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Ligand-Based Carbon-Nitrogen Bond Forming Reactions of Metal

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Ligand-Based Carbon-Nitrogen Bond Forming Reactions of Metal Ligand-Based CarbonÀNitrogen Bond Forming Reactions of Metal Dinitrosyl Complexes with Alkenes and Their Application to CÀH Bond Functionalization CHEN ZHAO, MARK R. CRIMMIN, F. DEAN TOSTE,* AND ROBERT G. BERGMAN* Department of Chemistry, University of California, Berkeley, Berkeley, California, 94720-1460, United States RECEIVED ON JULY 29, 2013 CONSPECTUS ver the past few decades, researchers have made substan- O tial progress in the development of transition metal com- plexes that activate and functionalize CÀH bonds. For the most part, chemists have focused on aliphatic and aromatic CÀH bonds and have put less effort into complexes that activate and function- alize vinylic CÀH bonds. Our groups have recently developed a novel method to functionalize vinylic CÀH bonds that takes advantage of the unique ligand-based reactivity of a rare class of metal dinitrosyl complexes. In this Account, we compare and discuss the chemistry of cobalt and ruthenium dinitrosyl complexes, emphasizing alkene binding, CÀH functionalization, and catalysis. Initially discovered in the early 1970s by Brunner and studied more extensively in the 1980s by the Bergman group, the cyclopentadienylcobalt dinitrosyl complex CpCo(NO)2 reacts rever- sibly with alkenes to give, in many cases, stable and isolable cobalt dinitrosoalkane complexes. More recently, we found that treatment with strong bases, such as lithium hexamethyldisilazide, Verkade's base, and phosphazene bases, deprotonates these complexes and renders them nucleophilic at the carbon r to the nitroso group. This conjugate anion of metal dinitrosoalkanes can participate in conjugate addition to Michael acceptors to form new carbonÀcarbon bonds. These functionalized cobalt complexes can further react through alkene exchange to furnish the overall vinylic CÀH functionalized organic product. This stepwise sequence of alkene binding, functionalization, and retrocycloaddition represents an overall vinylic CÀH functionalization reaction of simple alkenes and does not require directing groups. We have also developedanasymmetricvariantofthisreaction sequence and have used this method to synthesize C1-andC2-symmetric diene ligands with high enantioinduction. Building upon these stepwise reactions, we eventually developed a simple one-pot procedure that uses stoichiometric amounts of a cobalt dinitrosoalkane complex for both inter- and intramolecular CÀH functionalization. We can achieve catalysis in one-pot intramolecular reactions with a limited range of substrates. Our groups have also reported an analogous ruthenium dinitrosyl complex. In analogy to the cobalt complex, this ruthenium complex reacts with alkenes in the presence of neutral bidentate ligands, such as TMEDA, to give octahedral dinitrosoalkane complexes. Intramolecular functionalization or cyclization of numerous ruthenium dinitrosoalkane complexes proceeds under mild reaction conditions to give the functionalized organic products in excellent yields. However, despite extensive efforts, so far we have not been able to carry out intermolecular reactions of these complexes with a variety of electrophiles or CÀH functionalization reactions. Although additional work is necessary to further boost the catalytic capabilities of both cobalt and ruthenium dinitrosyl complexes for vinylic CÀH functionalization of simple alkenes, we believe this ligand-based vinylic CÀH functionalization reaction has provided chemists with a useful set of tools for organic synthesis. www.pubs.acs.org/accounts Vol. XXX, No. XX ’ XXXX ’ 000–000 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ A 10.1021/ar400176x & XXXX American Chemical Society Ligand-Based CarbonÀNitrogen Bond Forming Reactions Zhao et al. Introduction deprotonated, and the resulting carbanion may act as a nucleo- In recent years, the field of transition metal-catalyzed CÀH phile toward Michael acceptors (and other electrophiles). The À bond activation has undergone intensive development.1 10 net reaction is a direct vinylic CÀH functionalization of alkenes The ability to selectively activate and functionalize inert CÀH in the absence of a directing group. bonds has provided powerful new methods for the synthesis In this Account, we present a comprehensive comparison À of complex molecules.11 14 Of paramount importance to and discussion of the chemistry of cobalt and ruthenium À this synthetic revolution has been the work of a number of dinitrosyl complexes and their suitability for the C H func- groups to elucidate the intimate mechanisms of catalysts tionalization of alkenes. À capable of CÀH activation and functionalization.15 23 Metal Dinitrosyl Complexes Despite the fast growth of this field, the activation of