Takahiro
Iwamoto
*ab,
Takuya
Mitsubo
a,
Kosuke
Sakajiri
a and
Youichi
Ishii
*a
aDepartment of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan. E-mail: iwamoto@kc.chuo-u.ac.jp; yo-ishii@kc.chuo-u.ac.jp
bFaculty of Molecular Chemistry and Engineering, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. E-mail: tiwamoto@kit.ac.jp
First published on 28th May 2024
Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.
Transition-metal mediated vinylidene rearrangement from an η2-alkyne ligand to the corresponding vinylidene species represents a powerful method for alkyne transformations.26–31 The most common is vinylidene rearrangement of terminal alkynes because of the high migration ability of a hydrogen atom.32 Over the last few decades, the substrate scope has been expanded to internal alkynes to demonstrate that a variety of carbon and heteroatom groups can participate in this rearrangement as a migrating group (Fig. 1a).33–48 However, vinylidene rearrangement of borylalkynes has been largely unexplored; the successful vinylidene rearrangements are limited to just a few examples (Fig. 1b).49,50 Hill and coworkers reported the vinylidene rearrangement of HCCB(mida) (mida = N-methyliminodiacetato) (Fig. 1b, top).49b Subsequently, the first instance of vinylidene rearrangement of internal borylalkyne, p-C6H4MeCCB(pin) (pin = pinacolato), was reported by Ozerov and coworkers.49c Braunschweig also described an equilibrium between rhodium-borylvinylidene and -(hydrido)(borylalkynyl) complexes, the latter of which was derived from a terminal borylalkyne.49a However, although Hill proposed that the boryl group would serve as a 1,2-migration group, no mechanistic information on the 1,2-migration process of η2-borylalkyne is provided in these reports.
In this paper, we have investigated vinylidene rearrangements of internal borylalkynes by using [Ru(dppe)Cp]+, in which both tetra- and tricoordinate boryl groups are applicable (Fig. 1b, bottom). Isotopic labelling experiments provided the first experimental evidence showing that the boryl groups serve as a migrating group. Theoretical studies on a migration mode of the boryl group (i.e., either nucleophilic or electrophilic processes) is also reported. These results not only expand the scope of the vinylidene rearrangements but also add a new entry to the boryl group migration chemistry.
By using [RuCl(dppe)Cp] (1a) as the optimal precursor, we next conducted reactions of tricoordinate borylalkynes (Fig. 3). Treating PhCCB(dan) (dan = naphthalene-1,8-diaminato) under the same reaction conditions resulted in the complete consumption of 1a. However, a mixture of complex 2f and protodeboronated complex 3 was obtained in 68 and 30% yields, respectively. Through careful screening of the reagents and the reaction conditions, we found that use of anhydrous NaBArF4·6THF was effective to produce complex 2f in 70% isolated yield (96% NMR yield) along with formation of a small amount of 3. The susceptibility to the protodeboronation was more serious in the case of internal alkynes with B(pin). Treatment of PhCCB(pin) with [RuCl(dppe)Cp] in the presence of anhydrous NaBArF4·6THF provided a mixture of 2g and 3 in 59 and 40% NMR yield, respectively, and the high susceptibility toward H2O hampered isolation of complex 2g. Consequently, characterization of 2g was performed solely by NMR analyses, while several NMR signals characteristic of vinylidene complexes provided reliable information on the formation of vinylidene complex 2g (see the ESI†).
To gain insight into the formation of 3, we conducted brief studies on the protodeboronation. When a solution of PhCCB(dan) in C2H4Cl2 was treated with H2O (2.5 equiv.) at 70 °C for 1 h, PhCCH (4) was not formed, and the starting alkyne was fully recovered (Fig. 4a, up). In contrast, complex 2f underwent protodeboronation under similar conditions to give complex 3 in 89% yield. Protodeboronation of complex 2g proceeded much rapidly even at room temperature to afford 3 in 82% (Fig. 4a, bottom). Note that vinylidene complex 2a is stable under these reaction conditions. These results implied that the formation of complex 3 observed in the vinylidene rearrangements (Fig. 3) is derived from the protodeboronation of vinylidene complex 2, but not from the vinylidene rearrangement of terminal alkyne 4 (Fig. 4b). The remarkable susceptibility of vinylidene complex 2f and 2g to the protodeboronation likely results from their unique structure associated with the cationic vinylidene and the β-boryl functionalities.
