Highly Selective Palladium-Catalyzed Hydroborylative

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Cite This: J. Am. Chem. Soc. 2018, 140, 14324−14333

Highly Selective Palladium-Catalyzed Hydroborylative Carbocyclization of Bisallenes to Seven-Membered Rings Can Zhu,*,† Bin Yang,† Binh Khanh Mai, Sara Palazzotto, Youai Qiu, Arnar Gudmundsson, Alexander Ricke, Fahmi Himo,* and Jan-E. Bäckvall* Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden

J. Am. Chem. Soc. 2018.140:14324-14333. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/31/18. For personal use only.

S Supporting Information *

ABSTRACT: A highly selective palladium-catalyzed hydroborylative carbocyclization of bisallenes to afford seven-membered rings has been established. This ring-closing coupling reaction showed good functional group compatibility with high chemoand regioselectivity, as seven-membered ring 3 was the only product obtained. The extensive use of different linkers, including nitrogen, oxygen, malononitrile, and malonate, showed a broad substrate scope for this approach. A one-pot cascade reaction was realized by trapping the primary allylboron compound with an aldehyde, affording a diastereomerically pure alcohol and a quaternary carbon center by formation of a new C−C bond. A comprehensive mechanistic DFT investigation is also presented. The calculations suggest that the reaction proceeds via a concerted hydropalladation pathway from a Pd(0)-olefin complex rather than via a pathway involving a defined palladium hydride species. The reaction was significantly accelerated by the coordination of the pendant olefin, as well as the introduction of suitable substituents in the bridge, due to the Thorpe−Ingold effect. substances.10 Due to their unfavorable transannular interactions, the synthesis of medium-sized rings is still challenging.11 Therefore, the direct hydroborylative carbocyclization of unsaturated compounds would be an efficient and powerful strategy to realize both the construction of medium-sized rings and the introduction of boron functionality in a one-step manner (Scheme 1c). Bisallenes are a class of compounds bearing two allene moieties, which have been demonstrated as powerful building blocks in organic chemistry.12,13 In 2005, Ma et al. reported a highly efficient and selective Rh(I)-catalyzed approach to steroid scaffolds from bisallenes, which showed the unique character of bisallenes in organic synthesis.13g Herein, we disclose our recent observations on the highly selective palladium-catalyzed hydroborylative carbocyclization of bisallenes to seven-membered rings (Scheme 1d).14 A onepot cascade reaction was also developed by trapping of the primary allylboron product by an aldehyde, leading to an alcohol. The key challenge was to identify a suitable catalyst system, which works nicely in both the carbocyclization and the hydroboration reaction, and the control of the involved regio- and stereoselectivity.15 DFT calculations provide important information concerning the mechanism of the reaction and suggest that the formal hydropalladation step of the allene does not involve a defined palladium hydride.

I. INTRODUCTION Organoboranes are a class of highly useful compounds in organic synthesis, owing to the fact that they are fundamental building blocks for complex molecules.1,2 They have found wide applications in various carbon−carbon (C−C) bondforming reactions, in particular in Suzuki cross-coupling reactions as late-stage steps in synthetic chemistry.3−6 The direct hydroboration of unsaturated compounds (e.g., alkenes and alkynes) with boranes provides a direct and efficient approach to organoboranes, given the fact that these starting materials are readily accessible compounds (Scheme 1a).7 However, the reaction outcome of this chemistry is mostly limited to the anti-Markovnikov selectivity and syn-addition product(s). In recent years significant progress in the development of novel methodologies involving organoboranes has occurred, in particular on methods catalyzed by transition metals. This is exemplified by recent reviews on Suzuki−Miyaura coupling3 and organoboron compounds involved in photocatalysis.8 Important methods to prepare organoboranes that have received much attention include catalytic hydroboration, where regio- and stereoselectivity are controlled by the transition metal catalyst system (Scheme 1b).9 The metal hydride ([M]−H) is considered as the active species that promotes the hydroboration reaction. However, in this case the functional-group tolerance will be challenging, since many groups may react with the metal hydride. On the other hand, the construction of medium-sized rings has attracted increasing attention, owing to their broad occurrence as the core structure in various natural products and pharmacologically active © 2018 American Chemical Society

