Search

The monoalkylation of N-methoxypyridinium salts with alkyl radicals generated from alkenes (via hydroboration with catecholborane), alkyl iodides (via iodine atom transfer) and xanthates is reported. The reaction proceeds under neutral conditions since no acid is needed to activate the heterocycle and no external oxidant is required. A rate constant for the addition of a primary radical to N-methoxylepidinium >107 M−1 s−1 was experimentally determined. This rate constant is more than one order of magnitude larger than the one measured for the addition of primary alkyl radicals to protonated lepidine demonstrating the remarkable reactivity of methoxypyridinium salts towards radicals. The reaction has been used for the preparation of unique pyridinylated terpenoids and was extended to a three-component carbopyridinylation of electron-rich alkenes including enol esters, enol ethers and enamides., N-Methoxypyridinium salts are exceptionally reactive radical traps that can be used in efficient radical chain reactions with organoboranes.

Summary The selective functionalization of methane under ambient conditions remains a formidable challenge for chemists. While most studies have focused on transition metal complexes, much less attention has been devoted to molecular complexes based on non-metal elements, despite them being more sustainable and less environmentally impactful. Here, we report that an N-heterocyclic carbene-stabilized borenium complex can activate methane under relatively mild conditions. The resulting methylborenium complex can readily transfer the methyl group to catecholborane (HBcat), which can lead to a synthetic cycle for the conversion of methane to MeBcat. Both experimental and theoretical mechanistic studies suggest that the C–H bonds of methane are activated via a σ-bond metathesis pathway. Such direct aliphatic C–H borylation can be extended to other alkanes, such as ethane and octane. The formed alkylborenium complexes can react with terminal alkynes via 1,2-alkylboration to install both the alkyl and boryl functionalities onto organic scaffolds.

The P-stereogenic phosphinamides are a structurally novel skeletal class which has not been investigated as chiral organocatalysts. However, chiral cyclic 3-hydroxy ketones are widely used as building blocks in the synthesis of natural products and bioactive compounds. However, general and practical methods for the synthesis of such chiral compounds remain underdeveloped. Herein, we demonstrate that the P-stereogenic phosphinamides are powerful organocatalysts for the desymmetric enantioselective reduction of cyclic 1,3-diketones, providing a useful method for the synthesis of chiral cyclic 3-hydroxy ketones. The protocol displays a broad substrate scope that is amenable to a series of cyclic 2,2-disubstituted five- and six-membered 1,3-diketones. The chiral cyclic 3-hydroxy ketone products bearing an all-carbon chiral quaternary center could be obtained with high enantioselectivities (up to 98% ee) and diastereoselectivities (up to 99:1 dr). Most importantly, the reactions could be practically performed on the gram scale and the catalysts could be reused without compromising the catalytic efficiency. Mechanistic studies revealed that an intermediate formed from P-stereogenic phosphinamide and catecholborane is the real catalytically active species. The results disclosed herein bode well for designing and developing other reactions using P-stereogenic phosphinamides as new organocatalysts.

Amido-zincates containing hydrides are underexplored yet potentially useful complexes. Attempts to access this type of zincate through combining amido-organo zincates and pinacolborane (HBPin) via Zn–C/H–BPin exchange led instead to preferential formation of amide–BPin and/or [amide–BPin(Y)]− (Y = Ph, amide, H), when the amide is hexamethyldisilazide or 2,2,6,6-tetramethylpiperidide and the hydrocarbyl group was phenyl or ethyl. In contrast, the use of a dipyridylamide (dpa) based arylzinc complex led to Zn–C/H–BPin metathesis being the major outcome. Independent synthesis and full characterisation of two LnLi[(dpa)ZnPh2] (L = THF, n = 3; L = PMDETA, n = 1) complexes, 1 and 3, respectively, enabled reactivity studies that demonstrated that these species display zincate type reactivity (by comparison to the lower reactivity of the neutral complex (Me-dpa)ZnPh2, 4, Me-dpa = 2,2′-dipyridyl-N-methylamine). This included 1 performing the rapid deprotonation of 4-ethynyltoluene and also phenyl transfer to α,α,α-trifluoroacetophenone in contrast to neutral complex 4. Complex 1 reacted with one equivalent of HBPin to give predominantly PhBPin (ca. 90%) and a lithium amidophenylzincate containing a hydride unit, complex 7-A, as the major zinc containing product. Complex 7-A transfers hydride to an electrophile preferentially over phenyl, indicating it reacts as a hydridozincate. Attempts to react 1 with >1 equivalent of HBPin or with catecholborane led to more complex outcomes, which included significant borane and dpaZn substituent scrambling, two examples of which were crystallographically characterised. While this work provides proof of principle for Zn–C/H–BPin exchange as a route to form an amido-zincate containing a hydride, amido-organozincates that undergo more selective Zn–C/H–BPin exchange still are required., Careful tuning of the nature of the amide ligand in amido-zincates allows for selective Zn–C over Zn–N exchange with HBPin affording a hydride containing amido-zincate. The mixed hydrido-phenyl zincate preferentially transfers hydride over phenyl.

