Alkenes, tetrasubstituted

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

The cyclization to form very congested quaternary carbon centers involving the intramolecular insertion of di-, tri-, and tetrasubstituted alkenes is particularly useful for natural products synthesis[l36-138], In the total synthesis of gelsemine, the cyclization of 166 has been carried out, in which very severe steric hindrance is e.xpected. Interestingly, one stereoisomer 167... 

Hydration of alkenes by this method however is limited to monosubstituted alkenes and disubstituted alkenes of the type RCH=CHR Disubstituted alkenes of the type R2C=CH2 along with trisubstituted and tetrasubstituted alkenes do not form alkyl hydrogen sulfates under these conditions but instead react m a more complicated way with concentrated sulfuric acid (to be discussed m Section 6 21)... 

Tetrasubstituted alkene (Section 5 6) Alkene of the type R2C=CR2 in which there are four carbons directly bonded to the carbons of the double bond (The R groups may be the same or different)... 

It follows that tetrasubstituted alkenes do not normally give ozonides. However, they do give the normal cleavage products (ketones) by the other pathways. For the preparation of... 

Since edges (and presumably ledges) are now associated with double bond migration,98 and since apparent trans addition is a function of the double bond migration ability of various catalysts, perhaps such locations can produce both processes. The fact that tetrasubstituted alkenes hydrogenate much more slowly than tri-, di-, or monosubsituted alkenes would allow greater... 

Double bonds tend to migrate to more highly substituted positions within a substrate that is, terminal alkenes isomerize to disubstituted or trisubstituted alkenes, disubstituted alkenes tend to migrate to trisubstituted, and trisubstituted to tetrasubstituted alkenes. Of course, migration can go both ways, and adsorbed surface species may not exhibit the same thermodynamic stability as their desorbed relatives. (The rate of migration is strongly catalyst dependent for example, it frequently occurs rapidly on Pd and slowly on Pt.)... 

Path (b) leads to the highly stable tetrasubstituted alkene, and this is the path followed by most of the carbocations. 

Tetrasubstituted Alkenes. Tetrasubstituted alkenes lacking electron-withdrawing substituents undergo facile ionic hydrogenation to alkanes in very good yields. Simple examples include 2,3-dimethyl-2-butene,208,214 1,2-dimethyl-cyclopentene, 1,2-dimethylcyclohexene,229 and A9(10)-octalin.126,204,212... 

Tetrasubstituted alkenes, alkene to alkane reductions, 40-41 Thiolation ... 

In this context, alkynes have been investigated under carbometallation conditions in order to generate tri- or tetrasubstituted alkenes with very good stereochemical control.46 For the unactivated triple bonds, the major issue is the regioselectivity of the reaction. 

Recently, Oshima has reported a cobalt-catalyzed allylzincation of internal alkyne derivatives (Scheme 10). Optimization of the reaction leads to utilization of cobalt(ll) chloride in THF at room temperature. No traces of regio- and stereoisomers are obtained. The resulting alkenylzinc species 34 can be trapped with a large number of electrophiles in order to generate stereoselectively the tri- and tetrasubstituted alkenes 35-37. 

The regioselectivity of palladium-catalyzed additions of organoboronic acids to unsymmetrical alkynes is strongly dependent on steric and electronic effects (Scheme 12).62 Multi-component reaction has been reported for the synthesis of tetrasubstituted alkenes.58 The aryl group from an aryl iodide is generally added to the less hindered... 

Table 2. Formation of tri- and tetrasubstituted alkenes by RCM comparison of the efficiency of catalysts 1 and 24 (E=COOMe)a [19]...

The widely studied [RhCl(PPh3)3] complex, usually known as Wilkinson s catalyst, was discovered independently in 1965 by Wilkinson (a recipient of the Nobel Prize in 1973) and other groups [14]. This compound catalyzes the chemo-specific hydrogenation of alkenes in the presence of other easily reduced groups such as N02 or CHO, and terminal alkenes in the presence of internal alkenes [16]. The rate of hydrogenation parallels their coordination ability (Scheme 1.4), but tetrasubstituted alkenes are not reduced. 

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