Alkene internal unsymmetrical

The regioselective hydrozirconahon of internal unsymmetrical alkenes remains a challenge, as it could considerably expand the use of zirconocene complexes. Little is known about the mechanism of zirconium migration along an alkyl chain. 

The above inconveniences are circumvented by the application of substituted borane derivatives, e.g., Sia2BH and 9-BBN (see Fig. 1 for definitions), which react with high regioselectivity and sensitivity to steric factors. Thus, 1-alkenes and 1-alkynes are hydroborated at the terminal position. The internal unsymmetrically disubstituted alkenes are also selectively hydroborated. The clean transformation of 1-alkynes into vinylboranes—not possible with borane—can be achieved with these reagents (Fig. 2). 

Table 2. Asymmetric Hydroformylation of Internal Unsymmetrically Substituted Alkenes...

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

Tripylborane is an interesting reagent which resembles thexylborane. One of the important uses of thexylborane lies in the synthesis of unsymmetrical thexyldialkylboranes which can then be used in the synthesis of unsymmetrical ketones. However, the reaction is only successful if the alkene used in the first hydroboration step is an internal alkene. Simple terminal alkenes such as 1-hexene react too rapidly with the initially formed thexylmonoalkylborane to allow the reaction to be stopped at that stage. Therefore, mixtures of products result (ref. 27). 

Alkylboranes can be coupled by treatment with silver nitrate and base." Since alkylboranes are easily prepared from alkenes (15-16), this is essentially a way of coupling and reducing alkenes in fact, alkenes can be hydroborated and coupled in the same flask. For symmetrical coupling (R = R ) yields range from 60 to 80% for terminal alkenes and from 35 to 50% for internal ones. Unsymmetrical coupling has also been carried out, but with lower yields. Arylboranes react similarly, yielding biaryls. The mechanism is probably of the free-radical type. 

In a manner similar to that of the bis-silylation of internal alkynes shown above, the use of unsymmetrical disilanes with basic / //-phosphines is effective for intermolecular bis-silylation of terminal alkenes (Equations (32) and (33)).6 Although the regioselectivities need to be improved, clean bis-silylations of 1-octene, styrene, and norbornene have been achieved in high yields. 

Internal alkynes, which usually show low reactivities, are carbometalated to give trisub-stituted alkenes in high yields by applying this methodology. Unsymmetrical internal... 

The symmetric alkenes 11 and 15 are formed by homometathesis of the unsymmetric alkene 6. Alkene metathesis is an equilibrium reaction, and the homometathesis of internal alkene 6 may be a useful one only when separation of the products 11 and 15 from the starting alkene 6 is easy, namely when R] and R are clearly different functional groups. 

A very useful cross-metathesis is the reaction involving ethylene, which is called ethenolysis. Reaction of ethylene with internal alkenes produces the more useful terminal alkenes. Two terminal alkenes 45 and 42 are formed from the unsymmetric alkene 6 and ethylene. The symmetric alkenes 11 are converted to single terminal alkenes 45. The terminal dienes 46 are formed by ethenolysis of the cyclic alkenes 43. 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

The mechanism of the mercurydD-catalyzed alkyne hydration reactioi is analogous to the oxymercuration reaction of alkenes (Section 7.4). Elec trophilic addition of mercury(II) ion to the alkyne gives a vinylic cation which reacts with water and loses a proton to yield a mercury-containii eiio) intermediate. In contrast to alkene oxymercuration, no treatment witll NaBH is necessary to remove the mercury the acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen (Figure 8,3), A mixture of both possible ketones results when an unsymmetrically substituted internal alkyne (RCsCR ) is hydrated. The reaction is therefor ... 

Organolanthanide-catalyzed intermolecular hydrophosphination is a more facile process than intermolecular hydroamination. The reaction of alkynes, dienes, and activated alkenes with diphenylphosphine was achieved utilizing the ytterbium imine complex 9 (Fig. 8) as catalyst [185-188]. Unsymmetric internal alkynes react regioselectively, presumably due to an aryl-directing effect (48) [186]. 

The cis isomer of 9-tricosene is the sex pheromone of Musca domestica (housefly). It should be noted that cross-metathesis reactions involving unsymmetrical internal alkenes can lead to a complex product mixture, as self-metathesis and other cross-metathesis reactions also occur. 13-Heptacosene, the cis form of... 

When the alkene is internal and unsymmetrical, as in 3-methyl-1-cyclohexene (22), the reaction is more complicated. In addition to the two faces (top and bottom) of the molecule, there are two different and reactive sp carbons of the alkene. Hydroboration leads to four products, 23, 24, 25, and 26. With diborane... 

Hydroboration of unsymmetrical internal alkynes leads to a mixture of regioisomers and therefore is not generally considered useful for the synthesis of ketones, but hydrogenolysis of the hydroboration product provides an alternative route to cis alkenes (equation 9.76). ° ... 

Methyl-l-hexene is a terminal alkene, and formation of the more stable secondary cation leads to a single major product. However, oxymercu-ration of an unsymmetrical internal alkene leads to a mixture of products. Oxymercuration of 3,3-dimethylcyclopentene, for example, gives a mixture of 74 and 75. Although rearrangement did not occur, two regioisomers are formed because both of the possible carbocation intermediates are secondary. The two mercury-stabilized carbocations are essentially equal in stability, so both are formed and subsequent reaction with water leads to the mixture of alcohols shown after reduction with sodium borohydride. 

Attack at C2 is more sterically hindered due to the eminaZ-dimethyl group, so 75 may be formed in greater amormt. This observation is difficult to predict without experimental data, so in the oxymercuration ofunsymmetrical alkenes, anticipate a mixture of both possible alcohols. In other words, assume that oxymercuration of unsymmetrical internal alkenes will give a 1 1 mixture of two alcohols unless there is a compelling reason for one to predominate, such as electronic stabilization, severe steric hindrance, or resonance effects. Exceptions to this assumption will rarely be encountered in this book. 

The scope could be extended to the synthesis of products 94 from terminal alkenes 93 of all different types and containing most common functional groups. Importantly, the reaction also tolerates internal alkenes. ( I-Configured symmetrically substituted alkenes 95 provide the corresponding weso-diamines 96, whereas unsymmetrically substituted derivatives and cyclic alkenes give access to chiral diamines 96. 

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