9-BBN hydroboration

BBN hydroborates internal alkynes cleanly (Equation B1.13) and thus for this reaction it is superior to borane which tends to give intractable polymers when added to alkynes. The reaction is less useful for terminal alkynes as monohydroboration can only be achieved if an excess of alkyne is used. 

Hydroboration of allenes. With few exceptions, the 9-BBN hydroboration of allenes affords B-allylic-9-BBN derivatives. In contrast, the reactions of allenes with disiamylborane or dicyclohexylborane afiord predominantly vinylic boranes. With unsymmetrical allenes, 9-BBN binds to the less substituted carbon atom. Allene itself affords a 1,3-dibora derivative. The B-allylic-9-BBN derivatives are useful reagents for the allylic boration of carbonyl compounds. ... 

The rate of 9-BBN hydroboration of cis-2-pentene is 100 times slower than that of 1-hexene. 

Fig. 5.2 Reactive conformations in catalyzed and 9-BBN by hydroboration of N-tosyl-protected allylic amine derivatives, a Preferential orientation in catalyzed hydroboration of n-tosyl-protected allylic amine derivatives, b Preferential orientation in 9-BBN hydroboration of N-tosyl-protected allylic amine substrate (R = i-Pr)...

This enhanced bulk around nitrogen accentuates steric effects in hydroboration of these alkenes. Electronic parameters reinforce these steric effects in the catalyzed process for syn selectivity. Hindrance in 9-BBN hydroborations excludes NTsBn from the same face of the alkene as the borane, even from the outside position hence, these conversions are also syn selective. The borane, on the other hand, is smaller, so it can tolerate the NBnTs group from the outside position consequently anti selectivity is observed from this reaction conformation. 

Fig. 5.3 Relative conformations with catalyzed reagent, 9-BBN, and borane reveal (Table 5.6) [28] that a preferential orientation in catalyzed hydroboration is governed by steric and electronic effects, b preferential orientation in 9-BBN hydroboration is governed predominantly by steric effects, and c preferential orientation in BHj hydroboration is governed predominantly by electronic effects...

In sharp contrast to the behavior of 9-BBN, hydroboration of an internal allene with disiamylborane and dicyclohexylborane reveals preferential attack of the boron at the central carbon of the allenic chain, forming vinylboranes. For example, 2,4-dimethyl-2,3-pentadiene is converted [7] essentially, quantitatively, to the vinylborane with dicyclohexyl- and disiamylboranes, but exclusively to the allylborane with 9-BBN (Chart 5.24). In sharp contrast to acyclic allylborane, these allylborane products of 9-BBN do not react with acetone however, it undergoes usual alkaline hydrogen peroxide oxidation to afford the corresponding allylic alcohol. No protonolysis is observed on oxidation. This anomalous behavior of the allylborane is due to the steric bulk surrounding the boron atom, which prevents the coordination of oxygen of the acetone or water, necessary for allylboration or protonolysis [2, 8]. 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

BBN+4-methylpent-2-ene C2 and 9-BBN+4-methylpent-2-ene C3 are transition states for hydroboration by 9-BBN at the C2 and C3 positions, respectively. Which transition state is the lower energy Calculate the ratio of major to minor regioproducts at room temperature. Is this reaction likely to be more or less regioselective than the corresponding reaction involving BH3 Explain your reasoning. 

Bond density surface for 9-BBN-i4-methylpent-2-ene at C2 reveals to what extent bonds are formed in hydroboration transition state. 

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

Another reagent with high regioselectivity is 9-borabicyclo[3.3.1]nonane (9-BBN), which is prepared by hydroboration of 1,5-cyclooctadiene ... 

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol and this provides a convenient method for formation of a disubstituted Z-alkene.217... 

The reactions can be made enantioselective by using enantiomerically pure IpcBH2 for hydroboration of alkenes and then transforming the products to enantiomerically pure derivatives of 9-BBN by reaction with 1,5-cyclooctadiene.22... 

One such reported example is the synthesis of polypropylene-6-polymethyl-methacrylate (PP-6-PMMA) copolymers utilizing metallocene catalysis and the borane chemistry. In the initial step, PP with chain-end olefinic unsaturations was prepared using metallocene catalysts such as Et(Ind)2ZrCl2/MAO. The unsaturation sites were then hydroborated by 9-borabicyclo[3.3.1]nonane (9-BBN) to produce borane-terminated PP (43) (Fig. 30), which was selectively oxidized and interconverted to a... 

Figure 30 The hydroboration strategy of chain end olefinic unsaturation in polypropylene with 9-BBN for further conversion into block copolymers via polymer radicals. (Adapted from refs. 66 and 67.)...

A 0.5 M solution of 9-BBN in tetrahydrofuran was purchased from Aldrich Chemical Company, Inc., and was used without additional purification. The preparation2 of the reagent by hydroboration of 1,5-cyclooctadiene with boranaAetrahydrofuran complex is reported. 

In Baldwin s formal total synthesis of haliclamines A and B, a Suzuki coupling of 3-bromopyridine was the central operation [52], Chemoselective hydroboration of diene 66 employing 9-BBN occurred at the less hindered terminal olefin. Suzuki coupling of the resulting alkylborane with 3-bromopyridine then furnished alkylpyridine 67 as a common intermediate for the synthesis of haliclamines A and B. 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

Another example of great synthetic interest, involves the hydroboration reaction of alkenes [62], In general, the addition of borane to alkenes proceeds stepwise, the final product being the trialkylborane. However, hindered alkenes react slowly, especially when the dialkylborane precipitates from the medium. It was found that trialkyl bor-anes could be obtained rapidly under sonication, even with highly hindered substrates (Eq. 3.5). Applications of this useful modification were published, among which were the reduction-hydroxylation of vinyl groups by 9-BBN [63,64]. 

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