Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C−O Bond Cleavage

The high catalytic reactivity of homoleptic tris(alkyl) lanthanum La{C(SiHMe2)3}3 is highlighted by C−O bond cleavage in the hydroboration of esters and epoxides at room temperature. The catalytic hydroboration tolerates functionality typically susceptible to insertion, reduction, or cleavage reactions. Turnover numbers (TON) up to 10 000 are observed for aliphatic esters. Lanthanum hydrides, generated by reactions with pinacolborane, are competent for reduction of ketones but are inert toward esters. Instead, catalytic reduction of esters requires activation of the lanthanum hydride by pinacolborane

The Synthesis and Characterization of New, Robust Titanium (IV) Scorpionate Complexes

Titanium complexes possessing sterically encumbered ligands have allowed for the preparation of reactive moieties (imido, alkylidene and alkylidyne species) relevant to reactions such as olefin polymerization and alkyne hydroamination. For this reason, we have targeted robust scorpionate ancillary ligands to support reactive titanium centers. Thus, a series of titanium complexes were synthesized using an achiral oxazoline-based scorpionate ligand, tris(4,4-dimethyl-2-oxazolinyl)phenyl borate [To^M^]^-^ as well as the related chiral ligand, tris(4-isopropyl-2-oxazolinyl)phenyl borate [To^P^]^-^. The complex [Ti(κ^3^- To^M^)Cl~3~] was prepared in moderate yield (43%) by the rapid (<1 min at room temperature) reaction of Li[To^M^] and TiCl~4~ in methylene chloride; this new compound was characterized by ^1^H NMR spectroscopy as the expected C~3v~-symmetric species. One route to Ti (IV) alkyls involves salt metathesis; accordingly, syntheses of [To^M^]Ti alkyl complexes by interaction of [Ti(κ^3^-To^M^)Cl~3~] and one or three equivalents of alkylating agents, such as benzyl potassium (KCH~2~C~6~H~5~), trimethylsilylmethyl
lithium (LiCH~2~Si(CH~3~) ~3~), or neopentyl lithium (LiCH~2~C(CH~3~)~3~) are currently under investigation. The complexes [Ti(=NBut) (κ~3~-To^M^)(Cl)(Bu^t^py)] (Bu^t^py=4 tert-butylpyridine) and [Ti(=NBu^t^) (κ~3~-To^P^)(Cl)(Bu^t^py)] were synthesized by reaction of the known Ti imido [Ti(=NBu^t^)(Cl)~2~(Bu^t^py)~2~] with Li[To^M^] or Li[To^P^], respectively, by stirring overnight in methylene chloride at ambient temperature. The complexes were identified using ^1^H NMR spectroscopy, ^1^H-^13^C HMQC and ^1^H-^15^N HMBC correlation experiments

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

Salt metathesis reactions of Cp2(NR2)ZrX (X = Cl, I, OTf) and lithium hydrosilazides ultimately afford hydride products Cp2(NR2)ZrH that suggest unusual β-hydrogen elimination processes. A likely intermediate in one of these reactions, Cp2Zr[N(SiHMe2)t-Bu][N(SiHMe2)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

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 substrate’s 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

Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis(alkyl) compound Sm{C(SiHMe2)3}2THF2 (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This synthesis is similar to that of previously reported Yb{C(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostructural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to 2400 cm–1 assigned to νBH in the IR spectra and highly paramagnetically shifted signals in the 11B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions. Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is important for accessing the catalytically active species

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

An unusual β-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 β-elimination are not observed upon treatment of the zinc disilazide with Cl− or Li+separately

Remarkably Robust Monomeric Alkylperoxyzinc Compounds from Tris(oxazolinyl)boratozinc Alkyls and O2

Metal alkylperoxides are remarkable, highly effective, yet often thermally unstable, oxidants that may react through a number of possible pathways including O–O homolytic cleavage, M–O homolytic cleavage, nucleophilic O-atom transfer, and electrophilic O-atom transfer. Here we describe a series of zinc alkyl compounds of the type ToMZnR (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; R = Et, n-C3H7, i-C3H7, t-Bu) that react with O2 at 25 °C to form isolable monomeric alkylperoxides ToMZnOOR in quantitative yield. The series of zinc alkylperoxides is crystallographically characterized, and the structures show systematic variations in the Zn–O–O angle and O–O distances. The observed rate law for the reaction of ToMZnEt (2) and O2 is consistent with a radical chain mechanism, where the rate-limiting SH2 step involves the interaction of •OOR and ToMZnR. In contrast, ToMZnH and ToMZnMe are unchanged even to 120 °C under 100 psi of O2 and in the presence of active radical chains (e.g., •OOEt). This class of zinc alkylperoxides is unusually thermally robust, in that the compounds are unchanged after heating at 120 °C in solution for several days. Yet, these compounds are reactive as oxidants with phosphines. Additionally, an unusual alkylperoxy group transfer to organosilanes affords ToMZnH and ROOSiR3′

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(κ3-ToM)(CH2SiMe3)2(THF)], 1). THF exchange in 1 is rapid at room temperature, and activation parameters obtained by simulation of 1H NMR spectra acquired from 190 to 280 K are consistent with a dissociative mechanism (ΔS‡ = 30 ± 1 e.u., ΔG‡ = 11.9 kcal mol−1 at 243 K). The related phosphine oxide adduct [Y(κ3-ToM)(CH2SiMe3)2(OPPh3)] (2) also undergoes exchange via OPPh3 dissociation with a much higher barrier (ΔG‡ = 15.0 kcal mol−1 at 320 K). Compound 1 reacts with the amines tBuNH2, para-MeC6H4NH2, and 2,6-iPr2C6H3NH2 to provide six-coordinate [Y(κ3-ToM)(NHR)2(THF)] (3: R = tBu; 4: R = para-MeC6H4) and five-coordinate [Y(κ3-ToM)(NH-2,6-iPr2C6H3)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