Magnesium Catalysis of Imine Hydroboration

The β‐diketiminato magnesium alkyl complex [LMgnBu] (L=CH{CMe(NDipp)}₂, Dipp=diisopropylphenyl) is shown to be a highly effective precatalyst for the hydroboration of alkyl and aryl substituted aldimines and ketimines with pinacol borane (HBpin). Catalysis is proposed to occur through a sequence of Mg? N/B? H metathesis and rate‐determining Mg? H/N?C insertion steps, a proposal strongly supported by stoichiometric studies and kinetic analysis. The reactions are observed to proceed through the intermediacy of well‐defined magnesium amides, two examples of which have been isolated and structurally characterized. Mechanistic investigations suggest that the catalytic rate‐determining process occurs at an isolated magnesium center and requires the presence of two molecules of the imine substrate for effective turnover. This latter observation is rationalized as a requirement for the secondary substrate molecule to displace HBpin from the coordination sphere of the catalytic magnesium center.     

Mechanistic Insight into Hydroboration of Imines from Combined Computational and Experimental Studies

Boron Lewis acid-catalyzed and catalyst-free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N-(α-methylbenzylidene)aniline catalyzed by tris[3,5-bis(trifluoromethyl)phenyl]borane (BAr^F₃), our calculations show that the reaction proceeds through a boron Lewis acid-promoted hydride transfer mechanism rather than the classical Lewis acid activation mechanism. For the catalyst- and solvent-free hydroboration reaction of imine, N-benzylideneaniline, our calculations and experimental studies indicate that this reaction is difficult to occur under the reaction conditions reported previously. With a combination of computational and experimental studies, we have established that the commercially available BH₃ ⋅ SMe₂ can serve as an efficient catalyst for the hydroboration reactions of N-benzylideneaniline and similar imines. The hydroboration reactions catalyzed by BH₃ ⋅ SMe₂ are most likely to proceed through a hydroboration/B−H/B−N σ-bond metathesis pathway, which is very different from that of the reaction catalyzed by BAr^F₃.     

Aminoazanium of DABCO: An Amination Reagent for Alkyl and Aryl Pinacol Boronates

The aminoazanium of DABCO (H₂N‐DABCO) has been developed as a general and practical amination reagent for the direct amination of alkyl and aryl pinacol boronates. This compound is stable and practical for use as a reagent. Various primary, secondary. and tertiary alkyl?Bpin and aryl?Bpin substrates were aminated to give the corresponding amine derivatives. The amination is stereospecific. The anti‐Markovnikov hydroamination of olefins was easily achieved by catalytic hydroboration with HBpin and in subsequent situ amination using H₂N‐DABCO. Moreover, the combination of 1,2‐diboration of olefins, using B₂pin₂, with this amination process achieved the unprecedented 1,2‐diamination of olefins. The amination protocol was also successfully extended to aryl pinacol boronates.     

Boron Lewis Acid‐Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6F5)3 Cannot Do!

The transition‐metal‐free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BAr^F₃) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆F₅)₃) is reluctant to react. Unlike B(C₆F₅)₃, BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^FBpin and the electron‐deficient diboranes [H₂BAr^F]₂ and [(Ar^F)(H)B(μ‐H)₂BAr^F₂]. These in situ‐generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin‐derived anti‐Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.     

Alkaline‐Earth‐Catalyzed Dehydrocoupling of Amines and Boranes

Dehydrocoupling reactions between the boranes HBpin and 9‐borabicyclo[3.3.1]nonane and a range of amines and anilines ensue under very mild reaction conditions in the presence of a simple β‐diketiminato magnesium n‐butyl precatalyst. The facility of the reactions is suggested to be a function of the Lewis acidity of the borane substrate, and is dictated by resultant pre‐equilibria between, and the relative stability of, magnesium hydride and borohydride intermediates during the course of the catalysis.     

