Carbon−Heteroatom and Carbon−Carbon Bond‐Forming Reactions: Special Issue in Honor of the 2019 Wolf Prize Laureates in Chemistry, Professors Stephen L. Buchwald and John F. Hartwig

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Reaction of Diazoalkanes with Iron Phosphine Complexes Affords Novel Phosphazine Complexes

The crystal structures of novel products from the insertion of various diazoalkanes into the iron−phosphorus bond in FeCl_2L_2 (L = phosphine) complexes are presented. Specifically, ethyl diazoacetate and diphenyldiazomethane reacted with FeCl_2(PMe_2Ph)_2 to afford FeCl_2[N(PMe_2Ph)NC(H)CO_2Et]_2 (1) and FeCl_2[N(PMe_2Ph)NCPh_2] (2). Interestingly, ethyl diazoacetate inserted into both iron phosphine bonds of FeCl_2(d^ippe)_2 to afford the seven-membered metallacycle FeCl_2{N[NC(H)CO_2Et]P^iPr)_2CH_2CH_2P(^iPPr)_2N[NC(H)CO_2Et]}_2 (3)

Highly active iron imidazolylidene catalysts for atom transfer radical polymerization

Iron(II) halides possessing highly donating imidazolylidene ligands were found to be remarkably active and efficient catalysts for the atom transfer radical polymerization of styrene and methyl methacrylate

Metathesis of Electron-Rich Olefins: Structure and Reactivity of Electron-Rich Carbene Complexes

The addition of excess H_2C C(H)ER to (Pcy_3)_2Cl_2Ru C(H)R (1a,b) afforded a series of well-defined ruthenium carbene complexes, (Pcy_3)_2Cl_2Ru C(H)ER (ER = OEt (5), SEt (6), SPh (7), N(carbazole) (8), N(pyrrolidinone) (9)) in yields ranging from 66 to 90%. Such complexes containing an electron-donating group on the carbene carbon are often referred to as Fischer-type carbenes. Replacement of one phosphine ligand with 1,3-dimesitylimidazolylidene (IMes) afforded the respective mixed-ligand complexes (IMes)(Pcy_3)Cl_2Ru C(H)ER (11−14) in 48−89% yield. Alternatively, addition of H_2C C(H)OEt to (H_2IMes)(PCy_3)Cl_2Ru C(H)Ph (3a; H_2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) afforded (H_2IMes)(PCy_3)Cl_2Ru C(H)OEt (15) in 93% yield. The crystal structures of complexes 5, 7−9, and 11 were determined and found to be structurally similar to the parent ruthenium alkylidene ([Ru] C(H)R) complexes. In solution, the chemical shift of the [Ru] C(H)ER resonance in ^1H NMR spectra was found to be inversely related to the electronegativity of the α-heteratom; however, no trends were evident in the ^(31)P or ^(13)C NMR spectra. Intramolecular coordination of the pendant amide carbonyl group to the Ru center established a temperature-dependent equilibrium between complexes 9 and 14 and their cyclometalated forms. All Ru electron-rich complexes initiated the ring-opening metathesis polymerization (ROMP) of strained cyclic olefins and the ring-closing metathesis (RCM) of diethyl diallylmalonate. A general trend in the relative reactivities and thermal stabilities of the (PCy_3)_2Cl_2Ru C(H)ER complexes followed the order C > N > S > O. In addition, complexes coordinated with an N-heterocyclic carbene ligand (e.g., (IMes)(PCy_3)Cl_2Ru C(H)ER) displayed enhanced activities in olefin metathesis and were thermally more stable than their bis(phosphine) analogues. Finally, the thermal decomposition product of (PCy_3)_2Cl_2Ru C(H)OEt was isolated and determined by X-ray analysis to be (PCy_3)_2ClRu(H)CO (10)

Harnessing the Chemistry of CO2

Our research program is broadly focused on activating CO{sub 2} through the use of organic and organometallic based catalysts. Some of our methods have centered on annulation reactions of unsaturated hydrocarbons (and carbonyl substrates) to provide a diverse array of carbocycles and heterocycles. We use a combination of catalyst discovery and optimization in conjunction with classical physical organic chemistry to elucidate the key mechanistic features of the cycloaddition reactions such that the next big advances in catalyst development can be made. Key to all of our cycloaddition reactions is the use of a sterically hindered, electron donating N heterocyclic carbene (NHC) ligand, namely IPr (or SIPr), in conjunction with a low valent nickel pre-catalyst. The efficacy of this ligand is two-fold: (1) the high {delta}-donating ability of the NHC increases the nucleophilicity of the metal center which thereby facilitates interaction with the electrophilic carbonyl and (2) the steric hindrance prevents an otherwise competitive side reaction involving only the alkyne substrate. Such a system has allowed for the facile cycloaddition to prepare highly functionalized pyrones, pyridones, pyrans, as well as novel carbocycles. Importantly, all reactions proceed under extremely mild conditions (room temperature, atmospheric pressures, and short reaction times), require only catalytic amounts of Ni/NHC and readily available starting materials, and afford annulated products in excellent yields. Our current focus revolves around understanding the fundamental processes that govern these cycloadditions such that the next big advance in the cyclization chemistry of CO{sub 2} can be made. Concurrent to our annulation chemistry is our investigation of the potential for imidazolylidenes to function as thermally-actuated CO{sub 2} sequestering and delivery agents

Highly Active Metathesis Catalysts Generated In Situ from Inexpensive and Air-Stable Precursors

The preparation of well‐defined ruthenium alkylidene complexes bearing N‐heterocyclic carbene ligands such as 1,3‐dimesitylimidazol‐2‐ylidene (1) and 4,5‐dihydroimidazol‐2‐ylidene (2) have led to catalysts (such as 3 and 4; see Figure 1) which are highly active in ring‐closing metathesis (RCM), cross metathesis (CM), and ring‐opening metathesis polymerization (ROMP).1 These catalysts show increased thermal stability and similar tolerance to oxygen and moisture when compared to their parent bisphosphane complexes, [(PCy_3)_2Cl_2Ru=CHPh] (5, Figure 1).2 Since all synthetic routes to catalysts 3 and 4 proceed through the transformation of a ruthenium bisphosphane carbene,3 a direct route through readily available starting materials is still desirable

Highly Active Metathesis Catalysts Generated In Situ from Inexpensive and Air-Stable Precursors

The preparation of well‐defined ruthenium alkylidene complexes bearing N‐heterocyclic carbene ligands such as 1,3‐dimesitylimidazol‐2‐ylidene (1) and 4,5‐dihydroimidazol‐2‐ylidene (2) have led to catalysts (such as 3 and 4; see Figure 1) which are highly active in ring‐closing metathesis (RCM), cross metathesis (CM), and ring‐opening metathesis polymerization (ROMP).1 These catalysts show increased thermal stability and similar tolerance to oxygen and moisture when compared to their parent bisphosphane complexes, [(PCy_3)_2Cl_2Ru=CHPh] (5, Figure 1).2 Since all synthetic routes to catalysts 3 and 4 proceed through the transformation of a ruthenium bisphosphane carbene,3 a direct route through readily available starting materials is still desirable

Carbon−Heteroatom and Carbon−Carbon Bond‐Forming Reactions

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154904/1/ijch202000013_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154904/2/ijch202000013.pd