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Regioselective Diboration and Silaboration of Allenes Catalyzed by Au Nanoparticles Marios Kidonakis, and Manolis Stratakis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04084 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018
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ACS Catalysis
Regioselective Diboration and Silaboration of Allenes Catalyzed by Au Nanoparticles Marios Kidonakis and Manolis Stratakis* Department of Chemistry, University of Crete, Voutes 71003 Iraklion, Greece; E-mail:
[email protected] ABSTRACT: Under catalysis by commercially available supported Au nanoparticles on TiO2 (1 mol%), terminal allenes undergo diboration and silaboration exclusively on the terminal double bond, in high yields and stereoselectivity. In silaboration, the boron moiety (Bpin) is attached on the terminal carbon and the silyl group on the sp-C, a regioselectivity pattern that is unusual. No ligands or additives are required, while the catalyst is recyclable and reusable. The selectivity is rationalized in terms of a π-allyl type or an η-1 complex intermediate with an apparent allylic carbocationic character, as revealed through the use of cyclopropyl allenes as sensitive probes. KEYWORDS: Au nanoparticles, catalysis, allenes, diboration, silaboration.
The activation of σ heteroatom bonds by supported Au nanoparticles and other nanogold catalysts, such as hydrosilanes,1 disilanes,2 diboranes3 or silylboranes,4 and their subsequent addition to alkynes has been documented in recent years.5 Such addition reactions are typically catalyzed by Pd(0) or Pt(0) through an oxidative addition/reductive elimination mechanism.6 The origins of the catalytic activity of Au nanoparticles (Au NPs, Aun) in those reactions, which formally seem to involve the insertion of Aun on the σ heteroatom bond followed by delivery of the two moieties to the π system in a cis-fashion, is still unclear. The catalytic sites on Au NPs are probably the low coordinated atoms primarily at the corners of the gold clusters.7 Apart from addition to alkynes, the first example of the regioselective hydrosilylation of allenes catalyzed by Au NPs has been documented by our group.8 It was found that the addition of hydrosilane takes place on the more substituted double bond of a terminal allene with the silyl moiety bound on the former sp-carbon (Scheme 1).
Scheme 1. Regioselective hydrosilylation of allenes catalyzed by Au NPs.8
Expanding our interest on the Au nanoparticle-catalyzed addition reactions to π systems, we examined the reaction among diboron pinB-Bpin (pin: pinacolato) or silylborane pinB-SiMe2Ph with allenes in the presence of commercially available Au/TiO2. So far, the diboration or silaboration of allenes9 is known under several catalytic conditions (Scheme 2). The Pt(0)10 or Pd(0)-catalyzed11 diboration of terminal allenes occurs primarily or exclusively on the more substituted double bond, and high enantioselectivity has been achieved in
Scheme 2. Regioselectivity in the metal-catalyzed diboration or silaboration of a monosubstituted allene. the diboration of prochiral allenes using chiral ligands. The unusual diboration of terminal allenes on the less substituted double bond was reported using as catalyst a phosphine-free Pd complex requiring an alkenyl iodide as initiator.12 Α Ptcatalyzed diboration protocol on the terminal double bond was recently reported using the usymmetrical diboron compound pinB-Bdan, but only with 1,1-diarylallenes,13 as monosubstituted allenes provide a non-selective reaction. Finally, a specific example of a transition metal-free diboration on the less substituted double bond of an allene has been reported with moderate regioselectivity.14 Apart from diboration, the metalcatalyzed silaboration of allenes is also known. In the presence of Pd catalysts, the addition occurs on the more substituted double bond,15 with the boryl moiety attached on the sp-C atom of the allene. Several reports of Pd-catalyzed asymmetric silaboration were also reported.16 The only example of silaboration at the terminal double bond was shown in the presence of a phosphine free Pd catalyst conditions using an organic iodide as initiator.17 Notably, the reaction of allenes with pinBBpin or pinB-SiMe2Ph under Cu-catalysis conditions affords indirectly hydroboration or hydrosilylation, instead,18 or silacarboxylation if CO2 is present.19 To our delight, diboration and silaboration of terminal allenes proceeds readily in the
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presence of catalytic amounts of Au nanoparticles and occurs exclusively on the terminal double bond. Notably, the silaboration is taking place with an uncommon regioselectivity pattern (Scheme 2). Table 1. Regioselective diboration of terminal allenes with pinB-Bpin catalyzed by Au/TiO2.a O
R
O B B
+ R'
O O pinB-Bpin (1.2 equiv)
Au/TiO2 (1 mol% Au) R benzene, 70 oC, 3 h
Bpin
R'
Bpin
ing amounts of catalyst. Unambiguously, the catalytic sites are the Au nanoparticles, as in the presence of the support only, no reaction was seen. We point out that the catalytic system (Au/TiO2) is recyclable by simple filtration, and reusable in three consecutive runs without significant loss of activity (page S6, Supporting Information). Indeed, the powder XRD pattern of Au/TiO2 prior and after the reaction shows no changes (page S5). Table 2. Regioselective silaboration of terminal allenes with pinB-SiMe2Ph catalyzed by Au/TiO2.a
Bpin
Bpin Bpin
Bpin
Bpin MeO 1 (81%, Z/E=89/11)b Bpin
Bpin
2 (82%, Z/E>97/3)c
4 (87%, Z/E=88/12) Bpin
Bpin
Bpin
Bpin
MeO
3 (88%, Z/E=76/24)
Bpin
Bpin
Me 5 (85%, Z/E=85/15)
Cl 6 (88%, Z/E=88/12)
Bpin Bpin
Bpin
Bpin
Bpin
pinB Br 7 (82%, Z/E=81/19) Me
Bpin
Bpin
Me
F 8 (86%, Z/E=89/11) 9 (76%, Z/E=76/24)d
Bpin
Bpin 11 (88%)e
10 (84%) Ph
Ph Ph
Bpin Bpin
12 (82%)
Bpin
Ph
Bpin 13 (80%)
aIsolated
yields. b6 h. cThe yield was based on the fact that the starting material contains ~20% of the isomeric alkyne. d12 h. e7h.