Pincer-Supported Gallium Complexes for the Catalytic Hydroboration of Aldehydes,Ketones and Carbon Dioxide

Gallium hydrides stabilised by primary and secondary amines are scarce due to their propensity to eliminate dihydrogen. Consequently, their reactivity has received limited attention. The synthesis of two novel gallium hydride complexes HGa(THF)[ON(H)O] and H₂Ga[μ²-ON(H)O]Ga[ON(H)O] ([ON(H)O]²⁻=N,N-bis(3,5-di-tert-butyl-2-phenoxy)amine) is described and their reactivity towards aldehydes and ketones is explored. These reactions afford alkoxide-bridged dimers through 1,2-hydrogallation reactions. The gallium hydrides can be regenerated through Ga−O/B−H metathesis from the reaction of such dimers with pinacol borane (HBpin) or 9-borabicyclo[3.3.1]nonane (9-BBN). These observations allowed us to target the catalytic reduction of carbonyl substrates (aldehydes, ketones and carbon dioxide) with low catalyst loadings at room temperature.     

Aluminum Hydride Catalyzed Hydroboration of Alkynes

An aluminum‐catalyzed hydroboration of alkynes using either the commercially available aluminum hydride DIBAL‐H or bench‐stable Et₃Al?DABCO as the catalyst and H‐Bpin as both the boron reagent and stoichiometric hydride source has been developed. Mechanistic studies revealed a unique mode of reactivity in which the reaction is proposed to proceed through hydroalumination and σ‐bond metathesis between the resultant alkenyl aluminum species and HBpin, which acts to drive turnover of the catalytic cycle.     

Structural and Mechanistic Insights into s‐Block Bimetallic Catalysis: Sodium Magnesiate‐Catalyzed Guanylation of Amines

To advance the catalytic applications of s‐block mixed‐metal complexes, sodium magnesiate [NaMg(CH₂SiMe₃)₃] ( 1 ) is reported as an efficient precatalyst for the guanylation of a variety of anilines and secondary amines with carbodiimides. First examples of hydrophosphination of carbodiimides by using a Mg catalyst are also described. The catalytic ability of the mixed‐metal system is much greater than that of its homometallic components [NaCH₂SiMe₃] and [Mg(CH₂SiMe₃)₂]. Stoichiometric studies suggest that magnesiate amido and guanidinate complexes are intermediates in these catalytic routes. Reactivity and kinetic studies imply that these guanylation reactions occur via (tris)amide intermediates that react with carbodiiimides in insertion steps. The rate law for the guanylation of N,N′‐diisopropylcarbodiimide with 4‐tert‐butylaniline catalyzed by 1 is first order with respect to [amine], [carbodiimide], and [catalyst], and the reaction shows a large kinetic isotopic effect, which is consistent with an amine‐assisted rate‐determining carbodiimide insertion transition state. Studies to assess the effect of sodium in these transformations denote a secondary role with little involvement in the catalytic cycle.     

Cobalt-Catalyzed Regiodivergent Ring-Opening Dihydroboration of Arylidenecyclopropanes to Access Skipped Diboronates

Ligand-controlled regiodivergent cobalt-catalyzed ring-opening dihydroboration of arylidenecyclopropanes is developed to access synthetically versatile skipped diboronates with catalysts generated in situ from Co(acac)₂ and dpephos or xantphos. A variety of arylidenecyclopropanes reacted with pinacolborane (HBpin) to form the corresponding 1,3- or 1,4-diboronates in high isolated yields and with high regioselectivity. Skipped diboronate products from these reactions can undergo various transformations to allow selective installation of two different functional groups along alkyl chains. Mechanistic studies suggest that these reactions combine cobalt-catalyzed ring-opening hydroboration of arylidenecyclopropanes and hydroboration of homoallylic or allylic boronate intermediates.     

