Reports: ND355301-ND3: The Development of Cobalt Catalysts for the Deformylation of Aldehydes to Alkenes
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Work during this period focused on using the ligand N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)-1,2-cyclohexanediamine (bbpc, Scheme 1) to prepare stable Co(II) and Co(III)-peroxo complexes. We subsequently interrogated the reactivities of these species with various terminal oxidants and small molecule substrates and found that the bbpc ligand could support the formation of both electrophilic and nucleophilic oxidants from a shared Co(II) precursor.
Synthesis and Spectroscopic Characterization
A Co(II) species, [Co(bbpc)(MeCN)2](ClO4)2 (1) forms in 85% yield from mixtures of bbpc and Co(ClO4)2•6H2O. Unlike its iron-containing analog and many Co(II) complexes with other pyridylamines, 1 does not react rapidly with O2, even in the presence of substrates with weak allylic and benzylic C-H bonds.1-2
Complex 1 can be oxidized to [Co(bbpc)(O2)](ClO4) (2) upon reaction with slight excesses of H2O2 and Et3N. Upon oxidation, the dark red solutions of 2 persist at room temperature (RT) with no noticeable discoloration for several hours. The isolated 2 is diamagnetic, as assessed by 1H NMR. X-ray crystallography confirms that 2 is a Co(III)-peroxo species.
Reactivity
2 reacts with aldehydes in MeCN at 298 K. Cyclohexanecarboxyaldehyde (CCA) is converted to a mixture of cyclohexene and cyclohexanone in yields of 26 (±6)% and 63 (±10)%, respectively. With a large excess of CCA, the chromophore associated with 2 undergoes first-order decay. The reaction is second-order overall, with a k2 of 2.4 (±0.2) × 10-2 M-1 s-1. Complex 2 converts 2-phenylpropionaldehyde (2-PPA) to acetophenone in 81 (±12)% yield and a k2 of 1.7 (±0.3) × 10-2 M-1 s-1.
Complex 2 also degrades in the presence of benzaldehydes. Regrettably, the organic products were not identified; prior work in this area encountered similar difficulties.3 The reactivity with para-substituted benzaldehydes (Cl, H, F, Me) is consistent with the Co(III)-peroxo species acting as a nucleophile (ρ = 1.6).
Prior research from the Borovik and Ray groups found that Co(II) complexes could react with PhIO in the presence of a Lewis acid, such as Sc(III), to yield species capable of oxidizing 9,10-dihydroanthracene (DHA).4-6 Mixtures of complex 1 and PhIO likewise can activate weak C-H bonds; an additional Lewis acid beyond the Co(II), however, is not needed.
DHA and 1,4-cyclohexadiene (CHD) are converted exclusively into anthracene and benzene, respectively. The reactions complete in under 30 min. Oxygenated products, such as anthroquinone are not observed, and running the reaction under N2 has no significant impact on either the product distributions or yields. Xanthene is oxidized to xanthone. Under air but not N2, the yields of xanthone are higher and continue to increase past 30 min. Neither fluorene, cyclohexane, cyclohexene, cumene, nor toluene react with 1 and PhIO to observable degrees, even when 500 mM of these substrates is added.
The reactivity with DHA, CHD, and xanthene is notable in that it is catalytic, rather than stoichiometric, with respect to the cobalt. With excess PhIO, up to 12 equiv. of anthracene per equiv. of 1 can be produced, suggesting that the cobalt catalyst turns over 11 times. These substrates do not react with PhIO without a catalyst. The higher and increasing yields of xanthone under air suggests that this process partly proceeds through the propagation of organic radicals, rather than solely the regeneration of a metal-based oxidant, when O2 is present.
We investigated whether an additional Lewis acid could enhance the activity. To our surprise, we found that the addition of Sc(OTf)3 inhibited, rather than improved, the catalysis, for the yields of anthracene from DHA were noticeably lower when Sc(III) was present. The product distribution remained unaltered, with no oxygenated organic products.
meta-Chloroperbenzoic acid (MCPBA) can substitute for PhIO as the terminal oxidant, but the yields of the oxidized hydrocarbons are generally lower. MCPBA reacts directly with alkenes to yield epoxides, and 1,4-cyclohexene monoxide is observed in its reactions with 1 and CHD in addition to the anticipated benzene. We also investigated oxone, H2O2, tert-butylhydroperoxide, and air by itself as terminal oxidants but found no reactivity with organic substrates.
References
1. He, Y.; Goldsmith, C. R. Observation of a Ferric Hydroperoxide Complex during the Non-heme Iron Catalysed Oxidation of Alkenes and Alkanes by O2. Chem. Commun. 2012, 48, 10532-10534.
2. de Souza, I. C. A.; Faro, L. V.; Pinheiro, C. B.; Gonzaga, D. T. G.; da Silva, F. d. C.; Ferreira, V. F.; Miranda, F. d. S.; Scarpellini, M.; Lanznaster, M. Investigation of Cobalt(III)-Triazole Systems as Prototypes for Hypoxia-Activated Drug Delivery. Dalton Trans. 2016, 45.
3. Cho, J.; Sarangi, R.; Kang, H. Y.; Lee, J. Y.; Kubo, M.; Ogura, T.; Solomon, E. I.; Nam, W. Synthesis, Structural, and Spectroscopic Characterization and Reactivities of Mononuclear Cobalt(III)-Peroxo Complexes. J. Am. Chem. Soc. 2010, 132 (47), 16977-16986.
4. Hong, S.; Pfaff, F. F.; Kwon, E.; Wang, Y.; Seo, M.-S.; Bill, E.; Ray, K.; Nam, W. Spectroscopic Capture and Reactivity of a Low-Spin Cobalt(IV)-Oxo Complex Stabilized by Binding Redox-Inactive Metal Ions. Angew. Chem. Int. Ed. 2014, 53 (39), 10403-10407.
5. Pfaff, F. F.; Kundu, S.; Risch, M.; Pandian, S.; Heims, F.; Pryjomska-Ray, I.; Haack, P.; Metzinger, R.; Bill, E.; Dau, H.; Comba, P.; Ray, K. An Oxocobalt(IV) Complex Stabilized by Lewis Acid Interactions with Scandium(III) Ions. Angew. Chem. Int. Ed. 2011, 50 (7), 1711-1715.
6. Lacy, D. C.; Park, Y. J.; Ziller, J. W.; Yano, J.; Borovik, A. S. Assembly and Properties of Heterobimetallic CoII/III/CaII Complexes with Aquo and Hydroxo Ligands. J. Am. Chem. Soc. 2012, 134 (42), 17526-17535. ADDIN EN.REFLIST
Scheme 1
Figure 1. Plot kobs for the decay of 2 as a function of [CCA] (298 K, MeCN, air).
Figure 2. Hammett plot for reactions between 2.0 mM 2 and 400 mM para-X-benzaldehydes (298 K, MeCN).