Received: August 13, 2018 Published: October 3, 2018 14324

DOI: 10.1021/jacs.8b08708 J. Am. Chem. Soc. 2018, 140, 14324−14333

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Journal of the American Chemical Society Scheme 1. Previous Studies and This Work

Scheme 2. Initial Attempta

II. RESULTS AND DISCUSSION II.A. Development of Pd-Catalyzed Hydroborylative Carbocyclization of Bisallenes. Based on the concept shown in Scheme 1c and d, we initially chose bisallene 1a as the standard substrate to investigate the reactivity in this hydroborylative carbocyclization. When the reaction of 1a with HBpin (1.3 equiv) was run in toluene in the presence of Pd(OAc)2 (5 mol %), no product was detected after 3 h at 50 °C, and 1a was recovered in 94% yield. To our delight, when B2pin2 (1.3 equiv) together with MeOH (1.0 equiv) were used in place of the active borane (HBpin), the seven-membered carbocycle 3a was generated in 56% yield (Scheme 2). Surprisingly, the proposed isomers of type A and type B (Scheme 1d) were not formed during the reaction, showing the high selectivity of this approach. Finally, the formation of 3a was not detected when HBpin and MeOBpin were employed together.16 We next turned to optimizing the reaction conditions using bisallene 1a as the standard substrate. Under the initial reaction conditions at 50 °C, 3a was detected in 56% yield (Table 1, entry 1). Changing the temperature to 40 or 60 °C did not improve the outcome of the reaction, but rather led to a slight decrease in yield in both cases (Table 1, entries 2, 3).

a Yield was determined by 1H NMR analysis with anisole as the internal standard. bMeOH (1.0 equiv) and B2pin2 (1.3 equiv) were used. cHBpin (1.1 equiv) and MeOBpin (1.1 equiv) were used with THF as the solvent. HB = hydroboration.

To our delight, the use of AcOH in place of MeOH as the proton source improved the yield of the seven-membered carbocycle 3a to 67%, with much fewer side products detected (Table 1, entry 4). Solvent screening revealed that tetrahydrofuran (THF) was the best solvent, in which carbocycle 3a was produced in 70% yield (Table 1, entries 5−7). Moreover, when the amount of AcOH was kept at 1.2 14325


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Journal of the American Chemical Society Table 1. Optimization of the Reaction Conditionsa

entry

proton source (equiv)

B2pin2 (equiv)

solvent

yield of 3a (%)b

1 2c 3d 4 5 6e 7 8 9 10 11 12 13

MeOH (1.0) MeOH (1.0) MeOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (1.0) AcOH (0.9) AcOH (1.2) AcOH (1.3)

