New Tricks for an Old Dog: Aluminum Compounds as Catalysts in

This Perspective discusses recent results on the application of aluminum compounds in catalytic reduction processes, such as hydrogenation, transfer ...
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New Tricks for an Old Dog: Aluminum Compounds as Catalysts in Reduction Chemistry Georgii I. Nikonov* Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario, Canada L2S 3A1 ABSTRACT: This Perspective discusses recent results on the application of aluminum compounds in catalytic reduction processes, such as hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration, and others. Usually, simple aluminum salts such as halides or triflates have been employed. However, a growing number of well-defined molecular compounds, both cationic and neutral, are also being used in this chemistry.

KEYWORDS: aluminum, hydrosilylation, hydroboration, hydride transfer, hydrodefluorination

A

aluminum amidate complexes toward the MPV reduction of carbonyls with isopropanol (in toluene at 50 °C) and Oppenauer oxidation of alcohols using pivaldehyde as the oxidant, but the catalyst loads were relatively high (10% and 5%, respectively).16 The problem was solved by Krempner et al.,17 who prepared the well-defined aluminum bis(siloxide) complex 1 (Scheme 1) that serves as an efficient catalyst for the MPV reduction of a vast array of aldehydes and ketones at very low catalyst loads (0.05−0.5 mol %) and low temperatures (typically 25−50 °C). The efficiency of this catalytic system can likely be attributed to the size of the siloxane ligand, which prevents aggregation and may facilitate product dissociation from the coordination sphere of aluminum, thus overcoming the known mechanistic hurdle of conventional MPV systems.8

luminum, the most abundant metal in the earth’s crust (8%), is inexpensive and much less toxic than heavy metals, which makes it ideal for applications in catalysis. In fact, aluminum compounds were among the first metal catalysts to be discovered and applied in synthetic chemistry. Examples include the use of aluminum halides in Friedel−Crafts acylation,1 the use of stoichiometric amounts of AlEt3 in Ziegler’s aufbau reaction,2 and later the application of aluminum organyls as activators in Ziegler−Natta olefin polymerization.3 Aluminum alkoxides were the first catalysts for transfer (de)hydrogenation in the Meerwein−Ponndorf− Verley/Oppenauer (MPV/O) process.4−8 However, the subsequent development of alternative Lewis acids, soluble hydride sources (e.g., LiAlH4 and NaBH4), and transition metal catalysis overshadowed the practicability of aluminum in catalysis. The past decade has witnessed a remarkable resurrection of main-group catalysis,9−12 which was driven by the desire to develop less expensive and more environmentally benign alternatives to the typical transition metal systems. Aluminum fits the bill perfectly, and well-defined aluminum compounds have started to draw renewed interest in catalytic applications. This Perspective covers the recent progress in employing aluminum compounds as catalysts in the reduction of organic compounds and some related reactions. During the preparation of this Perspective, Roesky et al. published a related review on the application of soluble aluminum hydrides as catalysts in deprotonation, insertion, and activation reactions.13

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HYDROGENATION Although aluminum compounds historically have not been considered as good hydrogen splitters and therefore have not found applications in hydrogenation chemistry, they can in principle hydrogenate unsaturated substrates by a sequence of hydroalumination and H−H/Al−X σ-bond metathesis. Stephan et al. suggested this mechanism for the hydrogenation of imines (102 atm H2, 100 °C) catalyzed by HAliBu2 and AliBu3.18A similar scenario is likely realized in the hydrogenation of alkenes mediated by HAliBu2 and some other simple organoaluminum compounds, though this process operates under even harsher conditions (>200 °C, >100 bar H2).19 However, a recent report by Stair and co-workers on the hydrogenation of alkenes by a supported aluminum catalyst uncovers a new and important trait of this element. These

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TRANSFER HYDROGENATION The classical MPV/O reaction is mediated by simple aluminum alkoxides, often applied in stoichiometric quantities. Further research on improving the catalytic activity of the reaction up to 2006 has been summarized by Cha14 and Graves.15 More recently, the Graves group reported the application of © XXXX American Chemical Society

Received: July 24, 2017 Revised: September 3, 2017 Published: September 11, 2017 7257

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ACS Catalysis Scheme 1. Krempner’s Aluminum Bis(siloxide) Catalyst for MPV Reductions

