Cyclopentadiene1

[542-92-7]  · C5H6  · Cyclopentadiene  · (MW 66.10)

(undergoes a variety of cycloaddition reactions;5 can be deprotonated to form the anion which reacts with electrophiles27 and forms stable transition metal complexes;3 can be used to generate polymers31)

Physical Data: bp 41-42 °C; d 0.805 g cm-3.

Solubility: misc. EtOH, benzene.

Form Supplied in: obtained from dicyclopentadiene

Preparative Method: the volatile diene dimerizes readily and is prepared as required by depolymerization of technical dicyclopentadiene (widely available, bp 171-173 °C). The depolymerization is done by heating the dimer carefully under a fractionating column.2-4

Handling, Storage, and Precautions: toxic vapor; stench; highly flammable (flash point <25 °C); can dimerize explosively; forms unstable peroxides with oxygen; avoid skin contact. This reagent should be handled only in a fume hood.

Cycloadditions.

Diels-Alder reactions of cyclopentadiene with dienophiles leads predominantly to the endo products. In the presence of Lewis acids the endo selectivity is greatly increased (eq 1).5 The use of water as the reaction solvent,6 or the presence of modified clays7,8 or zeolites9 in organic solvents, also leads to a pronounced increase in endo selectivity together with increases in reaction rates.

Cyclopentadiene has also been widely used in asymmetric Diels-Alder reactions (eq 2).10 Chromium carbene complexes form cycloadducts with cyclopentadiene at increased reaction rates compared to the organic ester analogs (eq 3).11-13

Cyclopentadiene reacts with benzyne to afford Diels-Alder adducts in high yields (eq 4).14 This reaction is so efficient that it has been used as a diagnostic test for the formation of benzyne. Heteroatomic dienophiles such as imines,15,16 thioaldehydes,17 selenoaldehydes,18 and sulfenes (eq 5)19 also react with cyclopentadiene in a [4 + 2] manner.

Cycloaddition with cyclopentadiene has been used for protection of double bonds, which can be subsequently revealed upon pyrolysis (eq 6).20a This type of retrograde Diels-Alder reaction has been reviewed.20b,c

Cyclopentadiene undergoes a [4 + 3] cycloaddition upon reaction with 2-oxyallyl cations, providing bicyclo[3.2.1]oct-6-en-3-ones (eq 7).21 These types of products have been employed as important precursors to natural products.21

Cyclopentadiene reacts as a simple alkene with ketenes such as 2-carbonyl-1,3-dithiane (eq 8).22 Likewise, cyclopentadiene reacts chemoselectively with alkynes in the Pauson-Khand cycloaddition. The chemical yields are generally excellent, but the regioselectivity is only moderate (eq 9).23

Deprotonation.

Cyclopentadiene can be readily deprotonated by strong bases to give the stabilized (6p-electron) cyclopentadienyl anion. This anion is a ubiquitous ligand for transition metal complexes; examples are ruthenocene24 and ferrocene (eq 10).3

The cyclopentadienyl anion also reacts with ketones to generate fulvenes (eq 11).25 Aldehydes only polymerize under these conditions; however, they may be used to form the corresponding fulvenes by employing a modified method.26 Reaction of the anion with electrophiles often leads to mixtures of regioisomers due to facile ambient temperature 1,5-hydrogen shifts (eq 12).27a To circumvent these hydrogen shifts, the use of cyclopentadienylthallium at low temperature has been employed.27b

1,4-Addition Reactions.

Cyclopentadiene reacts with Bromine to afford the cis-1,4-adduct in high yield (eq 13).28 The cis-1,4-diol may be obtained by reaction with singlet oxygen and Thiourea (eq 14).29 The diacetate of this diol can be desymmetrized with enzymes to afford the enantiomerically enriched alcohol, which is an important chiral building block (eq 15).30

Polymerization.

Cyclopentadiene undergoes polymerization in the presence of 0.1 mol % of a tungsten nitrosyl Lewis acid catalyst (eq 16).31 The polymer is a mixture of 1,4- and 1,2-polycyclopentadiene.

Related Reagents.

5-Bromo-1,3-cyclopentadiene; 5-(Methoxymethyl)-1,3-cyclopentadiene.


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21. Hosomi, A.; Tominaga, Y. COS 1991, 5, 593.
22. Cossement, E.; Biname, R.; Ghosez, L. TL 1974, 997.
23. Khand, I. U.; Pauson, P. L.; Habib, J. A. JCR(M) 1978, 4418.
24. Bublitz, D. E.; McEwen, W. E.; Kleinberg, J. OSC 1973, 5, 1001.
25. Alper, H.; Laycock, D. E. S 1980, 799.
26. Stone, K. J.; Little, D. R. JOC 1984, 49, 1849.
27. (a) Rees, W. S., Jr.; Dippel, K. A. OPP 1992, 24, 527. (b) Corey, E. J.; Koelliker, U.; Neuffer, J. JACS 1971, 93, 1489.
28. Owen, L. N.; Smith, P. N. JCS 1952, 4035.
29. Kaneko, C.; Sugimoto, A.; Tanaka, S. S 1974, 877.
30. (a) Laumen, K.; Reimerdes, E. H.; Schneider, M. TL 1985, 26, 407. (b) Laumen, K.; Schneider, M. P. CC 1986, 1298. (c) Johnson, C. R.; Bis, S. J. TL 1992, 33, 7287.
31. Honeychuck, R. V.; Bonnesen, P. V.; Farahi, J.; Hersh, W. H. JOC 1987, 52, 5293.

A. Chris Krueger

Wayne State University, Detroit, MI, USA



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