134 8.1.3. Reductions with hydrosilanes ............................................................................................... 137 8.2. Cobalt-based catalysts: Transfer hydrogenations .......................................................................... 140 8.3. Nickel-based catalysts: Reductions with hydrosilanes .................................................................. 142 8.4. Other metals ................................................................................................................................. 143 9. Conclusions and outlook ...................................................................................................................... 144 10. Acknowledgments ............................................................................................................................ 147 11. Abbreviations ................................................................................................................................... 148 12. Author information ........................................................................................................................... 151 12.1. Corresponding author ............................................................................................................... 151 12.2. Orcid ......................................................................................................................................... 151 12.3. Biography ................................................................................................................................. 151 13. References ........................................................................................................................................ 152 3 1. Introduction: setting the scene Amines, and specifically aniline and its derivatives, represent (and surely will hold in the future) a huge market portion in the organic chemical industry.1,2 Aniline was isolated for the first time as product of the dry distillation of indigo by the German chemist Otto Unverdorben almost 200 years ago (1826). Fifteen years later, the Russian chemist Nikolay N. Zinin prepared for the first time aniline by reduction of nitrobenzene using sodium sulfides as the stoichiometric reductants (this transformation is generally known as Zinin reaction).3 Few years later (1851), Piria reported a two-step procedure for the synthesis of anilines from nitroarenes: the nitro group is converted in the first step into an aminosulfonic acid which is then transformed into the final product by hydrolysis with mineral acids in the second step.4 Later on (1854), the French chemist Pierre J. A. Béchamp demonstrated the use of metallic iron in acidic media as reducing agent for the production of aniline from nitrobenzene.5 For a long time, this method was the main manufacturing process for a large number of aniline-based molecules at reasonable costs. The success of dyestuff