Scheme 2. Hydrogenation of 1-Octene by a Polymer-Supported Aluminum Catalyst

authors found that a catechol-containing porous organic polymer (abbreviated as CatPOP A 2 B 1 ) can support tricoordinate aluminum alkyls that can catalytically hydrogenate alkyl- and aryl substituted alkenes and alkynes under relatively mild conditions (75 °C, 5 bar H2, Scheme 2).20 Moreover, these well-defined alkylaluminum sites exhibit recyclability. Ketones and aldimines are not reduced under these conditions, and interestingly, the catalytic activity is suppressed in aromatic solvents, likely because of π coordination. The identity of the reaction sites was established by a combination of X-ray photoemission spectroscopy (XPS), X-ray pair distribution function (PDF) studies, solid-state attenuated total reflectance IR (ATR-IR) spectroscopy, and 1H and 27Al magic-angle spinning (MAS) NMR spectroscopy, which revealed the presence of well-defined, tricoordinate monoaluminum species stabilized by the dianionic catecholate. This finding is in accord with the report by Copéret et al. that tricoordinate alumoxo sites present on the major (110) termination of γ- and δ-Al2O3 particles are sufficiently Lewis acidic to activate bonds as strong as H−H and H−CH3 and to coordinate N2.21

Scheme 3. Oertle and Wetter’s Mechanism for AlCl3Catalyzed Hydrosilylation of Tetrasubstituted Olefins

alkene to generate a carbocation (Scheme 4). It is noteworthy that such a Lewis acid-catalyzed process is similar to the Piers− Oestreich mechanism of borane-catalyzed hydrosilylation.26

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Scheme 4. Silylium Ion Mechanism for the Al-Catalyzed Hydrosilylation of Alkenes

HYDROSILYLATION Hydrosilylation is by far the most intensely studied reaction catalyzed by aluminum. Earlier research in this field was focused on the application of simple aluminum salts but later switched to the design of well-defined molecular compounds.10,22 In 1978, Finke and Moretto reported in the patent literature that aluminum halides and oxohalides are efficient catalysts for the hydrosilylation of alkenes and alkynes.23 Later, Oertle and Wetter applied this procedure to the hydrosilylation of tetrasubstituted olefins with HSiR2Cl (R = Me, Bu) and suggested that the silane activates aluminum by means of Cl/H exchange to generate the active hydride AlHCl2, which then adds to the alkene to form R−AlCl2 (hydroalumination).24 The cycle is completed by Al−C/H−Si metathesis (Scheme 3), which was supported by the observation that iBuAlCl2 reacts with HSiMe2Cl to give iBuSiMe2Cl. However, this proposal was challenged by Yamamoto and Takemae, who showed that hydrosilylation of methylcyclohexenes yielded the product of trans-hydrosilylation whereas hydroalumination is a cis-addition process.25 These authors suggested an alternative electrophilic mechanism based on the formation of an adduct between AlCl3 and the silane, i.e., ClMe2SiH·AlCl3, which then transfers the silylium ion to the

This mechanistic proposal was further elaborated by Jung et al., who showed that Lewis acid-catalyzed hydrosilylation of 1methylcyclohexene with triethylsilane at −20 °C gave cis-1triethylsilyl-2-methylcyclohexane, consistent with a trans-hydrosilylation pathway.27 Cycloalkenes bearing an alkyl group at the double bond were more reactive than nonsubstituted substrates, which correlates with the stability of the corresponding carbocations. Interestingly, the addition of ClSiEt3 to a mixture of cyclohexene and HSiEt3 increased the rate of catalysis, which can be rationalized by the formation of a silylium ion adduct R3Si+←Cl4Al−. Supporting this mechanistic proposal was the observation that in the case of ClSiMe2Et the silyl group in the product primarily comes from the trialkylchlorosilane and not from the hydrosilane. Of the three aluminum halides studied, AlBr3, AlCl3, and EtAlCl2, the last one was the least active because of its lower Lewis acidity, 7258

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ACS Catalysis

complex, which then abstracts the hydride from the silane to generate a silylium ion (Scheme 6a). Jung et al. challenged this

