Cyclopentenes, addition

A highly diastereoselective alkcnylation of c/s-4-cyclopentene-l,3>diols has been achieved with 0-protected (Z)-l-iodo-l-octen-3-ols and palladium catalyst (S. Torii, 1989). The ( )-isomers yielded 1 1 mixtures of diastcrcomcric products. The (Z)-alkenylpalladium intermediate is thought to undergo sy/i-addition to the less crowded face of the prochiral cyclopentene followed by syn-elimination of a hydropalladium intermediate. 

The oxidation of simple internal alkenes is very slow. The clean selectiv oxidation of a terminal double bond in 40, even in the presence of an internt double bond, is possible under normal conditions[89,90]. The oxidation c cyclic alkenes is difficult, but can be carried out under selected condition Addition of strong mineral acids such as HCIO4, H2S04 and HBF4 accelerate the oxidation of cyclohexene and cyclopentene[48,91], A catalyst system 0 PdSO4-H3PM06W6Oii(j [92] or PdCF-CuCF m EtOH is used for the oxidatioi of cyclopentene and cyclohexene[93]. 

A commercially important outlet in the fragrance industry is the methyl dihydrojasmonate [24851-98-7] (26) which is made by Michael addition of a malonate to 2-pentyl-2-cyclopenten-l-one [91791 -21 -8] (52) and which is used in perfumery for blossom fragrances, particularly jasmine (see Perfumes). 

Among the cases in which this type of kinetics have been observed are the addition of hydrogen chloride to 2-methyl-1-butene, 2-methyl-2-butene, 1-mefliylcyclopentene, and cyclohexene. The addition of hydrogen bromide to cyclopentene also follows a third-order rate expression. The transition state associated with the third-order rate expression involves proton transfer to the alkene from one hydrogen halide molecule and capture of the halide ion from the second ... 

In addition to cyclopentenes, other types of compounds may be formed upon heating of vinylcyclopropanes. For example pentadienes 6a/b may be formed by a competitive route from a diradical intermediate. 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

A dry 5(X)-mI flask equipped with a thermometer, pressure-equalizing dropping funnel, and magnetic stirrer is flushed with nitrogen and then maintained under a static pressure of the gas. The flask is charged with 50 ml of tetrahydrofuran and 13.3 ml (0.15 mole) of cyclopentene, and then is cooled in an ice bath. Conversion to tricyclo-pentylborane is achieved by dropwise addition of 25 ml of a 1 M solution of diborane (0.15 mole of hydride see Chapter 4, Section 1 for preparation) in tetrahydrofuran. The solution is stirred for 1 hour at 25° and again cooled in an ice bath, and 25 ml of dry t-butyl alcohol is added, followed by 5.5 ml (0.05 mole) of ethyl bromoacetate. Potassium t-butoxide in /-butyl alcohol (50 ml of a 1 M solution) is added over a period of 10 minutes. There is an immediate precipitation of potassium bromide. The reaction mixture is filtered from the potassium bromide and distilled. Ethyl cyclopentylacetate, bp 101730 mm, 1.4398, is obtained in about 75% yield. Similarly, the reaction can be applied to a variety of olefins including 2-butene, cyclohexene, and norbornene. 

The ring opening of cycloolefins is also possible with certain coordination catalysts. This simplified example shows cyclopentene undergoing a first-step formation of the dimer cyclodecadiene, and then incorporating additional cyclopentene monomer units to produce the solid, rubbery polypentamer ... 

When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only the trews stereoisomer of the dihalide addition product is formed rather than the mixture of cis and trans isomers that might have been expected if a planar carbocation intermediate were involved. We say that the reaction occurs with anti stereochemistry, meaning that the two bromine atoms come from opposite faces of the double bond—one from the top face and one from the bottom face. 

How does the formation of a bromonium ion account for the observed anti stereochemistry of addition to cyclopentene If a bromonium ion is formed as an intermediate, we can imagine that the large bromine atom might "shield" one side of the molecule. Reaction with Br ion in the second step could then occur only from the opposite, unshielded side to give trans product. 

