Reactions of Titanium Hydrazides with Silanes and Boranes: N–N Bond Cleavage and N Atom Functionalization

Reaction of Ti­(N₂^iPrN)­(NNPh₂)­(py) with Ph­(R)­SiH₂ (R = H, Ph) or 9-BBN gave reductive cleavage of the N_α–N_β bond and formation of new silyl- or boryl-amido ligands. The corresponding reactions of Cp*Ti­{MeC­(NⁱPr)₂}­(NNR₂) (R = Me or Ph) with HBPin or 9-BBN gave borylhydrazido-hydride or borylimido products, respectively. N_α and N_β atom transfer and dehydrogenative coupling reactions are also reported

Reactions of Titanium Hydrazides with Silanes and Boranes: N–N Bond Cleavage and N Atom Functionalization

Reaction of Ti­(N₂^iPrN)­(NNPh₂)­(py) with Ph­(R)­SiH₂ (R = H, Ph) or 9-BBN gave reductive cleavage of the N_α–N_β bond and formation of new silyl- or boryl-amido ligands. The corresponding reactions of Cp*Ti­{MeC­(NⁱPr)₂}­(NNR₂) (R = Me or Ph) with HBPin or 9-BBN gave borylhydrazido-hydride or borylimido products, respectively. N_α and N_β atom transfer and dehydrogenative coupling reactions are also reported

Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium <i>tert-</i>Butoxyimido Compound

We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti­{PhC­(NⁱPr)₂}­(NO^tBu) (<b>19</b>), with heterocumulenes, aldehydes, ketones, organic nitriles, Ar^F₅CCH, and B­(Ar^F₅)₃ (Ar^F₅ = C₆F₅). Compound <b>19</b> was synthesized via imide/alkoxyamine exchange from Cp*Ti­{PhC­(NⁱPr)₂}­(N^tBu) and ^tBuONH₂. Reaction of <b>19</b> with CS₂ and Ar′NCO (Ar′ = 2,6-C₆H₃ⁱPr₂) gave the [2 + 2] cycloaddition products Cp*Ti­{PhC­(NⁱPr)₂}­{SC­(S)­N­(O^tBu)} and Cp*Ti­{PhC­(NⁱPr)₂}­{N­(O^tBu)­C­(NAr′)­O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti­{PhC­(NⁱPr)₂}­{OC­(NTol)­N­(Tol)­C­(NO^tBu)­O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NO^tBu group transfer was observed with both ^tBuNCO and PhC­(O)­R, yielding the oxo-bridged dimer [Cp*Ti­{PhC­(NⁱPr)₂}­(μ-O)]₂ and either the alkoxycarbodiimide ^tBuNCNO^tBu or the oxime ethers PhC­(NO^tBu)­R (R = H (<b>25a</b>), Me (<b>25b</b>), Ph (<b>25c</b>)). DFT studies showed that in the reaction with PhC­(O)­R (R = H, Me) the product distribution between the <i>syn</i> and <i>anti</i> isomers of PhC­(NO^tBu)­R was under kinetic control. Reaction of <b>19</b> with ArCN gave the TiN_α insertion products Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar)­NO^tBu} (Ar = Ph (<b>28</b>), 2,6-C₆H₃F₂ (<b>27</b>), Ar^F₅ (<b>26</b>)) containing <i>tert</i>-butoxybenzimidamide ligands. Reaction of <b>19</b> or <b>26</b> with an excess of Ar^F₅CN gave Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar^F₅)­NC­(Ar^F₅)­N­(C­{Ar^F₅}­NO^tBu)} (<b>29</b>) following net head-to-tail coupling of 2 equiv of Ar^F₅CN across the TiN_α bond of <b>26</b>. Reductive N_α–O_β bond cleavage was observed with Ar^F₅CCH, forming Cp*Ti­(O^tBu)­{NC­(Ar^F₅)­C­(H)­N­(ⁱPr)­C­(Ph)­N­(ⁱPr)} (<b>30</b>). Addition of 2 equiv of [Et₃NH]­[BPh₄] to <b>19</b> in THF-<i>d</i>₈ resulted in protonolysis of the amidinate ligand, forming [PhC­(NHⁱPr)₂]­[BPh₄] and the cationic alkoxyimido complex [Cp*Ti­(NO^tBu)­(THF-<i>d</i>₈)₂]⁺. In contrast, reaction with B­(Ar^F₅)₃ resulted in elimination of isobutene and formation of Cp*Ti­{PhC­(NⁱPr)₂}­{η²-ON­(H)­B­(Ar^F₅)₃}

