Terminal monosubstituted

Terminal monosubstituted allenes afford primarily ( )- and (Z)-2-bromo-2-alkenes using either approach (equation 104).153-155 However, terminal disubstituted allenes give quite different products (equation 105).82-133-135... 

Stereoisomerism of polymers derived from 1,3-pentadiene and other unsymmetric terminally monosubstituted butadienes (CH2 = CH CH = CHR) is more complex. Stereoregular polymers can be formed from these monomers via their... 

A chiral Mt -O73-butenyl) species will give, depending on its structure and thus on the orientation of the incoming monomer, a new Mt ( -butenyl) species of the same chirality as the previous one (and hence an isotactic diad) or of the opposite chirality (and hence a syndiotactic diad) it is obvious that the tacticity may concern only 1,2-polymers of non-substituted or substituted butadiene and 1,4-polymers of terminally monosubstituted and symmetrically disubstituted butadiene. The mode of the formation of the butenyl group of the same or opposite chirality with respect to the preceding butenyl group is shown, for 1,3-butadiene insertion, in Figure 5.4 [7],... 

In contrast, trans olefins are often poor substrates for CPO. When the double bond is far from the chain terminus (i.e., from r/.v-3-alkenes upward), allylic hydro-xylation accompanies the epoxidation. In addition, with terminal monosubstituted olefins, heme alkylation occurs, thus producing inactivation of CPO. 1-Alkenes can be profitably oxidized to epoxides by CPO only when they are not monosubstituted. A detailed description of the yields and e.e. for CPO-catalyzed epoxidation of olefins has been reported by Adams and coworkers [23]. 

The palladium-catalyzed silylboration of typical allenes exhibited a strong tendency to occur at the internal double bond giving allylsilanes, which undergo allylation of aldehydes in the presence of a Lewis acid (Equation (44)).254 257 The selectivity can be varied by changing the substituents, but the boron atom always added to the central carbon. Enantioselective silylboration of terminal monosubstituted allenes was demonstrated by double asymmetric induction using chiral silylboranes and chiral catalysts (Equation (45)). 

Oxymercaration (2, 265-267 3. 194). Brown and Geoghegan have investigated the relative reactivities of a number of olefins in the oxymercuration reaction in a 20 80 (v/v) mixture of water and THF. The following reactivity is observed terminal disubstituted > terminal monosubstituted > internal disubstituted > internal trisub-stituted > internal leirasubstitu ted. Thus steric factors play a major role in the reactivity of olefins. Increased substitution on the double bond and increased steric hindrance at the site of hydroxyl or mercury substituent attachment decrease the rate of reaction. In the case of olefins of the type RCH CHR, cts-oleiins are more reactive than the corre.sponding rra/i.s-olefins. Inclusion of the double bond in ring systems causes a moderate rate increase which varies somewhat with structure cyclohcxenc > cyclo-pentenc cyclooctene norbornene bicyclo[2.2.2]-octene-2. 

Oxiranes of terminal monosubstituted and internal disubstituted olefins do not undergo the isomerization under standard conditions, but give aldehydes at elevated temperature. For the special substrates described in Sch. 35, different modes of reaction originated from intermediary carbocationic species, involve neighboring functional group participation, oxidation, etc. An improvement employing other silicon Lewis acids, for example Mc3SiI and McsSiBr, was developed by Kraus, Detty, and Sakurai [17,19f,62]. 

Thus far, discussion has centered around the reaction of alkenes with a source of electrophilic oxygen as a route to epoxides [the C=C + O protocol]. However, a second general approach is represented by the reaction of carbonyl compounds with amphophilic carbon centers [the C=0 + C protocol]. For example, sulfonium yhdes are known to convert aldehydes and ketones to epoxides much recent work has focused on asymmetric induction using this methodology, a topic which has been the subject of a concise review in the past year <04ACR611>. As an illustration, the D-mannitol derived chiral sulfide 42 serves as a useful chiral auxiliary in the sulfonium methylide epoxidation of aldehydes to provide terminal monosubstituted oxiranes (e.g., 44) in fair to excellent yield and good enantiomeric excess <04CC1076>. 

With terminal monosubstituted alkenes the problem of allylic rearrangement is marked. 1-Butene gives... 

Consider each of the following isomeric compounds with the molecular formula C, H jBr. Which ones will give only a terminal monosubstituted alkene when they undergo dehydrobromination by an E2 process ... 

Even though 3-bromo-l-butene — the product of 1,2-addition— still forms faster, it equilibrates to form the more stable l-bromo-2-butene. The position of the chemical equilibrium reflects thermodynamic stabihty, and we know that the disubstituted double bond of l-bromo-2-butene is more stable than the terminal, monosubstituted double bond in 3-bromo-l-butene. At a higher temperature, the reaction is under thermodynamic control. 

The structure of the transition state for nucleophilic attack of water on the mercurinium ion is closely related to the structure of this intermediate. Thus, the energy barrier is lower for attack of water at the more positive carbon atom of the intermediate. For a mercurinium ion of a terminal monosubstituted alkene such as 1-hexene, attack occurs at C-2, the more substituted site. 

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