Borane alkylborane formation

Perhaps the most interesting development of these types of reactions, the formation of a single product from terminal borylation of linear alkanes by a transition metal catalyzed reaction, comes from Hartwig. Cp Rh(jj -C6Me6) catalyzes the high-yield formation of linear alkylboranes from commercially available borane reagents under thermal conditions. Iridium catalysts are also effective. ... 

Formation of these BjHg dimer derivatives is possible RHBHjBHj, R2BH2BH2, RHBH2BHR, R2BH2BHR and R2BH2BR2. The dimer-monomer equilibria accounting for disproportionation are shifted to the dimer side, both in free compounds and in ether " . Only hindered alkylboranes, e.g., bis(2,3-dimethyl-2-butyl)borane (dithexylborane) or bis[l-(trimethylsilyl)-l-propyl]borane , are monomers. 

A typical reaction that illustrates Markovnikov addition is the reaction of HBr with 2-methyl-2-butene to give 2-bromo-2-methylbutane (1, sec. 2.10.A). This reaction proceeds by formation of the more stable carbo-cation, which reacts with the nucleophilic bromide ion. If the anti-Markovnikov bromide (the bromine resides on the less substituted carbon) is desired, a different mechanistic pathway must be followed. A typical anti-Markovnikov addition reaction is addition of borane to the alkene, giving primary alcohol (2) after oxidation of the intermediate alkylborane (sec. 5.4.A). This alcohol can be converted to the anti-Markovnikov bromide, 3, by treatment with PBr3. The key to controlling such reactions is a fundamental... 

Complex 31 also catalyses the high-yield formation of linear alkylboranes from alkanes under thermal conditions [22]. Scheme 10 depicts the dehydro-genative coupling of pinacolborane 33 with n-octane 35. In this reaction, 1-oc-tylpinacolborane 36 was the only product to be detected. No borane derivatives from functionalization at the internal positions were observed. 2-Methylhep-tane and methylcyclohexane provided only alkylboranes from primary C-H bond functionalization. The high selectivity for the termini of alkanes is kineti-cally determined and results from a steric preference for formation of a linear metal-alkyl complex. 

Borane product 56 must be the precursor to alcohol 52 and borane product 57 is the precursor to alcohol 53. Before alcohol 52 is formed as the major product, the reaction shows a preference for the formation of 56. There is no intermediate, so the transition states for each alkylborane product (54 and 55, respectively) must be examined to explain the preference for 56 over 57. This preference for the less substituted product is sometimes referred to as an anti-Markovnikov orientation (see Section 10.2.1). 

Purpose. The oxidation of an alkene to an alcohol is investigated via the in situ formation of the corresponding trialkylborane, followed by the oxidation of the carbon-boron bond with hydrogen peroxide. The conditions required for hydroboration (a reduction) of unsaturated hydrocarbons are explored. Alkylboranes are particularly useful synthetic intermediates for the preparation of alcohols. The example used in this experiment is the conversion of 1-octene to 1-octanol in which an anti-Markovrukov addition to the double bond is required to yield the intermediate, trioctylborane. Since it is this alkyl borane that subsequently undergoes oxidation to the alcohol, hydroboration offers a synthetic pathway for introducing substituents at centers of unsaturation that are not normally available to the anti-Markovnikov addition reactions that are based on radical intermediates. 

The addition of borane to alkenes is stereospecifically cis and leads to the formation of tri-alkylboranes. These may be oxidized to alcohols using the anion of hydrogen peroxide. Overall addition of water is achieved, in a c/s-stereospecific, anti-Markovnikov manner. Hydroboration/oxidation of alkynes gives ketones, after tautomerization of the enol formed. c/s-Dihydroxylation of alkenes is accomplished with catalytic OSO4 plus an oxidant such as NMO or K3[Fe(CN)g]. This contrasts with the formation of frans-diols by epoxidation of alkenes followed by the opening of the epoxide with hydroxyl ion. 

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