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Reduction of the Diazo Functionality of Α-Diazocarbonyl Compounds Into a Methylene Group by NH3BH3 Or Nabh4 Catalyzed by Au Nanoparticles

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Reduction of the Diazo Functionality of Α-Diazocarbonyl Compounds Into a Methylene Group by NH3BH3 Or Nabh4 Catalyzed by Au Nanoparticles nanomaterials Article Reduction of the Diazo Functionality of α-Diazocarbonyl Compounds into a Methylene Group by NH3BH3 or NaBH4 Catalyzed by Au Nanoparticles Marios Kidonakis and Manolis Stratakis * Department of Chemistry, University of Crete, 71003 Voutes, Heraklion, Greece; [email protected] * Correspondence: [email protected]; Tel.: +30-2810545087 Abstract: Supported Au nanoparticles on TiO2 (1 mol%) are capable of catalyzing the reduction of the carbene-like diazo functionality of α-diazocarbonyl compounds into a methylene group [C=(N2) ! CH2] by NH3BH3 or NaBH4 in methanol as solvent. The Au-catalyzed reduction that occurs within a few minutes at room temperature formally requires one hydride equivalent (B-H) and one proton that originates from the protic solvent. This pathway is in contrast to the Pt/CeO2- catalyzed reaction of α-diazocarbonyl compounds with NH3BH3 in methanol, which leads to the corresponding hydrazones instead. Under our stoichiometric Au-catalyzed reaction conditions, the ketone-type carbonyls remain intact, which is in contrast to the uncatalyzed conditions where they are selectively reduced by the boron hydride reagent. It is proposed that the transformation occurs via the formation of chemisorbed carbenes on Au nanoparticles, having proximally activated the boron hydride reagent. This protocol is the first general example of catalytic transfer hydrogenation of the carbene-like α -ketodiazo functionality. Keywords: α-diazocarbonyl compounds; transfer hydrogenation; boron hydrides; heterogeneous Citation: Kidonakis, M.; Stratakis, M. catalysis; supported Au nanoparticles Reduction of the Diazo Functionality of α-Diazocarbonyl Compounds into a Methylene Group by NH3BH3 or NaBH4 Catalyzed by Au 1. Introduction Nanoparticles. Nanomaterials 2021, 11, The reduction of the carbene-like diazo functionality into a methylene group 248. https://doi.org/10.3390/nano [C=(N2) ! CH2] in α-diazocarbonyl compounds is not a well-studied transformation 11010248 in the past, and there are scattered examples in the literature. Decades ago, the Wolfrom method [1] was reported, which employs hydriodic acid as the reducing agent and then a Received: 18 December 2020 protocol involving catalytic amounts of diethyl peroxydicarbonate in isopropanol [2]. The Accepted: 13 January 2021 α β β Published: 18 January 2021 interesting transformation of -diazo- -hydroxy esters into -keto esters [3,4] catalyzed by Rh2(OAc)4 could be seen as a relevant example and apparently involves an intramolecular Publisher’s Note: MDPI stays neutral transfer hydrogenation. Bu3SnH has been also used as reducing agent with Cu(acac)2 as with regard to jurisdictional claims in catalyst under irradiation [5], as well as a series of early transition metallocenes in the published maps and institutional affil- presence of H2O[6]. Finally, the relevant to this concept Pd/C-catalyzed hydrogenation of iations. a specific type of diazo compounds (α-diazo-β-hydroxy esters) [7] appears as a practical reductive method providing β-hydroxy esters in moderate to good yields. Recently, we presented the first example of Au-catalyzed hydrosilylation of α-diazocarbonyl compounds using recyclable and reusable Au nanoparticles on TiO2 as the active catalytic sites (Scheme1)[ 8]. This reaction proceeds in short reaction times in Copyright: © 2021 by the authors. α Licensee MDPI, Basel, Switzerland. anhydrous 1,2-dichloroethane (DCE) affording -silyl carbonyl compounds in high yields. This article is an open access article It is most likely that this transformation involves not only the activation of the diazo com- distributed under the terms and pounds on Au NPs forming chemisorbed Au-carbenes [9] but proximally chemisorbed hy- conditions of the Creative Commons drosilanes as well. The only side product of the hydrosilylation reaction is the replacement Attribution (CC BY) license (https:// of the diazo group by two hydrogen atoms [C=(N2) ! CH2]. This side pathway depends creativecommons.org/licenses/by/ on the amount of moisture in the solvent and apparently arises from the chemisorbed H 4.0/). species on the Au nanoparticle. This observation initiated our efforts to develop a new Nanomaterials 2021, 11, 248. https://doi.org/10.3390/nano11010248 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 248 2 of 10 on the amount of moisture in the solvent and apparently arises from the chemisorbed H species on the Au nanoparticle. This observation initiated our efforts to develop a new methodology for the reduction of the diazo functionality of α-diazocarbonyl compounds Nanomaterials 2021, 11, 248 2 of 10 into a methylene group. When we contacted the Au/TiO2-catalyzed hydrosilylation of di- azo compound 1a in DCE in the additional presence of H2O (0.5 equivalent), the product selectivity alteredmethodology, affording for the reduction the reduced of the diazo ester functionality 3a in 72% of α-diazocarbonyl relative yield