1.3 1.3 1.3 1.3 1.3 1.3 1.3 1.2 1.1 1.0 1.1 1.1 1.1

toluene toluene toluene toluene THF dioxane DCE THF THF THF THF THF THF

56 53 49 67 70 45 trace 70 71 66 65 75 (73) 72

87%, 90%, and 90% yield, respectively. When dicyclopentyland diphenyl-substituted bisallenes (1n and 1p) were employed, the seven-membered carbocycles 3n and 3p were obtained in 91% and 65% yield, respectively. However, when bisallene 1q, bearing a tetrasubstituted allene moiety, was employed, the corresponding product 3q was not formed, but direct hydroboration of one allene moiety was observed as the formation of 4 in 44% yield.17 Also a bisallene with R = H and R′ = nPr with X = NTs failed to give any product 3 (see Supporting Information p. S20 for bisallene 1r). It is noteworthy that products with seven-membered rings were the only products obtained under these reaction conditions.18 Finally, gram-scale synthesis of seven-membered carbocycle 3a was realized from bisallene 1a, which is readily obtained from commercially available compounds via a three-step synthesis without any chromatography purification (Scheme 4).19 Allylation of aldehydes using allyboron compounds has been developed as a powerful method for diastereoselective formation of alcohols.20 The high diastereoselectivity is the result of a six-membered ring transition state (Zimmerman− Traxler transition state). Since an allylboron unit is formed in products 3 (Scheme 3), we investigated whether these products could be trapped by an aldehyde, which should lead to diastereoselective formation of alcohols with a quaternary carbon in the β-position. After completion of the reaction of bisallene 1a to give allylboron product 3a under standard conditions (without removal of the solvent), pbromobenzaldehyde was added to the reaction mixture and the reaction was stirred for another 20 h at 50 °C. Workup of this one-pot cascade reaction afforded the corresponding alcohol 8a as a single diastereomer in 61% yield (Scheme 5).21 pChloro and p-fluoro benzaldehyde also proved to be compatible with the reaction conditions, and 8b and 8c were obtained in 65% and 62% yield, respectively. Electron-rich benzaldehydes, such as p-anisaldehyde and butyraldehyde, could be employed as shown by the formation of 8d and 8e in 61% and 56% yield, respectively. The reaction of bisallene 1b with two methyl groups afforded the corresponding product 8f in 54% yield. Finally, the employment of bisallenes with oxygen or malonate in the bridge also worked nicely, as shown by the formation of 8g−i in 52−64% yields. Control experiments showed that a catalytic amount of acid is essential for the allylation reaction of aldehydes with allyboron compounds (for details, see the Supporting Information). The one-pot reaction described in Scheme 5 provides a new quaternary carbon center by forming a new C−C bond, which should have potential applications in synthetic chemistry.22 II.B. Kinetic Isotope Effect. To gain a deeper insight into this Pd-catalyzed hydroborylative carbocyclization, the competitive kinetic isotope effect (KIE) was determined from the reaction of a 1:1 mixture of HOAc and acetic acid-d4 at 50 °C for 5 min (eq 1, Scheme 6). The product ratio 3a/3a-d measured was 4.05:1, while the conversion of the reaction was 20%. From these ratios the competitive KIE was determined to be kH/kD = 4.7 (for details, see the Supporting Information (SI)). Furthermore, parallel kinetic experiments using HOAc and acetic acid-d4 provided a parallel KIE (kH/kD, from the early stage of the reaction) value of 4.4 (for details, see the SI). These results indicate that the proton transfer from acetic acid to palladium or to the allene occurs during the ratedetermining step. Based on the competitive KIE, this step also has to be the first irreversible step.

a

Unless otherwise noted the reaction was conducted in the indicated solvent (1 mL) with 1a (0.2 mmol), B2pin2 (2), and proton source in the presence of Pd(OAc)2 (5 mol %) at 50 °C for 3 h. bYield determined by 1H NMR analysis using anisole as the internal standard. The number in parentheses is the isolated yield. cThe reaction was run at 60 °C. dThe reaction was run at 40 °C. e1a was recovered in 10% yield.

