Carbodicarbenes: Unexpected π-Accepting Ability during Reactivity

Aug 16, 2017 - An investigation of carbodicarbenes, the less explored member of the carbenic complex/ligand family has yielded unexpected electronic ...
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Carbodicarbenes: Unexpected π‑Accepting Ability during Reactivity with Small Molecules Wen-Ching Chen,*,† Wei-Chih Shih,† Titel Jurca,§ Lili Zhao,# Diego M. Andrada,¶ Chun-Jung Peng,† Chun-Chi Chang,† Shu-kai Liu,† Yi-Ping Wang,† Yuh-Sheng Wen,† Glenn P. A. Yap,‡ Chao-Ping Hsu,† Gernot Frenking,*,#,¶ and Tiow-Gan Ong*,†,⊥ †

Institute of Chemistry, Academia Sinica, No. 128, Sec. 2, Academia Road, Nangang, Taipei 11529, Taiwan, R.O.C. Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States # Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 211816, China ¶ Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse, D-35043 Marburg, Germany ‡ Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States ⊥ Department of Applied Chemistry, National Chiao Tung University, No. 1001, Ta Hsueh Road, Hsinchu 300, Taiwan, R.O.C. §

S Supporting Information *

ABSTRACT: An investigation of carbodicarbenes, the less explored member of the carbenic complex/ligand family has yielded unexpected electronic features and concomitant reactivity. Observed 1,2-addition of E−H bonds (E = B, C, Si) across the carbone central carbon and that of the flanking N-heterocyclic carbene (NHC) fragment, combined with single-crystal X-ray studies of a model Pd complex strongly suggests a significant level of π-accepting ability at the central carbon of the NHC moiety. This feature is atypical of classic NHCs, which are strong σ-donors, with only nominal π-accepting ability. The unanticipated π-acidity is critical for engendering carbodicarbenes with reactivity more commonly observed with frustrated Lewis pairs (FLPs) rather than the more closely related NHCs and cyclic (alkyl)(amino)carbenes (CAACs).

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INTRODUCTION

Scheme 1. Summary of Electronic Topology and Concomitant Reactivity for Common NHCs, CAACs, and CDCs

The pursuit of carbon-based species featuring unique electronic configurations and concomitant reactivity has been an active area of fundamental interest in the chemical sciences for decades. The culmination of focused effort and serendipity has ultimately overcome the octet-impasse with the isolation of a six-electron divalent carbon class; stable free phosphino-silyl carbenes1 and N-heterocyclic carbenes (NHCs).2 Owing to their unique topology, which affords robust σ-donation, and the advantage of facile modification to their peripheral architecture, NHCs have become ubiquitous ligands for transition metalbased and organocatalysts.3 Despite the significant advances in the field, which yielded rich libraries of structurally diverse analogues, the utility of NHCs remains constrained by their inherently weak π-acceptor properties.4 The recent landmark development of cyclic (alkyl)(amino)carbenes (CAACs) (Scheme 1) by the group of Bertrand detailed a new class of compounds, which not only exhibit strong σ-donation but also act as effective π-acceptors.5 This ambiphilicity affords CAACs greater leverage in stabilizing transition-metal and main-group elements in wide-ranging oxidation states, hitherto inaccessible by other ligand classes. © 2017 American Chemical Society

