Concerted C−N and C−H Bond Formation in a Magnesium-Catalyzed Hydroamination

Coordinatively saturated ToMMgMe (1; ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) is an active precatalyst for intramolecular hydroamination/cyclization at 50 °C. The empirical rate law of −d[substrate]/dt = k′obs[Mg]1[substrate]1 and Michaelis−Menten-type kinetics are consistent with a mechanism involving reversible catalyst−substrate association prior to cyclization. The resting state of the catalyst, ToMMgNHCH2CR2CH2CH═CH2 [R = Ph, Me, —(CH2)5—], is isolable, but isolated magnesium amidoalkene does not undergo unimolecular cyclization at 50 °C. However, addition of trace amounts of substrate allows cyclization to occur. Therefore, we propose a two-substrate, six-center transition state involving concerted C—N bond formation and N—H bond cleavage as the turnover-limiting step of the catalytic cycle

Magnesium-Catalyzed Mild Reduction of Tertiary and Secondary Amides to Amines

The first example of a catalytic hydroboration of amides for their deoxygenation to amines is reported. This transformation employs an earth-abundant magnesium-based catalyst. Tertiary and secondary amides are reduced to amines at room temperature in the presence of pinacolborane (HBpin) and catalytic amounts of ToMMgMe (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). Catalyst initiation and speciation is complex in this system, as revealed by the effects of concentration and order of addition of the substrate and HBpin in the catalytic experiments. ToMMgH2Bpin, formed from ToMMgMe and HBpin, is ruled out as a possible catalytically relevant species by its reaction with N,N-dimethylbenzamide, which gives Me2NBpin and PhBpin through C–N and C–C bond cleavage pathways, respectively. In that reaction, the catalytic product benzyldimethylamine is formed in only low yield. Alternatively, the reaction of ToMMgMe and N,N-dimethylbenzamide slowly gives decomposition of ToMMgMe over 24 h, and this interaction is also ruled out as a catalytically relevant step. Together, these data suggest that catalytic activation of ToMMgMe requires both HBpin and amide, and ToMMgH2Bpin is not a catalytic intermediate. With information on catalyst activation in hand, tertiary amides are selectively reduced to amines in good yield when catalytic amounts of ToMMgMe are added to a mixture of amide and excess HBpin. In addition, secondary amides are reduced in the presence of 10 mol % ToMMgMe and 4 equiv of HBpin. Functional groups such as cyano, nitro, and azo remain intact under the mild reaction conditions. In addition, kinetic experiments and competition experiments indicate that B–H addition to amide C═O is fast, even faster than addition to ester C═O, and requires participation of the catalyst, whereas the turnover-limiting step of the catalyst is deoxygenation

Mixed N‑Heterocyclic Carbene−Bis(oxazolinyl)borato Rhodium and Iridium Complexes in Photochemical and Thermal Oxidative Addition Reactions

