Ligand-Enabled NiH-Catalyzed Migratory Hydroamination: Chain Walking as a Strategy for Regiodivergent/Regioconvergent Remote sp 3 C–H Amination

Open AccessCCS ChemistryCOMMUNICATION1 Sep 2021Ligand-Enabled NiH-Catalyzed Migratory Hydroamination: Chain Walking as a Strategy for Regiodivergent/Regioconvergent Remote sp3C–H Amination Yulong Zhang†, Jun He†, Peihong Song, You Wang and Shaolin Zhu Yulong Zhang† State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 College of Chemistry, Chongqing Normal University, Chongqing 401331 †Y. Zhang and J. He contributed equally to this work.Google Scholar More articles by this author , Jun He† State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 †Y. Zhang and J. He contributed equally to this work.Google Scholar More articles by this author , Peihong Song State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 Google Scholar More articles by this author , You Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 Google Scholar More articles by this author and Shaolin Zhu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, Chemistry and Biomedicine Innovation Center (ChemBIC), School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000490 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Direct and positionally selective aliphatic C–H functionalization is an attractive means with which to streamline chemical synthesis. With many possible reaction sites, most traditional methods need a polar-directing group nearby to achieve high regio- and chemoselectivity and are often restricted to a single site of functionalization. Here, we report a nondirected, remote sp3C–H amination process with predictable and switchable regioselectivity. This reaction uses a nickel hydride–catalyzed remote relay hydroamination strategy in which an aliphatic amino group is installed at a position far from the original C=C bond present in all unsaturated hydrocarbon substrates. Depending on the choice of ligand, either terminal or benzylic functionalization products can be obtained with excellent levels of regioselectivity. We also show that an alkyl bromide could also be used as an olefin precursor when using Mn0 as a reductant. The utility of this transformation is further highlighted by the regioconvergent migratory hydroamination of isomeric mixtures of olefins forming single isomers of value-added benzylic or linear amines. Download figure Download PowerPoint Introduction The direct and site-selective functionalization of ubiquitous C–H bonds has the potential to streamline synthetic routes substantially.1 Despite notable recent efforts in C–H functionalization, the regioselective functionalization of alkyl chains, in particular, remains an important synthetic challenge. Many reported protocols for sp3C–H functionalization require the assistance of specialized polar-directing groups to achieve the desired regiochemical outcome and to improve reactivity. This limits their generality and applicability. Moreover, nondirected, remote aliphatic C–H functionalization protocols generally allow the introduction of a functional group only at a single position, which often must be electronically activated; functionalization of multiple sites based on ligand control has so far proven elusive. A general and efficient procedure for aliphatic C–H functionalization with switchable ligand-controlled selectivity, particularly one that operates under mild conditions with high functional group tolerance, has long been sought by the synthetic community. Given the prevalence of amines in pharmaceuticals, materials, and agrochemicals (Figure 1a),2 C–N bond-formation is of fundamental importance in a vast range of chemical applications. Over the past two decades, the use of ubiquitously available olefin-containing substrates in hydroamination3–12 has emerged as one of the most attractive strategies for C–N bond construction. In these processes, C–N bond formation generally only occurs at the original olefinic site. In a migratory hydroamination of alkenes, on the other hand, the installation of an amino group would occur in a distal position of the hydrocarbon chain,13,14 and this could permit the functionalization of distant, unactivated C–H bonds (Figure 1b). Recently, abundant nickel-catalyzed15 