Disubstituted cycloalkanes

A cycloalkane is a saturated cyclic hydrocarbon with the general formula C H2 . In contrast to open-chain alkanes, where nearly free rotation occurs around C, -C bonds, rotation is greatly reduced in cycloalkanes. Disubstituted cycloalkanes can therefore exist as cis-trans isomers. The cis isomer has both substituents on the same face of the ring the trans isomer has substituents on opposite faces. Cis-trans isomers are just one kind of stereoisomers—isomers... 

Cis-trans isomerism occurs in each of the above disubstituted cycloalkanes. 

Since bond rotation can t occur, the two 2-butenes can t spontaneously interconveTt they are different, isolable compounds. As with disubstituted cycloalkanes, we call such compounds cis-trans stereoisomers. The compound with substituents on the same side of the double bond is called c/3-2-butene, and the isomer with substituents on opposite sides is fra/iS-2-butene (Figure 6.3). 

A similar discussion can be offered for other 1,2-disubstituted cycloalkanes. 

Arylalkenes [23] and alkenes with electron withdrawing substituents [24] can be bis-alkylated across the alkene bond by electrochemical reaction with dflialoal-kanes giving 3- to 6-membered carbocyclic products in good yields. ITie best reaction conditions use an undivided cell with a nickel cathode and a sacrificial aluminium anode in dimethylformamide or N-methylpyrrolidone containing a tetraalkylammonium salt. Anodically generated aluminium ions are essential for the reaction. 1,2-Disubstituted alkenes, regardless of their stereochemistry, are converted to the tranj-substituted cycloalkane. 

Identify and draw cis and trans stereoisomers of disubstituted cycloalkanes. Problems 3-33,43, and 44... 

Considerable conformational complexity is found in disubstituted cycloalkanes,... 

Substituents in the two types of disubstituted cycloalkanes are connected in the same order but differ in spatial orientation. 

Fhe lack of rotation around carbon-carbon double bonds is of more than just theoretical interest it also has chemical consequences. Imagine the situation for a disubstituted alkene such as 2-butenc. Disubstituted means that two substituents other than hydrogen are bonded to the double-bond carbons.) The two methyl groups in 2-butcnc can be either on the same. side of the double bond or on opposite sides, a situation similar to that in disubstituted cycloalkanes (Section 4.2). 

Type I intramolecular ene reactions give mixtures of cis- and fra j-l,2-disubstituted cycloalkanes depending on whether the ene reaction proceeds through an endo or exo transition state. In general, steric rather than electronic effects determine which transition state is preferred. The reaction is often very selective, as indicated below. 

All disubstituted cycloalkanes with two groups bonded to different atoms have cis and trans isomers. 

Let us now turn our attention to disubstituted cycloalkanes, and draw all possible stereoisomers for 1,3-dibromocycIopentane. Because 1,3-dibromocyclopentane has two stereogenic centers, it has a maximum of four stereoisomers. 

To draw all possible stereoisomers, remember that a disubstituted cycloalkane can have two substituents on the same side of the ring (ds isomer, labeled A) or on opposite sides of the ring (trans isomer, labeled B). These compounds are stereoisomers but not mirror images of each other, making them diastereomers. A and B are two of the four possible stereoisomers. 

The concept of cIs and trans Isomers was first introduced for disubstituted cycloalkanes in Chapter 4. In both cases, a ring or a double bond restricts motion, preventing the rotation of a group from one side of the ring or double bond to the other. 

The terms cis and trans refer to the arrangement of groups in a particular compound, usually an alkene or a disubstituted cycloalkane. The terms syn and anti describe the stereochemistry of a process— for example, how two groups are added to a double bond. 

The p-toluenesulfonylhydrazones and alkenes with acyclic disubstituted double bonds are the E isomers See text for descriptions of the procedures Yield determined by gas chromatography Yield determined by proton magnetic resonance spectroscopy The cycloalkane was also formed in 32% yield... 

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