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Enantioenriched N-alkyl-imidazole-substituted pyridine dicarbene iron dialkyl complexes have been synthesized and characterized by 1H NMR and zero-field 57Fe Mossbauer spectroscopies as well as sin...

Summary Synthetic and mechanistic investigations into the C(sp2)-H borylation of various electronically diverse arenes catalyzed by bis(phosphine)pyridine (iPrPNP) cobalt complexes are reported. Borylation of various benzoate esters and arylboronate esters gave remarkably high selectivities for the position para to the functional group; in both cases, this regioselectivity was found to override the ortho-to-fluorine regioselectivity, previously reported for (iPrPNP)Co borylation catalysts, which arises from thermodynamic control of C(sp2)-H oxidative addition. Mechanistic studies support pathways that result in para-to-ester and para-to-boronate ester selectivity by kinetic control of B-H and C(sp2-H) oxidative addition, respectively. Borylation of a particularly electron-deficient fluorinated arylboronate ester resulted in acceleration of C(sp2)-H oxidative addition and concomitant inversion of regioselectivity, demonstrating that subtle changes in the relative rates of individual steps of the catalytic cycle can enable unique and switchable site selectivities.

Well-defined bis(silylene)pyridine cobalt(III) precatalysts for C(sp2)–H borylation have been synthesized and applied to the investigation of the mechanism of the catalytic borylation of furans and...

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2+2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners has established that this step occurs without redox involvement of the pyridine(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C(sp(3))–C(sp(3)) reductive coupling. A series of labeling experiments with pyridine(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C(sp(3))–C(sp(3)) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure-activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as to promote chemical recycling of cycloaddition polymers.

The mechanism of C(sp2)-H borylation of fluorinated arenes with B2Pin2 (Pin = pinacolato) catalyzed by bis(phosphino)pyridine (iPrPNP) cobalt complexes was studied to understand the origins of the uniquely high ortho-to-fluorine regioselectivity observed in these reactions. Variable time normalization analysis (VTNA) of reaction time courses and deuterium kinetic isotope effect measurements established a kinetic regime wherein C(sp2)-H oxidative addition is fast and reversible. Monitoring the reaction by in situ NMR spectroscopy revealed the intermediacy of a cobalt(I)-aryl complex that was generated with the same high ortho-to-fluorine regioselectivity associated with the overall catalytic transformation. Deuterium labeling experiments and stoichiometric studies established C(sp2)-H oxidative addition of the fluorinated arene as the selectivity-determining step of the reaction. This step favors the formation of ortho-fluoroaryl cobalt intermediates due to the ortho fluorine effect, a phenomenon whereby ortho fluorine substituents stabilize transition metal-carbon bonds. Computational studies provided evidence that the cobalt-carbon bonds of the relevant intermediates in (iPrPNP)Co-catalyzed borylation are strengthened with increasing ortho fluorine substitution. The atypical kinetic regime involving fast and reversible C(sp2)-H oxidative addition in combination with the thermodynamic preference for forming cobalt-aryl bonds adjacent to fluorinated sites are the origin of the high regioselectivity in the catalytic borylation reaction.

Intermediates relevant to cobalt-catalyzed alkene hydroformylation have been isolated and evaluated in fundamental organometallic transformations relevant to aldehyde formation. The 18-electron (R,R)-(iPr DuPhos)Co(CO)2 H has been structurally characterized, and it promotes exclusive hydrogenation of styrene in the presence of 50 bar of H2 /CO gas (1:1) at 100 °C. Deuterium-labeling studies established reversible 2,1-insertion of styrene into the Co-D bond of (R,R)-(iPr DuPhos)Co(CO)2 D. Whereas rapid β-hydrogen elimination from cobalt alkyls occurred under an N2 atmosphere, alkylation of (R,R)-(iPr DuPhos)Co(CO)2 Cl in the presence of CO enabled the interception of (R,R)-(iPr DuPhos)Co(CO)2 C(O)CH2 CH2 Ph, which upon hydrogenolysis under 4 atm H2 produced the corresponding aldehyde and cobalt hydride, demonstrating the feasibility of elementary steps in hydroformylation. Both the hydride and chloride derivatives, (X=H- , Cl- ), underwent exchange with free 13 CO. Under reduced pressure, (R,R)-(iPr DuPhos)Co(CO)2 Cl underwent CO dissociation to form (R,R)-(iPr DuPhos)Co(CO)Cl.

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH(2)SiMe(3))(2), have been developed. The first route consists of addition of two equivalents of LiCH(2)SiMe(3) to the iron dihalide complex, (CNC)FeBr(2), while the second relies on addition of the free CNC ligand to readily-prepared (py)(2)Fe(CH(2)SiMe(3))(2) (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type ((Ar)CNC)Fe(CH(2)SiMe(3))(2)(N(2)) ((Ar)CNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, ((tBu)ACNC)Fe(CH(2)SiMe(3))(2) ((tBu)ACNC = 2,6-((t)Bu-imidazol-2-ylidene)(2)pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the ((Ar)CNC)Fe(CH(2)SiMe(3))(2)(N(2)) derivatives to an H(2) atmosphere resulted in formation of the corresponding iron hydride complexes ((Ar)CNC)FeH(4). These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H(2), generating isotopologues and isotopomers of ((Ar)CNC)Fe(H(n))(D(4-n)) (n = 0–4). When (3,5-Me(2)(Mes)CNC)Fe(CH(2)SiMe(3))(2)(N(2)) (3,5-Me(2)(Mes)CNC = 2,6-(2,4,6-Me(3)-C6H2-imidazol-2-ylidene)(2)-3,5-Me(2)-pyridine) was treated successively with H(2) and then N(2), the corresponding reduced dinitrogen complex (3,5-Me(2)(Mes)CNC)Fe(N(2))(2) was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me(2)(Mes)CNC)Fe(CH(2)SiMe(3))(2)(N(2)).