compounds versus 28% of the anticipated hydrosilylationinto a methylene group. product When we 2a contacted. Although the Au/TiO the 2relative-catalyzed hydrosilylationyield of 3a ofcan be further diazo compound 1a in DCE in the additional presence of H2O (0.5 equivalent), the product increased byselectivity increasing altered, the affording equivalents the reduced of ester water3a in added 72% relative into yield the versus reaction 28% of themixture, at the same time, a anticipatedhigh excess hydrosilylation of hydrosilane product 2a should. Although be the used, relative as yieldhydrosilanes of 3a can be further react with H2O (a increased by increasing the equivalents of water added into the reaction mixture, at the reaction that is also catalyzed by Au/TiO2). Thus, we sought for an alternative and cheaper same time, a high excess of hydrosilane should be used, as hydrosilanes react with H2O reducing agent(a reaction to replace that is also hydrosilane catalyzed by Au/TiO and 2establish). Thus, we soughta more for anpractical alternative methodology and to achieve this reductivecheaper reducing transformation. agent to replace hydrosilane and establish a more practical methodology to achieve this reductive transformation. Scheme 1. Hydrosilylation of α-diazo carbonyl compounds catalyzed by Au/TiO ; reversal of chemoselectivity of the Scheme 1. Hydrosilylation of α-diazo carbonyl compounds2 catalyzed by Au/TiO2; reversal of attempted hydrosilylation of diazo compound 1a in the presence of 0.5 equivalents of H O. chemoselectivity of the attempted hydrosilylation of diazo2 compound 1a in the presence of 0.5 equivalents of H2O.Over the past two decades, supported Au nanoparticles (Au NPs) and other nano Au(0) materials have attracted significant attention due to their surprising ability to catalyze a plethora of organic transformations, despite the metallic character of Au atoms on Over thenanoparticles past two [10 decades–13], including, supported transfer hydrogenation Au nanoparticles reductive processes (Au [14 NPs),15]. Given and other nano Au(0) materialsthe heterogeneous have attracted nature ofsignificant Au NP-catalyzed attention processes due and that to they their can surprising be easily recycled ability to cata- and reused on the vast majority of the reported protocols, they can be considered as benign lyze a plethoragreen of catalysts. organic transformations, despite the metallic character of Au atoms on nanoparticles [10–13], including transfer hydrogenation reductive processes [14,15]. 2. Results Given the heterogeneous nature of Au NP-catalyzed processes and that they can be easily At the outset, we investigated the possible Au/TiO2-catalyzed reduction of the model recycled andcompound reused on1a intothe thevast desired majority product of3a the, using reported the readily protocols, available boron they hydride can be considered as benign greenreagents catalysts. NH3BH3 and NaBH4. Ammonia borane complex [16,17] as well as sodium borohydride [18] have been used efficiently in the past for the reduction of alkynes or nitro compounds using Au/TiO2 as catalyst. While no reaction occurs in a series of dry 2. Results aprotic solvents (dichloromethane, diethyl ether, hexane, or benzene), to our pleasure, we found that the treatment of 1a with NH3BH3 (0.4 molar equivalent) or NaBH4 (0.3 molar At the outset,equivalent) we in investigated MeOH for 15 min the at room possible temperature Au/TiO resulted2- incatalyzed the quantitative reduction formation of the model compound 1aof into the reduced the desired product 3a product(Scheme2). 3a In the, using absence the of Au/TiO readily2 or available in the presence boron of the hydride rea- support itself (TiO2, routile, or anatase), no transformation took place even after prolonged gents NH3BHreaction3 and timesNaBH (12 h)4. atAmmonia 50 ◦C, which borane is indicative complex that Au NPs [16 are,17] the catalyticas well sites. as sodium borohy- dride [18] have been used efficiently in the past for the reduction of alkynes or nitro com- pounds using Au/TiO2 as catalyst. While no reaction occurs in a series of dry aprotic sol- vents (dichloromethane, diethyl ether, hexane, or benzene), to our pleasure, we found that the treatment of 1a with NH3BH3 (0.4 molar equivalent) or NaBH4 (0.3 molar equivalent) in MeOH for 15 min at room temperature resulted in the quantitative formation of the reduced product 3a (Scheme 2). In the absence of Au/TiO2 or in the presence of the support itself (TiO2, routile, or anatase), no transformation took place even after prolonged reac- tion times (12 h) at 50 °C, which is indicative that Au NPs are the catalytic sites. The amounts of boron hydride reagents that are necessary to achieve a quantitative reduction indicate that one hydride equivalent from either NH3BH3 or NaBH4 is required; thus, the second hydrogen atom on the product arises from the protic solvent (methanol). To test this hypothesis, we performed the reduction of diazo compound 1b with NaBH4 in methanol-d4 (Scheme 2). Analysis of the reaction mixture revealed the presence of D Nanomaterials 2021, 11, 248 3 of 10 atoms in the products. The monodeuterated 3b-d1 was formed in 60% relative yield, along with dideuterated 3b-d2 (15%) and non-deuterated 3b-d0 (25%).
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