Borane selectivity

The reagent is obtained Hydroboration. Ocularly for regioselcctilvi are converted into aldi borane, selective hydro... 

Hydroboration. This borane is recommended for hydroboration of alkynes, particularly for regioselective hydroboration of unsymmetrical alkynes (equation I). 1 -Alkynes are converted into aldehydes in high yield. Since alkenes react only slowly with this borane, selective hydroboration of alkynes in the presence of alkenes is possible. 

Synthesis of Cyclononatetraenyl(di-R-propyl)borane 22 and Mechanism of Borotropic Migrations in this Borane. Selection Rules for the Sigmatropic Migrations of Dialkylboryl Groups... 

C = C triple bonds are hydrated to yield carbonyl groups in the presence of mercury (II) ions (see pp. 52, 57) or by successive treatment with boranes and H2O2. The first procedure gives preferentially the most highly substituted ketone, the latter the complementary compound with high selectivity (T.W. Gibson, 1969). 

Mono- and diaLkylboranes obtained by controlled hydroboration of hindered olefins and by other methods can serve as valuable hydroborating agents for more reactive olefins. Heterosubstituted boranes are also available and used for this purpose. These borane derivatives show differences in reactivity and selectivity. 

Table 1. Reactivities of Representative Functional Groups Toward Selected Boranes ...

Similai selectivity is observed in the synthesis of allylsilanes where X = CF3SO3 and Y = Si(CH3) 3 (304). Alkenyl- and alkynylborates containing a leaving group in the y-position rearrange to aUyhc and aUenic boranes, respectively (305). 

Fig. 1. Numbering conventions for selected borane polyhedra (7) discussed in text.

It has been estimated that using available neutron intensities such as 10 neutrons/(cm -s) concentrations of B from 10—30 lg/g of tumor with a tumor cell to normal cell selectivity of at least five are necessary for BNCT to be practical. Hence the challenge of BNCT ties in the development of practical means for the selective deUvery of approximately 10 B atoms to each tumor cell for effective therapy using short neutron irradiation times. Derivatives of B-enriched /oj o-borane anions and carboranes appear to be especially suitable for BNCT because of their high concentration of B and favorable hydrolytic stabiUties under physiological conditions. 

Amine boranes have been examined by a variety of spectroscopic methods (24—29). The boron-substituted alpha-amino acids have been utilized in animal model studies. These compounds along with their precursors and selected derivatives have been shown to possess antineoplastic, antiarthritic, and hypolipidemic activity (30—32). The boron amino acid analogues are also being evaluated for possible utility in boron neutron capture therapy (BNCT) (33). 

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

Dimethylborane+propene C2 and 2-propyldimethyl borane depict the regioisomeric transition state and addition product. Calculate the energies of these species relative to those of the alternative transition state and product. Given these energy differences, and the experimental observation that this addition is almost completely selective for the anti-Markovnikov product, does it appear that this reaction is under kinetic or thermodynamic control Explain. 

Applications of oxazaphospholidine-borane complexes, cyclic phosphoramides, compounds with N—P=0 function, and other P-heterocycles in enantio-selective catalysis 99SL377. 

Among the many chiral Lewis acid catalysts described so far, not many practical catalysts meet these criteria. For a,/ -unsaturated aldehydes, Corey s tryptophan-derived borane catalyst 4, and Yamamoto s CBA and BLA catalysts 3, 7, and 8 are excellent. Narasaka s chiral titanium catalyst 31 and Evans s chiral copper catalyst 24 are outstanding chiral Lewis acid catalysts of the reaction of 3-alkenoyl-l,2-oxazolidin-2-one as dienophile. These chiral Lewis acid catalysts have wide scope and generality compared with the others, as shown in their application to natural product syntheses. They are, however, still not perfect catalysts. We need to continue the endeavor to seek better catalysts which are more reactive, more selective, and have wider applicability. 

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 ... 

II. Selective Hydroborations Using Bis(3-methyl-2-butyl)borane (BMB)... 

Several studies of reactions of configurationally unstable a-substituted allylboranes have also been reported19,29. The reactions of dialkyl[( )-l-alkyl-2-butenyl]boranes and aldehydes at — 78 °C provide a mixture of syn- and an/i -diastereomers. reflecting reactions by both the Z-and /f-isomers. When generated and used at — I00°C, however, the ff/m-diastereomer is obtained with >95% diastercoselectivity and >90% selectivity for the /T-olefin isomer by way of a transition state analogous to 429. This result suggests that the allylboron isomerization is slow at —100 JC relative to carbonyl addition. 

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