Fig. 4 (a) Control experiments and (b) a proposed reaction mechanism for the protodeboronation ([Ru] = [Ru(dppe)Cp]+). |
Common organoboronate compounds are moderately stable toward hydrolysis. The high reactivity of 2f and 2g toward protodeboronation may be accounted for by two possible mechanisms: (1) addition of water at the boron atom followed by deprotonation and B–C bond cleavage, or (2) electrophilic addition of H2O to the vinylidene α-carbon followed by B−OH elimination like bora-Wittig-type reactions (Fig. 4b, bottom).51 Although a common protodeboronation mechanism (1) cannot be excluded, we assume that the latter mechanism (2) may be operative considering that the electron-deficient nature of the α-carbon in the cationic vinylidene complex 2 should be strongly enhanced by the Lewis acidic boryl group at the β-carbon.52 In any event, both mechanisms are in accordance with the actual reactivity order that the more Lewis acidic boryl group accelerates the protodeboronation.
To further assess the vinylidene rearrangements, we performed time course studies monitoring the formation of the vinylidene complexes by using 1H NMR analyses (Fig. 5). The vinylidene formation from PhCCB(pin) and 1a was found to proceed rapidly even at room temperature, and the vinylidene complex was formed almost quantitatively after 3 h. PhCCB(dan) also showed high activity, while the reaction rate was slightly lower compared with PhCCB(pin). In contrast, the reaction with PhCCB(mida) was sluggish at room temperature. Thus, the reaction rates of the vinylidene rearrangements are dramatically affected by the boryl groups and likely correlated with the Lewis acidity of the boryl groups (B(pin) > B(dan) > B(mida)).
To clarify the migrating group, 13C-labeling experiments were performed using 25% 13C-enriched alkyne, PhC13CB(mida) (Fig. 6). After a reaction under the optimal conditions, vinylidene complex 2–13Cα was obtained as the major product (2–13Cα:2–13Cβ = 22.6:1), and the migration ratio of B(mida) and Ph was calculated to be 99:<1 on the basis of the product ratio 2–13Cα:2–13Cβ determined by 13C{1H} NMR. In the case of an alkyne bearing B(dan) (19.5% enriched), the isomer ratio was determined after protodeboronation of the vinylidene complex because of a partial overlap of 13C NMR signals of the boryl vinylidene complex. Again, B(dan) group was found to serve as the migrating group with >99% selectivity. These results represent the first observations of 1,2-boryl migration in vinylidene rearrangement.
Our previous mechanistic studies on the vinylidene rearrangements of internal alkynes with two carbon substituents revealed that the rearrangement proceeds via nucleophilic migration pathway to highlight the importance of nucleophilicity of a migration group.34,36 However, the observation of the rapid rearrangements of less nucleophilic B(pin) and B(dan) over B(mida) is contradiction to this mechanistic scenario. Thus, we performed validation of the migration mode. In the previous nucleophilic vinylidene rearrangements, the rate of the rearrangement was revealed to be remarkably dependent on the electron-donating ability of a non-migrating group (Fig. 7).36 Thus we compared the reactivities of alkynes with an electronically different non-migrating group. In reactions of p-RC6H4CCB(mida), the rearrangement of (p-C6H4OMe)CCB(mida) with an electron donating aryl group was faster than that of PhCCB(mida), while electron withdrawing p-C6H4CF3 group retarded the reaction significantly (Fig. 7a). The observed electronic effects were entirely identical with our previous observations in nucleophilic vinylidene rearrangements.36 On the other hand, in reactions of p-RC6H4CCB(dan), no obvious migration preference depending on the electronic character of the non-migrating group was observed (Fig. 7b). This different structure-reactivity relationship is indicative of the distinct migration mode with the tricoordinate borylalkynes.53
