β-Carbon elimination
β-Carbon elimination (beta-carbon elimination) is a type of reaction in organometallic chemistry wherein an allyl ligand bonded to a metal center is broken into the corresponding metal-bonded alkyl (aryl) ligand and an alkene.[1] It is a subgroup of elimination reactions. Though less common and less understood than β-hydride elimination, it is an important step involved in some olefin polymerization processes and transition-metal-catalyzed organic reactions.[2]
Overview
[edit]Like β-hydride elimination, β-carbon elimination requires the metal to have an open coordination site cis to the alkyl group for this reaction to occur. β-carbon elimination is usually less favored than hydride elimination because the metal–hydride bond is stronger than the metal–carbon bond for most metals in catalytic reactions. The principles governing β-alkyl elimination are not well-established experimentally. One reason for this is that breaking C−C bonds in the presence of other reactive C−H bonds is a rare event, and systems designed to interrogate the reaction are more difficult to devise.[2]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/12/%CE%92-carbon_elimination.png/418px-%CE%92-carbon_elimination.png)
β-alkyl elimination
[edit]β-alkyl elimination is the most common and useful type among all β-carbon elimination reactions.
Classification/Driving force
[edit]β-alkyl elimination with early transition metal complexes
[edit]In terms of thermodynamics, more electron-deficient metal centers increase the likelihood of β-alkyl elimination. For example, β-alkyl elimination is more favorable than β-hydride elimination when it is bonded to electron-deficient early transition metals (Hf, Ti, Zr, Nb, etc.) with d0 configuration. Computational studies show a thermodynamic preference for β-Me elimination over β-H elimination in these complexes due to additional stability for the metal–alkyl species.[3] The origin of the additional bonding interaction comes from an orbital centered on the CH3 weakly π-donating to the LUMO of the d0 of the metal center which is analogous to the hyperconjugation effect (see figure on the right), thus increasing the stability of M−CH3 over M−H species. Their calculations predict that a more electrophilic metal ion enhances the −CH3 π-donation, which consequently increases the stability of M−CH3 over M−H species. Conversely, a more electron-rich metal ion will favor M−H formation (for example, using the more electron-donating Cp* ligand in Cp*2MX2).
In terms of kinetics, steric effects of ligands could play a role in increasing the energy barrier of β-H elimination relative to β-alkyl elimination, specifically when the ligand is Cp*. A model was proposed to illustrate this effect:[4] In both β-methyl elimination (A) and β-hydride elimination (B), the transferring group aligns perpendicular to the Cp*(centroid)−Zr−Cp*(centroid), allowing the σC−C or σC−H bond to overlap with the metal d-orbital. However, to achieve the prerequisite geometry for β-H elimination (B), the adjacent methyl group experiences a significant steric repulsion from the Cp* ligand, thereby elevating the barrier to hydride transfer. By contrast, transition state A for β-Me elimination experiences less steric interaction with the Cp* ligand.
β-alkyl elimination with middle and late transition metal complexes
[edit]In middle and late transition metal complexes, there is larger thermodynamic preference for β-H elimination over β-alkyl elimination, where the difference is usually >15 kcal/mol.[2] Examples involved middle and late transition metal complexes are either absent of β-hydrogens or use ring strain relief and aromaticity as driving forces to favor β-alkyl elimination over β-hydride elimination.[5]
Applications
[edit]Ring-opening polymerization (ROP)
[edit]Ring-opening polymerization that involves β-alkyl elimination can be catalyzed by Ti,[6] Zr,[7][8] Pd[9]-based catalyst, and some Lanthanide-based metallocene catalyst,[10][11] where different polymerization patterns vary when catalysts are different. Examples of copolymerization with alkene [10] or carbon monoxide[12][13] were also reported. The key step of this kind of ROP is string-driven β-alkyl elimination, which provides linear polymer with unsaturation in the polymer chain.
Organic synthesis
[edit]There is enormous amount of catalytic processes involving β-alkyl elimination that are synthetically useful. β-alkyl elimination in this case, however, is often considered as an alternative way of C–C bond cleavage while oxidative addition is the direct way.[14] One of the examples is β-alkyl elimination of tert-alcoholates which can generate from either addition of an organometallic reagent or ligand exchange.[15][16][17] The consequent organometallic species can undergo various downstream reactivities (reductive elimination, carbonyl insertion, etc.) to generate useful building blocks.
In addition to ring strain, aromaticity-driven β-Me elimination can be effectively employed to dealkylate steroid derivatives and some other cyclohexyl compounds.[18][19]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Aromaticity-driven_beta-alkyl_elimination.png/565px-Aromaticity-driven_beta-alkyl_elimination.png)
β-aryl elimination
[edit]β-aryl elimination is much less common and understood than β-alkyl elimination. Examples are reported to occur from metal alkoxide and amido complexes.[20][21][22] A theoretical study showed that these reactions are driven by consequent extensive conjugation system.[23] A very recent example of catalytic β-aryl elimination which leads to enantioselective synthesis of biaryl atropisomers is driven by release of distorted ring string.[24]
References
[edit]- ^ Smits, G.; Audic, B.; Wodrich, M. D.; Corminboeuf, C.; Cramer, N. (24 August 2017). "A β-Carbon elimination strategy for convenient in situ access to cyclopentadienyl metal complexes". Chemical Science. 8 (10): 7174–7179. doi:10.1039/C7SC02986A. ISSN 2041-6520. PMC 5635420. PMID 29081949. Wikidata Q42705934.
