Β-Carbon elimination explained

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.

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. 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.

Applications

Ring-opening polymerization (ROP)

See main article: Ring-opening polymerization.

Ring-opening polymerization that involves β-alkyl elimination can be catalyzed by Ti,[5] Zr,[6] [7] Pd[8] -based catalyst, and some Lanthanide-based metallocene catalyst,[9] [10] where different polymerization patterns vary when catalysts are different. Examples of copolymerization with alkene or carbon monoxide[11] [12] 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.[13] One of the examples is β-alkyl elimination of tert-alcoholates which can generate from either addition of an organometallic reagent or ligand exchange.[14] [15] [16] 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.[17] [18]

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.[19] [20] [21] A theoretical study showed that these reactions are driven by consequent extensive conjugation system.[22] 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.[23]

Notes and References

  1. Smits. G.. Audic. B.. Wodrich. M. D.. Corminboeuf. C.. Cramer. N.. Clémence Corminboeuf.
  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. 10.1021/acs.chemrev.6b00054. 27366938. 0009-2665.
  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. 10.1021/om00016a001. 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. 10.1021/om00037a061. 0276-7333.
  5. Rossi. R.. Diversi. P.. Porri. L.. May 1972. On the Ring-Opening Polymerization of Methylenecyclobutane. Macromolecules. 5. 3. 247–249. 10.1021/ma60027a004. 0024-9297. 1972MaMol...5..247R.
  6. 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. 10.1021/ja000810a. 0002-7863.
  7. 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. 10.1021/ja960811w. 0002-7863.
  8. 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. 10.1002/1521-3773(20010716)40:14<2685::AID-ANIE2685>3.0.CO;2-9. 1433-7851.
  9. 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. 10.1021/om030407n. 0276-7333.
  10. 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. 10.1021/ma00094a030. 0024-9297. 1994MaMol..27.4625Y.
  11. 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. 10.1002/pi.769.
  12. 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. 10.1021/ja017460s. 11817946. 0002-7863.
  13. Book: C–C Bond Activation. 2014. Springer Berlin Heidelberg. 9783642550546. Dong. Guangbin. Topics in Current Chemistry. 346. Berlin, Heidelberg. 10.1007/978-3-642-55055-3. 60212270.
  14. 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. 10.1246/bcsj.79.1315. 0009-2673. free.
  15. Seiser. Tobias. Cramer. Nicolai. 2009. Enantioselective metal-catalyzed activation of strained rings. Organic & Biomolecular Chemistry. 7. 14. 2835–40. 10.1039/b904405a. 19582290. 1477-0520.
  16. 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. 10.1021/ja00179a075. 0002-7863.
  17. 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. 10.1021/om00027a054. 0276-7333.
  18. 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. 10.1021/ja992987e. 0002-7863.
  19. 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. 10.1021/ja058550q. 16522075. 0002-7863.
  20. 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. 10.1021/ja054132+. 16104735. 0002-7863.
  21. 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. 10.1021/ja016914i. 11604000. 0002-7863.
  22. Xue. Liqin. Ng. Ka Chun. Lin. Zhenyang. 2009. Theoretical studies on β-aryl elimination from Rh(i) complexes. Dalton Transactions. 30. 5841–5850. 10.1039/b902539a. 19623383. 1477-9226.
  23. Deng. Ruixian. Xi. Junwei. Li. Qigang. Gu. Zhenhua. May 2019. Enantioselective Carbon-Carbon Bond Cleavage for Biaryl Atropisomers Synthesis. Chem. 5. 7. 1834–1846. 10.1016/j.chempr.2019.04.008. free.

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