RESUMEN
The zinc-catalyzed hydrosilylation and hydroboration of cyanamides have been described. Chemoselective reduction of cyanamides with Ph2SiH2 and partial or complete hydroboration of cyanamides with pinacolborane (HBpin) have been successfully carried out. The active catalyst/intermediate in the catalytic reactions, i.e., the bis-guanidinate zinc amidinate compound, has been isolated and structurally characterized.
RESUMEN
The first examples of tetrasubstituted conjugated bis-guanidinate (CBG) supported monomeric and thermally stable gallium dihalides [LGaX2], (X=Cl (Ga-Cl), I (Ga-I)) and dihydride (Ga-H) [LGaH2] (where L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr)}; Ar=2,6-Et2-C6H3) compounds are reported. The reaction of inâ situ generated LLi with 1.0â equiv. GaX3 (X=Cl, I) afforded compounds Ga-Cl and Ga-I. The reaction between Ga-Cl and Li[HBEt3] in benzene yielded the dihydride compound Ga-H. All reported compounds (Ga-Cl, Ga-I, and Ga-H) were characterized by NMR, HRMS, and single-crystal X-ray diffraction studies. Ga-H was probed for the hydroboration of carbodiimides (CDI), isocyanates, and isothiocyanates with HBpin. Compound Ga-H was also found effective for the catalytic hydroboration of imines, nitriles, alkynes, esters, and formates, affording the corresponding products in quantitative yields. Stoichiometric reactions with a CDI were performed to establish the catalytic cycle.
RESUMEN
The conjugated bis-guanidinate (CBG)-supported zinc hydride {LZnH}2; L = {(ArHN)(ArN)-CîN-Cî(NAr)(NHAr); Ar = 2,6-Et2-C6H3} (I) is utilized as a catalyst for the hydroboration of esters with pinacolborane (HBpin) under mild reaction conditions. Various aryl and alkyl substrates containing electron-donating, withdrawing, and cyclic groups of esters are effectively converted into alkoxy boronate esters as products upon hydroboration. Furthermore, stoichiometric experiments have been performed to understand the plausible reaction mechanism for the hydroboration of esters. Additionally, complex (I) was used for the hydroboration of carbonate, carboxylic acid, and anhydride substrates to showcase the broad substrate scope.
RESUMEN
The N,N'-chelated conjugated bis-guanidinate (CBG) supported zinc hydride (Zn-1) pre-catalyzed highly challenging chemoselective mono-hydrosilylation of a wide range of nitriles to exclusive N-silylimines and/or N,N'-silyldiimines is reported. Furthermore, the effectiveness of pre-catalyst Zn-1 is compared with another pre-catalyst analogue, i.e., DiethylNacNac zinc hydride (Zn-2), to know the ligand effect. We observed that pre-catalyst Zn-1 shows high efficiency and better selectivity than pre-catalyst Zn-2 for reducing nitriles to N-silylimines. Mechanistic studies indicate the insertion of the C≡N bond of nitrile into Zn-H to form the zinc vinylidenamido complexes (Zn-1' and Zn-2'). The active catalysts Zn-1' and Zn-2' are confirmed by NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. A most plausible catalytic cycle has been explored depending on stoichiometric experiments, active catalysts isolation, and in situ studies. Moreover, the synthetic utility of this protocol was demonstrated.
RESUMEN
The conjugated bis-guanidinate-stabilized zinc hydride complex (I)-precatalyzed chemoselective dehydroborylation of a wide array of terminal alkynes with excellent yields is reported. Further, precatalyst I is compared with a newly synthesized DiethylNacNac zinc hydride precatalyst (III) for selective dehydroborylation of terminal alkynes, and it is discovered that precatalyst I is more active than III. We have studied intra- and intermolecular chemoselective dehydroborylation of terminal alkynes over other reducible functionalities such as alkene, ester, isocyanide, nitro, and heterocycles. The highly efficient precatalyst I shows a turnover number of 48.5 and turnover frequency of up to 60.5 h-1 in the dehydroborylation of 1-ethynyl-4-fluorobenzene (1i). A plausible mechanism for selective dehydrogenative borylation of alkynes has been proposed based on active catalyst isolation and a series of stoichiometric reactions.
RESUMEN
In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO2 ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.
