Aaron D. Sadow

Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling of Organosilanes with Amines, Hydrazine, and Ammonia

We report magnesium-catalyzed cross-dehydrocoupling of Si-H and N-H bonds to give Si-N bonds and H(2). A number of silazanes are accessible using this method, as well as silylamines from NH(3) and silylhydrazines from N(2)H(4). Kinetic studies of the overall catalytic cycle and a stoichiometric Si-N bond-forming reaction suggest nucleophilic attack by a magnesium amide as the turnover-limiting step.

Magnesium-catalyzed hydroboration of esters: evidence for a new zwitterionic mechanism

A magnesium-catalyzed ester hydroboration reaction rapidly and efficiently (<0.5 mol% catalyst) provides alkoxy borane products via ester cleavage. Oxidized functional groups, such as cyano-, nitro-, cyclopropyl- and conjugated olefins, are unaffected by the ester reduction. Moreover, metal-catalyzed hydroboration reactions have been previously suggested to involve second-order interactions of hydroborane reagents and M–X (X = R and OR) for B–X bond formation. Catalytic kinetic studies rule out this traditional σ-bond metathesis mechanism for B–O bond formation, and instead a pathway involving a zwitterionic alkoxyborate is proposed.

Lewis Acid-Mediated β-Hydride Abstraction Reactions of Divalent M(C(SiHMe2)3)2THF2 (M = Ca, Yb)

The divalent calcium and ytterbium compounds M(C(SiHMe(2))(3))(2)THF(2) contain beta-agostic SiH groups, as determined by spectroscopy and crystallography. Upon thermolysis, HC(SiHMe(2))(3) is formed. However, the SiH groups are hydridic. The compounds M(C(SiHMe(2))(3))(2)THF(2) react with 1 and 2 equiv of the Lewis acid B(C(6)F(5))(3) to form MC(SiHMe(2))(3)HB(C(6)F(5))(3))THF(2) and M(HB(C(6)F(5))(3))(2)THF(2), respectively. These species contain the anion [HB(C(6)F(5))(3)](-) from hydride abstraction rather than [(Me(2)HSi)(3)CB(C(6)F(5))(3)](-) from alkyl abstraction. The 1,3-disilacyclobutane byproduct initially suggested beta-elimination [as the dimer of the silene Me(2)Si horizontal lineC(SiHMe(2))(2)], but the other products and reaction stoichiometry rule out that pathway. Additionally, Yb(C(SiHMe(2))(3))(2)THF(2) and the weak Lewis acid BPh(3) react rapidly and also give the H-abstracted products. Despite the strong hydridic character of the SiH groups and the low-coordinate, Lewis acidic metal center in M(C(SiHMe(2))(3)THF(2) compounds, beta-elimination is not an observed reaction pathway.

Acceptorless Photocatalytic Dehydrogenation for Alcohol Decarbonylation and Imine Synthesis

It has come to light: Renewed interest in conversions of highly oxygenated materials has motivated studies of the organometallic-catalyzed photocatalytic dehydrogenative decarbonylation of primary alcohols into alkanes, CO, and H(2). Methanol, ethanol, benzyl alcohol, and cyclohexanemethanol are readily decarbonylated. The photocatalysts are also active for amine dehydrogenation to give N-alkyl aldimines and H(2).

Conversion of a Zinc Disilazide to a Zinc Hydride Mediated by LiCl

An unusual beta-elimination reaction involving zinc(II) and LiCl is reported. LiCl and a coordinatively saturated disilazido zinc compound form an adduct that contains activated SiH moieties. In THF/toluene mixtures, this adduct is transformed into a zinc hydride and 0.5 equiv. cyclodisilazane. The Li(+) and Cl(-) ions apparently affect the reaction pathway of the disilazido zinc in a synergistic fashion. Thus, the zinc hydride and cyclodisilazane products of formal beta-elimination are not observed upon treatment of the zinc disilazide with Cl(-) or Li(+) separately.

Ligand Exchange Reactions and Hydroamination with Tris(oxazolinyl)borato Yttrium Compounds

Ligand substitution reactions and catalytic hydroamination/cyclization of aminoalkenes have been studied with a new oxazolinylborato yttrium compound, tris(4,4-dimethyl-2-oxazolinyl)phenylborato bis(trimethylsilylmethyl)yttrium ([Y(kappa(3)-To(M))(CH(2)SiMe(3))(2)(THF)], 1). THF exchange in 1 is rapid at room temperature, and activation parameters obtained by simulation of (1)H NMR spectra acquired from 190 to 280 K are consistent with a dissociative mechanism (DeltaS(++) = 30 +/- 1 e.u., DeltaG(++) = 11.9 kcal mol(-1) at 243 K). The related phosphine oxide adduct [Y(kappa(3)-To(M))(CH(2)SiMe(3))(2)(OPPh(3))] (2) also undergoes exchange via OPPh(3) dissociation with a much higher barrier (DeltaG(++) = 15.0 kcal mol(-1) at 320 K). Compound 1 reacts with the amines (t)BuNH(2), para-MeC(6)H(4)NH(2), and 2,6-(i)Pr(2)C(6)H(3)NH(2) to provide six-coordinate [Y(kappa(3)-To(M))(NHR)(2)(THF)] (3: R = (t)Bu; 4: R = para-MeC(6)H(4)) and five-coordinate [Y(kappa(3)-To(M))(NH-2,6-(i)Pr(2)C(6)H(3))(2)] (6). These oxazolinylborato yttrium compounds are precatalysts for the cyclization of aminoalkenes; the kinetics of catalytic conversion indicate zero-order substrate dependence and first-order catalyst dependence. Kinetic investigations of ligand exchange processes and hydroamination reactions indicate that the tris(oxazolinyl)borato-yttrium interaction is robust even in the presence of excess phosphine oxide and primary and secondary amines.

