Monohydroboration dienes

A series of books 7) and reviews 8) written on the utility of these reactions highlights the synthetic applicability of this field. However, there was no attempt to systematize the otherwise vast literature available for selective hydroboration, e.g., hydroboration of double bond in the presence of triple bond and vice versa, hydroboration of one of the double bonds of dienes, monohydroboration of alkynes, and selective hydroboration of carbon-carbon multiple bonds in the presence of many other functional groups. These topics are the aim of this review. Included here also is a brief account of the developing field of haloboration. 

Conjugated dienes exhibit a marked decrease in reactivity toward 9-BBN relative to alkenes and nonconjugated dienes. Structurally different double bonds, however, may allow clean monohydroboration.368 Homoallylic boranes are usually formed, but certain dienes yield allylic boranes. 

Thus, 1.7-octadiene (79), which was subjected to monohydroboration followed by asymmetric dihydroxylation of the remaining double bond to give triol 80 with approximately 80% ee. Further transformations then afforded the desired butenolide 81. Double asymmetric dihydroxylation of diene 83 and subsequent protection gave hydroxy lactone 84 [98], which was then converted into acetylenic bis(hydroxy)bistetrahydrofuran 82 as the required intermediate for the (+)-asimicin synthesis. Mitsunobu inversion at C-24 gave rise to the diastereomeric (+)-bullatacin precursor. 

Hydroboration of conjugated diene, e.g., monohydroboration of 1,3-cyclohexa-diene with disiamylborane95 yields the corresponding allylic boranes. A similar reaction with 9-BBN affords B-2-cyclohexenyl-9-BBN in 95% yield (Eq. 41). 

Monohydroboration of allenes with 9-BBN places the boron atom exclusively at the terminal position. Thus, 3-methyl-l,2-butadiene gives B-3-methyl-2-butenyl-9-BBN as the only product (Eq. 43)96). However, such a procedure for the preparation of allylic boranes by the hydroboration of conjugated dienes and allenes is not a general... 

Catalytic hydroboration is a new methodology of great synthetic potential. The reaction is usually carried out with catecholborane in the presence of rhodium, palladium, iridium and ruthenium compounds.2 In contrast to olefins, very little is known on catalytic hydroboration of conjugated dienes and enynes. Our earlier studies on the uncatalyzed monohydroboration of conjugated dienes,6 reports on the hydroboration of 1-decene with catecholborane catalyzed by lanthanide iodides,7 and monohydroboration of 1,3-enynes in the presence of palladium compounds,8 prompted us to search for other transition metal catalysts for monohydroboration of conjugated dienes and enynes 9 10... 

Table 2 Monohydroboration Products of Conjugated Dienes and Enyrtes with Catecholborane Catalyzed by NiCltfdppe) in THF at 20 °C...

In contrast to palladium and rhodium catalyzed monohydroboration of 1,3-dienes,20 no 1,4-addition and only 1,2-addition products were obtained with anri-Markovnikov regioselectivity. Acyclic dienes were more reactive than cyclic dienes, and styrene was unreactive. Among iron(II) and cobalt(II) chloride complexes with dppe, dppp and dppb only CoClz(dppp) showed catalytic activity albeit lower than NiCUIdppe). 

Hydroboration of acyclic, symmetrical, nonconjugated dienes with one equiv. of 9-BBN-H produces almost statistical mixtures of mono- and di-hydroborated species, but cyclic analogs may show substantial deviations from the statistical mixture. For example, monohydroboration of 1,5-cyclooctadiene occurs to the extent of 85% using 1 1 stoichiometry (equation 31), ° whereas disiamylborane gives predominantly dihydroboration product under such conditions. 

Unsymmetrical, nonconjugated dienes are generally easier to monohydroborate because of intrinsic differences between the two double bonds. Both 9-BBN-H and disiamylborane favor attachment to a terminal double bond rather than an internal double bond (e.g. equation 32). On the other hand, 2-methyl-1, S-hexadiene reacts predominantly at the 1-position with 9-BBN-H and almost exclusively at the 6-position with disiamylborane. The products of dihydroboration of a,o>-dienes with 9-BBN-H can be redistributed with borane-dimethyl sulfide to give boracyclanes. In this way, some of the problems sometimes associated with direct hydroboration of dienes with borane (see Section 3.10.2.1) may be overcome. 

Conjugated dienes are deactivated relative to isolated double bonds towards 9-BBN-H, so monohydroboration is difficult to achieve unless the second hydroboration stage is slowed by intrinsic lack of reactivity of the system or by steric crowding, when it is possible to produce allylic organoboranes e.g. equation 33). ... 

The rates of reaction of alkenes with disiamylborane vary over a much wider range than those with bo-rane-THF, and alkynes react more rapidly than comparable alkenes. The great selectivity shown allows ready distinction between different alkenes, or between two different double bonds in a nonconjugated diene e.g. equation 36). Selective monohydroboration of conjugated dienes is more troublesome. However, selective monohydroboration of some conjugated dienes, such as 1,3-cyclohexadiene, can be achieved with disiamylborane, as is also the case with 9-BBN-H. 