Received: July 30, 2017 Published: August 16, 2017 12830

DOI: 10.1021/jacs.7b08031 J. Am. Chem. Soc. 2017, 139, 12830−12836

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Journal of the American Chemical Society In addition to the broadly explored NHCs and CAACs, the concerted effort toward discovering stable carbenic species has also yielded carbodicarbenes (CDCs); as first realized theoretically and experimentally by Frenking, Tonner, and Bertrand.6 CDCs are carbones7 CL2 that feature a dicoordinated central carbon (0) atom possessing two lone pairs of electrons with NHCs as ligands L (Scheme 1 right). Two NHCs bound to the central carbon (0) supply the remaining four electrons necessary to achieve octet stabilization within the system. Due to the availability of two lone pairs of electrons on the central carbon,8 CDCs have been implicated as strong σdonating complementary surrogates to the well-established NHCs and CAACs for applications in coordination chemistry and catalysis.9 Importantly, numerous reports confirm that CDCs exhibit superior σ-donor properties to NHC ligands.9b,c,i To that effect, our group has recently reported the isolation of a highly Lewis-acidic dicationic hydrido boron complex supported by the electron rich CDC scaffold;9d a demonstrative case where similar stabilization could not be achieved by common NHC ligands. Additionally, our group9e and those of Meek9f,g and Stephan9h have successfully showcased CDCs as effective ligand scaffolds for a variety of metal-mediated catalytic transformations. However, unlike the case for NHCs and CAACs, the fundamental topological properties and concomitant reactivity of CDCs remain greatly underexplored. The unique properties of classical carbenes arise from their frontier orbitals, where the HOMO and LUMO are respectively comprised of a lone pair σ-orbital and a formally unoccupied low-lying orthogonal p-orbital.10 This topology renders the carbene central carbon atom ambiphilic and is responsible for their unique reactivity. As a result, CAACs and N,N′diaminocarbenes have been extensively studied for their ability to activate silanes, boranes, and other organic compounds.5b,11 Because of the ambiphilic nature at the central carbon atom, carbenes typically activate E−H bonds (e.g., E = Si, B) via a 1,1addition (Scheme 1, left). In marked contrast, there have been no similar reports on the reactivity of simple organic molecules (E−H) with CDCs. Such studies are invaluable for delineating the electronic properties of the CDC framework, which may stimulate novel and exciting applications akin to NHCs and CAACs. Expanding upon our recent success in diversifying the library of CDCs, we set forth to explore their reactivity toward E−H bonds (E = B, C, and Si). Although prior reports highlight the σ-donating ability and existence of two electron lone-pairs of this class of molecules,6b,8,9b,c,i our study provides evidence, for the first time, of a hidden or latent low-lying empty p-orbital embedded in the CDC framework. The ambiphilic nature of CDCs facilitates a synergistic 1,2-addition with organic molecules, with atom E interacting with the central CDC carbon (referred to as CA for the remainder of this report) and hydridic H with the central carbon of the flanking NHC moiety (referred to as CB for the remainder of this report). This 1,2addition pathway is markedly different from the 1,1-addition of conventional carbenes and reminiscent of reactivity observed with frustrated Lewis pairs (FLPs).12

Scheme 2. Reactions of 1a,b with Borane Substrates (a,b) and Silane Substrates (c,d)

(B-pin) in THF solution afforded isomers 2a (major) and 2a′ (minor) in good yield (85% total; ratio 4.3:1). The 1H NMR spectrum of 2a is notably different from its starting material with a new distinct singlet appearing at δ 6.18 ppm. A single crystal X-ray diffraction study confirmed the molecular structure of 2a (Figure 1, left), which features a trigonal planar

Figure 1. Solid-state structures of 2a (left) and 3b (right) with thermal ellipsoids set at 30% probability. Hydrogen atoms, with the exception of the C(8)H and C(2)H, respectively, are omitted. Selected bond lengths (Å) and angles (deg): 2a, B(1)−C(1) 1.5118(15), C(1)− C(21) 1.3972(15), C(1)−C(8) 1.5152(14), C(21)−C(1)−C(8) 120.69(9); 3b, B(1)−C(1) 1.491(3), P(1)−C(1) 1.7343(17), C(1)−C(2) 1.501(2), P(1)−C(1)−C(2) 118.34(12).

CA bearing the boron adduct (sum of angles = 359.8°) with a CA−B bond distance of 1.512 Å. The solid-state structure also aided in the assignment of the observed singlet at 1H NMR δ 6.18 ppm (vide supra) as the tertiary CB−H originating from the hydride of B-pin. The formation of 2a likely proceeds by nucleophilic attack of Lewis basic CA on electron deficient boron, while the hydride of B-pin selectively attacks the CB site (at the NHC fragment bearing the bulkier isopropyl side arms, while for 2a′ at CB of the NHC fragment with methyl side arms). This suggests that the CDC framework possesses a lowlying empty orbital at CB. Similar reactivity was observed with combinations of B-pin, borabicyclononane (9-BBN), and carbones 1a and 1b (bearing one NHC and one Ph3P unit) to generate a 1.6:1 isomeric mixture of 3a/3a′ (see Supporting

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RESULTS AND DISCUSSION The activation of E−H bonds by carbodicarbenes was first investigated with carbodicarbene 1a and borane compounds (E = B, Scheme 2a). The activation of B−H is a transformation of fundamental importance in organic and organometallic chemistry.13 Addition of carbodicarbene 1a to pinacol borane 12831