equiv with 1.1 equiv of B2pin2, the yield of 3a was improved further to 75% yield (Table 1, entries 8−13). Seven-membered carbocycle 3a was obtained as the only isomer, without any formation of type-A or type-B products (see Scheme 1d). Therefore, the optimized reaction conditions for additional studies were defined as Pd(OAc)2 (5 mol %), B2pin2 (1.1 equiv), and AcOH (1.2 equiv) in THF at 50 °C (Table 1, entry 12). Under the optimal reaction conditions, we next studied the scope of bisallenes in the reaction (Scheme 3). Me- or Bnsubstituted bisallene (1b or 1c) could also be employed to afford products 3b and 3c in 65% and 70% yield, respectively. Bisallene 1d, bearing two remote olefins, also worked well to produce 3d in 55% yield. The remote olefin group survived during the reaction, showing the high selectivity of this approach. Moreover, an ester as a functional group was also compatible with the reaction conditions as shown by the formation of 3e in 72% yield. Furthermore, a series of secondary substituents were investigated as well: when R is iPr, cyclopentyl, or cyclohexyl, the reaction afforded 3f−h in good yields with excellent selectivities. Next, we turned to investigating the effect of the linker connecting the two allene moieties. Bisallene 1i, without the olefin in the bridge, was therefore prepared. The reaction of 1i under standard conditions was very slow and gave only 20% yield of 3i after 3 h reaction time. However, 1i could be cyclized to 3i in 74% yield on prolongation of the reaction time to 30 h. Interestingly, the reactions of bisallenes 1j (X = NTs) and 1k (X = O) under the standard reaction conditions (3 h) gave the hydroborylated products 3j (76%) and 3k (70%), respectively, as the only product. Bisallene bearing the malononitrile (1l) or malonate (1m or 1o) moiety in the bridge produced the corresponding products 3l, 3m, and 3o in 14326


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Journal of the American Chemical Society Scheme 3. Substrate Scope

the activation barrier for the formation of Pd(0) is calculated to be 5.6 kcal/mol higher than TS-2Pre (see Figure S5 in the SI for optimized structures). This result clearly shows that the coordination of substrate 1a facilitates the formation of Pd(0) active catalyst. Interestingly, in Int-0, both of the allene moieties and also the olefin linker coordinate to the metal ion (see Figure 1). Structures of other coordination modes of Pd(0) with bisallene 1a are also optimized, which are calculated to be higher in energy than Int-0 (see Figure S6 in the SI for details). A structure in which an AcOH, the alkene, and only one of the allenes bind to Pd(0) could also be located. However, this structure was found to be 2.8 kcal/mol higher in energy than Int-0, and it is therefore not included in Scheme 7. In general, the expected route to reach the vinylpalladium intermediate Int-1 from Int-0 is through a Pd-hydride species from protonation of Pd(0) that can then undergo hydropalladation.24 However, the calculations show that this mechanism is associated with very high energies. The Pd(II)hydride intermediate is calculated to be 19.9 kcal/mol higher in energy than Int-0, and the barrier for the formal oxidative addition of the acetic acid to Pd(0) leading to the Pd(II)hydride is calculated to be 33.7 kcal/mol (see Figure S7 in the SI for details). Instead, the calculations indicate that Int-1 is reached from Int-0 in a single step, in which the proton of the

Scheme 4. Gram-Scale Synthesis of 3a

II.C. Computational Investigations. Density functional theory (DFT) calculations, using substrate 1a as a representative case, were carried out to gain further insight into the mechanism of the hydroborylative carbocyclization reaction (see the SI for the computational protocol). On the basis of these calculations the mechanism shown in Scheme 7 is proposed. The calculations first show that the Pd(0) active catalyst, Int-0, can be easily formed from the Pd(OAc)2 precatalyst.23 The details of this activation step, involving substrate coordination and transmetalation with B2pin2, are given in Figure S5 in the SI. Without the coordination of substrate 1a, 14327


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Journal of the American Chemical Society Scheme 5. One-Pot Cascade Reaction of Bisallene with B2pin2 and an Aldehydea

Scheme 7. Reaction Mechanism for the Palladium-Catalyzed Hydroborylative Carbocyclization Proposed on the Basis of Current DFT Calculations

a

For 8d: t2 = 44 h. bFor 8f: t1 = 4 h. cFor 8i: the reaction temperature was increased to 60 °C after the addition of the aldehyde.

