Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity.

Two series of zinc salts, [EtZn][A] and Zn[A]2, with weakly coordinating anions [A]- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me3NB12Cl11]- [1]-, [Pr3NB12H5Cl6]- [2]-, [Bu3NB12H4Cl7]- [3]-, and [Hex3NB12H5Cl6]- [4]- were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11]- and [HexCB11Cl11]-. The basicity of anion [4]- was determined as being close to that of the triflimide anion [N(SO2CF3)2]-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]2 are lower than those of the Lewis acids B(C6F5)3 and Zn[HexCB11Cl11]2. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

Towards Naked Zinc(II) in the Condensed Phase: A Highly Lewis Acidic ZnII Dication Stabilized by Weakly Coordinating Carborate Anions.

The employment of the hexyl-substituted anion [HexCB11 Cl11 ]- allowed the synthesis of a ZnII species, Zn[HexCB11 Cl11 ]2 , 3, in which the Zn2+ cation is only weakly coordinated to two carborate counterions and that is soluble in low polarity organic solvents such as bromobenzene. DOSY NMR studies show the facile displacement of at least one of the counterions, and this near nakedness of the cation results in high catalytic activity in the hydrosilylation of 1-hexene and 1-methyl-1cyclohexene. Fluoride ion affinity (FIA) calculations reveal a solution Lewis acidity of 3 (FIA=262.1 kJ mol-1 ) that is higher than that of the landmark Lewis acid B(C6 F5 )3 (FIA=220.5 kJ mol-1 ). This high Lewis acidity leads to a high activity in catalytic CO2 and Ph2 CO reduction by Et3 SiH and hydrogenation of 1,1-diphenylethylene using 1,4-cyclohexadiene as the hydrogen source. Compound 3 was characterized by multinuclear NMR spectroscopy, mass spectrometry, single crystal X-ray diffraction, and DFT studies.

Convenient Access to Gallium(I) Cations through Hydrogen Elimination from Cationic Gallium(III) Hydrides.

Although gallium hydrides XnGaH3-n (X = monoanionic substituent) are usually stable compounds, cationic arene-solvated species [H2Ga(arene)2]+ spontaneously eliminate dihydrogen at room temperature to afford the arene-solvated gallium(I) compounds [Ga(PhF)2][CHB11Cl11] (1) and [Ga(Ph3CH)][B(C6F5)4] (3). A key requirement appears to be the presence of a weakly coordinating anion. Use of the more basic triflimide anion, [NTf2]-, reverses the stability, i.e., the gallium(III) hydride H2GaNTf2 (4) is more stable than the gallium(I) compound GaNTf2 (5). The experimental results are supported by DFT calculations. Compounds 1 and 3 can be used as catalysts for the oligomerization of 2,4,4-trimethyl-1-pentene and the hydrosilylation of benzophenone and 1-hexene.

Chlorination of 1-Carba-closo-dodecaborate and 1-Ammonio-closo-dodecaborate Anions.

Fully chlorinated carborate and dodecaborate cages such as [CHB11Cl11]- and [Me3NB12Cl11]- are prominent examples of valuable and chemically rather inert weakly coordinating anions. While both anions can be obtained by chlorination of the precursors [CH12B11]- and [H3NB12H11]- with SO2Cl2 followed by methylation for the synthesis of [Me3NB12Cl11]-, best results were found using photochemical chlorination with SO2Cl2 for [CH12B11]- and thermal chlorination with SO2Cl2 for [H3NB12H11]-. The hexachlorinated anion [n-Pr3NB12H5Cl6]- was formed readily within 30 min by chlorination of [n-Pr3NB12H11]-, but attempts to synthesize isopropyl-substituted ammonio-dodecaborates with a higher chlorination number resulted in the formation of mixtures and partial decomposition. The silver and trityl salts of the anions [CHB11Cl11]-, [Me3NB12Cl11]-, and [n-Pr3NB12H5Cl6]- as well as the contact ion-pair [Et2Al][Me3NB12Cl11] were also prepared, and the compounds [Ag(NCMe)][Me3NB12Cl11], [Et2Al][Me3NB12Cl11], and [Et4N][i-Pr3NB12H5Cl6] were also characterized by X-ray crystallography.

