Diene complexes hydroboration

Scheme 26 1,4-Hydroboration of 1,3-diene derivatives with pinacolborane catalyzed by an iron complex...

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. 

NaOH-HiOz) or to carboxylic acids (with w-chloroperbenzoic acid).-190 Double bonds can be hydroborated in the presence of triple bonds if the reagent is 9-BBN.191 On the other hand, dimesitylborane selectively hydroborates triple bonds in the presence of double bonds.192 Furthermore, it is often possible to hydroborate selectively one particular double bond of a nonconjugated diene.191 When the reagent is catecholborane, hydroboration is catalyzed by rhodium complexes, such as Wilkinson s catalyst.194 Enantioselective hydro-boration-oxidation has been achieved by the use of optically active rhodium complexes.195... 

Dienes. The bifunctionality of dienes makes their hydroboration more complex than that of simple alkenes. Competing hydroboration of the two double bonds may lead to mixtures of products arising from mono- or diaddition. Additionally, cyclic or polymeric organoboranes may be formed. Differences in the reactivity of the two double bonds and the use of appropriate hydroborating agents, however, may allow selective hydroboration.29,330... 

Hydroboration proceeds without a catalyst, but hydroboration with less active catecholborane (225) is accelerated by catalysts. Usually 1,2-addition to conjugated dienes takes place, but the Pd-catalysed reaction of catecholborane (225) gives the 1,4-adduct 226. This reaction is not catalysed by Rh complexes [98], Hydroalumination of conjugated dienes catalysed by Cp2TiCl2 affords the allylic aluminium compounds 227 by 1,4-addition. The Pd-catalysed hydrostannation of isoprene with HSnBu3 affords the (Z)-2-alkenylstannane 228 with high regio- and stereoselectivities [99],... 

It has been observed that some conjugated polyenes can be selectively hydroborated with borane, 3) (Chart 3) probably due to the low reactivity of conjugated system in the intermediate organoborane. In some cases, the diene moiety within the polyene system has been protected by complexation 14-16). 

Probably the most important developments in this field over the past 10 years, however, have been in the area of enantioselective see Enantioselectivity) hydroborations using cationic rhodium complexes of the type [Rh(diene)L ]+ (L = chiral ligand). An excellent review on this topic has recently been published. New chiral see Chiral) catalyst systems are typically tested in hydroborations of vinyl arenes. Although catalyzed hydroboration of vinyl arenes can be used as a mild and efficient route to preparing 1-arylethanol... 

In contrast to olefins, little is known on catalytic hydroboration of conjugated dienes. Suzuki and Miyaura20 described a 1,4-addition of catecholborane to acyclic 1,3-dienes, catalyzed with tetrakis(triphenylphosphine)pa]ladium(0). An interesting Markovnikov type regioselectivity was observed in the enantioselective dihydroboration of (E)-1-phenyl-1,3-butadiene with catecholborane, catalyzed by chiral rhodium complexes.21 However, the scope of these reactions is not well known, and the choice of catalysts is very limited. 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

By applying a similar approach, Wu and coworkers [135] observed a highly double-stereoselective hetero Diels-Alder addition between diene 245 and ethyl glyoxylate. This reaction was catalyzed by the (salen)Co° complex 246 (Scheme 13.75). The major adduct 247 is hydroborated to give alcohol 248. Stereochemical inversion is achieved via oxidation and subsequent reduction to 249. Acid methanolysis followed by diol protection furnishes 250. Quenching of the lithium enolate of 250 with phenyl disulfide and subsequent oxidation with A-bromosuccinimide forms, after deprotection, (-f)-KDO (3-deoxy-D-mann6>-oct-2-ulosonic acid) [136a]. 

The cyclic hydroboration of a,oj-dienes with monoalkylboranes leads to boracycles, often producing these ring systems in high isomeric purities. An early example of this process is represented by the cyclic hydroboration of divinyl ether (1) with t-butylborane-trimethylamine complex (Equation (1)) <61JA2541>. 

Related techniques have been developed to prepare (Z-,Z)- (Z-, )- and ( -, )- dienes. Hydroboration of diacetylenes followed by protonolysis is a convenient route to (Z-,Z)-dienes, as in the conversion of 89 to 90. The requisite symmetrical diacetylenes are prepared by oxidative coupling with oxygen and cuprous chloride, as in the conversion of 1-cyclohexylethyne (78) to 89. Unsymmetrical conjugated dienes can be prepared by formation of a diacetylene ate complex, prepared from disiamylmethoxyborane by sequential reaction with different acetylides. A similar borane route to unsymmetrical diacetylenes uses dicyclohexyl methyl-thioborane. ... 

The presence of another unhindered carbon-carbon double or triple bond or functional group often affords a complex mixture of products resulting from competing hydroboration. As a result, the hydroboration of dienes, enynes, or functionally substituted alkenes produces a considerable amount of other products, which is highly undesirable for the subsequent utilization of the resulting organoborane. In addition, the stereoselective addition of borane is very poor in the absence of steric bulk, as shown ... 

---