DOI: 10.1021/jacs.7b08031 J. Am. Chem. Soc. 2017, 139, 12830−12836

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boron serves as the Lewis acid. In CDCs, however, the strongly nucleophilic CA serves as the Lewis basic site, and the carbon of the NHC component, CB, surprisingly exhibits significant Lewis acidic behavior. To further demonstrate the bifunctional or ambiphilic-like reactivity of carbones, we explored the C−H bond activation of phenyl acetylene (PhCCH); a common process for NHC → B(C6F5)3 FLPs.16a Reaction of 1b with PhCCH yielded compound 8 as the major product (46% yield) and compound 9 (24% yield) as the minor product (Scheme 3a). Respective molecular structures were confirmed

Information), 2b and 3b, respectively; as confirmed in the latter case by the appearance of characteristic doublets at 1H NMR δ 5.74 and δ 5.69 ppm, resulting from coupling with nearby phosphorus. Single crystal X-ray diffraction studies confirmed the molecular structures of 3a′ (see Supporting Information, Figure S2) and 3b (Figure 1, right). Structural features are largely similar to 2a, warranting no further discussion. The formation of 2 and 3 prompted exploration of potentially similar activation behavior for carbones with other E−H bonds (E ≠ B). Attention then turned to primary and secondary silanes (phenyl- and diphenylsilane); prototypical hydride donors in organic synthesis.14 The addition of phenylsilane and diphenylsilane to 1b occurred readily at ambient temperature, and the corresponding compounds 4 and 5 were obtained in high yield, 84% and 80%, respectively (Scheme 2c,d). The 13C NMR spectrum exhibits the loss of carbone signal (δ 64.4 ppm) and appearance of new doublets attributed to CA−Si at δ 10.9 (4) and 13.6 ppm (5). The 1H NMR spectra of 4 and 5 also reveal doublets at δ 4.82 and 5.06 ppm, respectively, attributed to the formation of a new CB−H moiety. The solidstate structures of 4 and 5 (Figure 2) were confirmed by single-

Scheme 3. Reactions (a−c) of 1b Highlighting Amphiphiliclike Reactivity

Figure 2. Solid-state structures of 4 (left), 5 (middle), and 6 (right) with thermal ellipsoids set at 30% probability. Hydrogen atoms, with the exception of the C(2)H, C(1)H, and C(2)H, respectively, are omitted. Selected bond lengths (Å) and angles (deg): 4, Si(1)−C(1) 1.8250(17), P(1)−C(1) 1.6913(17), C(1)−C(2) 1.491(2), P(1)− C(1)−C(2) 121.72(12); 5, Si(1)−C(10) 1.8315(12), P(1)−C(10) 1.7001(12), C(10)−C(1) 1.4921(16), P(1)−C(10)−C(1) 120.07(8).

crystal X-ray diffraction and support the structural assignment. Furthermore, they are consistent with a similar 1,2-oxidative addition as observed for boranes (vide supra). Interestingly, reaction of phenylsilane with carbodicarbene 1a led to the formation of 6 (confirmed by single-crystal X-ray, Figure 2); a different reaction pathway than that observed for 1b. The crystal structure of 6 reveals C−N bond breakage in one of the dimethyl-benzimidazoyl cores accompanied by a ring opening with insertion of the silylene fragment. We propose this occurs via a 2-fold hydride shift from the phenylsilane to CA and CB of 1a. Ring expansion with silane reagents has previously been reported for related NHCs.15 The 1,2-addition of Si−H or B−H bonds across the CA−CB bond of CDCs represents a reaction pathway that is remarkably different than the 1,1-addition reactivity reported for the ubiquitous NHC or CAAC frameworks. However, it resembles to a reasonable degree the type of bond activations commonly observed over the bifunctional sites of carbene-based FLPs;16 usually of the type NHC → B(C6F5)3, which have been investigated extensively. For such systems, the NHC carbon, as expected, is the Lewis basic site, and the electron-deficient

Figure 3. Solid-state structures of 8, 9, and 11 with thermal ellipsoids set at 30% probability. Solvent molecules and hydrogen atoms have been omitted for clarity. The compounds 8 and 9 result from the reaction of terminal alkyne with 1b. The compound 11 is a product derived from the reaction of isocyanate with 1b.

