Cyclopentenes from alkynes

Methylene-l-cyclopentenes.1 All attempts to effect cycloaddition of the acetate 1 (9,454 11,578) to alkynes are unsuccessful, but 1 does add to norbornadienes (prepared from alkynes) in the presence of a palladium(O) catalyst to form adducts that afford 4-methylene-1-cyclopentenes on flash vacuum thermolysis (equation I). 

Scheme 25) was observed when cyclic alkenes (e.g., 214) were treated with ruthenium carbene complex 18 in the presence of terminal alkynes (e.g., 215). A mechanism involving initial ROM, followed by alkyne insertion of the intermediate carbene complex, followed by ROM from intermediate 217, was proposed. In order to account for the unexpectedly high yield (the yield is higher than the anticipated E Z selectivity in the formation of 217) of the process, a second source of the observed product involving metathesis of an additional mole of cyclopentene from intermediate 217 was suggested. 

The PKR was first reported as a stoichiometric reaction between norbomadiene and a complex of acetylene bound to hexacarbonyl dicobalt (Equation 17.72). Pauson and coworkers also reported the first catalytic intermolecular formation of a cyclopentene from an alkyne, an olefin, and CO (Equation 17.73). The first intramolecular version of the PKR was reported almost 10 years later by Shore (Equation 17.74). At this point, the intramolecular PKR has been studied in more detail than the intermolecular PKR. 

A side reaction is normally the addition of the TMM complexes to the starting methylenecyclopropanes.Following these findings, it was shown that methylenecyclopropanes serve as potent precursors to form cyclopentene-containing products from alkynes, alkenes and allenes. ... 

Cross-metathesis of terminal alkyne 142 and cyclopentene gives cyclic compound 143 having a diene moiety [Eq. (6.114)]. ° Terminal ruthenium carbene generated from an alkyne and methylidene ruthenium carbene complex reacts with cyclopentene to afford two-carbon elongated cycloheptadiene 143 ... 

It is obvious from these trials that it is rather difficult to realize smooth transformation of alkynylcyclopropanes to cyclopentene derivatives. This difficulty presumably arises from the longer distance between the alkyne terminus and the cyclopropane ring compared with that between the latter and an alkene ter-... 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

EpoxycycIopentene (4) undergoes nucelophilic addition with the ethylaluminium alkynide (5) in a mixture of THF, toluene and hexane to give predominantly the isomer (6) in which the alkyne has been introduced at carbon 3, adjacent to the double bond (Scheme 18). When the reaction was run in toluene at low temperature, the unexpected 4-hydroxy-4-(l-hexynyl)cyclopentene (7) was obtained, which the authors suggest may arise from an aluminum -catalyzed rearrangement of the epoxide to cyclopent-3-enone, which then undergoes nucleophilic addition. 

The typical Nazarov strategy 51 followed by 53 disconnects the rest of the ring from the double bond, but you cannot be quite sure where the double bond will end up in the ring. Another special method, the Pauson-Khand reaction,24,25 differs in both respects. It adds the enone portion of the ring to the rest of the molecule and you can be quite certain where the new double bond will be. The reaction is between an alkene, say cyclopentene 97, an alkyne, and Co2(CO)8 to form a cyclopentenone26 98 in one step. A complex 99 is first formed between the alkyne and the cobalt atoms with the two n-bonds replacing two CO molecules (both are two-electron donors). You may see this complex drawn as 99a but it really has a tricyclic tetrahedrane structure 99b composed of three-membered rings and with a Co-Co bond. 

Cyclopentenes. Zirconacyclopentadienes prepared from two alkynes react with CO in the presence of BuLi to give substituted cyclopentenones. With CuCl as catalyst, the reaction of zirconacyclopentadienes with p-iodo-a,p-unsaturated carbonyl compounds provides cyclopentadienes. In the cases of 3-iodo-2-cycloalkenones, the products are spirocycles. 

Insertion reactions of alkylidene carbenes offer a useful entry to cyclopentene ring systems (4.81). Insertion is most effective with dialkyl-substituted alkylidene carbenes (R = alkyl), since rearrangement of the alkylidene carbene to the alkyne occurs readily when R = H or aryl. A number of methods have been used to access alkylidene carbenes. One of the most convenient uses a ketone and the anion of trimethylsilyl diazomethane. Addition of the anion to the ketone and eUmination gives an intermediate diazoalkene, which loses nitrogen to give the alkylidene carbene. For example, a synthesis of the antibiotic (-)-malyngolide started from the ketone 102 (4.82). The insertion reaction takes place with retention of configuration at the C—H bond. 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

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