whereas the bromide was more active than the chloride because of the higher solubility of the former in organic solvents. Chen and Chen reported that the very electrophilic alane Al(C6F5)3 catalyzes the hydrosilylation of 1-hexene (5 mol % catalyst load, 0.5 h, 98%) through the Yamamoto−Jung Lewis acid mechanism, proceeding via formation of the reactive silane complex [Et3Si−H···Al(C6F5)3] (isolated and studied by X-ray diffraction), which then delivers a silylium ion to the alkene to give a carbocation.28 The reaction is completed by the transfer of hydride from [HAl(C6F5)3]−. The related adduct Al(C6F5)3· toluene0.5 also catalyzes this reaction, albeit at a much reduced rate (the turnover frequency (TOF) drops from 39 to 19 h−1). Our group studied the hydrosilylation of olefins and alkynes mediated by the cationic complex [DippNacNacAlH]+ (2) (DippNacNac = CH{CMe(NDipp)}2, Dipp = 2,6-Pri2C6H3). Mechanistic studies revealed that olefins easily insert into the Al−H bond of 2 to give alkylaluminum derivatives but the latter do not react with silanes (e.g., HSiEt3) to furnish the hydrosilylation products even upon heating at 70 °C for 24 h. The fact that a stoichiometric reaction between 3,3-dimethyl1-butene and 2 gave a neohexylaluminum compound while the same reaction under catalytic conditions (5 mol % 2, room temperature (RT)) gave the rearranged hydrosilylation product 1-silyl-2,3-dimethylbutane suggests that catalysis likely proceeds via Lewis acid activation of the silane by the cationic aluminum center (Scheme 5). Stoichiometric reactions of the cationic

Scheme 6. (a) Yamamoto’s Mechanism for Hydrosilylation of Alkynes; (b) Jung’s Labeling Experiment for the Hydrosilylation of 1,6-Heptadiyne

Scheme 5. Divergent Reactions of [DippNacNacAlH]+ with Alkenes under Catalytic and Stoichiometric Conditions

proposal on the basis of a labeling reaction between 1,6heptadiyne and DSiMe2(CH2)2Ph, which delivered deuterium to the carbon at the 3-position of the ring. The result was interpreted in terms of initial activation of the silane by the Lewis acid, addition of a silylium ion to the diyne, cyclization, and finally abstraction of deuteride by an intermediate alkenyl cation (Scheme 6b).27 Also of note is that while 1,6-heptadiyne reacts with 4 equiv of HSiEt3 to give the cyclization product, the analogous reaction with 1,7-octadiyne gives the product of double hydrosilylation with silyl groups at the terminal positions.32 Aluminum-catalyzed reduction of carbonyls with silanes has also received significant attention. Group 13 and 15 triflates E(OTf)3 (E = Al, Ga, Bi; 2 mol % catalyst load; RT or 65 °C) catalyzed the reduction of benzaldehydes by silanes to produce dibenzyl ethers (ArCH2)2O and benzylated aromatic solvents ArCH2Ar′ (toluene and benzene, Ar′ = Ph or Tol), but little or no hydrosilylation products ArCH2OSiEt3 were observed.34 The corresponding reactions with acetophenone were less efficient, with dibenzyl ethers being the primary products in dichloroethane, whereas benzylated toluenes formed when toluene was used as the reaction medium. The formation of the latter product was explained in terms of Friedel−Crafts alkylation of the solvent by the incipient silylcarboxonium ion [Et3SiO−CHAr]+ (3). It was suggested that the dibenzyl ether was formed by a condensation between the silylcarboxonium ion and the hydrosilylation product ArCH2OSiEt3 to give a

alkyl complex [DippNacNacAlHex]+ with olefins revealed the reversible formation of 1,4-cycloaddition products similar to the lantern type of adducts between [DippNacNacAlMe]+ and alkenes reported by Jordan29 and Harder.30 Reactions between [DippNacNacAlH]+ and alkynes resulted in addition of the C C bond across the DippNacNacAl moiety to give related tripodal aluminum cations, which are also potent catalysts for the hydrosilylation of alkynes. Aluminum-catalyzed hydrosilylation of alkynes was studied by the research groups of Voronkov,31 Yamamoto,32 and Tanaka.33 In contrast to transition-metal-catalyzed Si−H addition to triple bonds, which usually proceeds in a cis manner, the hydrosilylation of alkynes with trialkylsilanes catalyzed by AlCl3 and EtAlCl2 gives the products of transaddition with high stereoselectivity.32 For terminal alkynes, the reaction is also highly regioselective, affording 1-silylalkenes. Et2AlCl was not effective in this reaction, likely because of its reduced Lewis acidity. The regio- and stereoselectivity of these reactions point to a silylium ion mechanism. However, unlike the related reactions with alkenes, which are believed to proceed via silane activation, Yamamoto et al.32 suggested that the Lewis acidic aluminum activates the alkyne by forming a π 7259

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ACS Catalysis Scheme 7. Al(OTf)3-Catalyzed Reductive Etherification of Carbonyls

