Hydroboration rearrangement

An alternative mechanism for the hydroboration rearrangement has been advanced by Brown et al. -529,534,535 considered the rearrangement to be... 

PG synthesis during the period under review. In one of these approaches, designed to examine the feasibility of joining the two optically active forms of (82) and (83) by a mixed hydroboration-rearrangement sequence, Corey and Ravindranathan have shown in a model system that the cyclopentene (85) couples smoothly with the rearranged borane (84) from cyclohexylthexyl-borane and bromoacetylene (83 R = ThP) to produce the silylated tetra-hydropyranyl derivative (86). Application of this approach to PG synthesis is in hand. 

The (partial) description of the synthesis and coupling of the five fragments starts with the cyclohexyl moiety C —C. The first step involved the enantio- and diastereoselective harpless epoxidation of l,4-pentadien-3-ol described on p. 126f. The epoxide was converted in four steps to a d-vinyl d-lactone which gave a 3-cyclohexenecarboxylate via Ireland-CIaisen rearrangement (cf. p. 87). Uncatalysed hydroboration and oxidation (cf. p. 131) yielded the desired trans-2-methoxycyclohexanol which was protected as a silyl ether. The methyl car-... 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

Other convenient routes to carboranes, selected from the growing number of recently reported syntheses, are as follows. Monocarbon carboranes can be prepared in good yield by the transition-metal catalysed hydroboration of alkenes followed by thermal rearrangement of the intermediate product, c.gP" ... 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The preparation of a functional segmented block copolymer was also investigated (scheme ll).15 First hydroboration polymerization of the oligomer using thexylborane was carried out. Then the obtained organoboron polymer was subjected to a chain-transformation reaction (DCME rearrangement). DCME and lithium alkoxide of 3-ethyl-3-pentanol in hexane was added to a THF solution of the polymer at 0°C. 

Hydroboration-oxidation permits the /z-Markovnikov and syn addition of H-and -OH without rearrangements. 

Isopinocampheol has been prepared by hydrogenation of irams-pinocarveol with a neutral nickel catalyst at 70-100°. The hydroboration reaction provides a convenient procedure for the conversion of olefins to alcohols without rearrangement and with a predictable stereochemistry. The reaction has been applied to a large number of olefins of widely different structures. The results obtained support the proposed generaliza-tit)ii that hydroboration involves an anborane from the less hindered side of the double bond. ... 

Hydration of alkenes can also be achieved either by oxymerciiration-reduction (Markovnikov addition of water) or hydroboration-oxidation (a ti-Markovnikov addition of water). Addition of water by oxymercura-tion-reduction or hydroboration-oxidation has two advantages over the acid-catalysed addition of water. These procedures do not require acidic condition, and carbocation rearrangements never occur. Thus, they give high yields of alcohols. 

The hydroboration amination sequence in diglyme is a general procedure for the conversion of olefins to primary amines without rearrangement and with predictable stereochemistry.5 An alternative procedure, using tetrahydrofuran as solvent and either hydroxylamine-O-sulfonic acid or chloramine, is applicable with terminal olefins and relatively unhindered internal and alicyclic olefins.6 O-Mesitylenesulfonylhydroxylamine also gave desired amines in comparable yield.7 Alternative procedures for the hydroboration of olefins use commercially available solutions of di-borane in tetrahydrofuran8 or dimethylsulfide.9... 

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... 

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