RESUMEN
The conjugated bis-guanidinate-supported zinc hydride [{LZnH}2; L = {(ArHN) (ArN)-CâN-Câ(NAr) (NHAr); Ar = 2,6-Et2-C6H3}] (I)-catalyzed highly demanding exclusive 1,2-regioselective hydroboration and hydrosilylation of N-heteroarenes is demonstrated with excellent yields. This protocol is compatible with many pyridines and N-heteroarene derivatives, including electron-donating and -withdrawing substituents. Catalytic intermediates, such as [(LZnH) (4-methylpyridine)] IIA, [(L'ZnH) (4-methylpyridine) IIA', where L' = CH{(CMe) (2,6-Et2C6H3N)}2)], LZn(1,2-DhiQ) (isoquinoline) III, [L'Zn(1,2-DhiQ) (isoquinoline)] III', and LZn(1,2-(3-MeDHQ)) (3-methylquinoline) V, were isolated and thoroughly characterized by NMR, HRMS, and IR analyses. Furthermore, X-ray single-crystal diffraction studies confirmed the molecular structures of compounds IIA', III, and III'. The NMR data proved that the intermediate III or III' reacted with HBpin and gave a selective 1,2-addition hydroborated product. Stoichiometric experiments suggest that V and III independently reacted with silane, yielding selective 1,2-addition of mono- and bis-hydrosilylated products, respectively. Based on the isolation of intermediates and a series of stoichiometric experiments, plausible catalytic cycles were established. Furthermore, the intermolecular chemoselective hydroboration reaction over other reducible functionalities was studied.
RESUMEN
A new example of a structurally characterized conjugated bis-guanidinate (CBG) supported zinc(I) dimer, i.e., LZnZnL (3) (L = {(ArNH)(ArN)-CîN-Cî(NAr)(NHAr)}; Ar = 2,6-Et2-C6H3) with a Zn-Zn bond is reported. Moreover, homoleptic (3) and heteroleptic (Cp*ZnZnL, 2, (Cp* = 1,2,3,4,5-pentamethyl cyclopentadienide)) zinc(I) dimers are used as precatalysts in the dehydroborylation of a wide array of terminal alkynes. Furthermore, the active catalyst, CBG zinc acetylide (LZn-CîC-Ph-4-Me)2, (5), is isolated which is confirmed by X-ray crystal structure analysis. A series of stoichiometric experiments have been performed to propose a plausible reaction mechanism.
RESUMEN
Herein, a remarkable conjugated bis-guanidinate (CBG) supported zinc hydride, [{LZnH}2 ; L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr); Ar=2,6-Et2 -C6 H3 }] (I) catalyzed partial reduction of heteroallenes via hydroboration is reported. A large number of aryl and alkyl isocyanates, including electron-donating and withdrawing groups, undergo reduction to obtain selectively N-boryl formamide, bis(boryl) hemiaminal and N-boryl methyl amine products. The compound I effectively catalyzes the chemoselective reduction of various isocyanates, in which the construction of the amide bond occurs. Isocyanates undergo a deoxygenation hydroboration reaction, in which the C=O bond cleaves, leading to N-boryl methyl amines. Several functionalities such as nitro, cyano, halide, and alkene groups are well-tolerated. Furthermore, a series of kinetic, control experiments and structurally characterized intermediates suggest that the zinc hydride species are responsible for all reduction steps and breaking the C=O bond.
RESUMEN
Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]2 (1), [LZnI]2 (2) and [LZnH]2 (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.
RESUMEN
Tetra-aryl-substituted symmetrical conjugated bis-guanidine (CBG) ligands such as L1-3 (3H) [L(3H) = {(ArHN)(ArHN)CâN-CâNAr(NHAr)}; Ar = 2,6-Me2-C6H3 (L1(3H)), 2,6-Et2-C6H3 (L2(3H)), and 2,6-iPr2-C6H3 (L3(3H))] have been employed to synthesize a series of four- and six-membered aluminum heterocycles (1-8) for the first time. Generally, aluminum complexes bearing N,N'- chelated guanidinate and ß-diketiminate/dipyrromethene ligand systems form four- and six-membered heterocycles, respectively. However, the conjugated bis-guanidine ligand has the capability of forming both four- and six-membered heterocycles possessing multimetal centers within the same molecule; this is due to the presence of three acidic protons, which can be easily deprotonated (at least two protons) upon treatment with metal reagents. Both mono- and dinuclear aluminum alkyls and mononuclear aluminum alkoxide, halide, and hydride complexes have been structurally characterized. Further, we have demonstrated the potential of mononuclear, six-membered CBG aluminum dialkyls in catalytic hydroboration of a broad range of aldehydes and ketones with pinacolborane (HBpin).