Synthesis and reactivity of indium(I) 1-carba-closo-undecachlorododecaborate.

The arene-solvated indium(I) species [In(C7H8)3][CHB11Cl11] (1) and [In(C6H5Br)1.5][CHB11Cl11] (2) were obtained by a redox reaction involving the silver salt Ag[CHB11Cl11] and indium powder at 80 °C in a toluene or bromobenzene solution. These thermally stable compounds react with triphenylphosphine and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene under reduction of indium(I) to indium metal and oxidation of the ligands to phosphonium and imidazolium cations contrary to the more commonly observed disproportionation reactions. The presence of 2 equiv of carbene led to deprotonation of the anion to give the dianion [CB11Cl11](2-). Interactions of In(+) with soft donor ligands such as phosphines, olefins, alkynes, and aromatics are weak, and a crystalline solid was only obtained with the nonvolatile phosphinoacetylene Mes2PC≡CPh (Mes = 2,4,6-Me3C6H2). The structure of this compound displays In···C interactions involving the triple bond and the π system of one mesityl group but no In···P contact. Solutions of 2 in fluorobenzene also showed moderate activity as the catalyst for intramolecular hydroamination of primary and secondary aminopentenes. The new compounds were characterized by multinuclear NMR spectroscopy and X-ray diffraction for compounds 1, 2, and 4-6.

A direct stereoselective preparation of a fish pheromone and application of the zinc porphyrin tweezer chiroptical protocol in its stereochemical assignment.

A two-step stereoselective preparation of a goldfish pheromone, 17α,20ß-dihydroxy-4-pregnen-3-one, is reported from the readily available cortexolone in 64% overall yield. The (20S)-epimer was also synthesized in three steps from cortexolone with an overall yield of 47%. A microscale chiroptical technique based on a host/guest complexation mechanism between the substrate and a dimeric metalloporphyrin host (tweezer) was used to confirm the stereochemical assignment, while Density Functional Theory (DFT) calculations were employed to explain the high stereoselectivity induced by the 17α-hydroxyl and C18-methyl groups.

Deoxygenative reduction of carbon dioxide to methane, toluene, and diphenylmethane with [Et2Al]+ as catalyst.

The strong Lewis acid [Et(2)Al](+) catalyzes the reduction of carbon dioxide with hydrosilanes under mild conditions to methane. In benzene solution, the side products toluene and diphenylmethane are also obtained through Lewis acid catalyzed benzene alkylation by reaction intermediates.

Cationic ethylzinc compound: a benzene complex with catalytic activity in hydroamination and hydrosilylation reactions.

The tight ion pair [EtZn(η(3)-C(6)H(6))][CHB(11)Cl(11)]·C(6)H(6) (1·C(6)H(6)) was obtained through ß-hydrogen abstraction and concomitant ethene elimination from Et(2)Zn with the trityl salt [Ph(3)C][CHB(11)Cl(11)]. This ionlike species shows catalytic activity in hydrosilylation and intramolecular hydroamination reactions. The amine adduct {CH(2)CHCH(2)C(Ph(2))CH(2)NH(2)}(3)ZnCB(11)Cl(11) (3), which features a rare transition metal-carborane σ bond, was isolated from a hydroamination experiment.

Synthesis of aryloxyaluminium hydrides and their conversion into aryloxyalumoxanes (ArOAlO)n.