Role Of CO2 As a Soft Oxidant For Dehydrogenation of Ethylbenzene to Styrene over a High-Surface-Area Ceria Catalyst

Catalytic performance and the nature of surface adsorbates were investigated for high-surface-area ceria during the ethylbenzene oxidative dehydrogenation (ODH) reaction using CO2 as a soft oxidant. The high surface area ceria material was synthesized using a template-assisted method. The interactions among ethylbenzene, styrene, and CO2 on the surface of ceria and the role of CO2 for the ethylbenzene ODH reaction have been investigated in detail by using activity test, in situ diffuse reflectance infrared and Raman spectroscopy. CO2 as an oxidant not only favored the higher yield of styrene but also inhibited the deposition of coke during the ethylbenzene ODH reaction. Ethylbenzene ODH reaction over ceria followed a two-step pathway: ethylbenzene is first dehydrogenated to styrene with H2 formed simultaneously, and then CO2 reacts with H2 via the reverse water gas shift. The produced styrene can easily undergo polymerization to form polystyrene, which is a key intermediate for coke formation. In the abse...

Zirconium-Catalyzed Desymmetrization of Aminodialkenes and Aminodialkynes through Enantioselective Hydroamination

The catalytic addition of alkenes and amines (hydroamination) typically provides α- or β-amino stereocenters directly through C-N or C-H bond formation. Alternatively, desymmetrization reactions of symmetrical aminodialkenes or aminodialkynes provide access to stereogenic centers with the position controlled by the substrates structure. In the present study of an enantioselective zirconium-catalyzed hydroamination, stereocenters resulting from C-N bond formation and desymmetrization of a prochiral quaternary center are independently controlled by the catalyst and reaction conditions. Using a single catalyst, the method provides selective access to either diastereomer of optically enriched five-, six-, and seven-membered cyclic amines from aminodialkenes and enantioselective synthesis of five-, six-, and seven-membered cyclic imines from aminodialkynes. Experiments on hydroamination of aminodialkenes testing the effects of the catalyst:substrate ratio, the absolute concentration of the catalyst, and the absolute initial concentration of the primary amine substrate show that the latter parameter strongly influences the stereoselectivity of the desymmetrization process, whereas the absolute configuration of the α-amino stereocenter generated by C-N bond formation is not affected by these parameters. Interestingly, isotopic substitution (H2NR vs D2NR) of the substrate enhances the stereoselectivity of the enantioselective and diastereoselective processes in aminodialkene cyclization and the peripheral stereocenter in aminodialkyne desymmetrization/cyclization.

Easily Prepared Chiral Scorpionates: Tris(2-oxazolinyl)boratoiridium(I) Compounds and Their Interactions with MeOTf

Optically active C 3-symmetric monoanionic ligands are uncommon in organometallic chemistry. Here we describe the synthesis of readily prepared tris(4 S-isopropyl-2-oxazolinyl)phenylborate [To (P)] and fluxional, zwitterionic four- and five-coordinate iridium(I) compounds [Ir(To (P))(eta (4)-C 8H 12)] ( 4) and [Ir(To (P))(CO) 2] ( 5). The highly fluxional nature of 4 and 5 makes structural assignment difficult, and the interaction between the iridium(I) center and the [To (P)] ligand is established by solid-state and solution (15)N NMR methods that permit the direct comparison between solution and solid-state structures. Although iridium cyclooctadiene 4 is a mixture of four- and five-coordinate species, the dicarbonyl 5 is only the five-coordinate isomer. The addition of electrophiles MeOTf and MeI provides the oxazoline N-methylated product rather than the iridium methyl oxidative addition product. N-Methylation was unequivocally proven by through-bond coupling observed in (1)H- (15)N HMBC experiments.

Nonclassical β-Hydrogen Elimination of Hydrosilazido Zirconium Compounds via Direct Hydrogen Transfer

Salt metathesis reactions of Cp(2)(NR(2))ZrX (X = Cl, I, OTf) and lithium hydrosilazides ultimately afford hydride products Cp(2)(NR(2))ZrH that suggest unusual β-hydrogen elimination processes. A likely intermediate in one of these reactions, Cp(2)Zr[N(SiHMe(2))t-Bu][N(SiHMe(2))(2)], is isolated under controlled synthetic conditions. Addition of alkali metal salts to this zirconium hydrosilazide compound produces the corresponding zirconium hydride. However as conditions are varied, a number of other pathways are also accessible, including C-H/Si-H dehydrocoupling, γ-abstraction of a CH, and β-abstraction of a SiH. Our observations suggest that the conversion of (hydrosilazido)zirconocene to zirconium hydride and silanimine does not follow the classical four-center mechanism for β-elimination.