Synthesis and structures of anionic rhenium polyhydride complexes of boron–hydride ligands and their application in catalysis

The rhenium complex, [K(DME)(18-c-6)][ReH₄(Bpin)(η²-HBpin)(κ²-H₂Bpin)] 1, comprising hydride and boron ligands only, has been synthesized by exhaustive deoxygenation of the commercially available perrhenate anion (ReO₄⁻) with pinacol borane (HBpin). The structure of 1 was analysed by X-ray crystallography, NMR spectroscopy, and DFT calculations. While no hydrides were located in the X-ray crystal structure, it revealed a trigonal arrangement of pinacol boron ligands. Variable-temperature NMR spectroscopy supported the presence of seven hydride ligands but further insight was hindered by the fluxionality of both hydride and boron ligands at low temperature. Further evaluation of the structure by Ab Initio Random Structure Searching (AIRSS) identified the presence of hydride, boryl, σ-borane, and dihydroborate ligands. This complex, either isolated or prepared in situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple operating procedures. It also acts as a reagent for the stoichiometric C–H borylation of toluene, displaying high meta regioselectivity in the borylated products. Reaction of 1 with 9-BBN resulted in HBpin substitution to form the new anionic tetra(dihydroborate) complex [K(DME)(18-c-6)][Re(κ²-H-9-BBN)₄] 4 for which the hydride positions were clearly identified by X-ray crystallography. The method used to generate these isolable yet reactive boron–hydride complexes is direct and straightforward and has potential utility for the exploitation of other metal oxo compounds in operationally simple catalytic reactions.

Exhaustive deoxygenation of perrhenate by pinacol borane forms a new Re anion of boron and hydride ligands only that undergoes borane ligand exchange, stoichiometric C–H boration, and catalytic pyridine hydroboration.     

Iron Catalyzed Double Bond Isomerization: Evidence for an FeI/FeIII Catalytic Cycle

Iron-catalyzed isomerization of alkenes is reported using an iron(II) β-diketiminate pre-catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H₃N⋅BH₃). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre-catalyst activation step that gives access to an η²-coordinated alkene Fe^I complex, followed by oxidative addition of the alkene to give an Fe^III intermediate, which then undergoes reductive elimination to allow release of the isomerization product.     

Catalytic hydroboration of aldehydes and ketones with sodium hydride: Application to chemoselective reduction of aldehydes over ketones

Sodium hydride-promoted catalytic hydroboration of aldehydes and ketones with pinacolborane (HBpin) was examined, and 10?mol% of NaH was found to cause the HBpin to participate in hydroboration in a convenient and efficient manner at mild reaction conditions. Further chemoselective hydroboration of aldehyde over ketone functionality was also analyzed. In addition, no hydroboration was observed form ester, acyl chloride, amide, nitrile, alkene, alkyne, alkyl halide and epoxide functional groups indicate that present system (HBpin, NaH) is highly selective for aldehydes and ketones.     

The transition metal catalyzed hydroboration of enamines

The addition of catecholborane (HBcat, cat = 1,2-O₂C₆H₄) to 9-vinylcarbazole can give either the branched or linear hydroboration product depending upon the judicious choice of metal catalyst used in these reactions. Analogous reactions with pinacolborane (HBpin, pin = 1,2-O₂C₂Me₄) and HBBzpin (Bzpin = 1,2-O₂C₂Ph₄) using catalytic amounts (5 mol%) of either Rh(acac)(dppb) or [Cp^∗IrCl₂]₂ gave the linear hydroboration product selectively. Hydroborations of 1-pyrrolidino-1-cyclopentene and 1-pyrrolidino-1-cyclohexene were complicated by a competing dehydrogenative borylation pathway. The branched isomer was not observed to any significant extent in these reactions, suggesting that the directing effect of the nitrogen atom is negligible. Although catalyzed additions of HBcat to 1-vinyl-2-pyrrolidinone gave complicated product distributions arising from competing reactions, addition of HBpin effectively generated the corresponding linear hydroboration product in good yields.     

Monomeric Magnesium Catalyzed Alkene and Alkyne Hydroboration

In this work, two monomeric magnesium alkyl complexes ( 1 and 2 ) were prepared using bis(phosphino)carbazole framework and among them 1 has been used as a catalyst for hydroboration of alkenes and alkynes with pinacolborane (HBpin). A broad variety of aromatic and aliphatic alkenes and alkynes were efficiently reduced. Anti-Markovnikov regioselective hydroboration of alkenes and alkynes was achieved, which was confirmed by deuterium-labelling experiments. The work represents the first example of the use of magnesium in homogeneous catalytic hydroboration of alkene with broad substrate scope. Experimental mechanistic investigations and DFT calculations provided insights into the reaction mechanism. Finally, the hydroboration protocol was extended to terpenes.     