The reaction of bulky phenols with H(3)Al x NMe(3) afforded the new primary phenoxyalanes Mes*OAlH(2) x NMe(3), 2 (Mes* = 2,4,6-(t)Bu(3)-C(6)H(2)-), 2,6-Dipp(2)C(6)H(3)OAlH(2) x NMe(3), 3 (Dipp = 2,6-(i)Pr(2)C(6)H(3)-), and 2,6-Trip(2)C(6)H(3)OAlH(2) x NMe(3), 4 (Trip = 2,4,6-(i)Pr(3)C(6)H(2)-) as colorless crystalline solids. Subsequent reactions of 2,6-(t)Bu(2)-4-MeC(6)H(2)OAlH(2) x NMe(3), 1, and 2 with oxygen sources such as (Me(2)HSi)(2)O or Me(2)SO gave the disiloxyalane 2,6-(t)Bu(2)-4-Me-C(6)H(2)OAl(OSiHMe(2))(2) x NMe(3), 5, and the alumoxane (Mes*OAlO.OSMe(2))(4), 6. The latter compound is a rare example of a species featuring an unsupported eight-membered (Al-O)(4) ring. The new compounds were characterized by (1)H and (13)C{(1)H} NMR and IR spectroscopy and elemental analyses, and compounds 3 and 6 were also characterized by single crystal X-ray diffraction.

Size matters: room temperature P-C bond formation through C-H activation in m-terphenyldiiodophosphines.

m-Terphenyl- and biphenyl-2-diiodophosphines, TerphPI 2 and BiphPI 2, have been obtained by halide exchange from the chloro derivatives TerphPCl 2 and BiphPCl 2 and excess LiI in a benzene solution at room temperature. Whereas BiphPI 2 compounds are stable, the TerphPI 2 species undergo intramolecular C-H activation at room temperature and cyclize to form unsymmetrical 9-iodo-9-phosphafluorenes 1-(3,5-dimethylphenyl)-6,8-dimethyl-9-iodo-9-phosphafluorene, 4; 1-(4-t-butylphenyl)-7-t-butyl-9-iodo-9-phosphafluorene, 5; and 1-(2-methylphenyl)-5-methyl-9-iodo-9-phosphafluorene, 6, albeit the latter reaction is rather slow due to unfavorable steric interactions. Cyclization of the alkyl-substituted 4,4'-di- tert-butyl-biphenyl-2-diiodophosphine, 11, is slow in refluxing benzene solution, but faster than that for the parent biphenyl-2-diiodophosphine. Ab initio and density functional theory calculations are in agreement with an electrophilic aromatic substitution mechanism that is facilitated by steric strains in the terphenyl compounds 2,6-(3,5-Me2C6H3)2C6H3PI2, 1; 2,6-(4-t-BuC6H4)2C6H3PI 2, 2; and 2,6-(2-MeC6H4)2C6H3PI 2, 3. All new compounds have been characterized by multinuclear NMR spectroscopy and direct analysis in real time mass spectrometry. 9-Iodo-9-phosphafluorene, 12, was also analyzed by X-ray diffraction.

Facile synthesis of monoazidotitanium isopropoxides.

The monoazidotitanium trisisopropoxide {(N(3))Ti(OiPr)(3)}(4) (2) was obtained by the reaction of (N(3))(2)Ti(OiPr)(2) (1) with Ti(OiPr)(4). Colorless 2 possesses an interesting tetrameric structure featuring bridging azide and isoproxide ligands and five- and six-coordinate titanium centers. It can be sublimed in vacuo at ca. 130 degrees C but decomposes rapidly above 180 degrees C. Controlled hydrolysis afforded the related yellow oxo cluster (iPrO)(8)Ti(4)(mu-OiPr)(2)(mu(4)-O)(mu-N(3))(4) (3).

Aluminumoxyhydride: improved synthesis and application as a selective reducing agent.

Aluminumoxyhydride (HAlO) has been obtained by the reaction of aluminum hydride with the siloxane (Me2HSi)2O or the stannoxane (Bu3Sn)2O as an amorphous colorless insoluble powder. The highest-purity product resulted from the reaction of H3Al.NMe3 with (Me2HSi)2O. However, HAlO suspensions in tetrahydrofuran (THF) of sufficient quality for synthetic applications can be prepared from commercially available reagents with only minor precautions. A LiAlH4 solution in THF was treated successively with Me3SiCl and (Me2HSi)2O, followed by heating at 60 degrees C for 20 h. The resulting suspensions are 0.4-0.5 M in active hydride content and selectively reduce aldehydes and ketones to the respective alcohols in the presence of any other common nonprotic functional group.

