Terminal monosubstituted alkene

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 ... 

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. 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Monosubstituted Alkenes. Simple unbranched terminal alkenes that have only alkyl substituents, such as 1-hexene,2031-octene,209 or ally Icy clohexane230 do not undergo reduction in the presence of organosilicon hydrides and strong acids, even under extreme conditions.1,2 For example, when 1-hexene is heated in a sealed ampoule at 140° for 10 hours with triethylsilane and excess trifluoroacetic acid, only a trace of hexane is detected.203 A somewhat surprising exception to this pattern is the formation of ethylcyclohexane in 20% yield upon treatment of vinylcyclohexane with trifluoroacetic acid and triethylsilane.230 Protonation of the terminal carbon is thought to initiate a 1,2-hydride shift that leads to the formation of the tertiary 1-ethyl-1-cyclohexyl cation.230... 

An interesting antibody-catalyzed intermolecular asymmetric 1,3-dipolar cycloaddition reaction between 4-acetamidobenzonitrile N-oxide and N,N-dimethylacrylamide generating the corresponding 5-acylisoxazoline was observed (216). Reversed regioselectivity of nitrile oxide cycloaddition to a terminal alkene was reported in the reaction of 4-A rt-butylbenzonitrile oxide with 6A-acrylamido-6A-deoxy-p-cyclodextrin in aqueous solution, leading to the formation of the 4-substituted isoxazoline, in contrast to the predominance of the 5-substituted regioisomer from reactions of monosubstituted alkenes (217). 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

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... 

Excellent enantioselectivity is observed in the CP0/H202-catalyzed epoxidation of short-chain (Z)-alkenes with a chain length of nine of fewer carbon atoms, except for monosubstituted alkenes, which often function as reversible suicide inhibitors of the enzyme [266-271]. (E)-Alkenes are highly unreactive substrates and are converted to epoxides in yields below 5%. A number of functionalized (Z)-2-alkenes have been successfully epoxidized by CPO using tert-butyl hydroperoxide as the terminal oxidant [272]. This procedure appears to be more effective, especially in large-scale reactions, due to the fairly high sensitivity of CPO to hydrogen peroxide. 

B-2. Which of the following statements best explains the greater acidity of terminal alkynes (RC=CH) compared with monosubstituted alkenes (RCH=CH2) ... 

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]. 

Hydrozirconation. The combination of zirconocene dichloride and t-butylmag-nesium chloride in C,H4-ether effects hydrozirconation of monosubstituted alkenes. The actual reagent may be HZrCp,Cl (Schwartz reagent, 6. 175-177) or t-BuZrCp,Cl. The reaction produces monoalkylzirconium derivatives with the metal attached to the terminal position of the alkene. 

Olefin metathesis is an equilibrium process and, with many alkene substrates, a mixture of starting material and two or more alkene products is present at equilibrium, making the reaction useless for preparative purposes. With terminal alkenes. however, one metathesis product is CH2=CH2 (a gas), which escapes from the reaction mixture and drives the equilibrium to the right. As a result, monosubstituted alkenes (RCH=CH2) and 2,2-disubstituted alkenes (R2C=CH2) are excellent metathesis substrates because high yields of a single alkene product are obtained, as shown in Equations [1] and [2]. 

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>. 

Propene is also a product in these reactions, which are reversible however, loss of this volatile alkene must favor formation of the new organomagnesium-halide reagents. The terminal (primary) reagent predominates. Addition to styrene provides an exception the secondary (but in this instance also the more stable) organomagnesium-halide is the major product. Note that addition is observed only to the monosubstituted double bond in Eq. (d). In fact, in the absence of other functional groups such additions are ordinarily observed only to monosubstituted alkenes. Isomerization of the 1-alkene to a 2-alkene is a competing reaction e.g., cis- and trans-2-pentene are formed in reactions of 1-pentene. ... 

A related complex lacking a THF of association, Cp 2YCH(TMS)2, is orders of magnitude less reactive than Cp 2YMe THF in reactions with monosubstituted alkenes. Addition of one molar equivalent of THF per equivalent of Cp 2YCH(TMS)2 restores catalytic activity to the level of the methyl complex [17]. Apparently, this THF also depresses the rate of a-bond metathesis (see below) relative to olefin insertion [10], with important ramifications for cycliza-tion/termination processes to be discussed later. Curiously, this THF effect appears unique to the Cp 2YR system. 

The AD reaction was central in the preparation of (+)-cw-sylvaticin 41,27 a natural product found to have potent anti-tumor activity. The ability of this compound to inhibit ATP production by blockade of the mitochondrial complex I was thought to be the origin of this biological outcome. The AD reaction, in this example, exploited the preference of this reaction for the oxidation of 1,2-frans-alkenes over monosubstituted alkenes. The E,E-isomer of tetradecatetraene 42 could be chemoselectively dihydroxylated at both internal alkenes, while the terminal alkenes remained untouched. Thus, 43 was generated in excellent chemical yield. 

Terminal alkenes, both mono- and disubstituted, have been prepared using the title reagent. In Wood s recently disclosed synthesis of welwitindolinone A isonitrile, he achieved a selective dehydration of the less hindered secondary alcohol in 32 to yield monosubstituted alkene 33. 13 Mori obtained terminal disubstituted alkene 35 in 93% yield from phosphorylated tertiary alcohol 34 in the penultimate step in his total synthesis of acoradiene (36), the aggregation pheromone of the broad-homed flour beetle.14... 

It is vital to control the itt-complex formation and insertion steps in order to direct the regioselectivity of the Mizoroki-Heck reaction, in which the organic R group will be either added to the internal carbon of the monosubstituted alkene, yielding an a-product, or the terminal, providing trans- or cw-/6-products (Figure 3.1, steps 2-4 and Figure 3.2) [16,41]. 

An enantiosdective version of this reaction was presented shortly thereafter, which uses the oxazohne ligand (Eq. (4.14)) [27]. The reaction results in good to excellent enantioinduction for a series of 2 -monosubstituted alkenes, while a dimethyl substitution at the terminal alkene carbon leads to 0% ee. This was interpreted by an overall mechanism for which the enantioselection is derived from the first step of alkoxypalladation followed by faster palladium loss from the corresponding quinone methide intermediate than nucleophilic attack by the second alkoxide. 

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