Diels-Alder cycloaddition reaction dimerization of cyclopentadiene

Cyclopentadiene is a very reactive diene and exists as its dimer that needs to be cracked (retro-Diels-Alder reaction) to prepare the diene. Cycloaddition with dienophiles forms bridged compounds of the bicyclo[2.2.1]heptane series. The reaction of cyclopentadiene with mono-and cw-disubstituted alkenes could give rise to two stereochemically distinct products, the endo- and ejco-bicyclo[2.2.1]heptene derivatives. It is found in practice, however, that the endo isomer predominates. 

Diels-Alder reactions like the one illustrated opposite are cycloadditions mobilising six electrons. The dimerization of cyclopentadiene 1.1 is another Diels-Alder reaction, but also illustrates its inherent reversibility—cracking the dimer 1.2 on heating is called a retro-cycloaddition or a cycloreversion. 

Diels-Alder reactions continue to be of value for the interception of highly reactive dienes or dienophiles. Thus, irradiation of 2-cyclo-octen-l-one or of 2-cyclohepten-l-one in the presence of excess cyclopentadiene affords the norbornene derivatives (173 n = 5 or 4 respectively), which undergo retro-[4 -I- 2] addition on distillation. Presumably the trans-2-cycloalken-l-ones are reactive intermediates in these processes. Elimination of acetic acid from bis(trifluoromethyl)cyclopentadienylcarbinyl acetate yields the reactive 6,6-bis(trifluoromethyl)fulvene, which rapidly dimerizes by [4 + 2] cycloaddition the fulvene intermediate can be intercepted by similar addition to cyclopentadiene. The adduct (174) is formed on reaction of cyclopentadiene with the imine produced by iodine-catalysed rearrangement of NN- 

Lanthanide /3-diketonates have been used as catalysts in Diels-Alder reactions. The first example of a lanthanide-catalyzed cycloaddition was the dimerization of spiro[2.4]hepta-4,6-diene by [Eu(tfn)3] (Morrill et al., 1975) (scheme 2). In the absence of the europiitm(lll) complex no dimerization took place. Because of the mild experimental conditions, this catalyst has potential in Diels-Alder reactions where acid labile components are combined. An example is the cycloaddition of cyclopentadiene with acrolein (Danishefsky and Bednarski, 1985). 

Product formation was postulated to arise from the spontaneous rearrangement of the initially formed hetero Diels-Alder adducts 127. The similar reaction when performed with an alkyl glyoxylate in the absence of water (neat) or in organic solvent (toluene) inevitably leads to cyclopentadiene dimerization instead of the expected cycloaddition. 

In Part B, the cycloaddition of 1,3-cyclopentadiene with maleic anhydride is examined (Eq. 12.5). This experiment provides an opportunity to investigate the stereoselectivity of the Diels-Alder reaction. Monomeric 1,3-cyclopentadiene cannot be purchased because it readily dimerizes at room temperature by a Diels-Alder reaction to give dicyclopentadiene (Eq. 12.8), which is commercially available. Fortunately, the equilibrium between the monomer and the dimer can be established at the boiling point of the dimer (170 °C, 760 torr) by a process commonly called cracking, and the lower-boiling 1,3-cyclopentadiene may then be isolated by fractional distillation. The diene must be kept cold in order to prevent its redimerization via a Diels-Alder reaction prior to use in the desired Diels-Alder reaction. 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction  

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

Pentalenes.—The X-ray structure of the tri-t-butylpentalene (60) has been determined the compound is planar and the double bonds are localised. Pentalenes (62), together with dimers, are produced by the action of acetylenes RC=CH (R = C02Me, CHO, or CN) on l,3-di-t-butyl-6-dimethylamino-fulvene (61). The aldehyde (62 R = CHO) dimerized to yield mainly compound (63), whereas the head-to-tail dimer (64) was isolated from the cyano-derivative (62 R = CN). The latter adds water under acidic conditions to give the alcohol (65), while dimethylamine affords the rearranged adduct (66). Cycloaddition reactions of the pentalene di-ester (67) proceed differently with dicyanoacetylene and cyclopentadiene the former yields the cyclobutene derivative (68), the latter the Diels-Alder product (69).  

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