Hydroboration/alkaline oxidation

Rationalizations of hydroboration stereoselectivity using models with dimeric boranes are thus not viable. Recent secondary isotope-effect measurements may suggest a model (Mann et al., 1986). Addition of a chiral dialkyl borane to 3-deuterio-cw-3-pentene, followed by alkaline oxidation, yields equal amounts of 3- and 4-deuterio-3-hexanols showing that the secondary isotope effects at these alkene sites are vanishingly small. Deuterium substitution at the allylic sites has a much larger effect. Thus similar treatment of 4,4-dideuterio-cw-5-decene yields a 2.86 1 mixture of 4,4-dideuterio- and 6,6-dideuterio-5-decanols. 

Zinc-mediated Barbier-t) e addition of 238, followed by Luche s procedure obtained a mixture of the homoallylic alcohol diastereomers 239 and 240. Alcohol 239 was carried through benzoylation, deketalization, silylation, and ozonolysis, to produce C-branched y-lactone 241. Benzoylation of 240 followed by hydroboration, PCC oxidation, debenzoylation, and alkaline-promoted cyclization directly formed C3-branching 2-deoxyfuranose 242 [87] (O Scheme 63). A novel method for stereoselective synthesis of 4 -a-carbon-substituted nucleosides, through epoxidation of 4, 5 -unsaturated nucleosides and SnCU-promoted epoxy ring opening, was... 

Acylsilanes may serve as synthetic equivalents of cai boxylic acids when treated with alkaline hydrogen peroxide. Especially attractive is the one-pot conversion of the ethynyl group of 1-alkynes into monosubstituted acetic acids via sequential silylation, hydroboration, and oxidation, as depicted below. 

Reaction of the ethyl thiolactam 501 with allyl bromide gave the salt 502 which on base-catalyzed rearrangement afforded 503 and a mixture of isomers. Hydroboration and alkaline oxidation of each isomer gave a mixture of alcohols (504 and 505) which were converted to quebrachamine (82) in the standard manner (Scheme 26). 

Hydroboration of styrenesf This borohydride adds to styrene to form a tetraalkylborate (1), which can be hydrolyzed to ethylbenzene (2) or converted into the Markownikoff mixed trialkylborane 3 by strong acids. Alkaline oxidation of 3 yields 1-phenylethanol (4). 

However, they are too slow to react with 9-BBN. Catecholborane and pinacolborane react with fluoroolefins in the presence of a transition metal catalyst. However, reactive boranes, such as dichloroborane-methyl sulpfide readily react with mono, and disubstituted terminal perfluoroaUcenes in the presence of BCI3. The latter coordinates to Me2S and releases the free dichloro-borane, which instantaneously hydroborates theperfluoroalkenes. Oxidation under alkaline conditions furnishes the primary alcohol in >98% regioselectivity (eq 8). Alternatively, the free dichloroborane prepared via Matteson s protocol can also be used for the hydroboration of fluoroolefins. 

A simple and efficient method for the stereochemical control of consecutive stereogenic center construction has been realized [16] by utilizing hydroboration of dialkenyl carbinol derivative. Consequently, hydroboration of 15 with 9-BBN, followed by alkaline oxidation yields 90% of anti, anti-isomer 16, whereas ThxBHj affords syn, anti-isomer 17 in 62% yield (Chart 6.27) [16]. 

The hydroboration step, being very sensitive to steric effects, yields only secondary alkylboranes from trisubstituted double bonds, whereas the less hindered alkylborane is formed predominantly from disubstituted steroidal double bonds. The diborane attack occurs usually towards the a-side and hence results in overall a-hydration of double bonds after alkaline hydrogen peroxide oxidation. ... 

The alkylboranes obtained by the hydroboration reaction are versatile intermediates for further transformations. The most important transformation is the oxidation to yield alcohols 17 it is usually carried out by treatment with hydroperoxide in alkaline solution. The group R migrates from boron to oxygen with retention of configuration ... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Hydroboration of 2-methyl-2-pentene at 25 °C followed by oxidation with alkaline H202 yields 2-methyl-3-pentanol/ but hydroboration at 160 °C followed by oxidation yields 4-methyl-l-pentanol. Suggest a mechanism. 

The R—0—B bonds are hydrolysed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of B-to-0 migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least substituted carbon of a double bond results in the alcohol being formed with regiochemistry which is complementary to that observed in the case of direct hydration or oxymercuration, that is, anti-Markownikoff. 138... 

Hydroboration with excess BH3 in THF followed by oxidation with alkaline H202 converts the (Z)-99 (n = 20, 22, and 24) into the (Z)-fused alcohols 100, and betweenanenes (101) (n = 20, 22, and 24) into the ( )-fused alcohols 102 55e). The reaction rates were found to be slightly affected by ring size, but almost no effect was observed between cis and trans isomers. 

Treatment with alkaline H202 oxidizes trialkylboranes to esters of boric acid. 1 This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles. The R group does not rearrange, and this reaction is a step in the hydroboration method of converting olefins to alcohols (5-12). The mechanism has been formulated as involving a rearrangement from boron to oxygen 311... 

The work of H. C. Brown has made hydroboration an enormously useful synthetic reaction. Oxidation of the adduct with alkaline hydrogen peroxide removes the boron smoothly without rearrangement and replaces it by a hydroxy group. The oxidation proceeds entirely with retention of configuration. For example, the product of Reaction 7.19 is converted by oxidation to trans-2-methylcyclopentanol in high yields (Equation 7.20). 

Hydroboration of 5[I, 9(1 l)-cholene (1) is possible. Oxidation of the product with alkaline hydrogen peroxide leads to an isomeric cholene, probably the A"-cholene (2) ... 

Oxidation of alkenylboranes by alkaline hydrogen peroxide gives aldehydes or ketones depending on the substituent pattern of the alkenyl group thus, when alkaline hydrogen peroxide oxidation is combined with alkyne hydroboration, the resulting two-step process is a procedure for converting alkynes to carbonyl compounds (Equations B2.7 and B2.8). 

Hydroboration of alkene 22 was realized49 by treatment with a large excess of diborane in anhydrous tetrahydrofuran, followed by oxidation with alkaline hydrogen peroxide. This procedure resulted in the formation of 3-deoxy-3-C-(hydroxymethyl)-l,2 5,6-di-0-iso-... 

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