In order to facilitate oxidative addition chemistry of fac-coordinated rhodium(I) and iridium(I) compounds, carbene–bis(oxazolinyl)phenylborate proligands have been synthesized and reacted with organometallic precursors. Two proligands, PhB(OxMe2)2(ImtBuH) (H[1]; OxMe2 = 4,4-dimethyl-2-oxazoline; ImtBuH = 1-tert-butylimidazole) and PhB(OxMe2)2(ImMesH) (H[2]; ImMesH = 1-mesitylimidazole), are deprotonated with potassium benzyl to generate K[1] and K[2], and these potassium compounds serve as reagents for the synthesis of a series of rhodium and iridium complexes. Cyclooctadiene and dicarbonyl compounds {PhB(OxMe2)2ImtBu}Rh(η4-C8H12) (3), {PhB(OxMe2)2ImMes}Rh(η4-C8H12) (4), {PhB(OxMe2)2ImMes}Rh(CO)2 (5), {PhB(OxMe2)2ImMes}Ir(η4-C8H12) (6), and {PhB(OxMe2)2ImMes}Ir(CO)2 (7) are synthesized along with ToMM(η4-C8H12) (M = Rh (8); M = Ir (9); ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate). The spectroscopic and structural properties and reactivity of this series of compounds show electronic and steric effects of substituents on the imidazole (tert-butyl vs mesityl), effects of replacing an oxazoline in ToMwith a carbene donor, and the influence of the donor ligand (CO vs C8H12). The reactions of K[2] and [M(μ-Cl)(η2-C8H14)2]2 (M = Rh, Ir) provide {κ4-PhB(OxMe2)2ImMes′CH2}Rh(μ-H)(μ-Cl)Rh(η2-C8H14)2 (10) and {PhB(OxMe2)2ImMes}IrH(η3-C8H13) (11). In the former compound, a spontaneous oxidative addition of a mesityl ortho-methyl to give a mixed-valent dirhodium species is observed, while the iridium compound forms a monometallic allyl hydride. Photochemical reactions of dicarbonyl compounds 5 and 7 result in C–H bond oxidative addition providing the compounds {κ4-PhB(OxMe2)2ImMes′CH2}RhH(CO) (12) and {PhB(OxMe2)2ImMes}IrH(Ph)CO (13). In 12, oxidative addition results in cyclometalation of the mesityl ortho-methyl similar to 10, whereas the iridium compound reacts with the benzene solvent to give a rare crystallographically characterized cis-[Ir](H)(Ph) complex. Alternatively, the rhodium carbonyl 5 or iridium isocyanide {PhB(OxMe2)2ImMes}Ir(CO)CNtBu (15) reacts with PhSiH3 in the dark to form the silyl compound {PhB(OxMe2)2ImMes}RhH(SiH2Ph)CO (14) or {PhB(OxMe2)2ImMes}IrH(SiH2Ph)CNtBu (17). These examples demonstrate the enhanced thermal reactivity of {PhB(OxMe2)2ImMes}-supported iridium and rhodium carbonyl compounds in comparison to tris(oxazolinyl)borate, tris(pyrazolyl)borate, and cyclopentadienyl-supported compounds

Reactions of Tris(oxazolinyl)phenylborato Rhodium(I) with C−X (X = Cl, Br, OTf) Bonds: Stereoselective Intermolecular Oxidative Addition

The achiral and enantiopure chiral compounds ToMRh(CO)2 (3) and ToPRh(CO)2 (4) (ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; ToP = tris(4S-isopropyl-2-oxazolinyl)phenylborate) were prepared to investigate stereoselective oxidative addition reactions and develop new catalytic enantioselective bond functionalization and cross-coupling chemistry. Reactivity at the rhodium center is first shown by the substitution of the carbonyl ligands in 3 and 4 in the presence of the appropriate ligand; thus treatment of ToMRh(CO)2 with P(OMe)3 provides ToMRh(CO)[P(OMe)3] (5). However, reaction of ToMRh(CO)2 and MeOTf (Tf = SO2CF3) affords the complex [{N-Me-κ2-ToM}Rh(CO)2]OTf (6), resulting from N-oxazoline methylation rather than oxidative addition to rhodium(I). In contrast, ToMRh(CO)2 reacts with allyl bromide and chloroform, forming the rhodium(III) species (κ3-ToM)Rh(η1-C3H5)Br(CO) (7) and (κ3-ToM)Rh(CHCl2)Cl(CO) (8), respectively. Interestingly, the chiral ToPRh(CO)2 and CHCl3 react to give one diastereomer of (κ3-ToP)Rh(CHCl2)Cl(CO) (9; 100:3 dr) almost exclusively. To evaluate the reactivity of these rhodium(I) compounds, the carbonyl stretching frequencies have been examined. The data for the mono- and trivalent rhodium oxazolinylborate compounds indicate that the electron-donating ability of [ToM]− is slightly greater than that of [ToP]−, and both ligands provide electronic environments that can be compared to the tris(pyrazolyl)borate ligand family

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 H2. A number of silazanes are accessible using this method, as well as silylamines from NH3 and silylhydrazines from N2H4. 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

