Alkenes, reduction hydroboration

In 2005, Renaud and co-workers reported a novel procedure for the formal hydrogenation of alkenes via hydroboration with an excess of catecholborane, followed by treatment of the intermediate boronic acid esters with methanol in the presence of air as a radical initiator [88]. A typical example, the reduction of 43 to 44, is shown in Scheme 38. Similar results were obtained for a wide range of primary, secondary, and tertiary alkylcatecholboranes. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Brown and coworkers have described an alternative synthesis of chiral alkylboronic esters. In this synthesis prochiral alkenes are hydroborated with monoisopinocamphenylborane to yield isopinocam-phenylalkylboranes which are then readily transformed to chiral alkyllraronic esters (Scheme 39). Homologation with dichloromethyllithium, followed by reduction with potassium triisopropoxyborohy-dride (KIPBH) and oxidation, finely yields B-chiral alcohols (Scheme 40). These alcohols are not easily prepared by other methods. Aldehydes can be prepared by homologation from chiral alkylboronic esters with LiCH(OMe)SPh and oxidation (Scheme 41). ... 

Three types of reactions are used to convert alkynes to alkenes and alkanes catalytic reduction, hydroboration-protonolysis, and dissolving-metal reduction. 

The net effect of hydroboration of an internal alkyne followed by treatment with acetic acid is reduction of the allgme to a ds-alkene. Thus, hydroboration-protonolysis and catalytic reduction over a Lindlar catalyst provide alternative schemes for conversion of an alkyne to a ds-alkene. 

Scheme 6.72. The hydroboration of an alkyne to yield alkene (reductively) or enol (oxidatively). The latter usually tautomerizes to ketone or aldehyde.

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

A catalytic cycle proposed for the metal-phosphine complexes involves the oxidative addition of borane to a low-valent metal yielding a boryl complex (35), the coordination of alkene to the vacant orbital of the metal or by displacing a phosphine ligand (35 —> 36) leads to the insertion of the double bond into the M-H bond (36 —> 37) and finally the reductive elimination to afford a hydroboration product (Scheme 1-11) [1]. A variety of transition metal-boryl complexes have been synthesized via oxidative addition of the B-H bond to low-valent metals to investigate their role in cat-... 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne, which avoids complications that occur with borane and lead to polymeric structures. Catechol borane is a particularly useful reagent for hydroboration of alkynes.212 Protonolysis of the adduct with acetic acid results in reduction of the alkyne to the corresponding cw-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

The formation of unsymmetrical ketones can also be done starting with IpcBCl2. Sequential reduction and hydroboration are carried out with two different alkenes. The first reduction can be done with (CH3)3SiH, but the second stage requires LiAlH4. 

The use of reductive alkylation conditions has been employed to access tricycles from the azide 353 <2002S242> (Equation 95). Hydroboration of the alkene double bond with dicyclohexylborane followed by reaction with the azide and subsequent elimination of nitrogen and cyclization gave the linear tricyclic diketopiperazine 354 and 355 as a mixture of diastereoisomers. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

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