remote hydrofunctionalization16–52 offers a new retrosynthetic protocol with which to achieve inert sp3C–H bond-selective functionalization. Retooling the facile β-hydride elimination and migratory insertion reactions into a strategically advantageous chain-walking process, nickel hydride53–57 chemistry could be employed to gain access to all possible alkylnickel(I) intermediates from alkene-starting materials with the C=C bond in an arbitrary position. If the reactivity of these alkylnickel(I) species could be controlled by the catalyst, then a convergent and regioselective functionalization process could be achieved. We wondered whether a suitable electrophilic amination reagent58–63 could be used to form the remote amination product. Given the previous application of readily prepared O-benzoylhydroxylamine reagents in transition metal chemistry, we envisioned that these could be used to capture an alkylnickel(I) intermediate, catalytically generated from the reaction of NiH and a remote olefin followed by chain walking, to form an unstable nickel amido(III) complex.64–67 This complex would then undergo reductive elimination to access the formal remote aliphatic C–H amination product under mild conditions. We wondered whether divergent regiochemical outcomes could be obtained from a single substrate, based on ligand control of the functionalization process (Figure 1c). In this paper, we report that ligand-controlled selectivity can indeed be achieved. In particular, judicious selection of ligands led to protocols that deliver the terminal and benzylic C–H amination products with high regioselectivity, starting from an arbitrary isomer (or isomeric mixtures) of the olefinic substrate. These protocols allow the synthesis of two privileged classes of amine products, benzylic amines and linear amines, each of substantial importance in medicinal and materials chemistry, from a single set of starting materials. Moreover, these transformations were realized under exceptionally mild reaction conditions, using a readily accessible monophosphine–nickel complex or a bipyridine–nickel complex as a catalyst. Figure 1 | Design plan: Regiodivergent migratory hydroamination enabled by nickel hydride. Download figure Download PowerPoint Figure 2 is a more detailed description of our proposed mechanism for this regiodivergent remote sp3C–H amination reaction. Initially, the active nickel(I) hydride species ( I) is generated from a Ni(II) precursor, an appropriate ligand, and a hydrosilane. In the presence of an alkene ( 1), an alkene ligated complex ( II) is formed, which then undergoes fast and reversible insertion of alkene, to form an alkylnickel(I) species. A series of isomeric alkylnickel(I) species ( III, III’, ….) is then accessed through iterative β-hydride elimination/migratory reinsertions. If nickel migration along the hydrocarbon chain is sufficiently rapid and the reaction of a particular alkylnickel(I) isomer with the aminating reagent ( 2) is favorable relative to other alkylnickel(I) isomers, one regioisomer of the product amine, either the benzylic amination product ( 3) or the terminal amination product, ( 4) will be formed with high regioselectivity, along with nickel benzoate ( V). The nickel(I) hydride species ( I) is then reformed in situ by a stoichiometric amount of the hydrosilane to complete the catalytic cycle for the remote hydroamination reaction. The key to the success of this process is the careful choice of the ligand to favor reactivity of either the terminal alkylnickel(I) or the benzylnickel(I) intermediate. Figure 2 | Envisioned mechanistic pathway of NiH-catalyzed regiodivergent migratory hydroamination. Download figure Download PowerPoint Importantly, a suitable catalyst must fulfill three requirements. First, the undesired competitive decomposition of hydroxylamine ester by nickel hydride must be slow. Second, the alkylnickel(I) chain walking should be faster and reversible compared with oxidative addition with hydroxylamine. Third, one isomer of the ligand-bound alkylnickel(I) intermediate ( III, III’, ….) must undergo amination with the hydroxylamine with significant selectivity—a less reactive alkylnickel(I) species should be converted into a more reactive alkylnickel(I) intermediate