Scheme 8. Hydrosilylation of Carbonyls Catalyzed by Complex 5

Scheme 9. Reduction of CO2 Catalyzed by Cation 6

further corroborated by the observation that 5 catalyzes H/D exchange between DSiEt3 and H2SiPh2. Finally, the observation that hydrosilylation of cyclopropyl methyl ketone provides an alkoxysilane without ring opening ruled out the possibility of a radical mechanism. Khandelwal and Wehmschulte36 showed that the lowcoordinate organoaluminum cation [Et2Al]+, generated by treatment of Et3Al with [Ph3C][CH6B11X6] (X = Cl, I) using the procedure developed by Reed et al.,37 catalyzes the reduction of CO2 by HSiEt3 to methane (90%), toluene (98%), and diphenylmethane (2%) in benzene solutions, with 58% conversion after 216 h at 80 °C using a 1 mol % catalyst load. H3SiPh and H2SiPh2 were found to be less effective as reductants because of facile redistribution of substituents at silicon, whereas HSiMe2tBu showed activity similar to HSiEt3 but provided much less methane and a higher yield of diphenylmethane. Mechanistic studies revealed that the catalytic activity of [Et2Al]+ is higher than that of [Et3Si]+ or [Me2tBuSi]+ generated in situ by reaction of the corresponding silane with [Ph3C][CH6B11X6]. No intermediates were observed during the reduction, but introduction of the suggested silyl formate intermediate, HCO2SiEt3, into the catalytic mixture resulted in fast conversion, consistent with the rate-limiting step being the initial reduction of CO2 to HCO2SiEt3, which then converts into the methoxysilane MeOSiEt3. It was shown that MeOSiEt3 is involved in both

silyloxonium ion (4), which then eliminated hexaethylsiloxane (Scheme 7). The difference between this catalytic scheme and the related B(C6F5)3-catalyzed hydrosilylation, resulting in silylated alcohols, was attributed to the different stability of the common silylcarboxonium intermediate 3, which was proposed to be longer-lived in the case of catalysis by Al(OTf)3. A control experiment showed that Et3SiO−CH2Ar alone does not convert into (ArCH2)2O under the action of Al(OTf)3 in the absence of a carbonyl compound. In addition to olefins, Al(C6F5)3 is also able to catalyze the hydrosilylation of ketones (e.g., acetophenone), although its activity is much reduced in comparison with that of B(C6F5)3 because of the much stronger Lewis acidity of the alane and hence its stronger coordination to ketones, which reduces the effective amount of Lewis acid catalyst available for activation of the silane.28 Koller and Bergman reported that the well-defined cationic complex [Tp*AlMe]+ (5) (Tp* = hydro-(1,3-dimethylpyrazol1-yl)borate) can serve as a catalyst for the hydrosilylation of aldehydes, ketones, δ-valerolactone, and aldimines at 75−100 °C (Scheme 8).35 Mechanistic studies showed that acetophenone does not react with 5, whereas addition of HSiEt3 results in the loss of J coupling between the Si−H proton and the methylene protons of the ethyl group. It was suggested that the aluminum cation coordinates and activates the silane in a fashion analogous to the borane-catalyzed reactions. This was 7260

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ACS Catalysis Scheme 10. Hydroboration of Carbonyls Catalyzed by the Neutral Aluminum Hydride 7

Scheme 11. Hydroboration of Carbonyls Catalyzed by

Scheme 12. Hydroboration of Alkynes Catalyzed by

Dipp

Dep

NacNacAlH(OTf)

NacNacAlH2

hydrides are generally more reactive than the related boranes because of the higher polarity of the Al−H bond and bigger size of the aluminum atom. However, they are much less stable, more dangerous (pyrophoric), and more difficult to handle. The complementary properties of these two elements, aluminum and boron, have recently been combined in an elegant way through aluminum-catalyzed hydroboration. In 2000, Woodward et al. reported the first instance of hydroboration of acetophenone catalyzed by BINOL-derived aluminum hydrides.40 The goal of this work was to develop a catalytic version of the asymmetric reduction by BINAL-H pioneered by Noyori.41 The aluminum hydride compounds were generated in situ by combining LiAlH4 with the chiral ligands 1,1′-(bi-2-naphthol) (BINOLH2), 2-hydroxy-2′-mercapto-1,1′-binaphthyl (MTBH2), and 2,2′-dimercapto-1,1′binaphthyl (DTBH2). Although hydroboration with catecholborane worked well, the enantioselectivity achieved with these aluminum catalysts was low (1−6%), whereas for the related gallium system, MTBH2/LiGaH4, a high ee of 72% was realized. Nembenna et al. described the efficient hydroboration of a variety of aldehydes and ketones catalyzed by the well-defined aluminum hydride 7 (Scheme 10).42 This process tolerates fluoro, chloro, bromo, and nitro substituents in aromatic substrates and allows for chemoselective reduction of aldehydes in the presence of ketones. It was suggested that the reaction proceeds via conventional insertion of the CO moiety into the Al−H bond followed by Al−O/H−B metathesis.43 The groups of Yang, Parameswaran, and Roesky showed that the aluminum hydride DippNacNacAlH(OTf) (8) (OTf = SO2CF3) is an excellent catalyst for the hydroboration of aldehydes (99% conversion with 1 mol % Al at RT) and ketones (>50% conversion with 1 mol % catalyst load at RT),