by single-crystal X-ray studies (Figure 3). Analysis of 8 revealed that two alkynes are incorporated onto the carbone scaffold; an alkenyl [PhCHCH]- at CA and an alkynyl PhCC- at CB. In accordance with our previously observed 1,2-additions (vide supra), we propose deprotonation of terminal alkyne by nucleophilic CA to furnish 7a. The stepwise attack by the anionic acetylenic fragment on the comparatively more electrophilic CB position of 7a generates 7b. A subsequent insertion of alkyne into 7b affords a major product 8 containing the PhCHCH- functionality.17 Some of intermediate 7b may isomerize facilely to 7c, which converts to minor product 9 via an intramolecular cyclization/insertion (Scheme 3a). 12832

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Journal of the American Chemical Society To better understand the mode of interaction for carbones toward small molecules, the reactivity of t-butylisocyanide with 1b was explored (Scheme 3b). In this instance, no new products were observed, indicating that reactivity of carbones is likely governed by a stepwise mechanism, which is not initiated through the electrophilic site, CB (Scheme 3b). In contrast, treatment of isocyanate, an electrophilic reagent, with 1b facilitated the formation of 11 (Scheme 3c) in excellent yield (88%). The X-ray structure of 11 revealed that two isocyanate molecules are incorporated into 1b, forming a six membered heterocarbocycle containing quaternary carbon, which annulated with the benzimidazole fragment (Figure 3). This result suggests a synergistic stepwise process originating with a nucleophilic attack by the central carbon to afford the proposed zwitterionic intermediate 10, followed by rapid cyclization with another isocyanate, initiated by the electrophilic carbon center at the benzimidazole unit, ultimately yielding 11 (Scheme 3c). The unique synergistic reactivity generated by the CDC framework shares certain characteristics with FLPs,18 but it also falls short of fitting precisely into the general definition for the classical FLP on two grounds: (i) It has no pronounced reactivity with electron-rich isocyanides, which means the electrophilic site of CDCs requires activation by synergistic nucleophilic attack at the center carbon. (ii) No reversible heterolytic cleavage of H2 is observed with hydrogen gas, a long-standing benchmark reaction for FLPs. The bond-activation activity of CDCs reported thus far (vide supra) strongly suggests a surprising level of π-accepting ability at the central carbon of the NHC moiety in the CDC framework (CB). The manifestation of this attribute for the interaction of CDCs with small organic molecules is best described as 1,2-reactivity across an unexpected ambiphilic site. To further illustrate the hidden π-accepting ability, we sought to explore a system where this topological feature would be highlighted exclusive of other addition/redox reaction processes. Introduction of 1a to PdCl2[P(OiPr)3] afforded complex 12 (Figure 4), as confirmed by single-crystal X-ray studies. Complex 12 features a square planar palladium center, bound to two cis-chlorides, and an η1-carbodicarbene and phosphite that were also cis with each other. Notably, we observe a short interatomic distance between O3 and C2 (2.890 Å) (Figure 4), a value that is smaller than the sum of van der Waals radii (3.27 Å),19 suggesting attractive interactions between the atoms. Combined with the observed reactivity toward small organic molecules, this provides strong evidence for the proposed activated π-acidity of CB located on the NHC unit of CDCs. Bertrand and co-workers have also previously observed similar nominal acceptor ability in a protonated CDCderivative salt. The protonated carbone based on two benzoxazol-2-ylidenes was shown not to be amenable to deprotonation by KOtBu, but rather the base added to the CB center.20 For unsaturated NHC, a 1,1-addition only happened with NH3−BH3 or H2 under metal catalysis.21 Nonetheless, no oxidative addition occurred for the reaction of 9-BBN or pinacol borane with unsaturated NHC, but a only normal Lewis acid−base adduct formation was observed, illustrating that the electrophilicity of the NHC is too low for oxidative addition of small molecules even after binding to electron deficient boron compounds.22 Quantum chemical calculations at the BP86(D3)/def2-SVP level of the geometry and analysis of the electronic structure of the palladium complex 12 with the QTAIM (Quantum Theory of Atoms in Molecules) method were performed.23 The

