Olefin mono-unhindered

Another study from the LERC laboratories illustrates the increasing inadequacy of this type of separation as the molecular weight of the oil is increased (7). On a high-resolution liquid chromatography (LC) column (10,000 theoretical plates) which gave a wide separation of n-hexadecane from n-hexadecene-1, a shale oil saturate-olefin concentrate from a light distillate (400°-600°F) showed only an inflection point for the saturate, hindered-olefin break. A sample of heavy gas oil (800°-1000°F) saturate-olefin concentrate did not show even that. We must conclude from these data that the silica gel separation probably separates only mono-unhindered olefins from the saturates and aromatics with any degree of effectiveness. 

Mono-, di-, and trialkylboranes may be obtained from olefins and the trifunctional borane molecule. Simple unhindered alkenes yield trialkylboranes and it is not possible to halt the reaction at the mono- or dialkylborane stage. With more hindered and trisubstituted alkenes the reaction can be controlled to stop at the dialkylborane stage. 

Borane may react sequentially with 3 mol of alkene to form mono-, di-, and trialk-ylboranes. Both the alkene structure and reaction conditions affect product distribution. Trialkylboranes are usually formed from terminal olefins [Eq. (6.57)] and unhindered disubstituted alkenes such as cyclopentene irrespective of the reactant ratio.340 The reaction cannot be stopped at the mono- or dialkylborane stage. In contrast, hindered disubstituted olefins (e.g., cyclohexene) and trisubstituted alkenes are converted mainly to dialkylboranes [Eq. (6.58)]. Careful control of... 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

In summary, only frorw-2,5-dimethylborolane, which is not readily available, consistently gives products of high optical purity with three major classes of olefins. In addition, relatively unhindered (Z)-olefms arc the only class handled well by diisopinocampheylborane. Consequently, the range of organoboranes directly produced in high optical purity by asymmetric hydroboration is rather limited. Fortunately, mono- and diisopinocampheylborane derivatives have several features which allow their optical purity to be upgraded to essentially 100% ee. 

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