Facile synthesis of unsymmetrical 9-phospha- and 9-arsafluorenes.

Unsymmetrical 9-chloro-9-phosphafluorenes (dibenzophospholes) and 9-chloro-9-arsafluorenes (dibenzoarsoles) have been obtained by simple thermolysis of m-terphenyldichlorophosphines and -arsines in close to quantitative yields. The reaction temperatures are about 200 degrees C for the phosphines and 140 degrees C for the arsine, and the reactions are complete within 5 min. Alternatively, these compounds can be synthesized through an AlCl3-catalyzed Friedel-Crafts type ring-closure reaction at low temperatures, but this method suffers from difficult workup procedures. The P(As)-Cl functionality is readily alkylated. Methylation of m-xylyl derivative 4 afforded 1-(3,5-dimethylphenyl)-6,8,9-trimethyl-9-phosphafluorene, 11. The latter compound formed the complexes 11 x Fe(CO)4, 12, and 11 x RuCl2(eta(6)-p-cymene), 13, indicating its good donor properties. The new compounds have been characterized by 1H, 13C{1H}, and 31P{1H} NMR spectroscopy; mass spectrometry; and single-crystal X-ray crystallography in the case of 11, 12, and 13.

[2,6-Mes(2)C(6)H(3)](2)Ga(+)Li[Al(OCH(CF(3))(2))(4)](2)(-) (Mes = 2,4,6-Me(3)C(6)H(2)): A compound containing a linear unsolvated two-coordinate gallium cation.

The title compound [2,6-Mes(2)C(2)H(3)](2)Ga(+)Li[Al(OCH(CF(3))(2))(4)](2)(-), 1, containing a linear two-coordinate gallium cation, has been obtained by metathesis reaction of [2,6-Mes(2)C(2)H(3)](2)GaCl with 2 equiv of Li[Al(OCH(CF(3))(2))(4)] in C(6)H(5)Cl solution at room temperature. Compound 1 has been characterized by (1)H, (13)C((1)H), (19)F, and (27)Al NMR spectroscopy and X-ray crystallography. Compound 1 consists of isolated [2,6-Mes(2)C(6)H(3)](2)Ga(+) cations and Li[Al(OCH(CF(3))(2))(4)](2)(-) anions. The C-Ga-C angle is 175.69(7) degrees, and the Ga-C distances are 1.9130(14) and 1.9145(16) A. The title compound is remarkably stable, is only a weak Lewis acid, and polymerizes cyclohexene oxide.

Monomeric Alanes: Synthesis, Structure, and Thermolysis of MesAl(H)N(SiMe(3))(2) and a One-Pot Synthetic Route to Mes(2)AlH (Mes = -C(6)H(2)-2,4,6-t-Bu(3)).

The reaction of (MesAlH(2))(2) (Mes = -C(6)H(2)-2,4,6-t-Bu(3)) with HN(SiMe(3))(2) affords the monomeric amidoarylalane MesAl(H)N(SiMe(3))(2), 1. This product can also be synthesized by the reaction of [MesAlH(2)](2) with LiN(SiMe(3))(2), which, in addition, yields the byproducts LiAlH(2){N(SiMe(3))(2)}(2), 3, and MesH. Thermolysis of 1 at 175-180 degrees C affords three different the related and the imide [MesAlN(SiMe(3))](n)(), 5. In addition, the previously reported monomeric alane Mes(2)AlH was synthesized in ca. 70% yield by a one-pot reaction between LiMes (generated in situ) and AlH(3).NMe(3). All products were spectroscopically characterized, and the structure of 1 was determined by X-ray crystallography. The Al-N distance (1.819(2) Å) in 1 is relatively long. However, it has a substantial, 18.5 kcal mol(-)(1), Al-N rotation barrier which is attributed to steric congestion rather than Al-N pi bonding.

