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Carboboration
Carboboration (organic chemistry reaction)

In organic chemistry, carboboration is a chemical reaction that adds both a carbon and a boron moiety across carbon-carbon multiple bonds such as alkenes and alkynes. This reaction forms organoborane compounds with both a new carbon group and a boron handle useful in organic synthesis. The carbon-boron bond enables diverse transformations in organoboron chemistry, including oxidation and the Suzuki Reaction, making carboboration valuable for pharmaceutical applications. Developed after hydroboration, carboboration often uses transition metal catalysis and involves regioselective addition modes such as 1,1- and 1,2-carboboration.

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1,1 Carboboration

1,1 carboboration delivers both the carbon-carbon bond and the carbon-boron bond to the same carbon in the substrate. It requires a 1,2-migration of a substituent from one carbon to the other in the double bond. The Wrackmeyer reaction is typically credited as being the pioneering example of 1,1 carboboration and utilizes a metal migrating group to help facilitate the transformation.1 However, there are several modern examples of carboboration with a variety of migrating groups.23

The Wrackmeyer reaction involves 1,1 carboboration of a 1-alkynylmetal compound to yield alkenylborane compounds. [M] can be silicon, germanium, tin, or lead compounds with various substituents or ligands. [M] and BR2 are typically cis to one another in the Wrackmeyer reaction, with some exceptions.4

Mechanism

Wrackmeyer-type 1,1 carboboration is proposed to go through a zwitterionic intermediate, and this intermediate has been isolated and characterized in some cases.567 However, the mechanism can be highly substrate and reagent dependent.

In a borane, the compound typically adopts a trigonal planar molecular geometry, making the boron atom an electrophilic center. The substituents can affect the strength of the borane as a Lewis acid.8 Boranes which are stronger Lewis acids are better electrophiles and therefore better able to facilitate carboboration. Boranes can be optimized to work on less activated substrates. Tris(pentafluorophenyl)borane [B(C6F5)3] is a strongly Lewis acidic borane which functions well in 1,1 carboborations with both activated and unactivated substrates, and it allows for the reaction to be facilitated with more mild conditions.9 An activated substrate such as an alkene or alkyne has an electron-withdrawing group directly attached to a carbon within the double or triple bond.10 Transition metal catalysts have been utilized to develop enantioselective 1,1 carboborations on unactivated alkenes. These reactions go through a catalytic cycle which may or may not go through a zwitterionic intermediate.11

Examples

1,2 Carboboration

1,2 carboboration delivers the carbon-carbon bond and the carbon-boron bond to adjacent carbons in the substrate. It is typically facilitated by transition metal catalysis, but transition-metal-free 1,2 carboborations have been developed and continue to be of interest to synthetic chemists.12 The benefit of utilizing transition metals is that the reactions can often have enantioselective control based on the ligands used on the metal complex. Common metals used are palladium, nickel, and copper, which are often coupled with an organoborane or a boron source with an electrophile or nucleophile.13

Mechanism

The mechanism of carboboration depends highly on the substrate and reagents utilized in the reaction. Shown below are examples of two types of Pd-catalyzed alkene 1,2 carboborations, Heck-type and the Wacker-type.14 However, the Cu- and Ni-catalyzed reactions can proceed through similar mechanisms. These two mechanisms mainly differ in the oxidation state of the active catalyst and how the carbon group is delivered to the substrate: whether the C–C bond is formed via migratory insertion from the catalyst (inner sphere) or attack by an external nucleophile (outer sphere). Wacker-type carboborations, catalyzed by PdII, are much rarer than Heck-type. The first example of a Wacker-type 1,2 carboboration was reported by the Engle group in 2019.15

Despite the common trend of utilizing transition metals, transition metal-free processes have also been developed, such as utilizing boronic acids16 or light-mediated radical initiation.17 These reactions usually lead to the boron substituent being at the terminus or less substituted side of the substrate, but anti-carborborations have also been developed which produce reverse regioselectivity.181920 Much work has also been done to render 1,2 carboboration enantioselective using various ligands on transition metal catalysts.

Examples

1,n Carboboration

A nickel-catalyzed 1,n arylboration was developed in 2019 by Yin and coworkers and remains the only example of a chain-walking arylboration.21 This was accomplished via a nitrogen-based ligand and a three-component coupling. The general scheme plus proposed mechanism is shown.

References

  1. Wrackmeyer, B. (1995). "1,1-Organoboration of alkynylsilicon, -germanium, -tin and -lead compounds". Coordination Chemistry Reviews. 145: 125. doi:10.1016/0010-8545(95)90220-1. /wiki/Doi_(identifier) ↩

  2. Kehr G.; Erker, G. (2016). "Advanced 1,1-carboboration reactions with pentafluorophenylboranes". Chemical Science. 7 (1): 56–65. doi:10.1039/c5sc03282b. PMC 5508682. PMID 28757997. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5508682 ↩

  3. Kehr G.; Erker, G. (2012). "1,1-Carboboration". Chemical Communications. 48 (13): 1839–1850. doi:10.1039/C1CC15628D. PMID 22116402. /wiki/Doi_(identifier) ↩

  4. Wrackmeyer, B. (1995). "1,1-Organoboration of alkynylsilicon, -germanium, -tin and -lead compounds". Coordination Chemistry Reviews. 145: 125. doi:10.1016/0010-8545(95)90220-1. /wiki/Doi_(identifier) ↩

  5. Bismuto, A.; Thomas, S. P.; Duarte, F; Cowley, M. J. (2012). "Characterization of the Zwitterionic Intermediate in 1,1-Carboboration of Alkynes". Chemical Communications. 48 (13): 1839–1850. doi:10.1039/C1CC15628D. PMID 22116402. /wiki/Doi_(identifier) ↩