To further obtain an insight into the migration mode, the boryl migrations of B(dan) and B(mida) group were evaluated by DFT calculations at B3LYP/6-311G(d) + SDD (Fig. 8). Both process from η2-alkyne to the corresponding vinylidene species is endothermic with a ΔG value of 10.1 kcal mol−1. An activation barriers of B(dan) migration is significantly lower than that of B(mida) migration. These results are consistent with the experimental observation that the vinylidene rearrangement of B(dan) proceeds smoothly at room temperature, while that of B(mida) needs a higher reaction temperature. At the transition state (TS) for the B(dan) migration, the C1–B bond length (1.703 Å) is obviously shorter than C2–B bond length (1.929 Å), which corresponds to the nearly generating C–B bond. A linear alignment of C1, C2, and C3 atoms (177°) and a nearly orthogonal conformation of the non-migrating phenyl group toward the C1–B bond (100°) indicates that π-orbitals of the non-migrating phenyl group can effectively interact with the C1–C2 π bond, which should be directly involved in the boryl migration. Of interest, this TS structure is similar with that in 1,3-boryl migration of a (boryl)(alkynyl)osmium complex.50a In the TS of B(mida) migration, C1–B bond (1.911 Å) is shorter than C2–B bond (2.067 Å), while the difference in these bond lengths is smaller than that in the B(dan) migration. Furthermore, in contrast to the B(dan) migration, the angle of C1–C2–C3 is slightly bent (167°). Note that, in the B(mida) migration, a dative B–N bond remains during the whole calculated processes from A to B (N–B = 1.711–1.717 Å).
NBO analyzes were performed at HF/6-311G(d)+SDD for A, TS, and B. In the TS of B(dan) migration, a definite interaction is estimated between the C1–C2 π bond as a donor and the p orbital of the boron center as an acceptor (Fig. 9a). The donor acceptor interaction supports the electrophilic migration mode of B(dan) group. On the other hand, B(mida) migration hardly includes the same type of interaction, because the corresponding p orbital of the boron center participates in the dative bond formation with the nitrogen atom. Fig. 9b shows NBO charges of A, TS, and B. In B(dan) migration, a positive charge of the boron atom increases from intermediate A (1.020) to TS (1.169). Although C2 has a positive charge in A and TS, the value decreases from A to TS by 0.021. These changes of the charge distributions are completely different with our previous results for the nucleophilic vinylidene rearrangements,36 yet consistent with the other theoretical calculations of electrophilic vinylidene rearrangements.54 Similar electron flows are calculated in the case of the B(mida) migration. Although we cannot conclude either electrophilic or nucleophilic migration mode of B(mida) group based on the present experimental and theoretical studies,55 these calculations suggest the B(dan) migration proceeds via the electrophilic mode. We believe that the migration of a tricoordinate boryl group, B(pin), also follows the mechanistic scenario similar to that of B(dan). This conclusion is consistent with the high migration aptitude of Lewis acidic boryl groups.
We finally examined substrate scope by using RCCB(mida) (Fig. 10). Reactions of p-RC6H4CCB(mida) (R = Me and F) proceeded smoothly to provide the corresponding vinylidene complexes in 70 and 71% yield, respectively. Boryl alkynes with an aryl group substituted at o- and m-positions also provided vinylidene complexes in good yields, while a sterically hindered substrate with an o-methyl group diminished the yield. As indicated by the ligand screening, the present rearrangement is rather sensitive toward the steric hindrance. Reactions of alkynes bearing biphenyl and naphthyl proceeded in moderate yields. Electron-rich heteroaromatic groups were applicable to afford the corresponding vinylidene complexes in moderate yields. Unfortunately, a reaction with alkylalkyne was found to be sluggish.