Reaction of the Primary Alane (2,4,6-t-Bu(3)H(2)C(6)AlH(2))(2) with Nitriles, Isonitriles, and Primary Amines.

The reactions of the sterically encumbered primary alane (MesAlH(2))(2) (Mes = C(6)H(2)-2,4,6-t-Bu(3)) with the nitriles t-BuCN, MesCN (Mes = C(6)H(2)-2,4,6-Me(3)) or MeCN lead eventually to dimeric amido alane products in which one of the ortho t-Bu groups of the Mes ligand is metalated and the nitrile is reduced to the amide ligand N(H)CH(2)R (R = t-Bu, Mes, or Me). The compounds (R = t-Bu, 2 (cis), 3 (trans); Mes, 4 (cis), 5 (trans); Me, 6 (cis)) have been isolated and characterized spectroscopically and also by X-ray crystallography in the cases of 4 and 5. The intermediate, dimeric iminato complex [MesAl(H){&mgr;(2)-NC(H)t-Bu}](2) (1), can also be isolated under carefully controlled, mild conditions. Reaction of (MesAlH(2))(2) with the isonitrile t-BuNC affords the cyclic species {MesAlN(t-Bu)CH(2)}(2) (7) featuring a six-membered (AlNC)(2) ring which, when heated, affords the cyclometalated species (8). Recognition that the products 2-6 and 8 were derivatives of primary amides led to an investigation of synthetic approaches to these product types via the direct reaction of (MesAlH(2))(2) with some primary amines. Treatment of (MesAlH(2))(2) with H(2)NCH(2)Mes affords the dimeric amido alane [Mes(H)Al{&mgr;(2)-N(H)CH(2)Mes}](2) as a mixture of trans (9) and cis (10) isomers. Further heating of 9 and 10 affords the ortho-metalated compounds 4 and 5. The reaction of (MesAlH(2))(2) with H(2)NSiPh(3) furnishes the bis amido aluminum compound MesAl{N(H)SiPh(3)}(2) (11) and [Mes(H)Al{N(H)SiPh(3)}](2). The latter yields the dimeric imide {MesAlNSiPh(3)}(2) (12) at elevated temperature.

Multiple Ga-Ga Bonding Character in Na2 [Ga(GaTrip2 )3 ], and a Comparison with Neutral Ga(GaTrip2 )3 (Trip=2,4,6-iPr3 C6 H2 ).

Delocalized over the Ga4 framework, the two electrons are lost in the oxidation of Na2 [Ga(GaTrip2 )3 ] to Ga(GaTrip2 )3 . This leads to a lengthening of the Ga-Ga bond by about 0.08 Å. Ar=Trip=2,4,6-iPr3 C6 H2 .

Synthesis and Characterization of Sterically Encumbered Derivatives of Aluminum Hydrides and Halides: Assessment of Steric Properties of Bulky Terphenyl Ligands.

The synthesis and characterization of several sterically encumbered monoterphenyl derivatives of aluminum halides and aluminum hydrides are described. These compounds are [2,6-Mes(2)C(6)H(3)AlH(3)LiOEt(2)](n)() (1), (Mes = 2,4,6-Me(3)C(6)H(2)-), 2,6-Mes(2)C(6)H(3)AlH(2)OEt(2) (2), [2,6-Mes(2)C(6)H(3)AlH(2)](2) (3), 2,6-Mes(2)C(6)H(3)AlCl(2)OEt(2) (4), [2,6-Mes(2)C(6)H(3)AlCl(3)LiOEt(2)](n)() (5), [2,6-Mes(2)C(6)H(3)AlCl(2)](2) (6), TriphAlBr(2)OEt(2) (7), (Triph = 2,4,6-Ph(3)C(6)H(2)-), [2,6-Trip(2)C(6)H(3)AlH(3)LiOEt(2)](2) (8) (Trip = 2,4,6-i-Pr(3)C(6)H(2)-), 2,6-Trip(2)C(6)H(3)AlH(2)OEt(2) (9), [2,6-Trip(2)C(6)H(3)AlH(2)](2) (10), 2,6-Trip(2)C(6)H(3)AlCl(2)OEt(2) (11), and the partially hydrolyzed derivative [2,6-Trip(2)C(6)H(3)Al(Cl)(0.68)(H)(0.32)(&mgr;-OH)](2).2C(6)H(6) (12). The structures of 2, 3a, 4, 6, 7, 9a, 10a, 10b, 11, and 12 were determined by X-ray crystallography. The structures of 3a, 9a, 10a, and 10b, are related to 3, 9, and 10, respectively, by partial occupation of chloride or hydride by hydroxide. The compounds were also characterized by (1)H, (13)C, (7)Li, and (27)Al NMR and IR spectroscopy. The major conclusions from the experimental data are that a single ortho terphenyl substituent of the kind reported here are not as effective as the ligand Mes (Mes = 2,4,6-t-Bu(3)C(6)H(2)-) in preventing further coordination and/or aggregation involving the aluminum centers. In effect, one terphenyl ligand is not as successful as a Mes substituent in masking the metal through agostic and/or steric effects.