Synthesis of Monomeric Fe(II) and Ru(II) Complexes of Tetradentate Phosphines

rac-Bis[{(diphenylphosphino)ethyl}-phenylphosphino]methane (DPPEPM) reacts with iron(II) and ruthenium(II) halides to generate complexes with folded DPPEPM coordination. The paramagnetic, five-coordinate Fe(DPPEPM)Cl2 (1) in CD2Cl2 features a tridentate binding mode as established by 31P{1H} NMR spectroscopy. Crystal structure analysis of the analogous bromo complex, Fe(DPPEPM)Br2 (2) revealed a pseudo-octahedral, cis-α geometry at iron with DPPEPM coordinated in a tetradentate fashion. However, in CD2Cl2solution, the coordination of DPPEPM in 2 is similar to that of 1 in that one of the external phosphorus atoms is dissociated resulting in a mixture of three tridentate complexes. The chloro ruthenium complex cis-Ru(κ4-DPPEPM)Cl2 (3) is obtained from rac-DPPEPM and either [RuCl2(COD)]2 [COD = 1,5-cyclooctadiene] or RuCl2(PPh3)4. The structure of 3 in both the solid state and in CD2Cl2 solution features a folded κ4-DPPEPM. This binding mode was also observed in cis-[Fe(κ4-DPPEPM)(CH3CN)2](CF3SO3)2 (4). Addition of an excess of CO to a methanolic solution of 1 results in the replacement of one of the chloride ions by CO to yieldcis-[Fe(κ4-DPPEPM)Cl(CO)](Cl) (5). The same reaction in CH2Cl2 produces a mixture of 5and [Fe(κ3-DPPEPM)Cl2(CO)] (6) in which one of the internal phosphines has been substituted by CO. Complexes 2, 3, 4, and 5 appear to be the first structurally characterized monometallic complexes of κ4-DPPEPM

Organometallic Complexes of Bulky, Optically Active, C3-Symmetric Tris(4S-isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (ToP*)

A bulky, optically active monoanionic scorpionate ligand, tris(4S-isopropyl-5,5-dimethyl-2-oxazolinyl)phenylborate (ToP*), is synthesized from the naturally occurring amino acid l-valine as its lithium salt, Li[ToP*] (1). That compound is readily converted to the thallium complex Tl[ToP*] (2) and to the acid derivative H[ToP*] (3). Group 7 tricarbonyl complexes ToP*M(CO)3(M = Mn (4), Re (5)) are synthesized by the reaction of MBr(CO)5 and Li[ToP*] and are crystallographically characterized. The νCO bands in their infrared spectra indicate that π back-donation in the rhenium compounds is greater with ToP* than with non-methylated tris(4S-isopropyl-2-oxazolinyl)phenylborate (ToP). The reaction of H[ToP*] and ZnEt2 gives ToP*ZnEt (6), while ToP*ZnCl (7) is synthesized from Li[ToP*] and ZnCl2. The reaction of ToP*ZnCl and KOtBu followed by addition of PhSiH3 provides the zinc hydride complex ToP*ZnH (8). Compound 8 is the first example of a crystallographically characterized optically active zinc hydride. We tested its catalytic reactivity in the cross-dehydrocoupling of silanes and alcohols, which provided Si-chiral silanes with moderate enantioselectivity

Magnesium-catalyzed hydrosilylation of a,b-unsaturated esters

ToMMgHB(C6F5)3 (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of ToMMgMe, PhSiH3, and B(C6F5)3. Unlike the transient ToMMgH formed from the reaction of ToMMgMe and PhSiH3, the borate adduct 1 persists in solution and in the solid state. Crystallographic characterization reveals tripodal coordination of the HB(C6F5)3 moiety to the six-coordinate magnesium center with a ∠Mg–H–B of 141(3)°. The pathway for formation of 1 is proposed to involve the reaction of ToMMgMe and a PhSiH3/B(C6F5)3 adduct because the other possible intermediates, ToMMgH and ToMMgMeB(C6F5)3, react to give an intractable black solid and ToMMgC6F5, respectively. Under catalytic conditions, silyl ketene acetals are isolated in high yield from the addition of hydrosilanes to α,β-unsaturated esters with 1 as the catalyst

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

Optically active C3-symmetric monoanionic ligands are uncommon in organometallic chemistry. Here we describe the synthesis of readily prepared tris(4S-isopropyl-2-oxazolinyl)phenylborate [ToP] and fluxional, zwitterionic four- and five-coordinate iridium(I) compounds [Ir(ToP)(η4-C8H12)] (4) and [Ir(ToP)(CO)2] (5). The highly fluxional nature of 4 and5 makes structural assignment difficult, and the interaction between the iridium(I) center and the [ToP] ligand is established by solid-state and solution 15N 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 1H−15N HMBC experiments

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