Cyclopentadiene . interacting double bonds

We have shown [13,79, 80] that the electronic spectra of c -l,3-buta-diene [81], cyclopentadiene, aromatic five-membered heterocycles [48,79], and norbornadiene [80] can be understood on the basis of a model with two interacting double bonds. Cyclopentadiene (CP) is the prime example of a ring-shaped molecule with a conjugated rr-electron system, and its structure can be related to that of short polyenes such as cM-1,3-butadiene (CB) and the simplest heterocycles, such as pyrrole (PY), furan (FU), and thiophene (TP). In the series, ci5-l,3-butadiene, cyclopentadiene, norbornadiene (NB), the latter is the most complex system, with the two ethylenic units coupled through indirect conjugation and 77, a interaction. One more system will be added here to the set of molecules with two interacting double bonds methylenecyclopropene (MCP) [(1) in Fig 3] the simplest cross-conjugated 77-electron system. 

Secondary orbital interaction had been proposed to explain predominant formation of endo attack prodncts in Diels Alder reaction of cyclopentadiene and dienophiles by Hoffmann and Woodward [22]. According to this rnle, the major stereoisomer in Diels-Alder reactions is that it is formed through a maximum accumulation of double bonds. In the Diels-Alder reactions, secondary orbital interaction consists of a stabilizing two-electron interaction between the atoms not involved in the formation or cleavage of o bonds (Scheme 19). 

Fig. 9a4) between cyclopentadiene and a C=C bond of the dumbbell-shaped part of the rotaxane. The dumbbell-shaped part contains two dicarbonyl stations (Fig. 9a3), one derived from fumaric acid (tram -CO-C H=CH-CO-. station 1), the other derived from succinic acid (—CO-CH2-CH2-CO-, station 2). The two diamide sites of the macrocycle can form four H-bonds with the two carbonyl groups of a given station (Fig. 9al for the interaction of the two carbonyl groups of fumaric-acid-derived station 1 with the four NH groups of the macrocycle through four H-bonds, see Fig. 9a2). Station 1 (derived from fumaric acid) has a tram C=C double bond due to its preorganization, this station interacts with the macrocycle better than the station 2. Consequently, the macrocycle is initially located at station 1 (Fig. 9a5). The Diels-Alder cycloaddition (80° C, 90% yield) of cyclopentadiene to the double bond of station 1 results in a mixture of diastereomers (Fig. 9a4) and causes displacement of the macrocycle from station 1 to station 2 (Fig. 9a6). The cycloaddition is reversible and the retro-Diels-Alder reaction occurs quantitatively (250°C, reduced pressure) when cyclopentadiene dissociates from the axle of the rotaxane this produces a displacement of the macrocycle from station 2 back to station 1. 

However, the cyclodimerization of 1,3-cyclohexadiene and also the addition of the cis,cis isomer of 2,4-hexadiene to 1,3-cyclohexadiene are only modestly stereoselective. The addition of cis,rra i-2,4-hexadiene to 1,3-cyclohexadiene is highly stereoselective for the addition to the lra s-propenyl group, but only modestly stereoselective for the addition to the cw-propenyl group. Further, the addition of a dienophile having a pendant, unsubstituted vinyl double bond to this diene is also highly endo stereoselective. The installation of a cis group at the terminus of the dienophilic moiety consistently appears to reduce the endo stereoselectivity to a more modest level. It has been proposed that the cis substituent attenuates the secondary interaction involving the endo double bond in the transition state for cycloaddition [47, 48]. The effect has been termed the cfs-propenyl effect . The addition of the ira j-anethole cation radical to both 1,3-cyclohexadiene and 1,3-cyclopentadiene is, however, only moderately diastereospecific (ca 3 1) [49]. 

