2- cyclopentane alkene

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

Hydrocarbons, compounds of carbon and hydrogen, are stmcturally classified as aromatic and aliphatic the latter includes alkanes (paraffins), alkenes (olefins), alkynes (acetylenes), and cycloparaffins. An example of a low molecular weight paraffin is methane [74-82-8], of an olefin, ethylene [74-85-1], of a cycloparaffin, cyclopentane [287-92-3], and of an aromatic, benzene [71-43-2]. Cmde petroleum oils [8002-05-9], which span a range of molecular weights of these compounds, excluding the very reactive olefins, have been classified according to their content as paraffinic, cycloparaffinic (naphthenic), or aromatic. The hydrocarbon class of terpenes is not discussed here. Terpenes, such as turpentine [8006-64-2] are found widely distributed in plants, and consist of repeating isoprene [78-79-5] units (see Isoprene Terpenoids). 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Compounds called cycloalkanes, having molecules with no double bonds but having a cyclic or ring structure, are isomeric with alkenes whose molecules contain the same number of carbon atoms. For example, cyclopentane and 2-pentene have the same molecular formula, C5H, but have completely... 

Cyclopentane has the low chemical reactivity which is typical of saturated hydrocarbons, while 2-pentene is much more reactive. Similarly, ring structures containing double bonds, called cyclo-alkenes, can be shown to be isomeric with alkynes. 

A simple test to distinguish between 2-pentene and cyclopentane is to add a few drops of a red Br2 solution to the unknown liquid. The reddish color will disappear if the liquid is an alkene or alkyne, e.g., 2-pentene, due to the addition of Br2 to the multiple bond. No such addition reaction occurs between Br2 and cyclopentane. 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

Dipolar addition is closely related to the Diels-Alder reaction, but allows the formation of five-membered adducts, including cyclopentane derivatives. Like Diels-Alder reactions, 1,3-dipolar cycloaddition involves [4+2] concerted reaction of a 1,3-dipolar species (the An component and a dipolar In component). Very often, condensation of chiral acrylates with nitrile oxides or nitrones gives only modest diastereoselectivity.82 1,3-Dipolar cycloaddition between nitrones and alkenes is most useful and convenient for the preparation of iso-xazolidine derivatives, which can then be readily converted to 1,3-amino alcohol equivalents under mild conditions.83 The low selectivity of the 1,3-dipolar reaction can be overcome to some extent by introducing a chiral auxiliary to the substrate. As shown in Scheme 5-51, the reaction of 169 with acryloyl chloride connects the chiral sultam to the acrylic acid substrate, and subsequent cycloaddition yields product 170 with a diastereoselectivity of 90 10.84... 

Radical [3 + 2 cycloaddition. Cyclopentanes can be prepared by addition of alkenes across vinylcyclopropanes catalyzed by phenylthio radicals formed from (C6H5S)2 and AIBN. A Lewis acid such as A1(CH3)3 can increase the rate and the stereoselectivity of this radical initiated cycloaddition. Thus the combination of the vinylcyclopropyl ester 1 with f-butyl acrylate (2) provides the four possible cyclo-... 

The first steps involve coordination and cycloaddition to the metal. Insertion of a third molecule of ethene leads to a more instable intermediate, a seven-membered ring, that eliminates the product, 1-hexene. This last reaction can be a (3-hydrogen elimination giving chromium hydride and alkene, followed by a reductive elimination. Alternatively, one alkyl anion can abstract a (3-hydrogen from the other alkyl-chromium bond, giving 1-hexene in one step. We prefer the latter pathway as this offers no possibilities to initiate a classic chain growth mechanism, as was also proposed for titanium [8]. The byproduct observed is a mixture of decenes ( ) and not octenes. The latter would be expected if one more molecule of ethene would insert into the metallocycloheptane intermediate. Decene is formed via insertion of the product hexene into the metallo-cyclopentane intermediate followed by elimination. 

Trans-1 -allyl-2-(trimethylsilyl)cyclopentane and trans-1 -allyl-2-(trimethylsilyl)-cyclohexane are formed from the reaction of la with cyclopentene and cyclohexene, respectively. A second allylsilylation reaction of these compounds with la also gives unusual allylsilylation products, 7-cyclopent-l-enyl-2,2-dimethyl-4-(trimethylsilyl-methyl)-2-silaheptane (30%) and 4-((cyclohex-l-enyl)methyl)-2,2,8,8-tetramethyl-2,8-disilanonane (39%). As observed in the allylsilylation of 4-(trimethylsilyl-methyl)-l-alkenes, these products are likely formed via intramolecular silyl rearrangements. In this case, the results strongly suggest that a 1,5-silyl shift and... 

Recently, the successful generation of PCU-8-vinylidenecarbene (4a) via reaction of 8-(dibromo-methylene)-PCU (3) with n-BuLi hs been reported [15]. When this reaction is performed in the presence of an alkene trapping agent (i.e., cyclohexene), a cage-functionalized erro-methylenecyclopropane, 5, is the only product. Compound 5 subsequently was characterized via conversion to the corresponding substituted dichlorospiro(cyclopentane), 6 (Scheme 2) the structure was established unequivocally via single-crystal X-ray structural analysis [15]. 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

The chiral diazo ester 27 was cychzed [16] with four commonly used rhodium car-boxylate catalysts (Tab. 16.2), wherein the rhodium pivalate [20] (entry 4) was most efficient for forming the cyclopentanes, and the rhodium trifluoroacetate (entry 1) was optimum for forming alkenes [21]. Furthermore, it was demonstrated that the yield of the cyclization and the diastereoselectivity could be improved at lower temperature using the pivalate-derived catalyst (entry 5). 

In a direct comparison, it was found that with substrate 6, rhodium(II) acetate directs the reaction toward C-H insertion, to give cyclopentane 7, whereas copper-bronze favors alkene insertion, yielding the cyclopropane 845-46. A similar selectivity for cyclopropanation has been achieved by using amide ligands on the rhodium catalyst4. ... 

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