The uncatalyzed addition of catecholborane to α-diimines has been examined. Reactions proceed smoothly at room temperature in the absence of a catalyst or additive, providing bis-hydroboration products with 3-coordinate boron centres or mono-hydroboration products with 4-coordinate boron centres, depending on the nature of the diimine reagent.

We describe the application of a mild, molecular-based, hydride metathesis protocol for the preparation of metastable germanium(II) dihydrides with compositions approaching [GeH2]n. The common starting material for this work [Ge(OtBu)2] was prepared in a high yield and shown to undergo OtBu/H exchange at Ge with the hydride sources pinacolborane (HBpin), catecholborane (HBcat), and diisobutylaluminum hydride (DIBAL-H) to give the [GeH2]n materials as yellow to orange solids. Heating one of these [GeH2]n materials to 200 °C affords a narrowing of the optical band gap (from 2.5 eV) and the generation of amorphous Ge. Reaction of [Ge(OtBu)2] with excess H3B·SMe2 in toluene at 70 °C provides a convenient route to thin films of amorphous Ge, including its deposition onto soft substrates, such as polyethyleneterephthalate (PET). Accompanying computations give insight into the energetics of OtBu/H exchange at Ge, and reveal a general thermodynamic preference for branched structures of [GeH2]n oligomers over linear forms as the Ge chain becomes longer. We also show that [Ge(OtBu)2] is a suitable pre-catalyst for the borylation of aldehydes.

It has been reported that either the boryl formate (HCOOBR2), bis(boryl)acetal (R2BOCH2OBR2), or methoxy borane (R2BOCH3) product of CO2 reduction is selectively afforded adopting the nickel (Ni) pincer catalyst by changing the properties of borane reductants. To obtain an insight into the influence of borane reductants on the selectivity of hydroboration of CO2 catalyzed by (tBuPCP)NiH, a density functional theory (DFT) computational study is performed with three different boranes (HBcat (catecholborane), 9-BBN (9-borabicyclo [3.3.1] nonane), and HBpin (pinacolborane)). The detailed formation and conversion of four-electron product R2BOCH2OBR2 were considered for the first time, combining with the three sequential catalytic cycle mechanism, and the calculated results demonstrate that the distinctions of the electronic process in some key intermediates and transition states lead to the selectivity on products for different borane reductants.

International audience; The pentamethylcyclopentadienyl N-heterocyclic carbene nickel complex [Ni(η5-C5Me5)Cl(IMes)] (IMes = 1,3-dimesitylimidazol-2-ylidene) efficiently catalyses the anti-Markovnikov hydroboration of alkenes with catecholborane in the presence of a catalytic amount of potassium tert-butoxide, and joins the very exclusive club of nickel catalysts for this important transformation. Interestingly, the regioselectivity can be reversed in some cases by using pinacolborane instead of catecholborane. Mechanistic investigations involving control experiments, 1H and 11B NMR spectroscopy, cyclic voltammetry, piezometric measurements and DFT calculations suggest an initial reduction of the Ni(II) precursor to a Ni(I) active species with the concomitant release of H2. The crucial role of the alkoxo-catecholato-borohydride species resulting from the reaction of potassium tert-butoxide with catecholborane in the formation of an intermediate nickel-hydride species that would then be reduced a Ni(I) active species, is highlighted.