acetic acid transfers to the terminal carbon of the allene.25 At the transition state (TS-1, Figure 1), the distances to the transferring hydrogen atom from the acetate oxygen atom and terminal carbon atom are 1.41 and 1.36 Å, respectively. Interestingly, the distance between the transferring hydrogen atom and Pd is 2.07 Å, which is too long for a palladium hydride. In TS-1, the second allene moiety loses its coordination to the metal (Figure 1). The barrier for this reaction is calculated to be as low as 14.3 kcal/mol (Figure 2), which is considerably lower than the pathway involving a

palladium hydride intermediate. The low barrier can be rationalized considering the fact that in the Pd(0) complex the strong back-donation makes the olefin electron-rich and therefore prone to protonation. These results are consistent with recent calculations on the palladium-catalyzed cycloisomerization of enynamide, in which a similar mechanism was obtained for this step.26 Here, it should be mentioned that another conformer of TS-1 was also located in which the allene moiety rather than the alkene linker coordinates to the Pd, leading directly to Int-2. However, this conformation was

Scheme 6. Kinetic Isotope Effect

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Journal of the American Chemical Society

Figure 1. Optimized structures of stationary points for the palladium-catalyzed hydroborylative carbocyclization. For clarity, the methyl groups on the Bpin ligands are omitted in all structures except Int-7. Bond distances are in Å.

energy than σ-allylpalladium complex Int-3, i.e., −37.4 kcal/ mol relative to Int-0. Subsequent transmetalation from Int-4 occurs to produce Int-6. This transmetalation consists of two steps: a nucleophilic attack (TS-4) and a B−B bond cleavage (TS-5). The energies of TS-4 and TS-5 are calculated to be +10.9 and +15.3 kcal/ mol relative to Int-4, respectively. At Int-6, an agostic interaction between the C−H bond of the nPr substituent and the Pd ion is observed. It is interesting to note that transmetalation directly from Int-3 was also considered and found to have an overall barrier of 16.8 kcal/mol (see Figure S8 in the SI for details). This barrier is much higher than the barrier (via TS-3) leading to π-allyl complex Int-4. The last step of the reaction is the reductive elimination of Int-6 to generate Int-7. The barrier of this step is also low,

found to be 3.7 kcal/mol higher than TS-1 (see Figure S11 in the SI for the optimized structure). For the carbocyclization to take place, the coordination to Pd in Int-1 has to change from the olefin linker to the allene moiety. This ligand change gives vinyl-allene palladium intermediate Int-2, which is 2.2 kcal/mol higher in energy than Int-1. Carbocyclization via TS-2 then occurs to generate the seven-membered-ring σ-allylpalladium intermediate, Int-3. The barrier for the carbocyclization is calculated to be 15.5 kcal/mol relative to Int-1 (Figure 2). The carbocyclization step is followed by a rearrangement to produce a more stable πallylpalladium intermediate, Int-4. The activation energy of this step is calculated to be 5.9 kcal/mol relative to Int-3, and the π-allylpalladium intermediate Int-4 is 12.9 kcal/mol lower in 14329


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Figure 2. Calculated free energy profile (kcal/mol) for the palladium-catalyzed hydroborylative carbocyclization.

Figure 3. Calculated free energy profiles (kcal/mol) for the palladium-catalyzed hydroborylative carbocyclization of substrates 1m and 1i.

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Figure 4. Optimized structures of the initial step for substrates 1i and 1m.

consistent with the experimental data showing that no product B was found in this reaction (see Figure S10 in the SI). Finally, we have also studied the influence of the bisallene linker on the energy profile by considering the reactions of substrates 1m (with a C(CO2Me)2 linker) and 1i (with a CH2 linker and also an iPr substituent instead of nPr). The obtained energy graphs are given in Figure 3. Experimentally, the overall reaction of 1i was observed to be about one order of magnitude slower than the reactions of 1a and 1m, which were equally fast (Scheme 3). As seen from Figure 3, substrates 1i and 1m follow the same reaction mechanism as 1a (Figure 2), with essentially the same energy profiles. Some interesting differences can, however, be observed, in particular in the energies of the initial step. Since the olefin linker is absent in 1i and 1m, intermediate Int-1 cannot be formed for these substrates, and therefore TS-1 results directly in Int-2 (the optimized structures for Int-0, TS1, and Int-2 for substrates 1i and 1m are shown in Figure 4). The energy of this intermediate is significantly higher for 1i and 1m compared to 1a (+2.3 and −1.1 kcal/mol for 1i and 1m, respectively, compared to −9.2 kcal/mol for 1a). The lack of coordination of the linker also leads to the barrier of the second step via TS-2 becoming lower for 1i and 1m compared to 1a (8.8 and 5.9 kcal/mol, respectively, vs 15.5 kcal/mol). In addition, the Thorpe−Ingold effect, present for 1m but not 1i, contributes further to the lowering of the energy of TS-2 for 1m. It is interesting to mention that at the hydropalladation TS1m the ester CO bond in 1m can coordinate to the Pd (analogously to the olefin linker in TS-1 for 1a). However, this conformation is calculated to be 7.4 kcal/mol higher than TS1m (see Figure S11 in the SI for the optimized structure).