Mechanistic Study of the Selectivity of Olefin versus Cyclobutene Formation by Palladium(0)-Catalyzed Intramolecular C(sp³)–H Activation

This study describes the mechanism and selectivity pattern of the Pd⁰-catalyzed C­(sp³)–H activation of a prototypical substrate bearing two linear alkyl groups. Experimentally, the use of the Pd/P­(<i>t</i>-Bu)₃ catalytic system leads to a ca. 7:3 mixture of olefin and benzocyclobutene (BCB) products. The C–H activation step was computed to be favored for the secondary position α to the benzylic carbon over the primary position β to the benzylic carbon by more than 4 kcal mol^–1, in line with previous selectivity trends on analogous substrates. The five-membered palladacycle obtained through this activation step may then follow two different pathways, which were computationally characterized: (1) decoordination of the protonated base and reductive elimination to give the BCB product and (2) proton transfer to the aryl ligand and base-mediated β-H elimination to give the olefin product. Experiments conducted with deuterated substrates were in accordance with this mechanism. The difference between the highest activation barriers in the two pathways was computed to be 1.2 kcal mol^–1 in favor of BCB formation. However, the use of a kinetic model revealed the critical influence of the kinetics of dissociation of HCO₃^– formed after the C–H activation step in actually directing the reaction toward either of the two pathways

Synthesis and Reactions of a Cyclopentadienyl-Amidinate Titanium <i>tert-</i>Butoxyimido Compound

We report the first detailed reactivity study of a group 4 alkoxyimido complex, namely Cp*Ti­{PhC­(NⁱPr)₂}­(NO^tBu) (<b>19</b>), with heterocumulenes, aldehydes, ketones, organic nitriles, Ar^F₅CCH, and B­(Ar^F₅)₃ (Ar^F₅ = C₆F₅). Compound <b>19</b> was synthesized via imide/alkoxyamine exchange from Cp*Ti­{PhC­(NⁱPr)₂}­(N^tBu) and ^tBuONH₂. Reaction of <b>19</b> with CS₂ and Ar′NCO (Ar′ = 2,6-C₆H₃ⁱPr₂) gave the [2 + 2] cycloaddition products Cp*Ti­{PhC­(NⁱPr)₂}­{SC­(S)­N­(O^tBu)} and Cp*Ti­{PhC­(NⁱPr)₂}­{N­(O^tBu)­C­(NAr′)­O}, respectively, whereas reaction with 2 equiv of TolNCO afforded Cp*Ti­{PhC­(NⁱPr)₂}­{OC­(NTol)­N­(Tol)­C­(NO^tBu)­O} following a sequence of cycloaddition–extrusion and cycloaddition–insertion steps. Net NO^tBu group transfer was observed with both ^tBuNCO and PhC­(O)­R, yielding the oxo-bridged dimer [Cp*Ti­{PhC­(NⁱPr)₂}­(μ-O)]₂ and either the alkoxycarbodiimide ^tBuNCNO^tBu or the oxime ethers PhC­(NO^tBu)­R (R = H (<b>25a</b>), Me (<b>25b</b>), Ph (<b>25c</b>)). DFT studies showed that in the reaction with PhC­(O)­R (R = H, Me) the product distribution between the <i>syn</i> and <i>anti</i> isomers of PhC­(NO^tBu)­R was under kinetic control. Reaction of <b>19</b> with ArCN gave the TiN_α insertion products Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar)­NO^tBu} (Ar = Ph (<b>28</b>), 2,6-C₆H₃F₂ (<b>27</b>), Ar^F₅ (<b>26</b>)) containing <i>tert</i>-butoxybenzimidamide ligands. Reaction of <b>19</b> or <b>26</b> with an excess of Ar^F₅CN gave Cp*Ti­{PhC­(NⁱPr)₂}­{NC­(Ar^F₅)­NC­(Ar^F₅)­N­(C­{Ar^F₅}­NO^tBu)} (<b>29</b>) following net head-to-tail coupling of 2 equiv of Ar^F₅CN across the TiN_α bond of <b>26</b>. Reductive N_α–O_β bond cleavage was observed with Ar^F₅CCH, forming Cp*Ti­(O^tBu)­{NC­(Ar^F₅)­C­(H)­N­(ⁱPr)­C­(Ph)­N­(ⁱPr)} (<b>30</b>). Addition of 2 equiv of [Et₃NH]­[BPh₄] to <b>19</b> in THF-<i>d</i>₈ resulted in protonolysis of the amidinate ligand, forming [PhC­(NHⁱPr)₂]­[BPh₄] and the cationic alkoxyimido complex [Cp*Ti­(NO^tBu)­(THF-<i>d</i>₈)₂]⁺. In contrast, reaction with B­(Ar^F₅)₃ resulted in elimination of isobutene and formation of Cp*Ti­{PhC­(NⁱPr)₂}­{η²-ON­(H)­B­(Ar^F₅)₃}

