Internal disubstituted alkene

The sluggish reaction of disiamylborane with tri- and tetrasubstituted alkenes as compared to terminal and most internal disubstituted alkenes, makes it one of the most versatile hydroborating agents. The relative reactivity of disiamylborane with alkenes, dienes, and alkynes varies over a range of 104 (Table 2)26), whereas that of diborane varies in the range of 20-30. 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

In the addition to this 1-alkyne, the boron bonds to the terminal carbon because it is attached to more hydrogens. The resulting vinylborane can be viewed as a disubstituted alkene and is less hindered than the vinylborane produced from an internal alkyne (a trisubstituted alkene). Because the vinylborane is less hindered, it adds a second boron to produce an alkane substituted on the end carbon with two boron groups. 

We have seen the relative reactivities of structurally different alkenes towards various dialkylboranes vary over a broad range of differences, e.g., disiamylborane 431 and 9-BBN481 vary in a range of 105. This makes selective hydroboration of the more reactive alkenes possible in the presence of less reactive one. Disubstituted internal (Z)-alkenes are more reactive to disiamylborane than their ( T)-isomers (Eq. 125). Recently, it has been observed that the selectivity is much higher with ThxBHCl SMe2... 

The asymmetric ene reaction with catalyst 98 is restricted to activated aldehydes as is indicated by the data in Table 16. The rates of the reaction are such that it is not applicable to internal olefins. A variety of 1,1-disubstituted alkenes can be used to give good asymmetric induction with the fastest rates observed with phenyl vinyl thioethers. Turnover can be realized with the more reactive aldehydes and/or alkenes but only in the presence of molecular sieves. The reaction of chloral with a-methyl-styrene shows that higher induction can be achieved with lower temperatures although the reaction is slower. The nature of the solvent affects the rate of the reaction. The reaction is much slower in toluene than in dichloromethane. 

Competition studies reported by Kuwajima, " which also complement the results of Nakai," illustrate the limitations of the 3-effect as a tool for predicting the outcome of vinylsilane-terminated cyclizations (Scheme 4). Acylium ion initiated cyclizations of (7a) and (7b) gave the expected cyclopentenones (8a) and (8b). However, compound (7c), upon treatment with titanium tetrachloride, gave exclusively the cyclopentenone proiduct (8c) arising fr the chemoselective addition on the 1,1-disubstituted alkene followed by protodesilylation of the vinylsilane. The reversal observed in the mode of addition may be a reflection of the relative stabilities of the carbocation intermediates. The internal competition experiments of Kuwajima indicate that secondary 3-silyl cations are generated in preference to secondary carbocations (compare Schemes 3 and 4), while tertiary carbocations appear to be more stable than secondary 3-silyl cari ations, as judged by the formation of compound (te). 

Semireduction of internal alkynes in the presence of a transition metal catalyst (e.g., Ni2B, Pd/C) provides disubstituted cw-alkenes. On the other hand, dissolving metal reduction of alkynes or reduction of propargylic alcohols with LiAlH4 or with Red-Al [sodium bis(2-methoxyethoxy)aluminum hydride] furnishes tran -disubstituted alkenes. ... 

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). 

With this reaction type, the formation of one or two quaternary carbon centers is possible. Thus, starting with a disubstituted internal (Z)-alkene, the ring substituents are both placed in the emfn-position of the resulting cyclohexadiene complex3. 

Hydrosilylation of alkenes.1 This lanthanide is an effective catalyst for hydrosilyla-tion of mono- and 1,1-disubstituted alkenes at 20° in CpHr, or C6H5CH3. Hydrosilylation of dienes with a terminal and an internal olefin is highly selective. 

Mercuration exhibits a carbocation-like pattern, but with the superposition of a large steric effect. For unsubstituted terminal carbons, the rate increases from ethene to propene to 2-methylpropene. This trend also holds for internal alkenes, as 2-methyl-2-butene is more reactive than 2-butene. However, steric effects become dominant for 2,3-dimethylbutene. This incursion of steric effects in oxymercuration has long been recognized and is exemplified by the results of Nelson and co-workers, who found separate correlation lines for mono- and disubstituted alkenes. Hydroboration by 9-BBN (structures) shows a different trend steric effects are dominant and reactivity decreases with substitution. Similar trends apply to rates of addition of dibromob-orane and disiamylborane. The importance of steric factors is no doubt due in part to the relatively bulky nature of these boranes. However, it also reflects a decreased electron demand in the hydroboration TS. 

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