through chain walking. Results and Discussion Reaction optimization With this hypothetical mechanism in hand, we set out to explore the proposed remote hydroamination reaction with 5-(4-methoxyphenyl)-2-pentene ( 1a) and O-benzoylhydroxylamine ( 2a). As shown in Figure 3, after extensive examination of a range of parameters, such as nickel sources, ligands, and solvents, the desired benzylic amination product ( 3a) was obtained using a combination of NiCl2 and Buchwald’s bulky biarylphosphine ligand SPhos68 under base free conditions in good isolated yield (75%) with excellent regioselectivity [regioisomeric ratio, rr = (major product:all other isomers) = 97∶3] at 25 °C (Figure 3, entry 1). Notably, use of another nickel source, NiBr2, produced only a low yield and moderate regioisomeric ratio (Figure 3, entry 2). Furthermore, replacement of the solvent DMA with THF led to a lower yield (Figure 3, entry 3). Evaluation of other silanes showed that dimethoxy(methyl)silane resulted in a slightly diminished yield and selectivity (Figure 3, entry 4). Similarly, a series of rigid and bulky biarylphosphine ligands bearing bis(cyclohexyl)phosphino groups were also carefully evaluated and were found to provide inferior yield and regioisomeric ratios (Figure 3, entries 5–8) compared with SPhos. This result illustrates the influence on the catalyst activity of both the size and electronic effect of the substituents on the nonphosphorus-containing ring of the dialkylbiarylphosphine ligand. Bidentate phosphine ligands such as Xantphos resulted in a complete loss of reactivity (Figure 3, entry 9). Encouraged by these results, we then sought conditions that could result in switching the site of C–H amination. After a thorough reevaluation of reaction parameters, we were able to find conditions for a terminal-selective amination that generated the linear amine with very good regioselectivity (rr = 95∶5) while using a simple C2-substituted bipyridine ligand ( L1) (Figure 3, entry 10). A key finding was that reactivity and regioselectivity can both be improved through the addition of 2-methylpyridine as a secondary ligand (Figure 3, entry 10 vs entry 11). Since the bipyridine ligand ( L1) is known to facilitate the chain-walking process, 2-methylpyridine was thought likely to play a role during the oxidative addition step. Inferior results were found with other pyridine-based additives (Figure 3, entry 10 vs entries 12 and 13), and it should be pointed out that the ortho-methyl groups on L1 were critical—use of the parent bpy led to no desired amination product (Figure 3, entry 14). Likewise, other nickel sources, amination reagents,69,70 silanes, and temperature all led to significantly lower yields (Figure 3, entries 15–18), thus showing the subtle interplay of reagents in this protocol. Figure 3 | Optimization of regiodivergent migratory hydroamination. aYields were determined by GC using n-dodecane as the internal standard. The yield in parentheses is the isolated yield and is an average of two runs (0.20 mmol scale). brr refers to the regioisomeric ratio, representing the ratio of the major product to the sum of all other isomers as determined by GC analysis. cNo amination product but decomposition of 2a’ was observed. PMP, p-methoxyphenyl; DMA, N,N-dimethylacetamide; THF, tetrahydrofuran; GC, gas chromatography. Download figure Download PowerPoint Substrate scope With the optimal conditions in hand, we next explored the generality of benzylic-selective remote hydroamination (Figure 4). An array of terminal aliphatic alkenes with a variety of substituents on the remote aryl ring ( 7c– 7e) underwent alkene isomerization-hydroamination smoothly (Figure 4a, Type I). Notably, heteroaromatic substrates, such as those containing a pyridine-linked aryl ring ( 7f), an indole ( 7g), a furan ( 7h), or a thiophene ( 7i) in place of the aryl group, were likewise suitable. Unactivated internal olefins were also suitable partners (Figure 4a, Type II). As expected, both Z ( 7m and 7n) and E ( 7o– 7q) alkenes, as well as E/Z mixtures ( 7j– 7l) were readily accommodated, regardless of the starting position of the C=C double bond. Remarkably, even with a heteroatomic substituent at the other terminus of the alkyl chain (e.g., an arylamine