Communication pubs.acs.org/JACS
Catalytic Carbonylative Rearrangement of Norbornadiene via Dinuclear Carbon−Carbon Oxidative Addition Douglas R. Hartline, Matthias Zeller, and Christopher Uyeda* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
stable η3-allyl complexes11 might be used to drive more challenging oxidative addition reactions that would be otherwise thermodynamically inaccessible. Here, we describe a well-defined and kinetically facile dinuclear C−C activation of norbornadiene (Figure 1). This reactivity provides the basis for a catalytic carbonylative transformation that rearranges the bridged norbornadiene framework into a fused bicyclo[3.3.0] product.
ABSTRACT: Single bonds between carbon atoms are inherently challenging to activate using transition metals; however, ring-strain release can provide the necessary thermodynamic driving force to make such processes favorable. In this report, we describe a strain-induced C−C oxidative addition of norbornadiene. The reaction is mediated by a dinuclear Ni complex, which also serves as a catalyst for the carbonylative rearrangement of norbornadiene to form a bicyclo[3.3.0] product. arbon−carbon σ-bonds make up the framework of all organic compounds yet are among the most challenging bonds to activate in transition metal catalysis.1 Nevertheless, reactions that cleave C−C bonds hold significant synthetic value, allowing abundant hydrocarbon feedstocks to be converted into products of greater complexity through skeletal rearrangements and/or the introduction of functional groups. In 1955, Tipper described a reaction between H2PtCl6 and cyclopropane, 2 later determined by Chatt to yield a platinacyclobutane,3 the product of C−C oxidative addition. This discovery has since inspired a wealth of catalytic processes that feature strain-induced activations of cyclopropanes and, to a lesser extent, cyclobutanes.4 The strain energy of norbornadiene exceeds that of cyclopropane,5 suggesting that small bicyclodienes may be similarly predisposed to undergoing ring-opening (Figure 1). Despite the apparent thermodynamic driving force, transition metal-mediated C−C activation reactions of norbornadiene are rare and generally stoichiometric in metal. 6,7 Indeed, norbornadiene is commonly employed in catalysis as an inert supporting ligand due to its propensity to form exceptionally stable π-interactions.8 Because much of the strain energy of norbornadiene is derived from the distorted alkenes (norbornane, its saturated counterpart, has a strain energy of only 14.4 kcal/mol),5b the net effect of binding to a strongly backbonding metal is to deactivate it toward ring-opening. Our group is interested in exploiting the unique electronic environment of metal−metal bonds to promote unconventional redox reactions not typically observed using mononuclear systems.9 In this context, we recently reported a low-valent [NDI]Ni2 complex (NDI = naphthyridine−diimine), which effects the rapid oxidative addition of allyl electrophiles through a cooperative dinuclear mechanism, whereby C−X bond cleavage occurs at one metal while the other forms a πinteraction with the incipient allyl system.10 This observation led us to question whether the ability of the Ni2 site to form
C
© 2017 American Chemical Society
Figure 1. Transition-metal catalyzed ring-opening reactions of monocyclic and bicyclic strained rings.
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STRAIN-DRIVEN DINUCLEAR C−C OXIDATIVE ADDITION The [i‑PrNDI]Ni2(C6H6) complex 1 is unreactive toward monoalkenes (e.g., ethylene, styrene, 1-octene, or norbornene) but undergoes a thermodynamically favorable ligand substitution with 1,4-cyclohexadiene (Keq = 120 at 22 °C) to generate the brown diamagnetic μ-η2:η2-(chd) adduct 2 (Figure 2). The affinity of the [NDI]Ni2 platform for 1,4-dienes can be rationalized on the basis of dinuclear orbital symmetry Received: August 15, 2017 Published: September 17, 2017 13672
DOI: 10.1021/jacs.7b08691 J. Am. Chem. Soc. 2017, 139, 13672−13675
Communication
Journal of the American Chemical Society