Figure 4. (top) Synthesis of 12 and schematic representation of πaccepting interaction observed therein; (bottom right) solid-state structure of 12 with thermal ellipsoids set at 30% probability and hydrogen atoms omitted for clarity. (bottom left) Laplacian distribution ∇2ρ(r) in the C1C2O3 plane of 12. Dashed red lines indicate areas of charge concentration (∇2ρ(r) < 0), while solid blue lines show areas of charge depletion (∇2ρ(r) > 0). The solid black lines (dashed for O3−C2) connecting the atomic nuclei are the bond paths and the small dots are the bond critical points (bcp).

optimized geometry is in excellent agreement with the experimental structure (Figure 4, right). Figure 4 (bottom left) shows the Laplacian distribution, ∇2ρ(r), in the C1C2O3 plane of 12. There is a bond critical point, C2−O3, which indicates attractive electronic interactions between oxygen and carbon. This should not be confused with a genuine chemical bond, but it does signal some binding interactions between O3 as donor and C2 as acceptor.24 We calculated the reaction courses for addition of 1a and 1b to B-pin and 9-BBN. Figure 5 (top) shows the result for the reaction 1a + B-pin → 2a/2a′. The other three addition reactions exhibit very similar reaction profiles (see Figures S11−S13, Supporting Information). The borane additions take place as two-step reactions. The first step is an exergonic process with very low activation barriers, which gives the carbon → borane adduct as intermediate. The second step is a hydride migration from boron to the central carbon atom of the NHC ligand. It has a slightly higher barrier than the first step of the reaction, but it is also exergonic. Two different isomers, 2a and 2a′, may be formed as reaction products, because the NHC ligands of 1a carry different substituents. The calculations suggest that the hydride migration to the more bulky NHC ligand, which carries isopropyl substituents at nitrogen leading to 2a, is kinetically and thermodynamically favored over the formation of 2a′. This is in agreement with the experimental data where the product ratio 2a/2a′ is 4.3:1. Table 1 shows the activation barriers and reaction energies of the four addition reactions. It holds for both steps of reactions 1−4 that there is no correlation between the height of the activation barriers and the reaction energies. Note the orientation of the B−H hydride moiety in the intermediate INT toward CB and the relatively low activation barriers for hydride migration. This supports the assertion that the NHC fragment in the carbones serves as an electron acceptor. The 12833

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Figure 5. (top) Calculated energy profile at the BP86+(D3BJ)/def2-SVP level for the addition of carbodicarbene 1a to the borane B-pin leading to the product 2a/2a′. Key interatomic distances are given in angstroms. Trivial hydrogen atoms have been omitted for clarity. (bottom) (a) Plot of the LUMO of 1a(H+). (b) Deformation density, Δρ1, of the transition state TS2 for the reaction 1b + 9-BBN. (c) Deformation density, Δρ2, of the transition state TS2 for the reaction 1b + 9-BBN. The direction of the charge flow is red → blue.

Table 1. Calculated Reaction Energies, ΔG (ΔE),a at BP86(D3)/def2-SVP

a

no.

reaction

educt

1 1′ 2 3 3′ 4

1a + B-pin → 2a 1a + B-pin → 2a′ 1b + B-pin → 2b 1a + 9-BBN → 3a 1a + 9-BBN → 3a′ 1b + 9-BBN → 3b

0.0 0.0 0.0 0.0 0.0 0.0

TS1 7.0 5.3 2.4 5.8 4.5 2.9

(−9.1) (−10.1) (−12.1) (−8.5) (−9.5) (−12.1)

INT −2.8 −2.9 −5.8 −7.2 −14.3 −18.2

TS2

(−20.4) (−21.7) (−24.3) (−27.4) (−36.1) (−37.3)

8.7 11.3 8.0 −2.5 −3.5 −7.4

(−5.6) (−2.6) (−5.8) (−20.8) (−21.5) (−22.6)

ΔTS2b 11.5 14.2 13.8 4.7 10.8 10.8

(14.8) (19.1) (18.5) (6.6) (14.6) (14.7)

product −18.8 −18.2 −30.0 −24.0 −22.3 −39.2

(−37.6) (−36.6) (−46.9) (−44.6) (−42.4) (−58.6)

Δproductc −16.0 −15.3 −24.2 −16.8 −8.0 −21.0

(−17.2) (−14.9) (−22.6) (−17.2) (−6.3) (−21.3)