as in 7o, a Boc carbamate in 7p, or an ether in 7q), migration toward the aryl group, and subsequent benzylic amination was still preferred. Additionally, styrenes themselves ( 7r– 7z) underwent hydroamination to produce the desired benzylic amine exclusively (Figure 4a, Type III). Interestingly, in an estrone-derived styrene, the sensitive ketone group was left intact under these exceptionally mild reaction conditions ( 7y). The reaction can also be extended to α-methyl styrene to provide the benzylic amine ( 7z) exclusively. Figure 4 | Substrate scope of remote hydroamination with benzylic selectivity. Under each product yields are given in percent, and either the regioisomeric ratio (rr) or the diastereomeric ratio (dr). Yield refers to isolated yield of purified product (0.20 mmol scale, average of two experiments). rr represents the ratio of the major product to the sum of all other isomers as determined by GC analysis, ratios reported as >95∶5 were determined by crude 1H NMR analysis. dr represents the diastereomeric ratio as determined by crude 1H NMR analysis. nPent, n-pentyl; Cy, cyclohexyl; TBS, tert-butyldimethylsilyl; Boc, tert-butoxycarbonyl. Download figure Download PowerPoint A survey of hydroxylamine esters revealed that a range of amino groups could be installed under the optimal conditions (Figure 4b). Since benzyl groups can be easily deprotected to give primary and secondary free amine products, benzyl-protected hydroxylamine esters were used in most cases. As depicted in Figure 4b, both electron-deficient ( 8b– 8f) and electron-rich ( 8g– 8i) hydroxylamine esters were suitable substrates. Moreover, a variety of functional groups were accommodated well, including a nitrile ( 8c), ethers ( 8e, 8g, and 8l), esters ( 8g and 8t), an aniline ( 8h), a thioether ( 8i), and a Boc carbamate ( 8u). Of particular interest is that potential coupling motifs, including a boronic acid pinacol ester ( 8d) and an aryl chloride ( 8f), remained intact. Moreover, a series of heterocycles frequently found in medicinally active agents, including furan ( 8j), thiophene ( 8k), pyridine ( 8l), and indole( 8m), were also compatible. Notably, a stereocenter adjacent to the nitrogen atom of the aminating reagent was left unchanged ( 8n). We next explored the nature of terminal-selective remote hydroamination of alkenes. The scope of alkene partners is shown in Figure 5a. As expected, both E ( 9a, 9b, and 9e) and Z ( 9d, 9f, and 9k) alkenes, as well as E/Z mixtures ( 9c, 9g– 9j, and 9l– 9r) were accommodated well, and high selectivity for amination at the terminal position was observed, regardless of the starting position of the C=C bond. Interestingly, when comparing different positions of the methyl group on the alkyl chain, migration toward the less sterically hindered methyl group was seen to be preferred ( 9h). Furthermore, substrates with a heteroatomic substituent at the other terminus of the alkyl chain, including an ether ( 9j), acetals ( 9k, 9m), and a Boc carbamate ( 9l), could also undergo a targeted reaction, producing linear amines in all cases. Remarkably, even with (hetero)aromatic rings ( 9m– 9r) at the other terminus of the alkyl chain, a high selectivity for amination at the terminal position was still observed. Next, we examined the scope of hydroxylamine (Figure 5b). Substrates bearing electron-poor ( 12b– 12e) and electron-rich ( 12f− 12h) aryl substituents were competent substrates. Moreover, a variety of functional groups, including ethers ( 12c, 12f, 12h, and 12k), an aniline ( 12h), a thioether ( 12g), an ester ( 12q), and a Boc carbamate ( 12r) were readily accommodated. Of particular interest is that potential coupling motifs, including a boronic acid pinacol ester ( 12d) and an aryl chloride ( 12e), remained intact and were available for further derivatization. Notably, a series of heterocycles frequently found in medicinally active agents, including furan ( 12i), thiophene ( 12j), pyridine ( 12k), and indole ( 12l), were all shown to be viable substrates. Figure 5 | Substrate scope of remote hydroamination with terminal selectivity. Under each product yields are given in percent, and either the regioisomeric ratio (rr). Yield and rr are as defined in Figure 3 