the subsequent reduction to methane and in the Lewis acidcatalyzed Friedel−Crafts alkylation of benzene. However, the activity of this cationic aluminum catalyst was rather low, which was further aggravated by its sensitivity and rapid deactivation. These problems were mitigated by the application of a more robust aluminum phenoxide cation, [(2,6Mes2C6H3O)2Al]+ (6) (Mes = 2,4,6-Me3C6H2), which was isolated as a salt with the noncoordinating counteranion [CHB11Cl11]− (Scheme 9).38 Complex 6 does not react with CO2, but when treated with a mixture of CO2 and Et3SiH in benzene-d6, it catalyzes the reduction of CO2 to a mixture of C6D5CH3 (60%), CH4 (18%), and (C6D5)2CH2 (2%) after 48 h at 85 °C (at 3 mol % catalyst load).

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ALLYSILYLATION The Jung group also reported on AlCl3-catalyzed allylsilylation of unactivated alkenes, diallylsilanes, conjugated dienes, and alkynes with allyltrimethylsilane.39 In these reactions, the silyl group adds to the external carbon of terminal alkenes and alkynes while the allyl group adds to the inner carbon, in agreement with the silylium ion mechanism. The process is related to hydrosilylation and is believed to follow a similar Lewis acid-catalyzed pathway, with the difference being that the substrate is activated by transfer of the allyl group, not the hydride, to aluminum.

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HYDROBORATION Hydroboranes are more sensitive compounds than hydrosilanes, but thanks to the better energetics for B−X bonds versus Si−X bonds (e.g., the energy of the B−O bond is 125 kcal mol−1 vs 110 kcal mol−1 for the Si−O bond), they can in some cases serve as superior reducing reagents. Aluminum 7261

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the hydroboration of terminal alkynes with HBpin at 80 °C (Scheme 14).51 The more sterically encumbered compound

likely because of the increased positive charge on the aluminum atom induced by the weakly coordinating triflate substituent (Scheme 11).44 The same catalyst mediates the silylcyanation of aldehydes and ketones by Me3SiCN.44,45 The mechanism of hydroboration was studied by density functional theory (DFT) calculations. The reaction was found to proceed via addition of the aldehyde to aluminum to form a five-coordinate intermediate that undergoes rate-determining intramolecular insertion of the carbonyl group into the Al−H bond to furnish an aluminum benzyloxide. The process is culminated by a 2 + 2 heterolytic splitting of the H−B bond on the Al−O bond, which regenerates the aluminum hydride and liberates the product of hydroboration. Yang et al. also described the hydroboration of terminal alkynes mediated by the dihydride DepNacNacAlH2 (9) (DepNacNac = HC(CMeNDep), Dep = 2,6-Et2C6H3) to give (E)-1-boryl alkenes (Scheme 12).46 Internal alkynes do not enter this reaction. In a stoichiometric experiment, it was observed that the compound NacNacAlH2 reacts with phenylacetylene to give the alkynyl derivative DepNacNacAl(H)(C CPh). This reaction was suggested to be the first step of a catalytic cycle that was probed by DFT calculations. However, the rate-determining step of this process, Al−C/H−C metathesis, was found to be unrealistically high (ΔG⧧ = 45.3 kcal mol−1), which suggests that a different mechanism may be operative. In fact, Scheme S5 in the Supporting Information for ref 46 shows that the compound DepNacNacAlH2 reacts with HBpin, although the text immediately preceding this scheme says that “[in] the stoichiometric reaction of 1 [i.e., DepNacNacAlH2] with HBpin at room temperature[,] no noticeable reaction was observed”. In the same report, dehydrogenative coupling of alcohols, thiols, and amines with boranes (HBPin and 9-BBN) catalyzed by DepNacNacAlH2 was described, and a mechanism based on Al−H bond protonolysis followed by Al− X/H−B metathesis (X = O, S, N) was proposed. However, Bertrand et al. showed that this reaction proceeded easily at room temperature without any catalyst.47 Cowley et al. found that the commercially available aluminum compounds iBu2AlH (DIBAL-H), Me3Al, Et3Al, and in particular bench-stable Et3Al·1,4-diazabicyclo[2.2.2]octane (Et3Al·DABCO) served as catalysts for the hydroboration of both terminal and internal alkynes by HBpin in good yields (60−90%; Scheme 13).48,49 Interestingly, alkenes

Scheme 14. Hydroboration of Alkynes Catalyzed by a Guanidinato-Supported Aluminum Hydride

{LDippNAlH2}2 showed reduced activity. Inoue et al. also reported that the more Lewis acidic monohydride triflate compounds {LMesNAlH(OTf)}2 and {LDippNAlH(OT)}2 catalyzed the hydroboration of aldehydes even at room temperature.