We study two metal-free catalysts for the reduction of CO2 with four different hydroboranes and try to identify mechanistically relevant intermediate species. The catalysts are the phosphinoborane Ph2P(CH2)2BBN (1), easily accessible in a one-step synthesis from diphenyl(vinyl)phosphine and 9-borabicyclo[3.3.1]nonane (H-BBN), and its formaldehyde adduct Ph2P(CH2)2BBN(CH2O) (2), detected in the catalytic reduction of CO2 with 1 as the catalyst but properly prepared from compound 1 and p-formaldehyde. Reduction of CO2 with H-BBN gave mixtures of CH2(OBBN)2 (A) and CH3OBBN (B) using both catalysts. Stoichiometric and kinetic studies allowed us to unveil the key role played in this reaction by the formaldehyde adduct 2 and other formaldehyde-formate species, such as the polymeric BBN(CH2)2(Ph2P)(CH2O)BBN(HCO2) (3) and the bisformate macrocycle BBN(CH2)2(Ph2P)(CH2O)BBN(HCO2)BBN(HCO2) (4), whose structures were confirmed by diffractometric analysis. Reduction of CO2 with catecholborane (HBcat) led to MeOBcat (C) exclusively. Another key intermediate was identified in the reaction of 2 with the borane and CO2, this being the bisformaldehyde-formate macrocycle (HCO2){BBN(CH2)2(Ph2P)(CH2O)}2Bcat (5), which was also structurally characterized by X-ray analysis. In contrast, using pinacolborane (HBpin) as the reductant with catalysts 1 and 2 usually led to mixtures of mono-, di-, and trihydroboration products HCO2Bpin (D), CH2(OBpin)2 (E), and CH3OBpin (F). Stoichiometric studies allowed us to detect another formaldehyde-formate species, (HCO2)BBN(CH2)2(Ph2P)(CH2O)Bpin (6), which may play an important role in the catalytic reaction. Finally, only the formaldehyde adduct 2 turned out to be active in the catalytic hydroboration of CO2 using BH3·SMe2 as the reductant, yielding a mixture of two methanol-level products, [(OMe)BO]3 (G, major product) and B(OMe)3 (H, minor product). In this transformation, the Lewis adduct (BH3)Ph2P(CH2)2BBN was identified as the resting state of the catalyst, whereas an intermediate tentatively formulated as the Lewis adduct of compound 2 and BH3 was detected in solution in a stoichiometric experiment and is likely to be mechanistically relevant for the catalytic reaction.

Chemoselective enantiodivergent reduction of N-benzyl-α-keto amides catalyzed by chiral oxido-vanadium(V) complexes derived from a reduced form of N-salicylidene L-tert leucinate can be achieved with good yields and good to high enantioselectivities. These chiral complexes were utilized in catalyzing the enantioselective synthesis of (R)-N-benzyl-mandelamide with pinacolborane and (S)-N-benzyl-mandelamide with catecholborane from the corresponding ketone precursors. A series of 12 different solvents and 8 different reducing agents were examined. By the use of pinacolborane (R isomer, 74–99% ee) or catecholborane (S isomer, 60–90% ee), a handy approach for complementary asymmetric reduction of α-keto amides can be attained.