A Highly cis‐Selective and Enantioselective Metal‐Free Hydrogenation of 2,3‐Disubstituted Quinoxalines

A wide range of 2,3‐disubstituted quinoxalines have been successfully hydrogenated with H₂ using borane catalysts to produce the desired tetrahydroquinoxalines in 80–99 % yields with excellent cis selectivity. Significantly, the asymmetric reaction employing chiral borane catalysts generated by the in situ hydroboration of chiral dienes with HB(C₆F₅)₂ under mild reaction conditions has also been achieved with up to 96 % ee, and represents the first catalytic asymmetric system to furnish optically active cis‐2,3‐disubstituted 1,2,3,4‐tetrahydroquinoxalines.     

Ammonia borane as a metal free reductant for ketones and aldehydes: a mechanistic study

Without a catalyst ketones and aldehydes were reacted in THF with ammonia borane (AB) to proceed hydroboration forming alkyl borates. Mechanistic studies revealed that dissociation of ammonia from AB occurred before the hydroboration step. When methanol was used as the solvent, metal free methanolysis of AB would take place with the ketone/aldehyde being directly hydrogenated by the MeOH·BH₃ complex.     

Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem CH Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.     

Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem C?H Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.     

Zinc catalysed electrophilic C–H borylation of heteroarenes

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat). An electrophile derived from [IDippZnEt][B(C₆F₅)₄] (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) combined with N,N-dimethyl-p-toluidine (DMT) proved the most active in terms of C–H borylation scope and yield. Using this combination weakly activated heteroarenes, such as thiophene, were amenable to catalytic C–H borylation using HBCat. Competition reactions show these IDipp–zinc cations are highly oxophilic but less hydridophilic (relative to B(C₆F₅)₃), and that borylation proceeds via activation of the hydroborane (and not the heteroarene) by a zinc electrophile. Based on DFT calculations this activation is proposed to proceed by coordination of a hydroborane oxygen to the zinc centre to generate a boron electrophile that effects C–H borylation. Thus, Lewis acid binding to oxygen sites of hydroboranes represents an under-developed route to access reactive borenium-type electrophiles for C–H borylation.

Cationic zinc Lewis acids catalyse the C–H borylation of heteroarenes using pinacol borane (HBPin) or catechol borane (HBCat).     

C?H Bond Activation/Borylation of Furans and Thiophenes Catalyzed by a Half‐Sandwich Iron N‐Heterocyclic Carbene Complex

A coordinatively unsaturated iron‐methyl complex having an N‐heterocyclic carbene ligand, [Cp*Fe(L^Me)Me] ( 1 ; Cp*=η⁵‐C₅Me₅, L^Me=1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), is synthesized from the reaction of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N′,N′‐tetramethylethylenediamine) with methyllithium and L^Me. Complex 1 is found to activate the C? H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(L^Me)(aryl)] (aryl=2‐furyl ( 2 ), 2‐thienyl ( 3 ), phenyl ( 4 )). The C? H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert‐butylethylene and a catalytic amount of 1 (10 mol % to HBpin). The borylation of the furan/thiophene or 2‐substituted furans/thiophenes occurs exclusively at the 2‐ or 5‐positions, respectively, whereas that of 3‐substituted furans/thiophenes takes place mainly at the 5‐position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2‐boryl‐furan and the borohydride complex [Cp*Fe(L^Me)(H₂Bpin)] ( 5 ). Heating a solution of 5 in the presence of tert‐butylethylene led to the formation of an alkyl complex [Cp*Fe(L^Me)CH₂CH₂tBu] ( 6 ), which was found to cleave the C? H bond of furan to produce 2 . On the basis of these results, a possible catalytic cycle is proposed.