legend. tBu, tert-butyl; iBu, iso-butyl; nOct, n-octyl. Download figure Download PowerPoint Reaction utilization The utility of this protocol was demonstrated by the employment as starting materials of isomeric mixtures of olefins, generally available from petroleum-derived feedstocks in bulk and substantially cheaper than the pure isomers (Figure 6a). Utilizing such mixtures in a regioconvergent remote hydroamination process on a large scale into value-added amines is of considerable interest. Gratifyingly, in both cases, amination on a 10 mmol scale was highly selective at either the benzylic or the terminal position and produced only one regioisomer in an effective fashion. Figure 6 | Regioconvergent and migratory cross-electrophile coupling experiments. Download figure Download PowerPoint To further demonstrate the robustness and synthetic utility of this migratory hydroamination protocol, a readily available and bench-stable alkyl bromide could, in the presence of a manganese reductant, be used directly instead of the alkene to produce the migratory amination product (Figure 6b). Assuming that the NiH and olefin intermediate could be generated in situ from an alkyl bromide38,40,42,49,52 with a manganese reductant under a nickel catalyst, we presumed that the remaining cascade chain-walking cross-coupling was analogous to that shown in Figure 2. Thus, this migratory cross-electrophile coupling process would provide an alternative pathway and a synthetically valuable addition to the current migratory hydroamination strategy. Conclusion In summary, we have developed a NiH-catalyzed migratory hydroamination process for the regiodivergent installation of a distal amino group at either a benzylic or a terminal position under mild conditions. This mild and efficient protocol utilizes easily accessible olefins and earth-abundant nickel salt as starting materials and catalysts. Moreover, it is usable for substrates containing a wide range of functional groups and provides remote sp3C–H amination products with good yield and high regioselectivity on scales up to 10 mmol. Finally, the practical value of this catalytic system is highlighted by the regioconvergent conversion of petroleum-derived feedstocks (isomeric olefinic mixture) to a single isomer of a value-added amine. The development of a versatile NiH catalyst toolbox for introducing other heteroatoms onto a distant sp3C–H bond is currently in progress. Supporting Information Supporting Information is available. Conflict of Interest The authors declare no competing interests. Acknowledgments Support was provided by the National Natural Science Foundation of China (21702102 and 21772087), NSF of Jiangsu Province (BK20190281), the Six Kinds of Talent Project of Jiangsu Province (JNHB-003), and the Program for High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province (Group Program). We thank Prof. Yi-Ming Wang (University of Pittsburgh) and Dr. Michael Pirnot (Massachusetts Institute of Technology) for helpful suggestions during the preparation of the manuscript. References 1. Yamaguchi J.; Yamaguchi A. D.; Itami K.C–H Bond Functionalization: Emerging Synthetic Tools for Natural Products and Pharmaceuticals.Angew. Chem. Int. Ed.2012, 51, 8960–9009. Google Scholar 2. Ricci A.Amino Group Chemistry, from Synthesis to the Life Sciences; Wiley-VCH: Weinheim, Germany, 2007. Google Scholar 3. Müller T. E.; Hultzsch K. 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Google Scholar Previous articleNext article FiguresReferencesRelatedDetails Issue AssignmentVolume 3Issue 9Page: 2259-2268Supporting Information Copyright & Permissions© 2020 Chinese Chemical SocietyKeywordsnickelchain walkingaminationregioselectivityC–H activationAcknowledgmentsSupport was provided by the National Natural Science Foundation of China (21702102 and 21772087), NSF of Jiangsu Province (BK20190281), the Six Kinds of Talent Project of Jiangsu Province (JNHB-003), and the Program for High-Level Entrepreneurial and Innovative Talents Introduction of Jiangsu Province (Group Program). We thank Prof. Yi-Ming Wang (University of Pittsburgh) and Dr. Michael Pirnot (Massachusetts Institute of Technology) for helpful suggestions during the preparation of the manuscript. Downloaded 2,859 times PDF DownloadLoading ...