RESUMEN
The reaction of LH [L = {(ArNH)(ArN)-C=N-C=(NAr)(NHAr)}; Ar =2,6-Et2-C6H3] with a commercially available alane amine adduct (H3Al·NMe2Et) in toluene resulted in the formation of a conjugated bis-guanidinate (CBG)-supported aluminum dihydride complex, i.e., LAlH2 (1), in good yield. The new complex has been thoroughly characterized by multinuclear magnetic resonance, IR, mass, and elemental analyses, including single-crystal structural studies. Further, we have demonstrated the aluminum-catalyzed hydroboration of a variety of nitriles and alkynes. Moreover, aluminum-catalyzed hydroboration is expanded to more challenging substrates such as alkene, pyridine, imine, carbodiimide, and isocyanides. More importantly, we have shown that the aluminum dihydride catalyzed both intra- and intermolecular chemoselective hydroboration of nitriles and alkynes over other reducible functionalities for the first time.
RESUMEN
A series of structurally characterized magnesium and zinc complexes of the form L4-tBuPh-M{N(SiMe3)2}2 [M = Mg (1) and Zn (2); L4-tBuPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-tert-butylphenyl)amidinate}], L4-iPrPh-M{N(SiMe3)2}2 [M = Mg (3) and Zn (4); L4-iPrPh = 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(4-isopropylphenyl)amidinate}], and L4-iPrPh-ZnEt2 (5) bearing a zwitterionic-type neutral amidinate or N-heterocyclic carbene-carbodiimide ("NHC-CDI") adduct and monoanionic amido or alkyl ligands have been reported. The synthesis of compounds 1-5 was achieved by the direct addition of a "NHC-CDI" adduct to a corresponding metal bis(amide) or dialkyl reagent. All compounds 1-5 exist as monomers in the solid state. In all cases, the metal (magnesium or zinc) centers adopt a distorted four-coordinate tetrahedral geometry bonded to one N,N'-chelated neutral zwitterionic ligand and two monoanionic amido or alkyl moieties. In contrast, sterically bulky zwitterionic amidinate 1,3-diethyl-4,5-dimethylimidazolium-2-{N,N'-bis(2,6-diisopropylphenyl)amidinate} (LDipp) upon treatment with lithium bis[(trimethylsilyl)amide], Li{N(SiMe3)2}, affords the NHC-lithium complex MeIEt-[Li{N(SiMe3)2}]2 (6), in which one molecule of NHC (MeIEt = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene) coordinates to one of the two lithium centers. In a similar way, the reaction between LDipp and Mg{N(SiMe3)2}2 allowed the formation of a NHC adduct of metal bis(amide), MeIEt-Mg{N(SiMe3)2}2 (7), instead of a zwitterionic adduct of metal bis(amide). Alternatively, the synthesis of both compounds 6 and 7 was achieved by the direct addition of 1 equiv of NHC, i.e., MeIEt to Li{N(SiMe3)2} (2.0 equiv) and Mg{N(SiMe3)2}2 (1.0 equiv) in benzene-d6, respectively. All compounds (1-7) were characterized by multinuclear {1H, 13C, and 29Si (for 1-4, 6, and 7) and 7Li (for compound 6)} magnetic resonance spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray structural analysis. In addition, preliminary reactivity studies of zwitterion-supported metal complexes have been investigated. Furthermore, density functional theory calculations have been carried out to obtain the energetics of zwitterion-supported lithium and magnesium complexes.
RESUMEN
The synthesis of novel heteroleptic organomagnesium(ii) amide complexes [IMesMg(Ar){N(SiMe3)2}]; (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), Ar = 2,6-Me2C6H3 (Xyl) (1) and 2,4,6-Me3C6H2 (Mes) (2) is reported. Both compounds 1 and 2 were confirmed by multinuclear (1H, 13C and 29Si) magnetic resonance spectroscopy, elemental analysis and single crystal X-ray structural analysis. Furthermore, the organomagnesium amide pre-catalyzed cross-dehydrogenative coupling of organosilanes with amines has been investigated.
