Allenes hydroboration

Computational studies of the formation of ( )-5-stannyl homoallylic alcohols by an allene hydroboration-aldehyde allylboration sequence show that the kinetic allene hydroboration product is less stable and isomerizes to the more sterically congested a-stannylallylborane with the C—Sn to boron a—n hyperconjugation interaction sufficiently stabilizing to override the steric congestion (Scheme 77). " ... 

Enamines can also be converted to amino alcohols via hydroboration. Allene-boranes react with aldehydes to give alkyne-alcohols. ... 

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

The hydroboration of substituted propargyl halides—a convenient synthesis of terminal allenes [135]. 

Hydroboration of allenes 65 with pinacolborane in the presence of Pt(DBA)2 and a trialkylphosphine provides either the allyl boronate 66 or the vinyl boronate 67 regioselectively, depending on the stereoelectronic factors of the phosphine employed (Equation 2) <1999CL1069>. Allyl and vinyl boronates are synthetically important because of their ability to undergo nucleophilic addition to carbonyl compounds as well as transition metal-catalyzed cross-coupling. 

Hydroboration of 1,3-dienes.3 This reaction, catalyzed by Pd(0) or Rh4(CO)12, provides (Z)-allylic boronates in about 85% yield. Hydroboration of a 1,3-enyne gives an allenic boronate (— 55% yield). 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

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... 

The separation of racemic mixture is the most difficult process and involves several steps. The selective hydroboration sequence provides a convenient procedure 199 200 for the resolution of 1,3-disubstituted allenes, while the process is tedious and lengthy by other methods201. Thus, the treatment of ( + )-1,3-dimethyl allene with 50 mol percent of (+)-IPC2BH provided unreacted (R)-(-)-l,3-dimethylallene (Eq. 128). The configuration of allenes is consistently R when resolved by ( + )-IPC2BH. As both forms of IPC2BH are available, it should be possible to obtain the allenes of opposite configuration. 

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. ... 

Hydroboration ofallenes Monohydroboration of cyclic allenes with catecholborane (1) followed by oxidation results in a single ketone arising from regiospecific attack of boron at the central carbon atom of the allene (equation I). The reaction with diborane or disiamylborane is not selective. 

Reactions n>itb alknes. Caserio et at. have reported that partial hydroboration of racemic 1,3-dimethylallene and 1,3-diphenylallene with ( + )-Pn4B2H2 [from (-)- -pincnc] gave recovered (—)-allcne of moderate activity. The method has been studied in greater detail by Moore et al. and they report that in every case examined the recovered alicne is enriched in the R enantiomer. Brown s reagent is thus useful for preparation of optically active allenes however, only moderate activities result and. of course, some of the alIcne is irrevocably lost in the process. 

Allylic-9-BBN derivatives are also available by hydroboration of allenes with 9-BBN-H. Unlike disiamylborane, 9-BBN-H shows a marked preference for attachment of boron to one of the termini of the allene system rather than to the central carbon atom. Allene itself is dihydroborated by 9-BBN-H, but substituted allenes give rise predominantly to allylboranes (e.g. equation 34). ... 

Allylboron compounds can sometimes be obtained via hydroboration of allenes or 1,3-dienes with dialkylboranes (e.g. see Section 3.10.4.2). Allylboron compounds are particularly readily hydrolyzed, and hydrolysis is accompanied by transposition of the double bond. For example, tricrotylborane reacts with water at room temperature to give 2 mol of 1-butene and 1 mol of crotyldihydroxyborane (Scheme 13). 

The hydroboration of heteroallyl derivatives often produces boron-heteroatom rings. These reactions are very closely related to the internal hydroborations discussed in Section 4.17.9.1.2. The hydroboration of 2-vinylpyrrole affords boron-nitrogen heterocycle 189 (Equation 29) <1997JOM181>. Two important hydroborations of allyl methyl sulfide <1971JA2823> and the allene 190 <1989JOC5814> are shown in Equations (30) and (31), respectively. The reduction/ hydroboration of 191 probably occurs via the intermediacy of 192 (Scheme 23) <1992JOC5288>. 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Energetically this reaction can be expected to be dipole-HO-controlled ( Lumo - Fhomo = 15.5 eV), but this should have little regiocontrol since the LUMO of the allene is so little polarised. The LUMO(dipole)/ HOMO ene) interaction ( lumo - homo = 16.75 eV) might come into play, since the allene HOMO is polarised, but the orbitals do not match up to explain the high level of regioselectivity, since the larger coefficient in the LUMO of diazomethane is on carbon. Finally, the hydroboration of allenes gives mainly the product with the boron on the central carbon 6.407.895 With boron as the electrophilic atom, frontier... 

In contrast to HjB-THF and other dialkylboranes, 9-BBN shows a strong tendency to form B-allylic-9-BBN derivatives in the hydroboration of aliphatic allenes. This tendency is retained even when the allene system is unfavorably substituted. Lower selectivity may be expected for cyclic allenes, for 1,2-cyclononadiene gives 17% of B-cyclononen-l-yl-9-BBN and 83% of B-cyclononen-2-yl-9-BBN. The reaction can be... 

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