Chemistry:β-Carbon elimination

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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

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]

Β-carbon elimination.png

β-alkyl elimination

β-alkyl elimination is the most common and useful type among all β-carbon elimination reactions.

Classification/Driving force

β-alkyl elimination with early transition metal complexes

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

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

Ring-opening polymerization (ROP)

Main page: Chemistry:Ring-opening polymerization

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

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]

Aromaticity-driven beta-alkyl elimination.png

β-aryl elimination

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

  1. ↑ Smits, G.; Audic, B.; Wodrich, M. D.; Corminboeuf, C.; Cramer, N. , Wikidata Q42705934
  2. ↑ 2.0 2.1 2.2 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. 
  3. ↑ 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. 
  4. ↑ Eshuis, Johan J. W.; Tan, Yong Y.; Meetsma, Auke; Teuben, Jan H.; Renkema, Jaap; Evens, George G. (January 1992). "Kinetic and mechanistic aspects of propene oligomerization with ionic organozirconium and -hafnium compounds: crystal structures of [Cp*2MMe(THT)+[BPh4]- (M = zirconium, hafnium)"]. Organometallics 11 (1): 362–369. doi:10.1021/om00037a061. ISSN 0276-7333. https://pure.rug.nl/ws/files/14631982/1992OrganometallicsEshuis.pdf. 
  5. ↑ Miura, Masahiro; Satoh, Tetsuya (2005-06-20), Tsuji, Jiro, ed., "Catalytic Processes Involving β-Carbon Elimination", Palladium in Organic Synthesis (Springer Berlin Heidelberg) 14: pp. 1–20, doi:10.1007/b104133, ISBN 9783540239826 
  6. ↑ Rossi, R.; Diversi, P.; Porri, L. (May 1972). "On the Ring-Opening Polymerization of Methylenecyclobutane". Macromolecules 5 (3): 247–249. doi:10.1021/ma60027a004. ISSN 0024-9297. Bibcode: 1972MaMol...5..247R. 
  7. ↑ 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. 
  8. ↑ 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. 
  9. ↑ 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. 
  10. ↑ 10.0 10.1 Jensen, Tryg R.; O'Donnel, James J.; Marks, Tobin J. (February 2004). "d 0 /f n -Mediated Ring-Opening Ziegler Polymerization (ROZP) and Copolymerization with Mono- and Disubstituted Methylenecyclopropanes. Diverse Mechanisms and a New Chain-Capping Termination Process". Organometallics 23 (4): 740–754. doi:10.1021/om030407n. ISSN 0276-7333. 
  11. ↑ 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. doi:10.1021/ma00094a030. ISSN 0024-9297. Bibcode: 1994MaMol..27.4625Y. 
  12. ↑ Kettunen, Mika; Abu-Surrah, Adnan S; Repo, Timo; Leskelä, Markku (November 2001). "Copolymerization of carbon monoxide with exo -methylenecycloalkane and dienes: synthesis of functionalized aliphatic polyketones: Functionalized aliphatic polyketone synthesis". Polymer International 50 (11): 1223–1227. doi:10.1002/pi.769. 
  13. ↑ 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. 
  14. ↑ Dong, Guangbin, ed (2014). C–C Bond Activation. Topics in Current Chemistry. 346. Berlin, Heidelberg: Springer Berlin Heidelberg. doi:10.1007/978-3-642-55055-3. ISBN 9783642550546. 
  15. ↑ 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. 
  16. ↑ Seiser, Tobias; Cramer, Nicolai (2009). "Enantioselective metal-catalyzed activation of strained rings". Organic & Biomolecular Chemistry 7 (14): 2835–40. doi:10.1039/b904405a. ISSN 1477-0520. PMID 19582290. 
  17. ↑ Huffman, Mark A.; Liebeskind, Lanny S. (November 1990). "Insertion of (.eta.5-indeny)cobalt(I) into cyclobutenones: the first synthesis of phenols from isolated vinylketene complexes". Journal of the American Chemical Society 112 (23): 8617–8618. doi:10.1021/ja00179a075. ISSN 0002-7863. 
  18. ↑ Halcrow, Malcolm A.; Urbanos, Francisco; Chaudret, Bruno (March 1993). "Aromatization of the B-ring of 5,7-dienyl steroids by the electrophilic ruthenium fragment "[Cp*Ru]+"". Organometallics 12 (3): 955–957. doi:10.1021/om00027a054. ISSN 0276-7333. 
  19. ↑ Older, Christina M.; Stryker, Jeffrey M. (March 2000). "The Mechanism of Carbon−Carbon Bond Activation in Cationic 6-Alkylcyclohexadienyl Ruthenium Hydride Complexes". Journal of the American Chemical Society 122 (12): 2784–2797. doi:10.1021/ja992987e. ISSN 0002-7863. 
  20. ↑ 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. 
  21. ↑ Zhao, Pinjing; Hartwig, John F. (2005-08-24). "β-Aryl Eliminations from Rh(I) Iminyl Complexes". Journal of the American Chemical Society 127 (33): 11618–11619. doi:10.1021/ja054132+. ISSN 0002-7863. PMID 16104735. 
  22. ↑ Terao, Yoshito; Wakui, Hiroyuki; Satoh, Tetsuya; Miura, Masahiro; Nomura, Masakatsu (October 2001). "Palladium-Catalyzed Arylative Carbon−Carbon Bond Cleavage of α,α-Disubstituted Arylmethanols". Journal of the American Chemical Society 123 (42): 10407–10408. doi:10.1021/ja016914i. ISSN 0002-7863. PMID 11604000. 
  23. ↑ Xue, Liqin; Ng, Ka Chun; Lin, Zhenyang (2009). "Theoretical studies on β-aryl elimination from Rh(i) complexes". Dalton Transactions (30): 5841–5850. doi:10.1039/b902539a. ISSN 1477-9226. PMID 19623383. http://xlink.rsc.org/?DOI=b902539a. 
  24. ↑ 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. 



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