New Routes to Synthetically Useful, Sterically Encumbered Arylaluminum Halides and Hydride Halides.

The synthesis and structural characterization of the compounds MesAlCl(2)(THF) (1), MesAlCl(2) (2), MesAl(H)Cl(THF) (3a), MesAl(H)Cl (4a), and (MesAlH(2))(2) (5) (Mes = 2,4,6-t-Bu(3)C(6)H(2)(-)) are described as well as those for two compounds 3b and 4b that are analogs of 3a and 4a but have H:Cl ratios that are less than 1:1. All compounds were characterized by (1)H, (13)C NMR, and IR spectroscopy, and 1, 2, 3a, and 4b were additionally characterized by X-ray crystallography. Compound 1 is best synthesized by the reaction of [(THF)(2)LiH(3)AlMes](2) (6) with 6 equiv of Me(3)SiCl. A more conventional route involving the addition of (THF)(2)LiMes to 2 equiv of AlCl(3) in toluene usually affords a mixture of 1 and AlCl(3).THF. Recrystallization of 1 from n-hexane results in a species that has less than 1 equiv of THF per MesAlCl(2). The THF free complex 2 may be obtained in quantitative yield by heating 1 for 20 min at 90 degrees C under reduced pressure. Compound 3a may be obtained by treating a 1:1 mixture of MesLi(THF)(2) and LiAlH(4) with 2 equiv of Me(3)SiCl or by the addition of slightly less than 4 equiv of Me(3)SiCl to 6. The THF can be removed from 3a by sublimation to give 4a. The related compounds 3b and 4b, which display an aluminum-bound H:Cl ratio that is deficient in H, can be obtained by reactions with slightly more than 2 equiv of Me(3)SiCl. Crystal data at 130 K with Cu Kalpha (lambda = 1.541 78 Å) radiation: 1, C(22)H(37)AlCl(2)O, a = 11.889(3) Å, b = 9.992(3) Å, c = 19.704(5) Å, orthorhombic, space group Pca2(1), Z = 4, R = 0.068 for 1556 (I > 2sigma(I)) data; 2, C(18)H(29)AlCl(2), a = 12.147(5) Å, b = 18.042(6) Å, c = 17.771(7) Å, beta = 95.77(3) degrees, monoclinic, space group P2(1)/n,Z = 8, R = 0.032 for 4610 (I > 2sigma(I)) data; 3a, C(22)H(38)AlClO, a = 16.887(7) Å, b = 16.333(6) Å, c = 8.739(3) Å, beta = 101.41(3) degrees, monoclinic, space group P2(1)/c, Z = 4, R = 0.073 for 2752 (I > 2sigma(I)) data; 4b, C(18)H(29.64)AlCl(1.36), a = 12.077(3) Å, b = 17.920(3) Å, c = 17.634(5) Å; beta = 95.21(2) Å, monoclinic, space group P2(1)/n,Z = 8, R = 0.070 for 4261 (I > 2sigma(I)) data.