  6. Wrackmeyer, B.; Khan, E. (2015). "1,1-Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds". European Journal of Inorganic Chemistry. 2016 (3): 300. doi:10.1002/ejic.201500727. /wiki/Doi_(identifier) ↩

  7. Wrackmeyer, B.; Kehr, G.; Suß, J.; Molla, E. (1999). "1,1-Organoboration of tetraynes—routes to new siloles, stannoles and fused heterocycles". Journal of Organometallic Chemistry. 577: 82. doi:10.1016/S0022-328X(98)01029-8. /wiki/Doi_(identifier) ↩

  8. Wrackmeyer, B.; Khan, E. (2015). "1,1-Carboboration through Activation of Silicon–Carbon and Tin–Carbon Bonds". European Journal of Inorganic Chemistry. 2016 (3): 300. doi:10.1002/ejic.201500727. /wiki/Doi_(identifier) ↩

  9. Kehr G.; Erker, G. (2012). "1,1-Carboboration". Chemical Communications. 48 (13): 1839–1850. doi:10.1039/C1CC15628D. PMID 22116402. /wiki/Doi_(identifier) ↩

  10. Costa, D. (2020). "Additions to non-activated alkenes: Recent advances". Arabian Journal of Chemistry. 13: 799. doi:10.1016/j.arabjc.2017.07.017. S2CID 102556829. https://doi.org/10.1016%2Fj.arabjc.2017.07.017 ↩

  11. Wang, W.; Ding, C.; Yin, G. (2020). "Catalyst-controlled enantioselective 1,1-arylboration of unactivated olefins". Nature Catalysis. 3 (11): 951. doi:10.1038/s41929-020-00523-8. S2CID 224783863. /wiki/Doi_(identifier) ↩

  12. Roscales, S.; Csákÿ, A. G. (2015). "Transition-Metal-Free Direct anti-Carboboration of Alkynes with Boronic Acids To Produce Alkenylheteroarenes". Organic Letters. 17 (6): 1605–1608. doi:10.1021/acs.orglett.5b00517. PMID 25738233. /wiki/Doi_(identifier) ↩

  13. Liu, Z.; Gao, Y.; Zeng, T.; Engle, K. M. (2020). "Transition-Metal-Catalyzed 1,2-Carboboration of Alkenes: Strategies, Mechanisms, and Stereocontrol". Israel Journal of Chemistry. 60 (3–4): 219–229. doi:10.1002/ijch.201900087. PMC 8006804. PMID 33785969. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8006804 ↩

  14. Liu, Z.; Gao, Y.; Zeng, T.; Engle, K. M. (2020). "Transition-Metal-Catalyzed 1,2-Carboboration of Alkenes: Strategies, Mechanisms, and Stereocontrol". Israel Journal of Chemistry. 60 (3–4): 219–229. doi:10.1002/ijch.201900087. PMC 8006804. PMID 33785969. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8006804 ↩

  15. Liu, Z.; Ni, H. Q.; Zeng, T.; Engle, K. M. (2018). "Catalytic Carbo- and Aminoboration of Alkenyl Carbonyl Compounds via Five- and Six-Membered Palladacycles". Journal of the American Chemical Society. 140 (9): 3223–3227. doi:10.1021/jacs.8b00881. PMC 6002770. PMID 29384373. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6002770 ↩

  16. Roscales, S.; Csákÿ, A. G. (2015). "Transition-Metal-Free Direct anti-Carboboration of Alkynes with Boronic Acids To Produce Alkenylheteroarenes". Organic Letters. 17 (6): 1605–1608. doi:10.1021/acs.orglett.5b00517. PMID 25738233. /wiki/Doi_(identifier) ↩

  17. Jin, S.; Larionov, O. V. (2018). "A Radical New Look for Alkene Carboboration". Chem. 4 (6): 1205. Bibcode:2018Chem....4.1205J. doi:10.1016/j.chempr.2018.05.022. S2CID 103226343. https://doi.org/10.1016%2Fj.chempr.2018.05.022 ↩

  18. Roscales, S.; Csákÿ, A. G. (2015). "Transition-Metal-Free Direct anti-Carboboration of Alkynes with Boronic Acids To Produce Alkenylheteroarenes". Organic Letters. 17 (6): 1605–1608. doi:10.1021/acs.orglett.5b00517. PMID 25738233. /wiki/Doi_(identifier) ↩

  19. Liu, Z.; Ni, H. Q.; Zeng, T.; Engle, K. M. (2018). "Catalytic Carbo- and Aminoboration of Alkenyl Carbonyl Compounds via Five- and Six-Membered Palladacycles". Journal of the American Chemical Society. 140 (9): 3223–3227. doi:10.1021/jacs.8b00881. PMC 6002770. PMID 29384373. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6002770 ↩

  20. Liu, Z.; Chen, J.; Lu, H. X.; Li, X.; Gao, Y.; Coombs, J. R.; Goldfogel, M. J.; Engle, K. M. (2019). "Pd(0)-Catalyzed Directed syn-1,2-Carboboration and -Silylation: Alkene Scope, Applications in Dearomatization, and Stereocontrol via a Chiral Auxiliary". Angewandte Chemie International Edition in English. 58 (47): 17068–17073. doi:10.1002/anie.201910304. PMC 7337986. PMID 31538388. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7337986 ↩

  21. Wang, W.; Ding, C.; Li, Y.; Li, Z.; Li, Y.; Peng, L.; Yin, G. (2019). "Migratory Arylboration of Unactivated Alkenes Enabled by Nickel Catalysis". Angewandte Chemie International Edition. 58 (14): 4612–4616. doi:10.1002/anie.201814572. PMID 30740847. S2CID 73450688. /wiki/Doi_(identifier) ↩