Efficient Pd⁰‑Catalyzed Asymmetric Activation of Primary and Secondary C–H Bonds Enabled by Modular Binepine Ligands and Carbonate Bases

New binepine ligands have been synthesized, and characterized and have been shown to induce high diastereo- and enantioselectivity in the intramolecular arylation of primary and secondary C­(sp³)–H bonds, giving rise to fused cyclopentanes. The ligands were obtained as bench-stable phosphonium tetrafluoroborate salts that can be directly employed in catalysis. It was shown that a ferrocenyl P-substituent on the ligand allows achievement of high stereoselectivities in combination with potassium carbonate for the arylation of primary C–H bonds under unprecedentedly low temperature (90 °C) and catalyst loading (1–2 mol % Pd/2–3 mol % ligand). Using a base-free precatalyst, carbonate was shown to be the active base and to provide higher stereoselectivities than acetate and pivalate. The more difficult arylation of secondary C–H bonds could also be achieved and required fine-tuning of the ligand structure and the carbonate countercation. This method allowed generation of fused tricyclic products containing three adjacent stereocenters as single diastereoisomers and with moderate to high enantioselectivity. Experimental data indicated that the enantiodetermining C–H activation step involves a monoligated species. DFT (PBE0-D3) calculations were performed with a prototypical binepine ligand to understand the origin of the enantioselectivity. The preference for the major enantiomer was traced to the establishment of a more efficient network of weak attractive interactions between the phosphine ligand and the substrate

Efficient Pd⁰‑Catalyzed Asymmetric Activation of Primary and Secondary C–H Bonds Enabled by Modular Binepine Ligands and Carbonate Bases

New binepine ligands have been synthesized, and characterized and have been shown to induce high diastereo- and enantioselectivity in the intramolecular arylation of primary and secondary C­(sp³)–H bonds, giving rise to fused cyclopentanes. The ligands were obtained as bench-stable phosphonium tetrafluoroborate salts that can be directly employed in catalysis. It was shown that a ferrocenyl P-substituent on the ligand allows achievement of high stereoselectivities in combination with potassium carbonate for the arylation of primary C–H bonds under unprecedentedly low temperature (90 °C) and catalyst loading (1–2 mol % Pd/2–3 mol % ligand). Using a base-free precatalyst, carbonate was shown to be the active base and to provide higher stereoselectivities than acetate and pivalate. The more difficult arylation of secondary C–H bonds could also be achieved and required fine-tuning of the ligand structure and the carbonate countercation. This method allowed generation of fused tricyclic products containing three adjacent stereocenters as single diastereoisomers and with moderate to high enantioselectivity. Experimental data indicated that the enantiodetermining C–H activation step involves a monoligated species. DFT (PBE0-D3) calculations were performed with a prototypical binepine ligand to understand the origin of the enantioselectivity. The preference for the major enantiomer was traced to the establishment of a more efficient network of weak attractive interactions between the phosphine ligand and the substrate