- ^ a b c O’Reilly, Matthew E.; Dutta, Saikat; Veige, Adam S. (2016-07-27). "β-Alkyl Elimination: Fundamental Principles and Some Applications". Chemical Reviews. 116 (14): 8105–8145. doi:10.1021/acs.chemrev.6b00054. ISSN 0009-2665. PMID 27366938.
- ^ Sini, Gjergji; Macgregor, Stuart A.; Eisenstein, Odile; Teuben, Jan H. (April 1994). "Why Is .beta.-Me Elimination Only Observed in d0 Early-Transition-Metal Complexes? An Organometallic Hyperconjugation Effect with Consequences for the Termination Step in Ziegler-Natta Catalysis". Organometallics. 13 (4): 1049–1051. doi:10.1021/om00016a001. ISSN 0276-7333.
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- ^ Miura, Masahiro; Satoh, Tetsuya (2005-06-20), Tsuji, Jiro (ed.), "Catalytic Processes Involving β-Carbon Elimination", Palladium in Organic Synthesis, vol. 14, Springer Berlin Heidelberg, pp. 1–20, doi:10.1007/b104133, ISBN 9783540239826
- ^ Rossi, R.; Diversi, P.; Porri, L. (May 1972). "On the Ring-Opening Polymerization of Methylenecyclobutane". Macromolecules. 5 (3): 247–249. Bibcode:1972MaMol...5..247R. doi:10.1021/ma60027a004. ISSN 0024-9297.
- ^ Beswick, Colin L.; Marks, Tobin J. (October 2000). "Metal-Alkyl Group Effects on the Thermodynamic Stability and Stereochemical Mobility of B(C 6 F 5 ) 3 -Derived Zr and Hf Metallocenium Ion-Pairs". Journal of the American Chemical Society. 122 (42): 10358–10370. doi:10.1021/ja000810a. ISSN 0002-7863.
- ^ Jia, Li; Yang, Xinmin; Seyam, Affif M.; Albert, Israel D. L.; Fu, Peng-Fei; Yang, Shengtian; Marks, Tobin J. (January 1996). "Ring-Opening Ziegler Polymerization of Methylenecycloalkanes Catalyzed by Highly Electrophilic d 0 /f n Metallocenes. Reactivity, Scope, Reaction Mechanism, and Routes to Functionalized Polyolefins". Journal of the American Chemical Society. 118 (34): 7900–7913. doi:10.1021/ja960811w. ISSN 0002-7863.
- ^ Takeuchi, Daisuke; Kim, Sunwook; Osakada, Kohtaro (2001-07-16). "Ring-Opening Polymerization of 1-Methylene-2-phenylcyclopropane Catalyzed by a Pd Complex To Afford Regioregulated Polymers". Angewandte Chemie International Edition. 40 (14): 2685–2688. doi:10.1002/1521-3773(20010716)40:14<2685::AID-ANIE2685>3.0.CO;2-9. ISSN 1433-7851.
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- ^ Yang, Xinmin; Seyam, A. M.; Fu, Peng-Fei; Mark, Tobin J. (August 1994). "exo-Methylene-Functionalized Polyethylenes via Ring-Opening Ziegler Polymerization. Product Control in Organolanthanide-Catalyzed Methylenecyclopropane Polymerization/Copolymerization". Macromolecules. 27 (16): 4625–4626. Bibcode:1994MaMol..27.4625Y. doi:10.1021/ma00094a030. ISSN 0024-9297.
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- ^ Kim, Sunwook; Takeuchi, Daisuke; Osakada, Kohtaro (February 2002). "Pd-Catalyzed Ring-Opening Copolymerization of 2-Aryl-1-methylenecyclopropanes with CO to Afford Polyketones via Alternating Insertion of the Two Monomers and C−C Bond Activation of the Three-Membered Ring". Journal of the American Chemical Society. 124 (5): 762–763. doi:10.1021/ja017460s. ISSN 0002-7863. PMID 11817946.
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- ^ Murakami, Masahiro; Makino, Masaomi; Ashida, Shinji; Matsuda, Takanori (September 2006). "Construction of Carbon Frameworks through β-Carbon Elimination Mediated by Transition Metals". Bulletin of the Chemical Society of Japan. 79 (9): 1315–1321. doi:10.1246/bcsj.79.1315. ISSN 0009-2673.
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- ^ Zhao, Pinjing; Incarvito, Christopher D.; Hartwig, John F. (March 2006). "Direct Observation of β-Aryl Eliminations from Rh(I) Alkoxides". Journal of the American Chemical Society. 128 (10): 3124–3125. doi:10.1021/ja058550q. ISSN 0002-7863. PMID 16522075.
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- ^ Deng, Ruixian; Xi, Junwei; Li, Qigang; Gu, Zhenhua (May 2019). "Enantioselective Carbon-Carbon Bond Cleavage for Biaryl Atropisomers Synthesis". Chem. 5 (7): 1834–1846. doi:10.1016/j.chempr.2019.04.008.