3- cyclopentene 1-alkyne

As evidenced by the treatment of three (Z)-(/l-arylsulfonyl-l-decenyl)phenyliodonium fluoroborates with triethylamine, the presence of electronically diverse substituents (H,p-N02, / -MeO) in the arylsulfonyl groups appears to have little impact on their migratory aptitudes (equation 239)32. Indeed, among six [(/1-alkyl-/l-arylsulfonyl)vinyl]iodonium salts that have been investigated, the cyclopentene/alkyne ratios show little variability (i.e. 70 30 to 80 20)32. Finally, the presence of an appropriately placed alcohol function in the side chain of a (/l-sulfonylvinyl)iodonium salt has been shown to completely suppress migration in favor of insertion (equation 240)32. 

The intramolecular [In + 2 rr] cycloaddition ofmethylenecyclopropane with the alkyne in 117 using isopropyl phosphite as a ligand affords the methylene-cyclopentene 118[55]. 

In the same way as arylcarbene complexes, alkenylcarbene complexes typically react with alkynes to provide [3C+2S+1C0] Dotz cycloadducts (see Chap. ccChromium-Templated Benzannulation Reactions , p. 123 in this book). However, some isolated examples involving the formation of five-membered rings through [3C+2S] cycloaddition processes have been reported [71]. In this context, de Meijere et al. found that /J-donor-substituted alkenylcarbene complexes react with alkynes to give cyclopentene derivatives [71a]. This topic is also discussed in detail in Chap.ccThe Multifaceted Chemistry of Variously Substituted a,/J-Unsaturated Fischer Metalcarbenes , p. 21 of this book. 

In a similar manner, terminal alkynes such as 1-14 participate in a Prins/pinacol reaction, resulting in a ring-expanding cyclopentene annulation to give compounds such as 1-15 in high yield (Scheme 1.5) [5]. 

It took another decade however before the idea of developing a rhodium-catalyzed olefin hydroboration process came to fruition. This occurred in 1985 when Mannig and Noth reported the first examples of such a process.8 They discovered that Wilkinson s catalyst 2 was effective for the addition of catecholborane 1 to a range of alkenes and alkynes, as exemplified by cyclopentene 4 (Scheme 2). 

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 meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

Saa and coworkers reported a remarkable cycloisomerization of alkynal to cycloalk-ene derivatives with loss of a CO molecule [30] some examples are shown in Table 6.3. Heating 5-alkynal 94a in AcOH (90 °C, 24h) with [Ru(Cp)(CH3CN)3]PF6 (5 mol%) afforded the cyclopentene derivative 9Sa in 90% yield. Ketone 94e afforded the cyclopentene 9Se in moderate yield, whereas the ester 94f gave the noncyclized product 96 with loss of one carbon unit (Table 6.3) (Scheme 6.32). 

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. 

Alkylidenes (alkylidene carbenes) are valence isomers of alkynes. They have been prepared by alkyne pyrolysis, by homologation of ketones, and by generation of alkenyl anions bearing oc-leaving groups. Generated by any of these means, an alkylidene will insert intramolecularly into a remote C- H bond to form a new C—C bond and thus a cyclopentene. A concerted two-electron process, this reaction proceeds with retention of absolute configuration at the C - H site. 

Cyclopentene-ynes 54a and 54b having as- or /ra r-substituents on the cyclopentene are reacted with Ic under ethylene gas to give pyrrolidine derivatives 55a and 55b, respectively, in high yields. The reaction proceeds in a highly stereoselective manner. However, under similar reaction conditions, cyclohexene-yne 54c having cis-substituents on the cyclohexene ring gives selectively triene 56, which is a CM product of an alkyne part and... 

Scheme 22 Alkyne relays in inter-intramolecular carbopalladation cascades leading to ring-annelated alkylidenecyclopenta-dienes and cyclopentenes. ...

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). 

SCS-MP2 and the new perturbative B2-PLYP density functional methods provide accurate reaction barriers and outperform MP2 and B3-LYP methods when applied to the 1,3-dipolar cycloaddition reactions of ethylene and acetylene.39 Phosphepine has been shown to catalyse the asymmetric 3 + 2-cycloaddition of allenes with a variety of enones (e.g. chalcones) to produce highly functionalized cyclopentenes with good enantiomeric excess.40 The AuPPh3SbF6 complex catalysed the intramolecular 3 + 2- cycloaddition of unactivated arenyne- (or enyne)-yne functionalities under ambient conditions.41 A review of the use of Rh(I)-catalysed 3 + 2-cycloadditions of diaryl-and arylalkyl-cyclopropenones and aryl-, heteroaryl-, and dialkyl-substituted alkynes to synthesise cyclopentadienones for use in the synthesis of natural products, polymers, dendrimers, and antigen-presenting scaffolds has been presented.42... 

As a combined reaction of (3-cleavage of a cyclopropylcarbinyl radical and intermolecular addition, treatment of vinylcyclopropane (266) and activated alkyne in the presence of PhSSPh and AIBN forms a cyclopentene skeleton (267), through the initial addition of a thiyl radical to the vinyl group, P-cleavage of the cyclopropylcarbinyl radical, addition of the carbon-centered radical to the alkyne, ring closure of a vinyl radical via 5-exo-trig manner, and finally subsequent P-elimination of the thiyl radical, as shown in eq. 3.107 [272-276]. Here, PhSSPh acts as a catalyst, since the thiyl radical is regenerated. Aliphatic disulfides such as... 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

Most reactions of this category involve the base-induced generation of alkylidene-carbenes (R2C = C ) which undergo an intramolecular 1,5-carbon-hydrogen insertion providing a useful route for the construction of substituted cyclopentenes a competing intramolecular pathway is rearrangement to alkynes. 

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