Combining Transition Metal Catalysis with Radical Chemistry: Dramatic Acceleration of Palladium‐Catalyzed CH Arylation with Diaryliodonium Salts

This paper describes a photoredox palladium/iridium‐catalyzed CH arylation with diaryliodonium reagents. Details of the reaction optimization, substrate scope, and mechanism are presented along with a comparison to a related method in which aryldiazonium salts are used in place of diaryliodonium reagents. The unprecedentedly mild reaction conditions (25 °C in methanol), the requirement for light and a photocatalyst, the inhibitory effect of radical scavengers, and the observed chemoselectivity trends are all consistent with a radical‐mediated mechanism for this transformation. This stands in contrast to the analogous thermal reaction with diaryliodonium reagents that is believed to proceed via an ‘ionic’ 2 e − pathway and requires a much higher reaction temperature (100 °C).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95414/1/adsc_201200738_sm_miscellaneous_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/95414/2/3517_ftp.pd

Cyclopropenium Salts as Cyclable, High‐Potential Catholytes in Nonaqueous Media

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136488/1/aenm201602027-sup-0001-S1.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136488/2/aenm201602027.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136488/3/aenm201602027_am.pd

New Insights into the Mechanism of Ruthenium-Catalyzed Olefin Metathesis Reactions

Over the past two decades, olefin metathesis has emerged as a mild and efficient method for the formation of carbon−carbon double bonds. In particular, (PCy_3)_2(Cl)_2RuCHPh (1)^2 has found extensive use in organic and polymer chemistry due to its high reactivity with olefins in the presence of a diverse array of functional groups. Recently, a new family of ruthenium-based olefin metathesis catalysts have been prepared by the substitution of a single PCy_3 ligand of 1 with an N-heterocyclic carbene. These new alkylidenes, particularly [Figure 1], exhibit dramatically increased activity over the parent system in ring-opening metathesis polymerization, ring-closing metathesis,4a and cross metathesis reactions. The mechanism of olefin metathesis reactions catalyzed by 1 has received intense investigation in our group and others and early studies established that phosphine dissociation is a crucial step along the reaction coordinate. As such, it has been suggested that the high activity of 2 and its analogues is due to their increased ability to promote this critical phosphine dissociation step. We report herein a detailed mechanistic study of phosphine exchange and initiation kinetics in alkylidenes 1 and 2. This study provides new and surprising evidence concerning the origin of the large activity differences between these two catalysts

Synthesis and Reactivity of Neutral and Cationic Ruthenium(II) Tris(pyrazolyl)borate Alkylidenes

A series of neutral and cationic ruthenium(II) alkylidenes containing the hydrotris(pyrazolyl)borate (Tp) ligand have been prepared. The complex Tp(PCy_3)(Cl)Ru=CHPh (2) was obtained by the reaction of (PCy_3)_2(Cl)_2Ru=CHPh (1) and KTp. Treatment of 2 with AgBF_4 or AgSbF_6 in the presence of a variety of coordinating solvents afforded [Tp(PCy_3)(L)Ru=CHPh]^+ (L = H_2O, CH_3CN, pyridine) in high yield. The dynamic NMR behavior of these new complexes is discussed, and the X-ray crystal structure of [Tp(PCy_3)(H_2O)Ru=CHPh]BF_4 (3) is reported. Alkylidenes 2−5 alone do not catalyze olefin metathesis reactions. However, complex 2 is activated for ring-closing metathesis by the addition of HCl, CuCl, and AlCl_3

Deoxyfluorination of (Hetero)aryl Aldehydes Using Tetramethylammonium Fluoride and Perfluorobutanesulfonyl Fluoride or Trifluoromethanesulfonic Anhydride

This Communication describes the conversion of (hetero)aryl aldehydes into the corresponding (hetero)aryl difluoromethyl products using anhydrous NMe4F in combination with perfluorobutanesulfonyl fluoride or trifluoromethanesulfonic anhydride.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154980/1/ijch201900066-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154980/2/ijch201900066_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154980/3/ijch201900066.pd

Synthesis, Structure, and Activity of Enhanced Initiators for Olefin Metathesis

A series of ruthenium olefin metathesis catalysts of the general structure (H_2IMes)(PR_3)(Cl)_2Ru CHPh (H_2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) have been prepared; these complexes are readily accessible in two steps from commercially available (H_2IMes)(PCy_3)(Cl)_2Ru CHPh. Their phosphine dissociation rate constants (k_1), relative rates of phosphine reassociation, and relative reaction rates in ring-opening metathesis polymerization (ROMP) and ring-closing metathesis (RCM) have been investigated. The rates of phosphine dissociation (initiation) from these complexes increase with decreasing phosphine donor strength. Complexes containing a triarylphosphine exhibit dramatically improved initiation relative to (H_2IMes)(PCy_3)(Cl)_2Ru CHPh. Conversely, phosphine reassociation shows no direct correlation with phosphine electronics. In general, increased phosphine dissociation leads to faster olefin metathesis reaction rates, which is of direct significance to both organic and polymer metathesis processes