The cyclopentene derivatives 9 undergo both 1,5- and 1,7-dipolar electrocyclizations to yield mixtures of pyrazoles 10 and benzodiazepines 11. In addition, loss of nitrogen results in the carbenes 12, which cyclize to the tetrahydrocyclopent[fl]indenes 13.117... 

The photochemical [2 + 2] cycloaddition of cyclopentene to pentaflu-oropyridine in cyclohexane gave a 1 1 adduct, which in excess olefin gave a single 1 2 adduct (37) (82JOC4462). The solvent has an important role, as in its absence two 1 2 adducts are obtained. With PhC CR in cyclohexane, the nature of R determined whether a triene (R = f-butyl) or tetraene (R = Me) was in the product mixture (89T1755). A mixture of 1 2 and two 1 1 adducts was obtained by [2 + 2] addition of but-2-yne in the absence of a solvent [87JFC(20)745]. 

Addition of cyclopentene to trifluoro-1,2,4-triazine gave products arising from addition of a second molecule of olefin. This was in contrast to trichloro-l,2,4-triazine where the eventual products were predominantly pyridine derivatives [82JCS(P1)1245]. 

The reaction of JV,iV-dimethylhydrazones (1-amino-1-azadienes) and alkenylcarbene complexes mainly produces [3C+2S] cyclopentene derivatives (see Sect. 2.6.4.5). However, a minor product in this reaction is a pyrrole derivative which can be considered as derived from a [4S+1C] cycloaddition process [75]. In this case, the reaction is initiated by the nucleophilic 1,2-addition of the nitrogen lone pair to the metal-carbon double bond followed by cyclisation and... 

An illustrative example of an alternative strategy (cf Fig. 11c) involving the use of a novel traceless linker is found in the multistep synthesis of 6-epi-dysidiolide (363) and several dysidiolide-derived phosphatase inhibitors by Waldmann and coworkers [153], outlined in Scheme 70. During the synthesis, the growing skeleton of 363 remained attached to a robust dienic linker. After completion of intermediate 362, the terminal olefin in 363 was liberated from the solid support by the final metathesis process with concomitant formation of a polymer-bound cyclopentene 364. Notably, during the synthesis it turned out that polymer-bound intermediate 365a, in contrast to soluble benzoate 365b, produced diene 367 only in low yield. After introduction of an additional linker (cf intermediate 366), diene 367 was released in distinctly improved yield by RCM. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

The regiochemistry of the Heck reaction is determined by the competitive removal of the (3-proton in the elimination step. Mixtures are usually obtained if more than one type of (3-hydrogen is present. Often there is also double-bond migration that occurs by reversible Pd-H elimination-addition sequences. For example, the reaction of cyclopentene with bromobenzene leads to all three possible double-bond isomers.146... 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

Johnson, C. R. el. al, Tetrahedron Lett., 1964, 45,3327 Preparation of the 4-bromo compound by partial debromination of crude 3,5-dibromo-cyclopentene by addition of its ethereal solution to the aluminate in ice-cold ether is hazardous. Explosions have occurred on 2 occasions about 1 h after addition of dibromide. 

Jahn combined the formation of the enolate 2-713 resulting from an intermolecu-lar Michael addition of 2-711 and 2-712 with a radical reaction (Scheme 2.157) [363]. The enolate 2-713 did not undergo any further transformations due to the lack of appropriate functionalities. However, after formation of a radical using a mixture of ferrocenium hexafluorophosphate (2-714) and TEMPO, a new reaction channel was opened which afforded the highly substituted cyclopentene 2-715a diastereoselec-tively. 

The research groups of Mariano and West developed a photoinduced electrocydi-zation/nucleophilic addition sequence. Thus, irradiation of N-alkylpyridinium perchlorates as 5-19 in an aqueous solution led to the aziridine cations 5-20, which react in a nucleophilic addition with OH to give the isolable azabicyclo[3.1.0]hex-2-enols 5-21. These can be further transformed by a nucleophilic ring-opening of the aziridine moiety under acidic conditions to lead to useful unsymmetrically trans,trans-trisubstituted cyclopentenes 5-22 (Scheme 5.5) [10]. 

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