Efficient Pd⁰‑Catalyzed Asymmetric Activation of Primary and Secondary C–H Bonds Enabled by Modular Binepine Ligands and Carbonate Bases

New binepine ligands have been synthesized, and characterized and have been shown to induce high diastereo- and enantioselectivity in the intramolecular arylation of primary and secondary C­(sp³)–H bonds, giving rise to fused cyclopentanes. The ligands were obtained as bench-stable phosphonium tetrafluoroborate salts that can be directly employed in catalysis. It was shown that a ferrocenyl P-substituent on the ligand allows achievement of high stereoselectivities in combination with potassium carbonate for the arylation of primary C–H bonds under unprecedentedly low temperature (90 °C) and catalyst loading (1–2 mol % Pd/2–3 mol % ligand). Using a base-free precatalyst, carbonate was shown to be the active base and to provide higher stereoselectivities than acetate and pivalate. The more difficult arylation of secondary C–H bonds could also be achieved and required fine-tuning of the ligand structure and the carbonate countercation. This method allowed generation of fused tricyclic products containing three adjacent stereocenters as single diastereoisomers and with moderate to high enantioselectivity. Experimental data indicated that the enantiodetermining C–H activation step involves a monoligated species. DFT (PBE0-D3) calculations were performed with a prototypical binepine ligand to understand the origin of the enantioselectivity. The preference for the major enantiomer was traced to the establishment of a more efficient network of weak attractive interactions between the phosphine ligand and the substrate

Linear-Selective Hydroarylation of Unactivated Terminal and Internal Olefins with Trifluoromethyl-Substituted Arenes

We report a series of hydroarylations of unactivated olefins with trifluoromethyl-substituted arenes that occur with high selectivity for the linear product without directing groups on the arene. We also show that hydroarylations occur with <i>internal</i>, acyclic olefins to yield linear alkylarene products. Experimental mechanistic data provide evidence for reversible formation of an alkyl­nickel–aryl intermediate and rate-determining reductive elimination to form the carbon–carbon bond. Labeling studies show that formation of terminal alkylarenes from internal alkenes occurs by initial establishment of an equilibrating mixture of alkene isomers, followed by addition of the arene to the terminal alkene. Computational (DFT) studies imply that the aryl C–H bond transfers to a coordinated alkene without oxidative addition and support the conclusion from experiment that reductive elimination is rate-determining and forms the anti-Markovnikov product. The reactions are inverse order in α-olefin; thus the catalytic reaction occurs, in part, because isomerization creates a low concentration of the reactant α-olefin

Monosubstituted Borane Ruthenium Complexes RuH₂(η²:η²‑H₂BR)(PR′₃)₂: A General Approach to the Geminal Bis(σ-B–H) Coordination Mode

A series of borane bis­(σ-B–H) ruthenium complexes RuH₂(η²:η²-H₂BR)­(PR′₃)₂ (R = alkyl, aryl; R′ = Cy, Cyp, ^<i>i</i>Pr) has been prepared by using two synthetic strategies. The first one is based on a simple substitution reaction by adding the corresponding monosubstituted H₂BR borane to the bis­(dihydrogen) ruthenium complex RuH₂(η²-H₂)₂(PCy₃)₂. The second one, more general, results from the reaction of the chloro complex RuHCl­(H₂)­(PR′₃)₂ (R′ = Cy, Cyp, ^<i>i</i>Pr) with the corresponding lithium monosubstituted borohydrides RBH₃Li (R = Mes, ^<i>t</i>Bu, Me, C₄H₃S, Ph). All the complexes have been characterized by multinuclear NMR, IR, and X-ray diffraction studies. DFT calculations have been used to better define the bonding mode of the borane ligand to the metal center as well as to establish the thermodynamic cycle that delineates the coordination process. The ^<i>t</i>Bu species displays a dynamic behavior evidencing an equilibrium between a borohydride and a σ-borane formulation. The thienyl case illustrates the competition between sulfur coordination and a bis­(σ-B–H) coordination mode