Allenes cycloalkenes

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

As was the case with chiral ( )-cycloalkenes, Cope and coworkers 62) were the first to prepare single-bridged allene in an optically active modification 63). 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

The chirality observed in this kind of substituted allene is a consequence of dissymmetry resulting from restricted rotation about the double bonds, not because of a tetrahedral atom carrying four different groups. Restricted rotation occurs in many other kinds of compounds and a few examples are shown in Table 13-3, which includes trans-cycloalkenes (Section 12-7), cycloalkyli-denes, spiranes, and ort/zo-substituted biphenyl compounds. To have enantiomers, the structure must not have a plane or center of symmetry (Section 5-5). 

On this ground, DCA was found a suitable sensitizer to induce the photooxygenation of a great variety of organic compounds such as alkenes [84,94-98], alkynes [99,100], sulfides [84,98,101], dienes [84], sulfoxides [102], cycloalkanes [103,104], cycloalkenes [105,106], epoxides [107,108], aziridines [109], allenes [110], dioxenes [111], p-dioxins [111,112], aromatic substrates [113], tertiary amines [114], and of great interest from the mechanistic point of view, sterically hindered olefines [97,115-117]. 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

Resolution of lj-cyclononadiene. Cope et al. examined the resolution of the cyclic allene 1,2-cyclononadiene (1) by way of the diastereomeric platinum complex containing this amine (compare resolution of cycloalkenes (2, 272-273)). A... 

Seven- and eight-membered cycloalkenes are available by intramolecular coupling of allenes. The reductive cyclizations (requiring SnCh) of o-nitrostyrenes and o-nitrobenzylidenimines afford indoles and 2//-indazoles, respectively. 

Table 16. Addition of carbenes to alkenes, cycloalkenes, allenes, and alken-ynes...

Chirality may exist in many molecules that do not possess a chiral center. Such compounds may possess a chiral plane or a chiral axis, and are said to be dissymetric with respect to either that plane or that axis. Certain optically active allenes, biaryls, alkylidenecyclohexanes, and spiranes provide examples of axially dissymmetric molecules (chiral axis), irons-Cycloalkenes exemplify planar dissymmetry in molecules. The configurations of these classes may be specified by the Cahn-Ingold-Prelog convention using the usual R and 5 descriptors. Special subrules, which we will not describe here, are applied to this purpose. The interested reader is referred to references 8 (see p. 43) and 9 for details. Scheme 2.1 presents some molecules that are optically active because of planar or axial dissymmetry, and for which the absolute configurations have been determined. 

Sodiumiliq. ammonia Cycloalkenes from cyclic allenes... 

Domino or cascade reactions provide valuable approaches, especially to various carbo- and heterocyclic systems with three, four, or even more annelated rings. The Heck reaction has successfully been employed in various inter-inter-, intra-inter-, inter-intra-, as well as all-intramolecular reaction cascades, hi this section, the termination of these processes by alkenes, arenes, and related ir-bond systems such as alkynes and allenes will be described. A cascade Heck reaction is considered to consist of an oxidative addition of a heteroatom-carbon bond to palladium (starter), carbopalladation of a nonaromatic carbon-carbon double or triple bond without immediate dehydropalladation (relay), one, two, or more fimher car-bopalladation(s) of a carbon-carbon double or triple bond, and eventually ensuing dehydropalladation. Crucial for a cascade reaction of this kind to occur is the blockage or retardation of a dehydropalladation at one of the intermediate stages by using 1,1-disubsti-tuted alkenes and appropriately substimted cycloalkenes, bicycloalkenes, or alkynes as relays since they give kinetically stable alkyl- or alkenylpalladium intermediates, respectively. 

Under the catalysis of Pd(PhCN)2Cl2, 2-alkynoates bearing a terminal allene moiety underwent smooth reductive cyclization to afford 3-(ethoxycarbonylmethylene)cycloalkenes (eq 124)7 ... 

Cyclic organic compounds alicychc cycloalkanes and cycloalkenes and aromatic hydrocarbons constitute a group of gases which are very toxic. Cycloalkenes (e.g. limonene, a—pinene) predominate the concentration of cycloalkanes (e.g. cyclobutane, cyclohexane). The concentration of the cycloalkanes in the gas formed in landfills in the United Kingdom ranged from 80 to 487 mg m (Allen et al. 1997). 

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