RESUMEN
Magnesium amide complexes such as Mg{N(SiMe3)2}2 (1) xylLMgN(SiMe3)2·THF (2) [xylL = ArNC(NiPr2)NAr; (Ar = 2,6-Me2-C6H3)] and dippLMgN(SiMe3)2·THF (3) [dippL = ArNC(NiPr2)NAr; (Ar = 2,6-iPr2-C6H3)] are reported as highly efficient pre-catalysts for the hydroboration of a wide range of esters using pinacolborane (HBpin) under mild reaction conditions. Moreover, we have shown compound 1 catalyzed chemoselective reduction of esters in the presence of other reducible functional groups such as alkene, alkyne and nitro.
RESUMEN
The well-defined aluminum monohydride compound [{(2,4,6-Me3-C6H2)NC(Me)}2(Me)(H)]AlH·(NMe2Et) (1) catalyzes hydroboration of a wide range of aldehydes and ketones under mild reaction conditions. Moreover, compound 1 displayed chemoselective hydroboration of aldehydes over ketones at rt.
RESUMEN
Victor Grignard's Nobel Prize-winning preparation of organomagnesium halides (Grignard reagents) marked the formal beginning of organometallic chemistry with alkaline earth metals. Further development of this invaluable synthetic route, RX+MgâRMgX, with the heavier alkaline earth metals (Ca and Sr) was hampered by limitations in synthetic methodologies. Moreover, the lack of suitable ligands for stabilizing the reactive target molecules, particularly with the more electropositive Ca and Sr, was another obstacle. The absence in the literature, until just recently, of fundamental alkaline earth metal complexes with M-H, M-F, and M-OH (where M is the Group 2 metal Mg, Ca, or Sr) bonds amenable for organometallic reactions is remarkable. The progress in isolating various unstable compounds of p-block elements with ß-diketiminate ligands was recently applied to Group 2 chemistry. The monoanionic ß-diketiminate ligands are versatile tools for addressing synthetic challenges, as amply demonstrated with alkaline earth complexes: the synthesis and structural characterization of soluble ß-diketiminatocalcium hydroxide, ß-diketiminatostrontium hydroxide, and ß-diketiminatocalcium fluoride are just a few examples of our contribution to this area of research. To advance the chemistry beyond synthesis, we have investigated the reactivity and potential for applications of these species, for example, through the demonstration of dip coating surfaces with CaCO(3) and CaF(2) with solutions of the calcium hydroxide and calcium fluoride complexes, respectively. In this Account, we summarize some recent developments in alkaline earth metal complex chemistry, particularly of Mg, Ca, and Sr, through the utilization of ß-diketiminate ligands. We focus on results generated in our laboratory but give due mention to work from other groups as well. We also highlight the closely related chemistry of the Group 12 element Zn, as well as the important chemistry developed by other groups using the complexes we have reported. Although Mg and Ca are more abundant in living organisms, no other metal has as many biological functions as Zn. Thus Zn, the nontoxic alternative to the heavier Group 12 elements Cd and Hg, occupies a unique position ripe for further exploration.
RESUMEN
The preparation of a series of amidinato and guanidinato zinc halide complexes incorporating ligands of varying steric bulk is described, and their thermal stabilities compared. Salt elimination reactions between [M(Giso)] (M = K or Li; Giso = [(ArN)(2)CNCy(2)](-), Ar = 2,6-diisopropylphenyl, Cy = cyclohexyl) and ZnX(2) (X = I or Br) have yielded the monomeric complexes [(Giso)ZnI] and [(Giso)Zn(mu-Br)(2)Li(OEt(2))(2)]. Both have been crystallographically characterised and the former shown to slowly decompose in solution at ambient temperature to give the carbodiimide, ArN[double bond, length as m-dash]C[double bond, length as m-dash]NAr. In contrast, reactions between alkali metal complexes of a less bulky guanidinate, [M(Priso)] (Priso = [(ArN)(2)CNPr(i)(2)](-)) and ZnX(2) have yielded [(IZn)(2)(mu-NPr(i)(2)){mu-N,N'-(NAr)(2)CH}] and [(Priso)Zn(mu-Br)(2)Li(OEt(2))(2)]. The latter decomposes in solution at ambient temperature, generating ArN[double bond, length as m-dash]C[double bond, length as m-dash]NAr, which was also produced in the preparation of the former. Analogies are drawn between the decomposition of [(Priso)Zn(mu-Br)(2)Li(OEt(2))(2)] and the carbonic anhydrase catalysed dehydration of bicarbonate. Two bulky amidinato zinc complexes, [{(Piso)Zn(mu-Br)}(2)] and [Zn(Piso)(2)] (Piso = [(ArN)(2)CBu(t)](-)) have been prepared, structurally characterised and shown to be markedly more thermally stable than the zinc guanidinate compounds. Attempts to reduce several of the zinc(ii) halide complexes to dimeric zinc(i) compounds were so far unsuccessful, in all cases leading to the deposition of zinc metal.
