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Review
. 2019 Aug 15;9(44):25554-25568.
doi: 10.1039/c9ra02839k. eCollection 2019 Aug 13.

Transition metal catalyzed [6 + 2] cycloadditions

Amit Anand  1 , Prabhpreet Singh  2 , Vipan Kumar  2 , Gaurav Bhargava  3
Affiliations
Review

Transition metal catalyzed [6 + 2] cycloadditions

Amit Anand et al. RSC Adv. .

Abstract

The [6 + 2] cycloaddition reactions are one of the important synthetic transformations to construct eight membered carbo-/heterocyclic systems. The present review is an attempt to update readers on transition metal catalyzed [6 + 2] cycloaddition reactions of various 6π-contributing substrates such as cycloheptatrienes (CHT), cyclooctatetrenes (COTT), allenals, vinylcyclobutanones, fulvene etc. employing rhodium, cobalt, titanium, copper, platinum, ruthenium, rhenium and diphenylprolinolsilyl ethers etc. as catalysts. The transition metal catalyzed [6 + 2] cycloaddition reactions with a variety of functionalized substrates provide straightforward access to eight membered cyclic and/or 5/8, 6/8 etc. condensed carbo-/heterocyclic molecules in moderate to good yields.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Carbocycles in natural products.
Scheme 1
Scheme 1. Rh(i) catalysed [6 + 2] cycloaddition reactions of tethered alkenes with cyclobutanones.
Fig. 2
Fig. 2. Plausible mechanism for synthesis of eight-membered condensed carbocycles.
Scheme 2
Scheme 2. Rh(i)-catalyzed [6 + 2] cycloaddition of alkene tethered allenal.
Scheme 3
Scheme 3. Rh(i)-catalyzed [6 + 2] cycloaddition of alkyne tethered allenal.
Fig. 3
Fig. 3. Proposed mechanism for rhodium catalysed [6 + 2] cycloaddition reactions of allenals.
Scheme 4
Scheme 4. Intermolecular [6 + 2] cycloadditions.
Scheme 5
Scheme 5. Intermolecular [6 + 2] cycloadditions using various Rh(i) complexes.
Scheme 6
Scheme 6. Intermolecular [6 + 2] cycloadditions of allenals and alkynes.
Scheme 7
Scheme 7. Rh(i) catalysed [6 + 2] reactions of unfunctionalised alkyne-allenyl cyclopropane/butane.
Scheme 8
Scheme 8. Cycloadditions of alkyne-allenyl cyclobutane 13 in different Rh(i) complexes.
Fig. 4
Fig. 4. Proposed mechanism for the formation of 14.
Scheme 9
Scheme 9. [6 + 2] cycloaddition of CHT with various alkynes.
Fig. 5
Fig. 5. Proposed Mechanism for rhodium catalysed [6 + 2] cycloaddition reaction of CHT with internal alkyne.
Scheme 10
Scheme 10. Cobalt(ii) catalyzed [6 + 2] cycloaddition reactions of CHT with terminal alkynes.
Fig. 6
Fig. 6. Proposed Mechanism for cobalt catalysed [6 + 2] cycloadditions of CHT with terminal alkynes.
Scheme 11
Scheme 11. Cobalt catalysed [6 + 2] reactions of CHT with substituted allenes.
Scheme 12
Scheme 12. Cobalt(ii) catalysed [6 + 2] cycloaddition reactions of COTT with terminal allenes.
Scheme 13
Scheme 13. Cobalt(ii) catalysed [6 + 2] cycloaddition reactions of COTT with cyclic allene.
Scheme 14
Scheme 14. Cobalt-catalyzed [6 + 2] cycloadditions of cyclooctatetraene with terminal alkynes.
Scheme 15
Scheme 15. Cobalt-catalyzed [6 + 2] cycloadditions of cyclooctatetraene with symmetrical alkynes.
Scheme 16
Scheme 16. Cobalt-catalyzed [6 + 2] cycloadditions of cyclooctatriene (COT) with terminal alkynes.
Fig. 7
Fig. 7. Valance tautomeric forms of COT/COTT and their participation in cycloadditions with alkynes.
Fig. 8
Fig. 8. Proposed mechanism for cobalt catalysed cycloadditions of COTT26/COT 32 with terminal alkynes 30.
Scheme 17
Scheme 17. [6 + 2] cycloadditions of various enol silyl ether with dicobalt acetylene complex.
Scheme 18
Scheme 18. Ti-catalyzed [6 + 2] cycloadditions of alkynes to 7-substituted 1,3,5-cycloheptatrienes.
Scheme 19
Scheme 19. Ti-catalyzed [6 + 2] cycloadditions of silicon containing alkynes to 7-substituted 1,3,5-cycloheptatrienes (CHT).
Scheme 20
Scheme 20. Ti-catalyzed [6 + 2] cycloadditions of 1,2-dienes to 7-substituted 1,3,5-cycloheptatrienes (CHT).
Scheme 21
Scheme 21. Ti-catalyzed [6 + 2] cycloadditions of 1,2 cyclononadiene 7-substituted 1,3,5-cycloheptatrienes (CHT).
Scheme 22
Scheme 22. Ti-catalyzed [6 + 2] cycloadditions of silicon containing alkynes with bis-1,3,5-cycloheptatrienes (CHT).
Scheme 23
Scheme 23. PtCl2-catalyzed [6 + 2] cycloadditions of alkynes tethered to cycloheptatrienes 54.
Scheme 24
Scheme 24. PtCl2-catalyzed [6 + 2] cycloadditions of trienynes 54a–e.
Scheme 25
Scheme 25. PtCl2-catalyzed [6 + 2] cycloadditions of heteroatom tethered trienynes 57a–g.
Fig. 9
Fig. 9. Proposed mechanism for cycloisomerisations of trienynes.
Scheme 26
Scheme 26. Formation of ruthenacycles 67via oxidative cyclisations.
Fig. 10
Fig. 10. Proposed mechanism for formation of ruthenacycles 65.
Scheme 27
Scheme 27. Reactivity of ruthenacycle 65.
Scheme 28
Scheme 28. Cu(OTf)2 catalysed [6 + 2] cycloadditions reactions of 71 and 72.
Fig. 11
Fig. 11. Structure of yuremamine.
Scheme 29
Scheme 29. [6 + 2] cycloadditions of CHT with ethyl acrylate catalysed by solid supported chromium(0) complex.
Scheme 30
Scheme 30. [6 + 2] cycloadditions of CHT with ethyl acrylate catalysed by solid supported chromium(0) complex.
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