1H NMR (acetone-d6): δ 7.79 (s, 8H, o-H of BArF4), 7.75 (m, 4H, o-H of Ph in dppe), 7.67 (s, 4H, p-H of BArF4), 7.45 (m, 4H, p-H of Ph in dppe × 2), 7.35 (m, 8H, m-H of Ph in dppe × 2), 7.24 (m, 4H, o-H of Ph in dppe), 7.01 (m, 3H, m- and p-H of RuCCPh), 6.88 (m, 2H, o-H of RuCCPh), 5.67 (s, 5H, Cp), 3.99 (d, 2H, 2JHH = 17.3 Hz, CH2 of mida), 3.39 (m, 2H, CH2 of dppe), 3.24 (d, 2H, 2JHH = 16.8 Hz, CH2 of mida), 3.03 (m, 2H, CH2 of dppe), 2.72 (s, 3H, CH3 of mida). 31P{1H} NMR (acetone-d6): δ 79.5 (s, dppe). 13C{1H} NMR (acetone-d6): δ 339.2 (t, 2JCP = 15.4 Hz, RuCC), 167.7 (s, CO of mida), 162.6 (q, 1JCB = 50.0 Hz, ipso-C of BArF4), 138.0 (m, ipso-C of Ph in dppe), 135.5 (br, o-C of BArF4), 134.9 (m, ipso-C of Ph in dppe), 133.9 (virtual t, o-C of Ph in dppe), 132.0 (m, o- and p-C of Ph in dppe), 131.5 (s, p-C of Ph in dppe), 130.8 (s, o-C of RuCCC6H5), 130.0 (brq, 2JCF = 34.4 Hz, m-C of BArF4), 129.7 (m, m-C of Ph in dppe × 2 and m-C of RuCCC6H5), 128.8 (s, ipso-C of RuCCC6H5), 127.3 (s, p-C of RuCCC6H5), 125.4 (q, 1JCF = 272.9 Hz, CF3 of BArF4), 118.4 (m, p-C of BArF4), 92.0 (s, Cp), 62.8 (s, CH2 of mida), 47.2 (s, CH3 of mida), 28.5 (m, PCH2). The signal assignable to the RuCC could not be found, probably because it is overlapped with other signals. Elemental analysis calcd for C76H53O4B2F24P2NRu·0.5CH2Cl2: C, 53.19; H, 3.09; N, 0.81. Found: C, 53.13; H, 3.06; N, 0.80.
1H NMR (CDCl3, see ESI† for the atom numbering): δ 7.74 (s, 8H, o-H of BArF4), 7.52 (s, 4H, p-H of BArF4), 7.38 (m, 12H, p- ×2, m- and o-H of Ph in dppe), 7.02 (m, 15H, o- and m-H of Ph in dppe and H3, H4, H6, H7), 6.62 (d, 2H, 3JHH = 6.9 Hz, H2), 5.90 (br, 2H, H5), 5.43 (s, 5H, Cp), 4.73 (br, 2H, NH of dan), 3.05 (m, 2H, CH2 of dppe), 2.87 (m, 2H, CH2 of dppe). 31P{1H} NMR (CDCl3): δ 78.0 (s, dppe). 13C{1H} NMR (CDCl3): δ 335.6 (t, 2JCP = 14.8 Hz, RuCC), 161.8 (q, 1JCB = 50.0 Hz, ipso-C of BArF4), 140.0 (s, C10), 136.2 (br, C8), 135.0 (m, o-C of BArF4 and ipso-C of Ph in dppe), 133.1 (m, ipso-C of Ph in dppe), 132.0 (m, p- × 2 and o-C of Ph in dppe), 131.4 (m, o-C of Ph in dppe), 129.0 (m, m-C × 2 of Ph in dppe, and C2, C3, and m-C of BArF4), 127.3 (s, C6), 127.0 (s, C4), 125.8 (s, C1), 124.7 (q, 1JCF = 273.6 Hz, CF3 of BArF4), 119.6 (s, C9), 118.4 (s, C7), 117.6 (s, p-C of BArF4), 117.1 (br, RuCC), 106.5 (s, C5), 91.3 (s, Cp), 27.8 (m, PCH2). High resolution mass measurement and elemental analysis have failed due to the high susceptibility toward hydrolysis.