Energies in kcal/mol. bActivation barrier for the second step ΔTS2 = INT − TS2. cReaction energy of the second step Δproduct = INT − product.

calculated migration step in reaction 3a/3a′ also suggests that the hydride movement toward the bulkier NHC ligand yielding 3a is kinetically and thermodynamically favored over the formation of 3a′, which concurs with the experimental finding (see Supporting Information). We analyzed the electronic structures of the educts and transition states for factors which support the notion that the NHC ligand of the carbones serves as an electron acceptor once the central carbon binds to borane. The obvious species is the reaction intermediate INT, which is the educt for the

hydride migration. The vacant orbitals of the intermediates are fairly delocalized over the π system. We therefore calculated protonated 1a where the proton serves as a model for a Lewis acid. Figure 5 (bottom, a) shows that the LUMO of 1a(H+) indeed has the largest coefficients at atom CB of the NHC ligands. We also analyzed the transition state TS2 for hydride migration with the EDA-NOCV method,25 which makes it possible to visualize the change in the electronic structure that is associated with the bond formation between chosen fragments in a molecule. This is done by plotting the 12834

DOI: 10.1021/jacs.7b08031 J. Am. Chem. Soc. 2017, 139, 12830−12836

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deformation density, which is connected to the pairwise orbital interactions. The calculation was carried out for TS2 of reaction 4 (1b + 9-BBN). The chosen fragments are 1b and 9-BBN with the frozen geometries of TS2. Thus, the deformation densities come mainly from the C → B bond in TS2 and only a smaller part comes from the nascent C−H bond. The plots in Figure 5 (bottom, b and c) show the two largest deformation densities ρ1 and ρ2, where the direction of the charge flow is red → blue. Although they are not strictly separated, it becomes obvious that ρ1 comes mainly from the C → B bond while ρ2 is associated with the emerging C−H bond. The shape of the latter clearly indicates that there is a charge flow from the B−H bond toward atom CB of the NHC ligand. The full set of the EDA-NOCV calculations is given in Table S11.

Gernot Frenking: 0000-0003-1689-1197 Tiow-Gan Ong: 0000-0001-9817-6300 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science & Technology of Taiwan (MOST-104-2628-M-001-005-MY4 grant and MOST-105-2113-M-001-027-MY2) and Academia Sinica Career Development Award (104-CDA-M08). The theoretical work was supported by Nanjing Tech University (Grant Numbers 39837132 and 39837123) and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials.

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CONCLUSION In summary, we have demonstrated a new reaction path for carbodicarbenes; the less explored members of the carbenic complex/ligand family. The observed 1,2 addition of E−H bonds (E = B, C, Si) across the carbone central carbon (CA) and that of the flanking NHC fragment (CB) suggests ambiphilic-type activity. Observed reactivity toward small organic molecules, combined with single-crystal X-ray studies of a model Pd complex, strongly suggests a significant level of hidden π-accepting ability at the central carbon of the NHC moiety of the carbone. Classical NHCs are strong donors, with only nominal π-accepting ability, the manifestation of which is typically as acceptors of π-back bonding from the element to which the NHC is σ-donating.26 Herein we observe what is formally an NHC fragment within the carbone framework displaying hidden π-accepting ability from a σ-lone pair. This unforeseen π-acidity is critical for facilitating the 1,2-addition of E−H to carbones. Further elaboration of this nonclassical 1,2 reactive behavior is compelling for the development of novel organocatalytic reaction pathways and frames the focus of ongoing studies in our group.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08031. Detailed experimental procedures and 1H and 13NMR spectra of all compounds, X-ray crystallographic data, and computational calculations (PDF) Crystallographic structure of 2a (CIF) Crystallographic structure of 3a′ (CIF) Crystallographic structure of 3b (CIF) Crystallographic structure of 4 (CIF) Crystallographic structure of 5 (CIF) Crystallographic structure of 6 (CIF) Crystallographic structure of 8 (CIF) Crystallographic structure of 9 (CIF) Crystallographic structure of 11 (CIF) Crystallographic structure of 12 (CIF)

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DOI: 10.1021/jacs.7b08031 J. Am. Chem. Soc. 2017, 139, 12830−12836

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DOI: 10.1021/jacs.7b08031 J. Am. Chem. Soc. 2017, 139, 12830−12836