The previously reported dimeric NHI aluminum dihydrides 1 a,b, as well as the bis(NHI) aluminum dihydride salt 9 +[OTs]-, the bis(NHI) boron dihydride salt 10 +[OTs]-, and the "free" bis(NHI) ligand 12 were investigated with regard to their activity as a homogenous (pre)catalyst in the hydroboration (i. e. catalytic reduction) of carbon dioxide (CO2) in chloroform under mild conditions (i. e. room temperature, 1 atm; NHI=N-heterocyclic imine, Ts=tosyl). Borane dimethylsulfide complex and catecholborane were used as a hydride source. Surprisingly, the less sterically hindered 1 a exhibited lower catalytic activity than the bulkier 1 b. A similarly unexpected discrepancy was found with the lower catalytic activity of 10 + in comparison to the one of the bis(NHI) 12. The latter is incorporated as the ligand to the boron center in 10 +. To elucidate possible mechanisms for CO2 reduction the compounds were subjected to stoichiometric reactivity studies with the borane or CO2. Aluminum carboxylates 4, 6, and 7 + with two, four, and one formate group per two aluminum centers were isolated. Also, the boron formate salt 11 +[OTs]- was characterized. Selected metal formates were subjected to stoichiometric reactions with boranes and/or tested as a catalyst. We conclude that each type of catalyst (1 a,b, 9 +, 10 +, 12) follows an individual mechanistic pathway for CO2 reduction.

The synthesis of two bifunctional compounds bearing a terminal hydroxy-bisphosphonic function (HBP) was achieved following a linear and a convergent strategy. In the linear approach, the free hydroxy-bisphosphonic function was introduced in the last step of the synthesis, under neutral conditions using an Arbuzov reaction with tris(trimethylsilyl) phosphite and a carboxylic acid precursor activated in situ with catecholborane. In the convergent approach, Huisgen type cycloaddition was studied starting from an HBP-functionalized alkyne partner to obtain the targeted bifunctional molecule. These complementary approaches allow for the preparation of complex bone-targeting molecules as potential prodrug candidates.

We demonstrate an efficient protocol for the chemoselective hydroboration of organic nitriles with pinacolborane (HBpin) and catecholborane (HBcat) using aluminum alkyl complex [κ2‐{2‐F‐C6H4NP(Se)Ph2}2Al‐(Me)] as a pre‐catalyst to afford diboryl amines under solvent‐free and mild conditions (60°C) in high yield. The aluminum complex was prepared by the reaction of [2‐F‐C6H4NHP(Se)Ph2] and trimethylaluminum in toluene. The solid‐state structure of Al complex is established. Nitriles with a wide array of electron‐withdrawing and electron‐donating functional groups were easily converted to the desired products through the formation of aluminum hydride as an active species. A kinetic study of the catalytic reaction is also reported.

A catalytic enantioselective reduction of α-trifluoromethylated imines by a BINOL-derived boro-phosphate employing catecholborane as hydride source has been developed. This method provides an efficient route to prepare synthetically useful chiral α-trifluoromethylated amines in high yields and with excellent enantioselectivities (up to 98% yield and 96% ee) under mild conditions.

The borylation of ligated dinitrogen by 1,3-B-H bond addition over a W-N[triple bond, length as m-dash]N unit using various hydroboranes has been examined. In a previous study, we have shown that Piers' borane (1) reacted with the tungsten dinitrogen complex 2 to afford a boryldiazenido-hydrido-tungsten species. The ease and mildness of this reaction have encouraged us to extend its scope, with the working hypothesis that 1 could potentially catalyse the 1,3-B-H bond addition of other hydroboranes. Under productive reaction conditions, dicyclohexylborane (HBCy2) and diisopinocampheylborane (HBIpc2) underwent retro-hydroboration to give cyclohexylborane (H2BCy) or isopinocampheylborane (H2BIpc), respectively; these monoalkylboranes act as N2-borylating agents in the presence of a catalytic amount of 1. Under similar conditions, 9-borabicyclononane (9-BBN) slowly adds over the W-N[triple bond, length as m-dash]N unit without rearrangement to a monoalkylborane. Catecholborane (HBcat) undergoes the 1,3-B-H bond addition without the need for a catalyst. We were not able to build more than one covalent B-N bond between the terminal N of the N2 ligand and the boron reagent with this methodology.

