Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
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Reductive Elimination to Form C(sp3)−N Bonds from Palladium(II) Primary Alkyl Complexes D. Matthew Peacock,†,§ Quan Jiang,‡ Thomas R. Cundari,*,‡ and John F. Hartwig*,† †
Department of Chemistry, University of California, Berkeley, California 94720, United States Department of Chemistry and Center for Advanced Scientific Computing and Modeling, University of North Texas, Denton, Texas 76203, United States
‡
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ABSTRACT: Reductive eliminations to form alkyl−nitrogen bonds are rare, and examples of this reaction from isolated complexes containing simple, unstabilized primary alkyl groups have not been observed. We report the synthesis of stable neopentylpalladium(II) anilido and methyleneamido complexes that undergo reductive elimination to form the C(sp3)−N bonds in N-neopentyl anilines and Nneopentyl imines, respectively. The synthesis and isolation of these complexes were enabled by weak chelation of palladium by P,O ancillary ligands. DFT calculations suggest that neopentylpalladium(II) complexes undergo reductive elimination by a concerted mechanism resembling a migration of the alkyl ligand to the nitrogen either following initial dissociation of the oxygen donor or in concert with lengthening of the Pd−O bond, depending on the identities of the reacting and ancillary ligands.
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Scheme 1. Reductive Elimination from Alkylpalladium Complexes to Form C(sp3)−N Bonds
eductive eliminations are fundamental reactions that constitute the bond-forming step of many catalytic processes.1 However, well-characterized complexes that undergo reductive elimination to form C(sp3)−N bondsa prevalent moiety in all applications of chemistry2−6are rare. Most examples of this reaction occur through an SN2-type attack of a nitrogen nucleophile on an alkyl ligand. For example, methylplatinum(IV) and benzylpalladium(II) complexes containing electrophilic alkyl groups react by this mechanism.7−9 High-valent Pd(IV) (Scheme 1, top) and Ni(III) or -(IV) complexes bearing primary alkyl groups react by related SN2-type mechanisms, but they only form Nalkylamine products in high yield when they are stabilized through cyclometalation of the alkyl ligand.10−15 Formation of an alkyl−nitrogen bond by concerted coupling between alkyl and nitrogen-based ligands has been reported only recently.16,17 This concerted process occurred from palladium(II) anilido complexes containing the secondary 3methylnorborn-2-yl alkyl ligand (Scheme 1, middle). Although these complexes were the first to undergo this type of reductive elimination, the required complexity of the secondary alkyl ligand limits the applicability of this reaction to catalytic processes and casts doubt on the potential generality of concerted reductive eliminations to form alkyl−nitrogen bonds, particulary the potential of this reaction to join an amido group with a less hindered primary alkyl group. Published synthetic methods suggested that primary alkylpalladium(II) amido complexes might undergo reductive elimination as part of the catalytic cycle, but these proposed intermediates were not characterized.18 Moreover, previously reported neopentylpalladium(II) complexes, which contain the © XXXX American Chemical Society
simplest primary alkyl group that cannot undergo β-hydrogen elimination, did not reductively eliminate N-neopentyl amines,19,20 and initial experiments in one of the authors’ groups exploring reactions of complexes with commercial ancillary ligands did not lead to identification of a discrete neopentylpalladium(II) amido complex that underwent reductive elimination in high yield (e.g., Scheme S-2).9 Received: August 28, 2018
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DOI: 10.1021/acs.organomet.8b00617 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics Here, we report the synthesis of stable neopentylpalladium(II) anilido and methyleneamido complexes that undergo reductive elimination to form N-neopentyl amines and imines, respectively (Scheme 1, bottom). These results were enabled by preparing complexes containing the weakly chelating P,O ancillary ligands Ad2(2-OCH3-5-(CF3)C6H3)P (L1) and Ad2PCH2CH2OCH3 (L2).17,21,22 These ligands simultaneously increase the yields of the synthesis of neopentylpalladium(II) amido and methyleneamido complexes, facilitate the isolation of stable complexes in pure, crystalline form, and cause reductive elimination to occur from simple, primary alkylpalladium complexes under mild conditions to form alkyl−nitrogen bonds. We began our studies by synthesizing the complexes (Lκ2P,O)Pd(neopentyl)(NHAr) (2a,b; Ar = 4-F-2-(OCH3)C6H3) by a two-step procedure from (COD)Pd(neopentyl)Cl (Figure 1a). Pd(II) chloride complexes 1a,b were isolated as
of 2b to [(L2-κP)Pd(neopentyl)NHAr]2 (3) occurred upon isolation in solid form (Figure S-1).24 Complex 2a containing the more rigid ligand L1 underwent reductive elimination at 60 °C with 2 equiv of L1 in C6D6 to form neopentylamine 4 in 59% yield after 6 h (Figure 1a). NAryl pivaldehyde imine 5 formed as a side product (ca. 5%, see Scheme S-4). Under the same reaction conditions, the mixture of monomeric 2b and dimeric 3 bearing the more flexible ligand L2 reacted to produce 4 and 5 in 48 and 11% yields, respectively, after 8 h. To expand the scope of palladium complexes that reductively eliminate to form C(sp3)−N bonds from primary alkyl complexes, we prepared neopentylpalladium species containing the anions of benzophenone imines. These imines are synthetically valuable ammonia surrogates that participate in reductive eliminations with palladium-bound aryl groups.25,26 However, reductive elimination to form an C(sp3)−N bond from an alkylmetal methyleneamido complex has not been reported. The L2-ligated complex 1b reacted with a series of benzophenone imines and base to form the corresponding methyleneamido complexes (L2-κ2P,O)Pd(neopentyl)(NCAr2) (6a−c) (Figure 2a). In contrast to the
Figure 1. (a) Synthesis of Pd(II) anilido complexes 2a,b and reductive elimination to form N-neopentylamine 4. (b) ORTEP drawing of (L1-κ2P,O)Pd(neopentyl)(NHAr) (2a) with 50% probability ellipsoids. Hydrogen atoms and a solvent molecule are omitted for clarity. Legend: (a) isolated yield; (b) yield determined by 19 F NMR spectroscopy; (c) combined isolated yield for 79:21 mixture of 2b + 3.
Figure 2. (a) Synthesis of Pd(II) methyleneamido complexes 6a−c and reductive elimination to form N-neopentylimines 8a−c. (b) ORTEP drawing of (L2-κ2P,O)Pd(neopentyl)(NCPh2) (6a) with 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Legend: (a) conditions (i) 1.2 equiv of NaO-t-Bu in THF and (ii) 1.0 equiv of LiHMDS in Tol-d8 with 2 equiv of L2, 0 °C, with (L2)2Pd0 (