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp3)−C and C(sp3)−O bond‐forming reactions at high‐valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral NiIV complex ligated by tris(pyrazolyl)borate and a cationic octahedral NiIII complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp3)−C(sp2) bond‐forming reductive elimination occurs from both centers, but the NiIII complex reacts up to 300‐fold faster than the NiIV, depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO−), the NiIV complex exclusively undergoes C(sp3)−OAc bond formation, while the NiIII analogue forms the C(sp3)−C(sp2) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M−C(sp3) bonds, and thus their relative reactivity towards outer‐sphere SN2‐type bond‐forming reactions.The high point: This report describes a systematic comparison of factors impacting the relative rates and selectivities of C(sp3)−C and C(sp3)−O bond‐forming reactions at high‐valent Ni centers as a function of oxidation state (NiIII versus NiIV). Two Ni complexes are compared: a cationic octahedral NiIV complex ligated by tris(pyrazolyl)borate and a cationic octahedral NiIII complex ligated by tris(pyrazolyl)methane.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/150547/1/anie201903638.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150547/2/anie201903638-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/150547/3/anie201903638_am.pd

Realization of an Asymmetric Non‐Aqueous Redox Flow Battery through Molecular Design to Minimize Active Species Crossover and Decomposition

This communication presents a mechanism‐based approach to identify organic electrolytes for non‐aqueous redox flow batteries (RFBs). Symmetrical flow cell cycling of a pyridinium anolyte and a cyclopropenium catholyte resulted in extensive capacity fade due to competing decomposition of the pyridinium species. Characterization of this decomposition pathway enabled the rational design of next‐generation anolyte/catholyte pairs with dramatically enhanced cycling performance. Three factors were identified as critical for slowing capacity fade: (1) separating the anolyte–catholyte in an asymmetric flow cell using an anion exchange membrane (AEM); (2) moving from monomeric to oligomeric electrolytes to limit crossover through the AEM; and (3) removing the basic carbonyl moiety from the anolyte to slow the protonation‐induced decomposition pathway. Ultimately, these modifications led to a novel anolyte–catholyte pair that can be cycled in an AEM‐separated asymmetric RFB for 96 h with >95 % capacity retention at an open circuit voltage of 1.57 V.Applied molecular design! This study presents a mechanism‐based approach to the molecular design of electrolytes for implementation in an asymmetric non‐aqueous redox flow battery.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/1/chem202000749-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/2/chem202000749.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154972/3/chem202000749_am.pd

Synthesis and Reactivity of Neutral and Cationic Ruthenium(II) Tris(pyrazolyl)borate Alkylidenes

A series of neutral and cationic ruthenium(II) alkylidenes containing the hydrotris(pyrazolyl)borate (Tp) ligand have been prepared. The complex Tp(PCy_3)(Cl)Ru=CHPh (2) was obtained by the reaction of (PCy_3)_2(Cl)_2Ru=CHPh (1) and KTp. Treatment of 2 with AgBF_4 or AgSbF_6 in the presence of a variety of coordinating solvents afforded [Tp(PCy_3)(L)Ru=CHPh]^+ (L = H_2O, CH_3CN, pyridine) in high yield. The dynamic NMR behavior of these new complexes is discussed, and the X-ray crystal structure of [Tp(PCy_3)(H_2O)Ru=CHPh]BF_4 (3) is reported. Alkylidenes 2−5 alone do not catalyze olefin metathesis reactions. However, complex 2 is activated for ring-closing metathesis by the addition of HCl, CuCl, and AlCl_3

A Versatile Precursor for the Synthesis of New Ruthenium Olefin Metathesis Catalysts

The ruthenium complex (IMesH_2)(Cl)_2(C_5H_5N)_2Ru═CHPh [IMesH_2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene] (3) was prepared by the reaction of (IMesH_2)(PCy_3)(Cl)_2Ru═CHPh (2) with an excess of pyridine. Complex 3 contains substitutionally labile pyridine and chloride ligands and serves as a versatile starting material for the synthesis of new ruthenium benzylidenes