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DEHYDROGENATION OF FORMIC ACID Hydrogenation of CO2 and the reverse reaction, dehydrogenation of methanol and formic acid, are hot topics in modern reduction chemistry. Myers and Berben found that aluminum compounds ligated by the bis(imino)pyridine ligand Ph dimpyrAlX(THF) (10) (Phdimpyr = H3C5N(2,6-C(Ph) =NDipp)2; X = H, Me) catalyzed the dehydrogenation of formic acid (taken in the form of a 5:2 mixture with Et3N) to afford H2 and CO2 with an initial TOF of 5200 h−1 (Scheme 15).52 Kinetic studies by Myers and Berben showed that the Scheme 15. Dehydrogenation of Formic Acid Catalyzed by dimpyr Compound 10

reaction is first-order in the catalyst and approximately firstorder in formic acid, but the rate reaches saturation at higher concentrations of the acid (>7.6 M). The entropy of activation was measured to be large and negative, ΔS⧧ = −84.8 J mol−1 K−1, indicative of an ordered transition state. Further mechanistic studies revealed that the first products from the interaction between PhdimpyrAlX(THF) and HCO2H are hydrogen gas and methane for X = H and Me, respectively, suggesting protonolysis of the Al−X bond. Low-temperature NMR experiments showed that this protonolysis occurred through aluminum−ligand cooperative activation proceeding via initial protonation of the NDipp group of Phdimpyr and formation of the aluminum formate complex (PhdimpyrH)AlX(O2CH), followed by subsequent double protonation to release HX and form [(PhdimpyrH2)Al(O2CH)2]+, with one of the CN arms hydrogenated to give PhdimpyrH2. The possibility of a hydride shift between the formate and aluminum was established next. The reverse reaction, insertion of CO2 into the Al−hydride bond of PhdimpyrAlH(THF) under neutral conditions, was facile and produced the formate compound Ph dimpyrAl(κ2-O2CH). Upon addition of the weak acid H2NTs, the latter aluminum species liberated CO2 and formed the hydride (PhdimpyrH)Al(NTs)H with a monoprotonated ligand. The analogous reaction with PhdimpyrAl(κ2-O2CD) resulted in

Scheme 13. Hydroboration of Alkynes Catalyzed by Et3Al· DABCO

were inert under these conditions, allowing chemoselective addition of the H−B bond to the triple bond to be accomplished. On the basis of the results of stoichiometric experiments, these authors suggested an alternative mechanism of hydroboration involving alkyne hydroalumination followed by Al−C/H−B metathesis. In contrast to the aforementioned report by Yang et al., mechanistic studies did not support protonolysis of the Al−H bond in the case of terminal alkynes. Inoue et al. investigated the catalytic activity of the guanidinato-supported aluminum hydrides {LMesNAlH2}250 and {LDippNAlH2}2 (LMesN = 1,3-dimesitylimidazolin-2-imino, LDippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-imino) in 7262

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Scheme 16. Simplified Reaction Pathway Calculated for the Dehydrogenation of Formic Acid by Berben’s Hydride Ph dimpyrAlH (Free Energies in kcal mol−1 Are Shown)

The first well-defined molecular catalyst was reported by Koller and Bergman, who showed that that phenylenediamine aluminum compound 13 is a catalyst for intramolecular hydroamination of aminoalkenes (Scheme 17), but the catalyst