In the presence of Au/TiO2 (1 mol%) a series of monosubstituted and 1,1-disubstituted allenes react smoothly with 1.2 equiv of bis(pinacolato)diboron (pinB-Bpin) in dry benzene as solvent at 70 oC, providing in high yields diborated adducts exclusively on the terminal double bond within 3 h (Table 1). Surprisingly, chlorinated solvents such as DCE are not compatible at all with the Au-catalyzed diboration, as also had been observed in the analogous diboration of alkynes,3b while the reaction in THF, ethyl acetate or MeOH is also unproductive. Even non-dried solvent can be used, however additional amounts pinB-Bpin are required to compensate its partial oxidation into pinBOBpin by water. Monosubstituted allenes provide mainly or exclusively Z-adducts as also observed in the only so far known example of this type of regioselectivity,12 and the E,Z-stereoselectivity is independent on the load-
Following the diboration studies, we focused on the reaction among silylborane pinB-SiMe2Ph and allenes. Previously, we had shown that in the presence of catalytic amounts of Au NPs, this silylborane adds readily to alkynes at ambient conditions with an abnormal regioselectivity,4 and additionally uncovered the unprecedented silaboration of epoxides and oxetanes.20 To our delight, the addition of pinB-SiMe2Ph (1.5 equiv) to terminal allenes takes place at room temperature (1 mol% Au/TiO2), exclusively on the terminal double bond, with the Bpin moiety attached on the terminal carbon atom (Table 2). Such silaboration products can be also synthesized by a Rh(I)-catalyzed stereoselective double-bond migration of
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ACS Catalysis 2-silyl-1-alkenylboronates.21 The synthetic utility of the produced silylated allylic boronates has been already exemplified.17,21 In some specific cases, minor amounts of diboration adducts (up to 15%) are also formed, due to the partial disproportionation of silylborane into pinB-Bpin and disilane under the reaction conditions.3b,4 For reasons that are not understood phenyl-bearing 1,1-disubstituted allenes are unreactive (Table 2). The most suitable solvent of silaboration is dry dichloromethane, as the reaction in THF or MeOH does not take place; in benzene the reaction rate is lower, while the amounts of diboration side-products increase. As noted earlier, typically, the Pd(0)-catalyzed silaboration of terminal allenes occurs on the internal double bond. The only exception which occurs with the same regioselectivity as in our case, is a modified Pd(0)-catalyzed protocol which requires an iodide as initiator.17 The diboration of cyclopropyl-substituted allene 25 (Scheme 3) provided substantial information regarding the possible mechanism. Treatment of 25 with 1.2 equiv of pinBBpin, cleanly afforded after 3 h the diborated product 26 as a single geometric isomer in which the cyclopropyl ring has undergone a rearrangement. No other products are formed. Analogous exclusive ring-opening rearrangement pathway had been observed in the Au/TiO2-catalyzed hydrosilylation of the same allene.8 Since 25 is unreactive with pinB-SiMe2Ph, we synthesized the monosubstituted analogous allene 27 which underwent smooth silaboration in the presence of Au/TiO2 forming an inseparable mixture of rearranged 28 and unrearranged 29, in a relative ratio 28/29=1/1.22
abnormal regioselectivity in the Au/TiO2-catalyzed silaboration of alkynes.4 Intermediates of type I or II undergo ring opening isomerization in the case of cyclopropyl allenes 25 and 27, respectively, as shown in the bottom of Scheme 4. The non-complete rearrangement regarding the silaboration of 27, can be rationalized in terms of the less favorable formation of a secondary phenylcyclopropyl carbocation, relative to the analogous more stable tertiary carbocation III.
pinB pinB pinB
Aun
R Bpin
R
Bpin
Aun
Au Bpin
Bpin delivery at - Bpin the less hindered Au allylic position I +
R
Bpin
R major or only diastereomer
R'3Si - Bpin Au II R +
preferential Bpin versus SiR'3 delivery at the less hindered allylic position
R'3Si
Au Bpin
R Aun SiR'3 Bpin
R
Ph pinB Au Bpin III
?