Pinacolborane, catecholborane, triethylsilane, triphenylsilane, dimethylphenylsilane, 1,1,1,3,5,5,5-heptamethyltrisiloxane, triethylgermane, triphenylgermane, and triphenylstannane deuterated at the heteroatom position have been catalytically prepared in 50–70% isolated yield, through H/D exchange between the D2 molecule and the respective boranes and hydrides of the group 14 elements, in the presence of the rhodium(I)-monohydride catalyst precursor RhH{κ3-P,O,P-[xant(PiPr2)2]} (xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene)., Financial support from the MINECO of Spain (Projects CTQ2017-82935-P and RED2018-102387-T (AEI/FEDER, UE)), Gobierno de Aragón (Group E06_20R, project LMP148_18, and predoctoral contract to A.M.), FEDER, and the European Social Fund is acknowledged.

Complex OsH6(PiPr3)2 is an efficient catalyst precursor for the addition of pinacolborane and catecholborane to the C–N triple bond of alkyl nitriles. In this way, a variety of N,N-diborylamines have been isolated and fully characterized, including 13 derivatives not described so far. The range of nitriles used is wide and comprises substrates having unfunctionalized linear and branched chains, and functionalized chains with methoxide, trifluoromethyl, aryl, pyridyl, benzoyl, or cyanide groups. Kinetic studies demonstrate that the overall process consists of two consecutive irreversible reactions: the catalytic metal-promoted monohydroboration of the nitrile to afford the borylimine and the metal-free stoichiometric hydroboration of the latter to give the diborylamine. The mechanism of the hydroboration has been established by combining the kinetic analysis of the catalysis, stoichiometric reactions, and DFT calculations. The rate-determining step of the catalysis is the insertion of the C–N triple bond of the nitrile into the Os–B bond of an osmium-σ-borane intermediate and is regiodirected by the nucleophilicity of the nitrogen atom and the electrophilicity of the carbon atom of the nitrile., Financial support from the MINECO of Spain (Projects CTQ2017-82935-P (AEI/FEDER, UE), CTQ2016-78205-P, PID2019-106184GB-I00, and RED2018-102387-T), Gobierno de Aragón (Group E06_20R and project LMP148_18), FEDER, and the European Social Fund is acknowledged.

The title compound is a demonstration of hydro­boration of ynamines with borandiol ester. The bond lengths of the resulting push–pull olefin are discussed., N-[(Z)-2-(2H-1,3,2-Benzodioxaborol-2-yl)-2-phenyl­ethen­yl]-N-(propan-2-yl)aniline, C23H22BNO2, contains a C=C bond that is conjugated with a donor and an acceptor group. An analysis that included similar push–pull olefins revealed that bond lengths in their B—C=C—N core units correlate with the perceived acceptor and donor strength of the groups. The two phenyl groups in the mol­ecule are rotated with respect to the plane that contains the BCCN atoms, and are close enough for significant π-stacking. Definite characterization of the title compound demonstrates, for the first time in a reliable way, that hydro­boration of ynamines with borandiol esters is feasible. Compared to olefin hydro­boration with borane, the ynamine substrate is activated enough to undergo reaction with the less active hydro­boration reagent catecholborane.

Compounds containing trivalent boron (TB) as the electron-deficient site(s) find numerous practical uses ranging from Lewis bases in organic synthesis to high-tech industry, with a number of novel applications anticipated. We present an experimental and theoretical study of the gas-phase valence photoionization (VUV-PES), core photoionization (XPS) and photoexcitation (NEXAFS) spectra of a representative TB compound catecholborane (CB). For modelling and assigning the spectra we used the ΔDFT and restricted single excitation space TD-DFT methods for the XPS and NEXAFS, and OVGF and EOM-CCSD for the VUV-PES. The vibrationally resolved structure was computed in the Franck–Condon (FC) and Herzberg– Teller (FCHT) approximations generally resulting in a good agreement with the observed spectral features. For the prediction of core-electron binding energies (CEBEs) several density functionals were tested. The best performance overall was furnished by ωB97X-D suggesting that including the dispersion correction is beneficial. The FCHT vibronic intensities are in clear discrepancy with the B 1s NEXAFS spectrum if the harmonic approximation is used for the B–H wagging mode both in the ground and in the first core-excited state. Instead, a much better agreement is obtained if the excited state potential is approximated to a symmetric double- well. The observed vibronic pattern could be a general fingerprint of the presence of TB centre(s), specifically, the transfer of the (core) density to the vacant boron p-orbital in the excited state.