1H NMR (CDCl3): δ 7.74 (s, 8H, o-H of BArF4), 7.53 (s, 4H, p-H of BArF4), 7.48–7.28 (m, 12H, p- ×2, m- and o-H of Ph in dppe), 7.24 (m, 4H, m-H of Ph in dppe), 7.10 (m, 7H, o-H of Ph in dppe and m- and p-H of RuCCPh), 6.69 (m, 2H, o-H of RuCCPh), 5.22 (s, 5H, Cp), 2.98 (m, 4H, CH2 of dppe × 2), 0.93 (s, 12H, CH3 of pin). 31P{1H} NMR (CDCl3): δ 78.3 (s, dppe). 13C{1H} NMR (CDCl3): δ 347.4 (t, 2JCP = 15.4 Hz, RuCC), 161.8 (q, 1JCB = 50.0 Hz, ipso-C of BArF4), 136.1 (m, ipso-C of Ph in dppe), 134.9 (br s, o-C of BArF4), 132.8 (m, o-C of Ph in dppe), 132.7 (m, ipso-C of Ph in dppe), 131.8 (s, p-C of Ph in dppe), 131.6 (s, p-C of Ph in dppe), 131.0 (m, o-C of Ph in dppe), 129.2 (m, m-C of Ph in dppe), 129.0 (brq, 2JCF = 29.7 Hz, m-C of BArF4), 128.9 (m, m-C of Ph in dppe and m- and o-C of RuCCC6H5), 126.7 (s, p-C of RuCCC6H5), 124.6 (q, 1JCF = 271.0 Hz, CF3 of BArF4), 117.5 (s, p-C of BArF4), 91.5 (s, Cp), 84.0 (s, CO of pin), 28.9 (m, PCH2), 24.4 (s, CH3 of pin). The signal assignable to the RuCC and ipso-C of RuCCC6H5 could not be found, probably because it is overlapped with other signals. High resolution mass measurement and elemental analysis have failed due to the high susceptibility toward hydrolysis.
A J Young NMR tube was charged with [RuCl(dppe)Cp] (30.0 mg, 0.0500 mmol), PhCCB(dan) (13.6 mg, 0.0507 mmol), and NaBArF4·6THF (81.7 mg, 0.0619 mmol) in CDCl3 (0.5 mL). After 7 h at room temperature, vinylidene complex 2f was formed in 94% NMR yield. After solvent was removed under reduced pressure, H2O (1.8 mg, 0.10 mmol) and 1,2-dichloroethane (0.5 ml) were added. The mixture was heated 70 °C for 1 h. The black solution was dried in vacuo and analyzed by 1H NMR spectrum. The NMR yield was determined by using 1,3,5-trimethoxybenzene as an internal standard. The same procedure was used for the protodeboronation of vinylidene complex 2g.
1H NMR (CDCl3): δ 7.76 (s, 8H, o-H of BArF4), 7.53 (s, 4H, p-H of BArF4), 7.40 (m, 16H, p- ×2, m- ×2 and o-H of Ph in dppe), 7.15 (m, 4H, o-H of Ph in dppe), 6.92 (m, 1H, p-H of RuCCPh), 6.86 (m, 2H, m-H of RuCCPh), 6.23 (m, 2H, o-H of RuCCPh), 5.52 (s, 5H, Cp), 4.68 (s, 1H, RuCCH), 3.00 (m, 2H, CH2 of dppe), 2.71 (m, 2H, CH2 of dppe). 31P{1H} NMR (CDCl3): δ 77.3 (s, dppe). 13C{1H} NMR (CDCl3): δ 354.1 (t, 2JCP = 16.1 Hz, RuCC), 161.9 (q, 1JCB = 50.0 Hz, ipso-C of BArF4), 135.0 (br, o-C of BArF4), 134.7 (m, ipso-C of Ph in dppe), 133.6 (m, ipso-C of Ph in dppe), 132.1 (m, p- ×2 and o-C of Ph in dppe), 131.4 (virtual t, o-C of Ph in dppe), 129.4 (m, m-C of Ph in dppe × 2), 129.0 (brq, 2JCF = 34.0 Hz, m-C of BArF4), 128.8 (s, m-C of RuCCC6H5), 126.9 (s, p-C of RuCCC6H5), 125.8 (s, o-C of RuCCC6H5), 125.1 (s, ipso-C of RuCCC6H5), 124.7 (q, 1JCF = 273.5 Hz, CF3 of BArF4), 117.9 (s, RuCC), 117.6 (m, p-C of BArF4), 92.0 (s, Cp), 27.2 (m, PCH2). HRMS m/z: [M + Na]+ calcd for RuP2C39H35+: 667.12575; found 667.12595.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4dt01042f |
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