conversion within 30 min at room temperature and proceeds cleanly from 1 to 3 without the buildup of an observable intermediate π-complex. Complex 3 possesses an effective magnetic moment of 0.75 μB (Evans method, 295 K), which is intermediate between the expected values for a singlet and a triplet state. The 1H NMR resonances display well-resolved splitting patterns but span a −2.6 to 18.0 ppm range at 295 K, becoming increasingly paramagnetically shifted at higher temperatures. Together, these data suggest that 3 is diamagnetic at the low-temperature limit but is capable of thermally populating a low-lying triplet state.13 A singlet−triplet gap of 4.4 kcal/mol was estimated by modeling the 1H NMR chemical shift data as a function of temperature (191 to 320 K). XRD analysis of crystalline 3 reveals cleavage of the C(sp3)− C(sp2) bond, with the C1 and C6 atoms now separated by 2.876(5) Å (see Figure 2c for the atom numbering scheme). Akin to the previously reported [i‑PrNDI]Ni2(allyl)Cl complex,10 the C1−C2−C3 allylic system is symmetrically coordinated across the Ni1−Ni2 bond. Additionally, the pendant Ni1-bound alkenyl group is engaged in a secondary π-interaction with Ni2. Given the relatively low barrier for the formation of 3, we next examined whether the C−C oxidative addition is sufficiently close to thermoneutral to be reversible. In our previous studies, we identified alkynes as strongly coordinating ligands that could potentially displace norbornadiene following C−C reductive elimination (Figure 2a). Accordingly, addition of 1-phenylpropyne to solutions of 3 resulted in the release of free norbornadiene and the formation of the previously characterized [i‑PrNDI]Ni2(PhCCMe) complex 4.14 With an equimolar stoichiometry of 1-phenylpropyne and 3, the reaction does not reach full conversion but establishes a stable equilibrium after approximately 5 min (Keq = 2.1). By DFT, the oxidative addition of norbornadiene to form 3 is exothermic by 14.8 kcal/mol and has an activation barrier of only 10.6 kcal/mol, consistent with the fast reaction rate observed experimentally (Figure 3b). The hypothetical C−C oxidative addition of 1,4-cyclohexadiene, by contrast, is unfavorable by 17.6 kcal/mol relative to the characterized adduct 2. The difference in the thermodynamics of these two
Figure 2. (a) Strain-induced, reversible oxidative addition of norbornadiene. (b) Solid-state structure of 2; Ni−Ni: 2.4785(7) Å. (c) Solid-state structure of 3; Ni−Ni: 2.4892(6) Å.
properties.12 In the d9−d9 configuration, the Ni−Ni(σ*) combination is unfilled and capable of accepting electron density from one of the π-bonding orbitals of the diene (b2 in C2v symmetry). This ligand-to-metal donation is supplemented by back-bonding from the Ni−Ni(δ) and Ni−Ni(δ*) combinations into the two diene π* orbitals (Figure 3a). Norbornadiene possesses a high degree of ring strain due to the geometric constraint imposed by the bridging methylene group.5b Rather than forming a stable diene adduct, norbornadiene (1.0 equiv) undergoes a rapid oxidative addition reaction with the [i‑PrNDI]Ni2(C6H6) complex 1 to generate a green C1-symmetric product (3). The reaction reaches full
Figure 3. DFT models (BP86/6-311G(d,p)) for 1,4-diene complexation and C−C oxidative addition. (a) Orbital interaction diagram describing the bonding between the [i‑PrNDI]Ni2 and 1,4-cyclohexadiene fragments of 2 (symmetry labels in C2v). (b) Energetics of the norbornadiene oxidative addition (singlet surface: black; triplet surface: blue) and a hypothetical reaction of 1,4-cyclohexadiene (singlet surface: gray). Energies are relative to the diene adducts in their singlet state. 13673
DOI: 10.1021/jacs.7b08691 J. Am. Chem. Soc. 2017, 139, 13672−13675
Communication
Journal of the American Chemical Society processes (32.4 kcal/mol) is approximately equal to the calculated strain energy of norbornadiene (34.7 kcal/mol).5b
Table 1. Optimization Studies of the Catalytic Carbonylative Rearrangementa
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STOICHIOMETRIC AND CATALYTIC CARBONYLATIVE REARRANGEMENT Having identified a stoichiometric C−C activation of norbornadiene, we turned our attention to exploring insertion reactions of small molecules. We reasoned that a migratory insertion−reductive elimination sequence with CO15 could provide two potential products: the bridged bicyclo[2.3.1] product 5, which would correspond to a direct insertion into the bond being activated, or the fused bicyclo[3.3.0] product 6, in which the allylic system has been transposed (Scheme 1). Scheme 1. Stoichiometric Carbonylative Rearrangement of Norbornadiene
entry
CO source
additiveb
yield
1 2 3 4 5 6 7 8 9 10 11 12
Mn2(CO)10 (0.5 equiv) Co2(CO)8 (0.5 equiv) Mo(CO)6 (0.5 equiv) W(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (0.5 equiv) Cr(CO)6 (1.0 equiv)
− − − − − bpy (1.0 equiv) phen (1.0 equiv) DBU (1.0 equiv) Et3N (1.0 equiv) TMEDA (1.0 equiv) TMEDA (0.5 equiv) TMEDA (2.0 equiv)