A phosphinite-based dinickel pincer complex, {2,3,5,6-(iPr2PO)4C6}Ni2Cl2, is synthesized by heating a mixture of 1,2,4,5-tetrahydroxybenzene, iPr2PCl, NiCl2, and Et3N at 150 °C in a microwave reactor or refluxing 1,2,4,5-(iPr2PO)4C6H2 with NiCl2 and Et3N at 175–250 °C in a flask. Substitution of chlorides by hydrides (from LiAlH4 or LiEt3BH) and methyl groups (from MeLi) generates {2,3,5,6-(iPr2PO)4C6}Ni2H2 and {2,3,5,6-(iPr2PO)4C6}Ni2Me2, respectively. Protonation of the methyl complex with HCO2H gives {2,3,5,6-(iPr2PO)4C6}Ni2(OCHO)2. Spectroscopic and crystallographic data of these Janus POCOP pincer complexes are compared to those of the analogous mononuclear complexes {2,6-(iPr2PO)2C6H3}NiX (X = Cl, H, Me, and OCHO). The hydride complex {2,3,5,6-(iPr2PO)4C6}Ni2H2 reacts with phenylacetylene and CO2 to give insertion products. It also catalyzes the reduction of CO2 with catecholborane (HBcat) to yield CH3OBcat and catBOBcat with a turnover frequency of 7.5 min–1. The stoichiometric and catalytic activi...

The stoichiometric reactions of the alkylfluorides 1-fluoroadamantane (Ad-F), fluorocyclohexane (Cy-F), 1-fluoropentane (Pent-F) and benzyl fluorides with secondary boranes pinacolborane (HBpin), catecholborane (HBcat), 9-borabicyclo(3.3.1)nonane (9-BBN) and Piers' borane (HB(C6 F5 )2 ) are described. While HBcat, 9-BBN and HB(C6 F5 )2 reduce Ad-F to Ad-H, the latter borane was shown to react with secondary and primary fluoroalkanes, affording C-F borylation, while benzyl fluorides undergo Friedel-Crafts chemistry.

Several pincer ligated nickel mercapto complexes, [2,6-(R2 PCH2 )2 C6 H3 ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R2 PO)2 C6 H3 ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu2 PO)2 C6 H2 ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH3 ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH2 -μ2 -S(B2 H5 ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

The reactivity of a bicyclic (alkyl)(amino)carbene (BICAAC) towards different boranes has been examined in the present work. The reactions with boranes BX3·SMe2 (X = H, Cl, Br), BF3·OEt2 and BCl3 yield Lewis adducts [BICAAC·BH3] (1), [BICAAC·BHCl2] (2), [BICAAC·BH2Cl] (3), [BICAAC·BF3] (4), [BICAAC·BCl3] (5) and [BICAAC·BBr3] (6) respectively, whereas more hydridic boranes, 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (HBcat), enable the insertion of the carbene carbon into the B–H bond to form [BICAAC(H)-(9-BBN)] (7) and [BICAAC(H)-Bcat] (8). These complexes are the first examples of BICAAC–boron compounds and have been characterized using IR, multinuclear NMR spectroscopy, HRMS spectrometry and single crystal X-ray diffraction. Computational analyses were also performed to gain insight into the mechanism of B–H bond activation and adduct formation. Furthermore, the reactions of the BICAAC with boranes have been compared with the known reactions of CAACs and NHCs.

Catalytic sp3 C–O bond cleavage using B(C6F5)3/HBcat is reported. This method is first demonstrated on simple ethers and silyl protected alcohols, which exhibit reactivity parallel to the known B(C6F5)3/HSiR3 system. In more complex carbohydrate derivatives, however, unique selectivities that are not possible with hydrosilane reductants have been achieved, including regioselective cyclizations. Preliminary computational studies suggest that diboryl oxonium ions are disfavored and that four-coordinate boronium ions may contribute to selectivity.