Ph
Me
pinB Au Bpin
Aun 26
Me
Scheme 4. Possible mechanism of diboration and silaboration of a terminal allene catalyzed by Au nanoparticle (Aun).
Scheme 3. Diboration and silaboration of cyclopropylsubstituted allenes 25 and 27, respectively, catalyzed by Au/TiO2.
As in the case of the Au nanoparticle-catalyzed allene hydrosilylation,8 we propose in Scheme 4 an analogous mechanism, with two plausible similar scenarios as working hypotheses, either an η-1 complex or a π-allyl gold intermediate. In our analysis we consider the η-1 complexes I for diboration and II for silaboration. In intermediate I, the pinB is selectively delivered on the terminal carbon atom for steric reasons. For the same steric reasons silaboration occurs at the same position, but the selectivity in terms of C-B versus C-Si bond formation from the decomposition of intermediate II is attributed to thermodynamics. Formation of the C-B bond is more favorable compared to the C-Si by approximately 24 kcal/mol, which is a larger difference if comparing the dissociation energies among the Au-B and Au-Si bonds (~13.0 kcal/mol). An analogous argument was invoked to explain the
In conclusion, we present herein the first examples of Aucatalyzed diboration and silaboration of terminal allenes, using a simple commercially available, recyclable and reusable catalyst. No ligands or additives are necessary. In both reactions the addition occurs exclusively on the terminal double bond, which is an uncommon mode of reactivity using other noble metal catalysts. The regioselectivity pattern of silaboration is also uncommon, with boron moiety attached on the terminal carbon atom, with only one precedent in the literature. Using cyclopropyl allenes as sensitive probes, it is proposed that the addition occurs through an intermediate with a profound allylic carbocationic character. Corresponding Author
[email protected] ASSOCIATED CONTENT 1
Supporting Information. Copies of H,
13
C NMR of all reaction products. This material is available free of charge via the Internet at http://pubs.acs.org.
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ACKNOWLEDGMENTS This work was partially funded by the Special Account for Research Funds of the University of Crete (program 4460). MK thanks ELIDEK program from the Hellenic Foundation for Research and Innovation for a scholarship. We also thank ProFI (FORTH, Heraklion, Greece) for obtaining the HRMS spectra. REFERENCES (1) Selected examples: a) Corma, A.; Gonzalez-Arellano, C.; Iglesias, M.; Sanchez, F.; Angew. Chem. Int. Ed. 2007, 46, 7820-7822. b) Lykakis, I. N.; Psyllaki, A.; Stratakis, M. J. Am. Chem. Soc. 2011, 133, 10426-10429. c) Ishikawa, Y.; Yamamoto, Y.; Asao, N. Catal. Sci. Technol. 2013, 3, 2902-2905. d) Saridakis, I.; Kidonakis, M.; Stratakis, M. ChemCatChem 2018, DOI: 10.1002/cctc.201701526. (2) a) Gryparis, C.; Stratakis, M. Chem. Commun. 2012, 48, 1075110753. b) Gryparis, C.; Kidonakis, M.; Stratakis, M. Org. Lett. 2013, 15, 6038-6041. (3) a) Chen, Q.; Zhao, J.; Ishikawa, Y.; Asao, N.; Yamamoto, Y.; Jin, T. Org. Lett. 2013, 15, 5766-5769. b) Kidonakis, M.; Stratakis, M. Eur. J. Org. Chem. 2017, 4265-4271. (4) Gryparis, M.; Kidonakis, M.; Stratakis, M. Org. Lett. 2014, 16, 1430-1433. (5) Selected review articles: a) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Chem. Rev. 2012, 112, 2467-2505. b) Stratakis, M.; Garcia, H. Chem. Rev. 2012, 112, 4469-4506. c) Takale, B. S.; Bao, M.; Yamamoto, Y. Org. Biomol. Chem. 2014, 12, 2005-2027. (6) Beletskaya, I.; Moberg, C. Chem. Rev. 2006, 106, 2320-2354. (7) a) Stenlid, J. H.; Brinck, T. J. Am. Chem. Soc. 2017, 139, 11012-11015. b) Schneider, W.-D.; Heyde, M.; Freund, H.-J. Chem. Eur. J. 2018, DOI: 10.1002/chem.201703169. (8) Kidonakis, M.; Stratakis, M. Org. Lett. 2015, 17, 4538-4541. (9) Selected review articles: a) Oestreich, M.; Hartmann, E.; Mewald, M. Chem. Rev. 2013, 113, 402-441. b) Takaya, J.; Iwasawa, N. ACS Catal. 2012, 2, 1993-2006. (10) a) Ishiyama, T.; Kitano, T.; Miyaura,N. Tetrahedron Lett. 1998, 39, 2357-2360. b) Wang, M.; Cheng, L.; Wu, Z. Organometallics 2008, 27, 6464-6471.
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