RESUMEN
The preparation and characterization of a series of magnesium(II) iodide complexes incorporating beta-diketiminate ligands of varying steric bulk and denticity, namely, [(ArNCMe)(2)CH](-) (Ar=phenyl, ((Ph)Nacnac), mesityl ((Mes)Nacnac), or 2,6-diisopropylphenyl (Dipp, (Dipp)Nacnac)), [(DippNCtBu)(2)CH](-) ((tBu)Nacnac), and [(DippNCMe)(Me(2)NCH(2)CH(2)NCMe)CH](-) ((Dmeda)Nacnac) are reported. The complexes [((Ph)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(OEt(2))], [((Dmeda)Nacnac)MgI(OEt(2))], [((Mes)Nacnac)MgI(thf)], [((Dipp)Nacnac)MgI(thf)], [((tBu)Nacnac)MgI], and [((tBu)Nacnac)MgI(DMAP)] (DMAP=4-dimethylaminopyridine) were shown to be monomeric by X-ray crystallography. In addition, the related beta-diketiminato beryllium and calcium iodide complexes, [((Mes)Nacnac)BeI] and [{((Dipp)Nacnac)CaI(OEt(2))}(2)] were prepared and crystallographically characterized. The reductions of all metal(II) iodide complexes by using various reagents were attempted. In two cases these reactions led to the magnesium(I) dimers, [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)]. The reduction of a 1:1 mixture of [((Dipp)Nacnac)MgI(OEt(2))] and [((Mes)Nacnac)MgI(OEt(2))] with potassium gave a low yield of the crystallographically characterized complex [((Dipp)Nacnac)Mg(mu-H)(mu-I)Mg((Mes)Nacnac)]. All attempts to form beryllium(I) or calcium(I) dimers by reductions of [((Mes)Nacnac)BeI], [{((Dipp)Nacnac)CaI(OEt(2))}(2)], or [{((tBu)Nacnac)CaI(thf)}(2)] have so far been unsuccessful. The further reactivity of the magnesium(I) complexes [((Mes)Nacnac)MgMg((Mes)Nacnac)] and [((tBu)Nacnac)MgMg((tBu)Nacnac)] towards a variety of Lewis bases and unsaturated organic substrates was explored. These studies led to the complexes [((Mes)Nacnac)Mg(L)Mg(L)((Mes)Nacnac)] (L=THF or DMAP), [((Mes)Nacnac)Mg(mu-AdN(6)Ad)Mg((Mes)Nacnac)] (Ad=1-adamantyl), [((tBu)Nacnac)Mg(mu-AdN(6)Ad)Mg((tBu)Nacnac)], and [((Mes)Nacnac)Mg(mu-tBu(2)N(2)C(2)O(2))Mg((Mes)Nacnac)] and revealed that, in general, the reactivity of the magnesium(I) dimers is inversely proportional to their steric bulk. The preparation and characterization of [((tBu)Nacnac)Mg(mu-H)(2)Mg((tBu)Nacnac)] has shown the compound to have different structural and physical properties to [((tBu)Nacnac)MgMg((tBu)Nacnac)]. Treatment of the former with DMAP has given [((tBu)Nacnac)Mg(H)(DMAP)], the X-ray crystal structure of which disclosed it to be the first structurally authenticated terminal magnesium hydride complex. Although attempts to prepare [((Mes)Nacnac)Mg(mu-H)(2)Mg((Mes)Nacnac)] were not successful, a neutron diffraction study of the corresponding magnesium(I) complex, [((Mes)Nacnac)MgMg((Mes)Nacnac)] confirmed that the compound is devoid of hydride ligands.