Without the addition of any additives and production of any small molecules, C3-borylated indoles and transfer hydrogenated indolines have been simultaneously achieved by a B(C6F5)3-catalyzed disproportionation reaction of a broad range of indoles with catecholborane. This catalyst system exhibits excellent catalytic performance for practical applications, such as easy scale-up under solvent-free conditions and long catalytic lifetime over ten sequential additions of starting materials. A combined mechanistic study, including isolation and characterization of key reaction intermediates, analysis of the disproportionation nature of the reaction, in situ NMR of the reaction, and analysis of detailed experimental data, has led to a possible reaction mechanism which illustrates pathways for the formation of both major products and byproducts. Understanding the reaction mechanism enables us to successfully suppress side reactions by choosing appropriate substrates and adjusting the amount of catecholborane nee...

The reaction of [RhCl(CS)(PPh3)2] with excess catecholborane affords the cumulenic carbido complex [Rh2(μ-C)Cl2(PPh3)4] which undergoes phosphine and halide substitution to afford a range of complexes in which the Rh[double bond, length as m-dash]C[double bond, length as m-dash]Rh spine remains intact. Amongst these, the reactions with K[L] (L = H2B(pz)2, H2B(pzMe2)2, HB(pz)3; pz = pyrazol-1-yl) afford [Rh2(μ-C)(PPh3)2(L)2] whilst with K[HB(pzMe2)3] the unsymmetrical complex [Rh2H(μ-C)(μ-C6H4PPh2-2){HB(pzMe2)3}2] is obtained in which the carbido ligand spans d6-Rh(iii) and d8-Rh(i) centres.

Scandium terminal imido complex [(NNNN)Sc═NDIPP] (2; NNNN = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2NMe2)]−, DIPP = 2,6-iPr2C6H3) reacts with 9-borabicyclononane (9-BBN) to give scandium borohydride [(NNNN(B)H)Sc(N(H)DIPP)] (3; NNNN(B)H = [MeC(N(DIPP))CHC(Me)(NCH2CH2NMeCH2CH2N(Me)CH2(BBN)]2–), and C(sp3)–H bond borylation of the NNNN ligand occurs during this reaction. In contrast, the reaction between complex 2 and catecholborane (CatBH) gives scandium catecholate [(NNNN)Sc(Cat)] (4), and B–O bond cleavage happens during this reaction. Both 3 and 4 have been well-characterized including the single-crystal X-ray diffraction analysis. Reaction of 2 with bis(catecholato)diboron (CatB–BCat) also gives a B–O bond cleavage product.

The first examples of stoichiometric dehydrogenative B-H/C(sp3 )-H benzylic borylation reactions, which are of relevance to catalytic methylarene (di)borylation, are reported. These unusual transformations involving a (κ2 -P,N)Pt(η3 -benzyl) complex, and either pinacolborane or catecholborane, proceed cleanly at room temperature. Density functional calculations suggest that borylation occurs via successive σ-bond metathesis steps, whereby a PtII -H intermediate engages in C(sp3 )-H bond activation-induced dehydrogenation.

We report the hydroboration of CO2 with catecholborane catalyzed by a series of bis(phosphinite) pincer ligated nickel thiolate complexes. Turnover frequencies (TOFs) up to 2400 h-1 were achieved at room temperature under an atmospheric pressure of CO2. This represents the highest TOF value known to date for the reduction of CO2 to the methoxide level under mild conditions.

New well-defined amphiphilic polymethylene-b-poly(vinyl acetate) (PM-b-PVAc) diblock copolymers were synthesized via a tandem strategy combining polyhomologation of ylides, chain-end functionalization and a visible light induced degenerative iodine transfer polymerization using decacarbonyldimanganese (Mn2(CO)10). Firstly, an iodo terminated polymethylene (PM-I) was prepared by polyhomologation of ylides initiated by a new organic borane based on catecholborane followed by functional group transformation using iodine as the end-capping reagent. Then, a series of amphiphilic PM-b-PVAc diblock copolymers with controlled molecular weight (Mn = 5530–17 020 g mol−1) and relatively narrow molecular weight distribution (Đ = 1.31–1.45) were obtained efficiently through a visible light induced degenerative iodine transfer polymerization of vinyl acetate (VAc) using PM-I as macroinitiator in the presence of Mn2(CO)10 under weak visible light irradiation at 40 °C. The micelles of such an amphiphilic diblock copolymer formed in tetrahydrofuran were observed by transmission electron microscopy (TEM). Moreover, the fabrication of porous films using PM-b-PVAc diblock copolymers via a static breath-figure process was investigated.