The barrier to converting the s-trans conformation to the s-cis conformation contributes to the overall activation barrier for Diels-Alder reactions. Structural factors that increase the proportion of diene in its s-cis conformation increase the rate of the Diels-Alder reaction, and factors that increase the proportion of diene in its s-trans conformation decrease the rate of the reaction. Cyclopentadiene is one of the best dienes for the Diels-Alder reaction partly because it cannot rotate out of its s-cis conformation. In fact, cyclopentadiene undergoes [4 + 2] cycloaddition to itself so readily that it lasts only a few hours at 0 °C. o-Xylylenes are especially good dienes both because of their enforced s-cis geometry and because a nonaromatic starting material is transformed into an aromatic product. By contrast, dienes in which one of the double bonds is cis are poor substrates for Diels-Alder reactions because steric interactions between the in substituents in the s-cis conformation are particularly severe, and dienes whose s-trans conformation is enforced do not ever undergo the Diels-Alder reaction. 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

Disubstituted cyclopropenes undergo addition to tetraarylcyclopentadienones to produce mixtures of exo- and emfo-adducts in which the endo-isomer predominates (Table 12).187188 The predominance of enr/o-product has been explained in terms of secondary orbital interactions, involving the methylene group of the cyclopropene.147 3-Methyl-3-vi-nylcyclopropene reacted with 2,3,4,5-tetraphenylcyclopenta-2,4-dien-l-one at the cyclopropene double bond and a [4+2] cycloadduct of unspecified stereochemistry was isolated (41% mp 231-234 °C) together with decarbonylated products reaction of 2-oxo-4,5-diphenyl-l,3-bis(methoxycarbonyl)cyclopentadiene with 3-methyl-3-vinylcyclopropene was more complicated and apparently attack occurred at both 71-bonds.1... 

Experience has shown that cyclopentadiene annulatlon of 2,3-dimethylenebicyclo[2.2.2]octanes can be efficiently realized by means of the Skattebtfl procedure.7 However, the added strain in 2,3-dimethylenenorbornanes reroutes the rearrangement instead into vinylallene formation. This phenomenon has been attributed to an inability on the part of the torsionally-constrained empty carbene p orbital to interact with the flanking double bond. This structural inhibition 1s entirely alleviated by positioning the cyclopropyl carbene completely external to the norbornene ring as in the present example. The heightened conformational maneuverability of the carbenoid center 1s conducive to exclusive cyclopentadiene ring formation. 

The double bonds of cyclopentadiene are constrained in a planar cisoid conformation and this diene therefore reacts easily with a variety of dienophiles. 1,3-Cyclohexadiene is also reactive, but with an increase in the size of the ring, the reactivity of cyclic dienes decreases rapidly. In large rings, the double bonds can no longer easily adopt the necessary coplanar configuration because of non-bonded interaction of methylene groups in the planar molecule. Cis,cis- and cis,trans-l,3-cyclo-octadienes form only copolymers when treated with maleic anhydride and cis,cis- and cis,trans-1,3-cyclodecadienes similarly do not form adducts with maleic anhydride. Dienes with 14- and 15-membered rings react with dienophiles but only vmder relatively severe conditions. 

Sol 14. The double bonds of the cyclopentadiene are held in the s-cis conformation. This makes the cyclopentadiene so reactive in the Diels—Alder reaction that it dimerizes at room temperature. One molecule acts as the diene and the other as the dienophile to form dicyclopentadiene. The dicyclopentadiene formed has the ring of dienophile in an endo orientation to the cyclopentadiene ring that acts as the diene. Usually, substituents on the dienophile are found to be endo in the adduct if the substituents contain the 7T-bonds due to favorable secondary interactions. 

Reports of [4 + 2] cycloadditions of cyclobutenes bearing electron-withdrawing groups on the double bond have appeared. Predominantly exo products are formed from (709 X = CN, CO2R, or COCl) and cyclopentadiene, whereas (709 X = COjH) gives mainly (88%) the cndo-isomer. This reversal of stereochemical course is rationalized by the existence of an intramolecular H-bond and its influence on the non-bonded interactions in the transition state. A new synthesis of benzo-cyclobutenes employs the condensation of (709 X = COjMe) with butadiene and subsequent oxidative decarboxylation and dehydrogenation. This sequence has also been applied to the synthesis of the annelated benzocyclobutene (710). " ... 

The chiral catalyst 142 achieves selectivities through a double effect of intramolecular hydrogen binding interaction and attractive tt-tt donor-acceptor interactions in the transition state by a hydroxy aromatic group [88]. The exceptional results of some Diels-Alder reactions of cyclopentadiene with substituted acroleins catalyzed by (R)-142 are reported in Table 4.21. High enantio- and exo selectivity were always obtained. The coordination of a proton to the 2-hydroxyphenyl group with an oxygen of the adjacent B-0 bond in the nonhelical transition state should play an important role both in the exo-endo approach and in the si-re face differentiation of dienophile. 

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