Alkenes highly substituted

An effect that results when two or more atoms or groups rnteract so as to alter the electron drstnbutron rn a system rs called an electronic effect The greater stability of more highly substituted alkenes is an example of an electronic effect... 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

Reaction of HOCl, formed from calcium hypochlorite and CO2, with highly substituted alkenes in CH2CI2 is a convenient route to aHyUc chlorides (111). Ketones are chlorinated to a-chloroketones by reaction with HOCl Acetone initially gives CH2COCH2CI (112). Methyl ethyl ketone gives 78% CH3CHCICOCH3, 15% CH3CH2COCH2CI, and 7% dichlorides (113). 

The regioselectivity of dehydrohalogenation of alkyl halides follows the Zaitsev rule p elimination predominates in the direction that leads to the more highly substituted alkene. 

Quaternary ammonium hydroxides undergo elimination on being heated. It is an anti elimination of the E2 type. The regioselectivity of the Hofmann elimination is opposite to that of the Zaitsev rule and leads to the less highly substituted alkene. 

Markovnikov regiochemislry occurs, with H adding to the less highly substituted alkene carbon and halogen adding to the more highly substituted carbon. 

According to Zaitsev s rule, formulated in 1875 by the Russian chemist Alexander Zaitsev, base-induced elimination reactions generally (although not always) give the more stable alkene product—that is, the alkene with more alkyl substituents on the double-bond carbons. In the following two cases, for example, the more highly substituted alkene product predominates. 

In the elimination of HX from an alkyl halide, the more highly substituted alkene product predominates. 

Here s an example how might we prove that E2 elimination of an alkyl halide gives the more highly substituted alkene (Zaitsev s rule, Section 11.7) Does reaction of 1-chloro-l-methylcyclohexane with strong base lead predominantly to 1-methyl cyclohexene or to methylenecyclohexane ... 

Zaitsev s rule (Section 11.7) A rule stating that E2 elimination reactions normally yield the more highly substituted alkene as major product. 

NaH, NaNH2) or with smaller amounts of RLi, but in these cases side reactions are common and the orientation of the double bond is in the other direction (to give the more highly substituted alkene). The reaction with Na in ethylene glycol is called the BamfordStevens reaction. For these reactions two mechanisms are possible—a... 

The protic catalytic conditions are also compatible with trapping of the radicals formed after cyclization with acrylates or acrylonitriles prior to their reduction with Cp2TiCl . In this manner highly substituted alkenes for the potential preparation of modified steroids can be accessed (Scheme 19) [97]. 

In competition, the less substituted alkenes displace more highly substituted alkenes (this can be virtually 100%, for example, 2,3-dimethylcy-clohexene versus 1,2-dimethyl cyclohexene.)... 

Zaitsev rule an elimination occurs to give the most stable, more highly substituted alkene... 

A different P-hydrogen can be removed from the carbocation, so as to form a more highly substituted alkene than the initial alkene. This deprotonation step is the same as the usual completion of an El elimination. (This carbocation could experience other fates, such as further rearrangement before elimination or substitution by an S l process.)... 

Buchwald et al. realized asymmetric hydrogenation of tetrasubstituted alkenes with a cationic zirconocene catalyst prepared by the protonolysis method (Scheme 8.53). The reactions were quite slow with these highly substituted alkenes, but some very good enantiomeric excesses were obtained [96],... 

Hydroalumination. Monosubstituted alkenes undergo hydroalumination with dichloroalane in the presence of (C2H5)3B or C6H5B(OH)2. Hydroalumination of more highly substituted alkenes proceeds reluctantly. The reaction is regiose-lective, and the products react with a variety of electrophiles under mild conditions. 

Normally, hydride can be readily a-abstracted from alkyl complexes when the metal is in a low oxidation state and bound to good n-accepting ligands. In complexes with p-hydrogen, P-elimination might become the main reaction, specially when highly substituted alkenes are formed (Figure 3.10). 

Asymmetric allylic C-H activation of more complex substrates reveals some intrinsic features of the Rh2(S-DOSP)4 donor/acceptor carbenoids [135, 136]. Cyclopropanation of trans-disubstituted or highly substituted alkenes is rarely observed, due to the steric demands of these carbenoids [16]. Therefore, the C-H activation pathway is inherently enhanced at substituted allylic sites and the bulky rhodium carbenoid discriminates between accessible secondary sites for diastereoselective C-H insertion. As a result, the asymmetric allylic C-H activation provides alternative methods for the preparation of chiral molecules traditionally derived from classic C-C bond-forming reactions such as the Michael reaction and the Claisen rearrangement [135, 136]. 

Phenyl-l-propene, PhCH=CHCH, is a more highly substituted alkene and therefore more stable than l-phenyl-2-propene, PhCH2CH=CH,. Even more important, it is more stable because the double bond is conjugated with the ring. 

Epoxidation. 1,1,3,3-Tetrachloroacetone can mediate the epoxidation of olefins by hydrogen peroxide in the same way as hcxafluoroacctonc (9, 244-245). It has the advantage that it is probably less toxic, and it is inexpensive and commercially available. The actual oxidant presumably is the hydroperoxide 1, which is converted during the epoxidation into the unstable hydrate (2), from which the tctrachloroacetone can be recovered in >70% yield. Monosubstitutcd alkenes are epoxidized in low yields by this method more highly substituted alkenes are epoxidized in 65 85% yield (VPC).1... 

Allylic chlorides Actually the reaction of HOG with highly substituted alkenes is a convenient route to allylic chlorides if CH2G2 is used as the organic cosolvent. The reagent is prepared by addition of dry ice to calcium hypochlorite (70%) in water. The reaction of 1-methylcyclohexene is typical (equation I). Chlorohydrins arc the main products only in the case of 1-alkenes and 1,2-disubstituted alkenes. 

The phosphetanes are rather scarce and present an open field of research. The parent phosphetane and its phosphorus derivatives (P-CN = 3 or 4) are still unknown. The only synthesis, first developed in 1962 (62JOC606), involves addition of PC13 in the presence of AICI3 to a number of highly substituted alkenes in CH2CI2 at 0 to -10 °C (equation 72). 

---