BMB is superior to diborane for dihydroboration of terminal dienes to give, after oxidation, a.w-diols.- Thus 1,5-hexadiene is converted into 1,6-hexanediol in 93% yield, whereas with diborane the yield is 69% and the remainder is a mixture of isomeric diols. With disiamylborane, Zweifel, Nagase, and Brown were able to eifect monohydroboration of cyclic dienes in reasonable yield. This finding was used to advantage by a Merck group for selective conversion of the diene (I) to the primary alcohol (2). 

The combination of versatile chemistry and similarity to aluminum makes boron an attractive choice for protecting group chemistry. This has been amply demonstrated in a series of papers by Chung et al. from work initially carried out at Exxon. Patents published in 1988 disclose the use of boron-functionalized monomers in both homo- and copolymerizations using heterogeneous TiCla (pretreated with aluminum alkyl) AlEt2Cl formulations. - Monomers were prepared by monohydroboration of dienes (1,4-pentadiene, 1,7-octadiene, etc.) with 9-borabicyclo-[3.3.1]nonane (9-BBN). Near quantitative yields of the homopolymers of 7-octenyl-9-BBN and 5-hexenyl-9-BBN could be obtained (eq 6). 

Conjugation usually reduces the reactivity of dienes to hydroboration. Symmetrical acyclic 1,3-dienes can often be monohydroborated successfully with 9-BBN (equation III) unsymmctrical acyclic 1,3-dienes usually undergo mono-hydroboration without difficulty (equations IV and V). 

The hydroboration of 1-halo-l-alkynes is a key step in the synthesis of trans-alkenes [18], tram,trans-dienes [19], and cis-l-halo-l-alkenes [20]. In contrast to the hydroboration of 1-alkynes, 9-BBN reacts with 1-halo-l-alkyne in an equimolar ratio, with monohydroboration occurring at the 1 position (Eq. 5.31) [21]. 

The rate of hydroboration coupled with relative reactivities of alkenes toward hydroboration with 9-BBN are used directly to predict hydroboration of un-symmetrical nonconjugated dienes. 1-Hexene is about 100 times more reactive than is ds-2-pentene and 116 times more reactive than is 2-methyl-2-bu-tene. Similarly, 2-methyl-1-pentene is 194 times more reactive toward 9-BBN than ds-2-pentene is. The data, thus, reveal that selective monohydroboration of terminal C=C of nonconjugated dienes can be achieved, as are illustrated in Chart 5.19 [1-3]. 

Monohydroboration achieved in other dienes is shown in Chart 5.20. 

The hydroboration of symmetrical nonconjugated dienes, such as 1,5-hexa-diene with 9-BBN, in a 1 1 molar ratio proceeds in an essentially statistical manner, giving approximately 25% residual diene, 50% monohydroboration product, and 25% dihydroboration product. On the other hand, conjugated dienes, for example, 1,3-butadiene, behaves differently and affords equal amounts of residual diene and 1,4-dihydroboration product. Conjugation markedly decreases the reactivity of diene toward hydroboration. Consequently, nonconjugated dienes like 1,4-hexadiene are selectively hydroborated with 9-BBN in the presence of 1,3-pentadiene (Scheme 5.6) [3]. 

The 1,3-cyclopentadiene reacts slowly with 9-BBN, and its dimerization competes seriously, resulting in a complex mixture of products [3,4]. The high yield of monohydroboration is realized only in the six-membered ring diene system. As the ring size increases, monohydroboration decreases and dihydroboration increases (Chart 5.21). The high yield of monohydroboration realized in the six-membered ring system is the result of exceptional inertness due to steric requirements of the puckered six-membered ring [5]. 

Brown and Campbell have reported new ways in which alkenylboranes can be coupled to form symmetrical 1,3-dienes of defined geometry. On treatment with the 1 1 adduct of dibromoborane and dimethyl sulphide, both terminal and internal acetylenes undergo highly regioselective monohydroboration with remarkable facility to give alkenyldibromoboranes. In common with dialkenylchloroboranes, the alkenyldibromoboranes dimerize to form high yields of symmetrical , -1,3-dienes when treated with three molar equivalents of methylcopper e.g. Scheme 73). Alkenylcopper species, which are known to... 

Liotta, R. Brown, H. C. 1977. Hydroboration. 48. Effect of structure on selective monohydroboration of representative nonconjugated dienes by 9-borabicyclo[ 3.3.1]nonane. J. Org. Chem. 42, 2836-2839. 

A detailed study has been made of the hydroboration of cyclic dienes with representative reagents and has shown that high yields of monohydroborated products are obtained with six-menibered diene systems. As ring size increases,however,the proportion of this... 

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