The preparation of a new series of dihydridoboranes supported by N,N-chelating ligands, [R2NCH2CH2NAr]- (R = alkyl, Ar = aryl), is reported. These new boranes react selectively with carbonyls, imines, and a series of electron-deficient fluoroarenes. The reactivity is complementary to recognized reagents such as pinacolborane, catecholborane, NHC-BH3, and borane (BH3) itself. Selectivities are rationalized by invoking both open- and closed-chain forms of the reagents as part of equilibrium mixtures.

A compound with a boron-boron triple bond is shown to undergo stepwise hydroboration reactions with catecholborane to yield an unsymmetrical hydro(boryl)diborene and a 2,3-dihydrotetraborane. Abstraction of H- from the latter compound produces an unusual cationic, planar tetraborane with a hydrogen atom bridging the central B2 moiety. Spectroscopic and crystallographic data and DFT calculations support a "protonated diborene" structure for this compound, which can also be accessed via direct protonation of the corresponding diborene.

The reduction of CO2 is of great importance. In this paper, different types of bis(phosphinite) (POCOP) pincer nickel complexes, [2,6-(R2PO)2C6H3]NiX (R = tBu, iPr, Ph, X = SH, N3, NCS), were applied to the catalytic hydroboration of CO2 with catecholborane (HBcat). It was found that pincer complexes with tBu2P or iPr2P phosphine arms are active catalysts for this reaction in which CO2 was successfully reduced to a methanol derivative (CH3OBcat) with a maximum turnover frequency of 1908 h&minus, 1 at room temperature under an atmospheric pressure of CO2. However, complexes with phenyl-substituted phosphine arms failed to catalyze this reaction&mdash, the catalysts decomposed under the catalytic conditions. Complexes with iPr2P phosphine arms are more active catalysts compared with the corresponding complexes with tBu2P phosphine arms. For complexes with the same phosphine arms, the catalytic activity follows the series of mercapto complex (X = SH) &asymp, azido complex (X = N3) &gt, &gt, isothiocyanato complex (X = NCS). It is believed that all of these catalytic active complexes are catalyst precursors which generate the nickel hydride complex [2,6-(R2PO)2C6H3]NiH in situ, and the nickel hydride complex is the active species to catalyze this reaction.

An intramolecular cyclopropanation reaction involving B-(1-chloroalkyl)catecholborane intermediates generated from 1,4-dienes through hydroboration with catecholborane and Matteson homologation was developed. This sequential procedure leading to bicyclo[3.1.0]hexanes involves the formation of three new sigma C-C bonds at the same carbon atom. A mechanistic study supports the involvement of carbocationic intermediates.

The generation of 1-bromo and 1-chloroalkyl radicals from organoboranes has been investigated. The direct approach involving the hydroboration of halogenated alkenes is impeded by partial dehalogenation taking place during the hydroboration process. An indirect method involving the generation of B-(1-chloroalkyl)catecholborane by homologation of B-alkylcatecholborane with dichloromethyllithium was developed. A reaction sequence involving a hydroboration reaction, a Matteson homologation, and a radical allylation process has been performed as a one-pot process that takes advantage of three different reactivities of organoboron species. Starting from styrene derivatives, it was possible to prepare B-(1-chloro-2-arylpropyl)catecholboranes that are excellent precursors to 1-chloro-2-arylpropyl radicals. A concise approach for the synthesis of an optically active α-methylene-γ-lactone from p-chlorostyrene has been developed on the basis of a two-step sequence involving an enantioselective hydroboration-homologation-cyclization reaction followed by a hydrolysis-lactonization process.