À À Cobalt Dinitrosyls. Initially reported in the early vinylic C H bonds to generate metal vinyl complexes À 1970s,39 41 the cyclopentadienylcobalt dinitrosyl complex directly from the corresponding alkene is underexplored. [CpCo(NO)2](1) is a reactive intermediate that has been In 1995, the Trost and Murai groups independently reported À studied by in situ spectroscopic techniques42 44 and compu- the ruthenium catalyzed alkylation of enones and enals via a tational chemistry.45 As depicted in Scheme 2, four different directed vinylic CÀH bond activation.24,25 More recently, the synthetic methods have been developed for the generation vinylic CÀH activation of alkenes conjugated with imine and of [CpCo(NO)2](1). These include (i) the reaction of CpCo(CO)2 ketoxime directing groups catalyzed by Rh(I) has been reported 39À41 26À30 with nitric oxide (NO) gas, (ii) the reversible reaction of by the Bergman and Ellman groups and the Cheng À the cobalt dimer 2 with NO,42 44 (iii) the salt metathesis of 31 2 À group. Nevertheless, methods to activate sp C Hbondsof κ2 À [( -TMEDA)Co(NO)2][BPh4](3) with group 1 cyclopentadienyl alkenes in the absence of a directing group are sparse,32 34 salts,46 and (iv) the thermal or UV-light promoted retrocycload- possible due to the high concentration of olefin present, espe- dition of alkenes from cobalt dinitrosoalkane complexes. cially under catalytic conditions, which favors π-coordination Although it has been detected spectroscopically, attempts À of the olefin over oxidative addition of the C H bond. to isolate the reactive cobalt dinitrosyl complex by tuning both the electronic and steric properties of the Cp ligand À À SCHEME 1. Metal Dinitrosyl-Mediated Direct Vinylic C H Bond have to date proved unsuccessful.39 41,45,46 In the absence Functionalization of a suitable alkene trap, [CpCo(NO)2] decomposes to a complex mixture including 2. In related studies, we have shown that replacing the cyclopentadienyl ligand with the tris(pyrazolyl)borate, Tp*, allowed the isolation of the mono- nitrosyl complex [Tp*CoNO].45,46 In line with expectations furnished by the Cp system, this paramagnetic species gave an intermediate believed to be Tp*Co(NO)2 upon exposure to an atmosphere of NO. Although attempts to isolate the cobalt dinitrosyl again failed, Tp*Co(NO)2 could be trapped with strained alkenes such as norbornadiene to yield the corresponding metal dinitrosoalkane complexes. SCHEME 2. Synthetic Entry Points to [CpCo(NO)2] In 2009, in a joint study, our groups documented a novel approach for the direct activation and functionalization of vinylic CÀH bonds using the ligand-based reactivity of a metal À dinitrosyl complex.35 38 The reaction proceeds via reversible alkene binding to the nitrosyl ligands of the complex to form a metal dinitrosoalkane complex in which the sp3 CÀH bonds proximal to the metal nitroso moiety are acidified (Scheme 1). Under basic conditions, one of these CÀHbondsmaybe B ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 000–000 ’ XXXX ’ Vol. XXX, No. XX Ligand-Based CarbonÀNitrogen Bond Forming Reactions Zhao et al. À Ruthenium Dinitrosyl. In 2011, we reported the synthe- (Scheme 4).35 44 The binding event represents a rare example sis of the ruthenium dinitrosyl complex 4 from the reaction of a reaction of a metal dinitrosyl complex with an alkene to of NO and dichloro-(p-cymene)ruthenium(II) dimer in 88% directly form two new carbonÀnitrogen bonds. In general, yield (Scheme 3).47 Unlike 1, ruthenium complex 4 is an electron-rich and strained alkenes bind to 1 much more isolable compound. A single-crystal X-ray diffraction study readily than electron-poor and unstrained alkenes. Under demonstrated that 4 is a five-coordinate complex possess- the conditions we have examined, other unsaturated func- ing a linear nitrosyl ligand in the basal plane of square- tional groups, such as alkynes, allenes, carbonyls, and R,β- based pyramidal geometry and a bent nitrosyl occupying unsaturated enones and ynones, do not react with 1 to form the apical position. Hence complex 4 may be described as a isolable dinitrosoalkane complexes. neutral, 16-electron complex with a vacant coordination site Mechanistic studies demonstrated that alkene binding to opposite the apical nitrosyl. A related ruthenium complex, 1 is reversible and stereospecific (Scheme 5), allowing olefin [RuCl(NO)2(PPh3)2]PF6, bearing both a linear and bent NO exchange of one cobalt dinitrosoalkane complex with an À ligand, was reported by Eisenberg and co-workers in 1970.48 50 external alkene to form a different complex under either À thermal or photochemical conditions.42 44 SCHEME 3. Synthesis of Ruthenium Dinitrosyl Complex 4 SCHEME 5. Reversible and Stereospecific Alkene Binding to Cobalt Complex 1 Alkene Binding and Exchange Reactions of Metal Dinitrosyl Complexes Reactions of Alkenes with [Cp2Co(NO)2]
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