Branched Alkenes

Because one might expect steric hindrance to be important, it is worth mentioning that the metathesis of alkenes branched at the double bond has been reported. Thus, isobutene gives (small) quantities of 2,3-dimethy 1-2-butene and ethene (16, 17) ... 

When chlorination or bromination of alkenes is carried out in the gas phase at high temperature, addition to the double bond becomes less significant and substitution at the allylic position becomes the dominant reaction.153-155 In chlorination studied more thoroughly a small amount of oxygen and a liquid film enhance substitution, which is a radical process in the transformation of linear alkenes. Branched alkenes such as isobutylene behave exceptionally, since they yield allyl-substituted product even at low temperature. This reaction, however, is an ionic reaction.156 Despite the possibility of significant resonance stabilization of the allylic radical, the reactivity of different hydrogens in alkenes in allylic chlorination is very similar to that of alkanes. This is in accordance with the reactivity of benzylic hydrogens in chlorination. 

Alkene branching has a considerable effect on the cobalt-catalyzed alkene isomerization and subsequent hydroformylation. In a study of various methylheptenes, Haymore and coworkers found that there was very little hydroformylation at the carbon center with the branch, even if it was part of the double bond. Scheme 4 shows data for the hydroformylation of two methylheptenes and percentage of aldehyde formed at each site. Note that isomerization past the branching carbon is not a dominant reaction. Once again, terminal aldehydes are the favored products. 

Alkenes can be hydroformylated " by treatment with carbon monoxide and hydrogen over a catalyst. The most common catalysts are cobalt carbonyls (see below for a description of the mechanism) and rhodium complexes, " but other transition metal compounds have also been used. Cobalt catalysts are less active than the rhodium type, and catalysts of other metals are generally less active. " Commercially, this is called the 0x0 process, but it can be carried out in the laboratory in an ordinary hydrogenation apparatus. The order of reactivity is straight-chain terminal alkenes > straight-chain internal alkenes > branched-chain alkenes. With terminal alkenes, for example, the aldehyde unit is formed on both the primary and secondary carbon, but proper choice of catalyst and additive leads to selectivity for the secondary product " or primary... 

Linear Alkene Branched Alkene Temp. (°C) Percent Hydrogenation Linear Branched ... 

Boiling Point As a Function of Alkene Branching and Molecule Size... 

In aqueous/organic biphasic medium the reaction rate for the hydrogenation of linear and cyclic alkenes with several Ru(II) complexes including [HRuCl-(TPPMS)2]2, [HRuC1(TPPMS)2(L)2] and [HRuCl(TPPTS)2(L)2] (L = aniline or tet-rahydroquinoline) followed the order linear C2 f-6 linear C7-C10 > cyclic alkenes > branched alkenes [54]. This reactivity pattern is similar to the case of al-kene hydrogenations with [HRuCl(PPh3)3], i.e., the least-substituted double bonds are hydrogenated the fastest. 

The only alkenes which can be reduced by ionic hydrogenation are those capable of generating a stabilized carbocation, such as branched alkenes, alkylcyclopropenes, and also substituted styrenes6, . Unbranched alkenes, as well as alkenes branched at other than the alkenic carbon, are not reduced. This allows the selective reduction of highly substituted double bonds in the presence of unsubstituted double bonds. This is the opposite regioselectivity to that observed. 

Linear alkenes Branched alkenes Cycloalkenes Alkynes (Acetylenes)... 

Ru ion, arising from RuCl2(dmso)4, to assemble in tail-to-tail, orthogonal positions, both coordinating teipyridines each of them with two alkene branches. Thus, the resulting Ru complex 40 on reaction with catalyst 1 led... 

The lUPAC name for an alkene is determined by first identifying the longest chain containing the double bond. As with the alkanes, the longest chain provides the stem name, but the suffix is -ene rather than -ane. The carbon atoms of the longest chain are then numbered from the end nearer the carbon-carbon double bond, and the position of the double bond is given the number of the first carbon atom of that bond (the smaller number). This number is written in front of the stem name of the alkene. Branched chains are named the same way as the alkanes. The simplest alkene, CH2=CH2, is called ethene, although the common name is ethylene. 

In the case of the system natural rubber-chlorine the reaction is complex. Fully chlorinated natural rubber contains about 65% chlorine compared with the 51% that would be obtained if the only reaction were one of addition. Hence at some stage some substitution is necessary. This is not altogether surprising since although linear alkenes give predominantly addition products (A), alkenes branched at the double bond give predominantly allylic substitution, for example as with 2-methyl-2-butene (B). 

The above cascade of reactions does not occur with the aromatic enediynyl azide 3.731, since conjugation with the benzene ring deactivates the alkyne [336]. Both aromatic and nonaromatic enediynes are unstable even in ambient conditions. The aromatic enediynyl azide 3.731 in boiling benzene in the presence of 1,4-cyclohexadiene for 5 hours produces the expected intramolecular cycloaddition to the side of alkene branch to form a bridged bicyclic triazoline enediyne 3.733 (42%, Scheme 3.84) [336]. The cycloaddition was monitored by the disappearance of the characteristic bands of the azide group at 2120 cm in the... 

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