the deuteride compound (PhdimpyrH)Al(NTs)D, thus establishing the possibility of β-hydride abstraction from the formate. It was further suggested that the β-H shift proceeds as an “outer sphere β-hydride abstraction” in which the carbonyl end of the formate forms a hydrogen bond to the amide proton, thus closing a six-membered ring. This mechanistic picture was further elaborated by DFT calculations by Fu et al., who elucidated the role of the noninnocent dimpyr ligand in the H−O activation step and the details of the decarboxylation step.53 Addition of formic acid to Ph dimpyrAlH proceeds via H−O bond cleavage on the Al−N moiety to give the aluminum formate hydride (PhdimpyrH)AlH(κ1-O2CH) (Scheme 16), which is the rate-determining step in the lowest-energy pathway (ΔG⧧ = 12.9 kcal mol−1). Direct elimination of hydrogen from the NH and AlH units requires a higher barrier of ΔG⧧ = 15.3 kcal mol−1, whereas a more favorable pathway proceeds via addition of a second equivalent of formic acid to give (PhdimpyrH)Al(κ1-O2CH)(κ2O2CH) and can be classified as HO2CH-assisted Al−H bond cleavage. Protonation of the imine carbon takes place upon addition of the third molecule of formic acid to furnish the tris(formate) derivative (PhdimpyrH2)Al(κ1-O2CH)3 (11). The cationic analogue, [(PhdimpyrH2)Al(O2CH)2]+, suggested by Berben et al., lies 26.3 kcal mol−1 higher in energy. Dehydrogenation of formic acid then proceeds via β-H elimination from (PhdimpyrH2)Al(κ1-O2CH)3 to give a hydride intermediate 12, which can then directly react with formic acid to generate 11. In contrast, attempts to locate a cyclic transition state involving metal−ligand cooperation, as postulated by Berben et al., failed. The actual catalytic cycle thus includes only 11 and the hydride diformate complex 12. Analogous decarbonylation of (PhdimpyrH)Al(O2CH)2 requires a much higher barrier of 40.6 kcal mol−1 and is thus not relevant to catalysis.

Scheme 17. Intramolecular Hydroamination Catalyzed by Phenylenediamine-Based Compound 13

load was relatively high (10 mol %) and the conditions were harsh (150 °C).55 It was proposed that the reaction proceeded via Al−NMe2/H−NHR metathesis to give an aluminum olefin−amide, followed by cyclization with the pendant olefin group. Koller and Bergman also reported on the use of an OCO pincer complex of aluminum in the intramolecular hydroamination of 2,2-diphenylpent-4-en-1-amine under conditions comparable to the previous example (10 mol % catalyst load, 150 °C), albeit with a longer reaction time (100 vs 38 h).56 The same group prepared guanidinate-supported complexes, such as 14, and studied them in catalytic hydroamination.57 Although intramolecular cyclization of pentenamines did not work, these aluminum guanidinates can be applied toward the hydroamination of carbodiimide, a socalled guanylation reaction, which leads to substituted guanidines (Scheme 18).58 The reaction occurred within minutes at room temperature, required only a 1 mol % load of the catalyst, and showed a high functional group tolerance. Similarly, aminoguanidines could be prepared via hydrazination of various carbodiimides with 1,1-disubstituted hydrazines, although much higher temperatures (120 °C) were required. Khandelwal and Wehmschulte investigated the activity of the cationic aluminum species [Et2Al][CHB11H5I6], [Et2Al][CHB11H5Cl6], [DcpAlEt][CHB11H5Cl6] (Dcp = 2,6-(2,6Cl2C6H3)2C6H3), and [Dipp*AlEt][CHB11H5I6] (15) (Dipp*

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HYDROAMINATION Hydroamination is an electron-neutral process in the sense that one carbon atom is oxidized while the other is reduced. Nevertheless, it is related to other hydroelementation reactions discussed above, and therefore, it is appropriate to be considered here. Li et al. reported that AlCl3 and other Lewis acids can catalyze the intermolecular hydroamination of norbornene and suggested a Lewis acid mechanism.54 7263

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ACS Catalysis Scheme 18. Hydroamination of Carbodiimide Catalyzed by 14

Scheme 19. Hydroamination of Aminopentenes Catalyzed by Cation 15

= 2,6-Dipp2C6H3) in the intramolecular hydroamination of primary and secondary aminopentenes (Scheme 19).59 The reactions were carried out in C6D6 with a 10 mol % catalyst load at 135 °C, i.e., under slightly better conditions than those used with the neutral catalyst 13. It was suggested that the reaction proceeds via formation of a metal amide complex, with subsequent attack of the amide nitrogen at the double bond, which may be activated by coordination to the metal center. The corresponding neutral aluminum compounds, Et3Al, DcpAlEt2, and Dipp*AlEt2, were 25 times less reactive than the corresponding cations. The cyclization of secondary amines, such as benzylaminopentenes, with [Et2Al][CHB11H5I6] was also studied. The substrate bearing a methyl group at the 2position of the olefin reacts much slower than the unsubstituted analogue (64% conversion at 11 h vs >99% after 0.5 h). However, the effect of sterics is not straightforward, as the methyl-substituted benzylaminopentene reacted slower than its primary counterpart whereas the substituted benzylaminopentene N-benzyl-2,2-diphenylpent-4-en-1-amine reacted faster than the corresponding unsubstituted aminopentene 2,2diphenylpent-4-en-1-amine. Gunnoe et al. showed that Al(OTf)3 is a precatalyst for the hydroamination of unactivated primary and secondary alkenylamines between 110 and 150 °C.60 However, unlike the reactions discussed above, mechanistic studies suggested that the actual catalyst was triflic acid generated in situ.