amounting to 10.6 kcal/mol. Finally, to close the catalytic cycle, a ligand exchange can take place, releasing product 3a and binding a new bisallene substrate (1a). The calculations show that the overall cycle is exergonic by as much as 76.5 kcal/mol. As seen from the full free energy profile given in Figure 2, the reactions proceeding via TS-1, TS-2, and TS-5 have quite similar energy barriers of 14.3, 15.5, and 15.3 kcal/mol, respectively. It is therefore not possible on the basis of only the calculations to single out which one of these steps is rate-determining. However, for substrate 1a, the KIE measurements discussed above (Scheme 6) clearly show that TS-1 is the rate-determining step, which indicates that the energy of this TS is somewhat underestimated compared to the other two TSs in the calculations. The fact that there are three transition states associated with barriers that are close in energy is indeed a very interesting finding, because it indicates that even small alterations of the substrate (e.g., the use of a different linker or different substituents) can lead to a change in the nature of the rate-determining step. Here it is important to mention that we also explored the possibility that the seven-membered-ring product A in Scheme 1d could be formed from Int-6. This requires a rearrangement of Int-6 to the other σ-allyl, Int-6′ (see Figure S9 in the SI). The barrier for reductive elimination from Int-6′ via transition state TS-6′ is calculated to be 18 kcal/mol, and Int-6′ is 6.3 kcal/mol higher than Int-6, which makes the overall barrier from Int-6 to be 24.3 kcal/mol. The latter barrier is 13.7 kcal/ mol higher in energy than the barrier via TS-6 (see Figure S9 in the SI for details). This result is consistent with the fact that product A could not be observed experimentally. Furthermore, we also considered the formation of the six-membered-ring product B (see Scheme 1d) from Int-2 via carbocyclization transition state (TS-3′). The energy of this transition state was calculated to be 4.3 kcal/mol higher than TS-2, which is also 14331


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Journal of the American Chemical Society Notes

III. CONCLUSIONS In conclusion, we have developed an efficient palladiumcatalyzed carbocyclization−borylation of bisallenes to sevenmembered rings in good to excellent yields (up to 91% yield). The reaction is highly selective, as shown by the exclusive formation of one isomer of the seven-membered-ring product from bisallenes. In addition to the olefin unit, different substituents including nitrogen, oxygen, malononitrile, and malonate in the bridge also resulted in a fast hydroborylative carbocyclization. The introduction of suitable substituents in the bridge, including nitrogen, oxygen, malononitrile, and malonate, also favored such transformations due to the Thorpe−Ingold effect. A one-pot cascade reaction was realized by addition of an aldehyde to the in-situ-formed allylboron product, providing diasteroselective formation of sevenmembered-ring alcohols bearing a new quaternary carbon center. The detailed mechanism of the carbocyclization−borylation was investigated by means of DFT calculations, which revealed that the initial step takes place via a concerted hydropalladation. The more expected oxidative addition of the acetic acid leading to an intermediate Pd(II)-hydride was found to be associated with much higher energy barriers. These results might at first sight seem somewhat counterintuitive. However, the low barrier for the concerted protonation mechanism can be understood in terms of the Pd(0) providing a strong backdonation that makes the olefin electron-rich and therefore prone to protonation. Similar results were obtained by Anderson and co-workers for the palladium-catalyzed cycloisomerization of enynamide.26 Finally, due to the success of our approach in the selective synthesis of seven-membered rings bearing a boron functionality, this method will be highly attractive in synthetic chemistry. Further studies on the scope and asymmetric variants of the new reactions are currently underway in our laboratory.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swedish Research Council (201603897, 2018-00830), the Berzelii Center EXSELENT, and the Knut and Alice Wallenberg Foundation is gratefully acknowledged.

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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.8b08708.

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Experimental procedures and compound characterization data, including the 1H/13C NMR spectra, computational details, additional figures of intermediates and transition states, absolute energies and energy corrections, and Cartesian coordinates of optimized structures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] (C. Zhu) *[email protected] (F. Himo) *[email protected] (J.-E. Bäckvall) ORCID

Can Zhu: 0000-0001-6604-6173 Binh Khanh Mai: 0000-0001-8487-1417 Fahmi Himo: 0000-0002-1012-5611 Jan-E. Bäckvall: 0000-0001-8462-4176 Author Contributions †

C. Zhu and B. Yang contributed equally to this work. 14332


Article

Journal of the American Chemical Society 390. (b) Vandewalle, M.; De Clercq, P. Tetrahedron 1985, 41, 1765. (c) Yet, L. Chem. Rev. 2000, 100, 2963. (d) Blanchard, N.; Eustache, J. In Metathesis in Natural Product Synthesis: Strategies, Substrates and Catalysts; Cossy, J.; Arseniyadis, S.; Meyer, C., Eds.; Wiley-VCH: Weinheim, 2010; pp 1−43. (11) Anslyn, E. V.; Dougherty, D. A., Eds. Modern Physical Organic Chemistry; University Science Books: Herndon, VA, 2005. (12) For a highlight review of bisallenes, see: Chen, G.; Jiang, X.; Fu, C.; Ma, S. Chem. Lett. 2010, 39, 78. (13) For selected examples on bisallenes, see: (a) Jiang, X.; Cheng, X.; Ma, S. Angew. Chem., Int. Ed. 2006, 45, 8009. (b) Lian, X.; Ma, S. Chem. - Eur. J. 2010, 16, 7960. (c) Cheng, J.; Jiang, X.; Ma, S. Org. Lett. 2011, 13, 5200. (d) Inagaki, F.; Narita, S.; Hasegawa, T.; Kitagaki, S.; Mukai, C. Angew. Chem., Int. Ed. 2009, 48, 2007. (e) Kim, S. M.; Park, J. H.; Kang, Y. K.; Chung, Y. K. Angew. Chem., Int. Ed. 2009, 48, 4532. (f) Volla, M. R.; Bäckvall, J.-E. ACS Catal. 2016, 6, 6398. (g) Ma, S.; Lu, P.; Lu, L.; Hou, H.; Wei, J.; He, Q.; Gu, Z.; Jiang, X.; Jin, X. Angew. Chem., Int. Ed. 2005, 44, 5275. (h) Zhu, C.; Yang, B.; Qiu, Y.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2016, 55, 14405. (i) Naidu, V. R.; Posevins, D.; Volla, C. M. R.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2017, 56, 1590. (14) For reviews on seven-membered rings, see: (a) Nguyen, T. V.; Hartmann, J. M.; Enders, D. Synthesis 2013, 45, 845. (b) Kantorowski, E. J.; Kurth, M. J. Tetrahedron 2000, 56, 4317. (15) (a) For a review on the control of regio- and stereoselectivity in allene chemistry, see: Ma, S. Pure Appl. Chem. 2007, 79, 261. (b) Yuan, W.; Ma, S. Adv. Synth. Catal. 2012, 354, 1867. (16) For detailed comparative experiments using different reagent(s) for the hydroboration of bisallenes, see the SI. One reviewer suggested that bis(pinacolato)diboron (B2pin2) may react with acetic acid to give H-B(pin) and AcO-B(pin). The reactions of B2pin2 with acetic acid in THF at 50 °C in the absence or presence of Pd(OAc)2 did not give any new boron species and B2pin2 was recovered in 96% and 100% yield, respectively (see the SI). (17) When bisallene 1q, bearing a tetrasubstituted allene moiety, was employed under the standard reaction conditions, the direct hydroboration of one allene moiety was observed as the formation of 4 in 44% yield. The corresponding product 3q was not detected.