Scheme 20. Hydrodefluorination of Aliphatic Fluorides by [iBu2Al]+

hydride from the hydroalane to regenerate the catalyst. Aromatic substrates, such as C6H5F and C6F6, which are more common in transition-metal-catalyzed hydrodefluorination, gave only small conversions, a fact that can be related to the stability of the corresponding carbocations. As such, these results parallel the findings by Ozerov et al. on silylium ion abstraction of fluoride from fluorinated organic substrates.63 The Ozerov group also reported that the related aluminum cation [Et2Al]+, paired with the weakly coordinating carborane anion [HCB11Br6H5]−, catalyzed the methyldefluorination of monofluoroalkanes, gem-difluorocyclopentane, and aromatic and aliphatic compounds with the CF3 group, with AlMe3 as the alkyl source (Scheme 21).64 The catalyst load was very low (0.33 mol %), and the conditions were mild, mostly at ambient temperature. Only C(sp3)−F bonds underwent this reaction; C(sp2)−F bonds remained unaffected. When AlEt3 or AliBu3 was utilized as the alkyl source, significant hydrodefluorination of the C−F bonds occurred, likely because of competing hydride transfer from the organoaluminum compound. The silane/alane adduct Et3SiH·Al(C6F5)3 effectively catalyzed the hydrodefluorination of Ph3CF with a TOF of 600 h−1.28 It was suggested that the resting state of the catalyst is the adduct Et3SiF·Al(C6F5)3, which reacts with Ph3CF to produce [Ph3C]+[FAl(C6F5)3]− and free Et3SiF. The trityl salt then abstracts hydride from Et3SiH to give a silylium ion, which reacts with [FAl(C6F5)3]− to regenerate the catalyst.

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HYDRODEFLUORINATION Hydrodefluorination (HDF) is an important reduction technique providing access to partially fluorinated organic molecules, many of which find applications in the agrochemical and pharmaceutical industries.61 Rosenthal et al. disclosed that the aluminum cation [iBu 2 Al]+ , generated by hydride abstraction from iBu2AlH with the trityl cation, catalyzes HDF of nonactivated C−F bonds of C6H5CF3, n-C5H11F, and n-C6H13F at room temperature in cyclohexane (for aliphatic fluorides) or o-dichlorobenzene (for C6H5CF3) using iBu2AlH as the reductant (Scheme 20).62 Weakly coordinating counteranions such as [B(C6F5)4]−, [Al(C6F5)4]−, and [Al{OC(CF3)3}4]− were employed to ensure a low-coordinate environment around the aluminum center. Mechanistically, the reaction is believed to proceed via abstraction of fluoride by the aluminum cation to give a carbocation, which then abstracts

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CONCLUSIONS The use of aluminum offers obvious benefits, such as low cost and low toxicity. Although the use of aluminum is not new to catalysis, in the second half of the 20th century its applications 7264

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ACS Catalysis Scheme 21. Methyldefluorination of C(sp3)−F Bonds Catalyzed by [Et2Al][HCB11Br6H5]

were largely overshadowed by the burgeoning field of transition metal catalysis. Over the past decade, however, there has been a significant renaissance in the catalytic chemistry of aluminum. In the field of reduction chemistry, the interest is steadily shifting from the use of simple aluminum salts, such as halides and triflates, to the employment of tailor-made catalysts. The range of applications of aluminum compounds has increased from transfer hydrogenation (MVP process) and hydrosilylation to include hydroboration, hydroamination, hydrodefluorination, and more recently even hydrogenation, which increases both the synthetic repertoire and the scope of substrates that can be engaged in reduction. What has been missing to date is the use of chiral aluminum compounds in stereoselective catalysis, but one can easily predict that this niche will be filled in the near future. Another remaining challenge for aluminum catalysis is to expand the substrate scope beyond the common alkenes, alkynes, aldehydes, and ketones to include more difficult substrates such as imines, esters, carbonic acids, nitriles, and heterocycles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Georgii I. Nikonov: 0000-0001-6489-4160 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS G.I.N. thanks NSERC for generous financial support (Discovery Grant RGPIN-2017-05231) and Dr. T. Chu for useful discussions.

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