reported. For selected reviews on the relative asymmetric allylation, see: (a) Yus, M.; González-Gómez, J. C.; Foubelo, F. Chem. Rev. 2011, 111, 7774. (b) Diner, C.; Szabó, K. J. J. Am. Chem. Soc. 2017, 139, 2. (23) (a) Zhu, C.; Yang, B.; Jiang, T.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2015, 54, 9066. (b) Zhu, C.; Yang, B.; Bäckvall, J.-E. J. Am. Chem. Soc. 2015, 137, 11868. (c) Qiu, Y.; Yang, B.; Zhu, C.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2016, 55, 6520. (d) Persson, A. K. Å.; Jiang, T.; Johnson, M.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2011, 50, 6155. (e) Jiang, T.; Bartholomeyzik, T.; Mazuela, J.; Willersinn, J.; Bäckvall, J.-E. Angew. Chem., Int. Ed. 2015, 54, 6024. (24) (a) Gauthier, D.; Lindhardt, A. T.; Olsen, E. P. K.; Overgaard, J.; Skrydstrup, T. J. Am. Chem. Soc. 2010, 132, 7998. (b) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508. (c) Konnick, M. M.; Decharin, N.; Popp, B. V.; Stahl, S. S. Chem. Sci. 2011, 2, 326. (d) López-Serrano, J.; Duckett, S. B.; Lledós, A. J. Am. Chem. Soc. 2006, 128, 9596. (e) Ma, S.; Jiao, N.; Ye, L. Chem. - Eur. J. 2003, 9, 6049. (f) Ma, S.; Guo, H.; Yu, F. J. Org. Chem. 2006, 71, 6634. (25) (a) Zheng, X.-W.; Nie, L.; Li, Y.-P.; Liu, S.-J.; Zhao, F.-Y.; Zhang, X.-Y.; Wang, X.-D.; Liu, T. RSC Adv. 2017, 7, 5013. (b) Li, H.; Ma, X.-L.; Zhang, B.-H.; Lei, M. Organometallics 2016, 35, 3301. (c) Han, L.-L.; Liu, T. Org. Biomol. Chem. 2017, 15, 5055. (26) Mekareeya, A.; Walker, P. R.; Couce-Rios, A.; Campbell, C. D.; Steven, A.; Paton, R. S.; Anderson, E. A. J. Am. Chem. Soc. 2017, 139, 10104.

(18) The crude 1H NMR spectrum for the reaction forming 3l is also provided to show the involved selectivity of the reaction. For details, see the SI. (19) For details, see the SI. (20) (a) Hoffman, R. W. Pure Appl. Chem. 1988, 60, 123. (b) Yus, M.; González-Gómez, J. C.; Foubelo, F. Chem. Rev. 2011, 111, 7774. (21) The relative configuration of compounds 8 was determined by the NMR studies of 8f (for details, see the SI). Furthermore, the possible transition states (TS) were described as follows. Therefore, TS-A would be favored for the reaction toward alcohols (8). For a

related example, see: Deng, H.-P.; Eriksson, L.; Szabó, K. J. Chem. Commun. 2014, 50, 9207. (22) Attempts to obtain an asymmetric version in the one-pot reaction led only to 15% ee in the best case using a chiral phosphoric acid (for details, see the SI). Catalytic asymmetric allylation of aldehydes using fully substituted allylboron compounds has not been 14333


Highly Selective Palladium-Catalyzed Hydroborylative

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