Boranes redistribution

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Monohalogenoboranes are conveniendy prepared from borane—dimethyl sulfide and boron trihahdes (BX where X = Cl, Br, I) by redistribution reaction, eg, for monochloroborane—dimethyl sulfide [63348-81-2] (9) (81—83). Other methods are also known (84—87). 

Primary dialkylboranes react readily with most alkenes at ambient temperatures and dihydroborate terminal acetylenes. However, these unhindered dialkylboranes exist in equiUbtium with mono- and ttialkylboranes and cannot be prepared in a state of high purity by the reaction of two equivalents of an alkene with borane (35—38). Nevertheless, such mixtures can be used for hydroboration if the products are acceptable for further transformations or can be separated (90). When pure primary dialkylboranes are required they are best prepared by the reduction of dialkylhalogenoboranes with metal hydrides (91—93). To avoid redistribution they must be used immediately or be stabilized as amine complexes or converted into dialkylborohydtides. 

Dihalogenoboranes are conveniently prepared by the redistribution of borane—dimethyl sulfide with boron trihaUde—dimethyl sulfide complexes (82,83), eg, for dibromoborane—dimethyl sulfide [55671-55-1] (14). 

The two redistribution products, monochlorodiborane and dichloro-borane, could be isolated by low-temperature gas chromatography (216). Monochlorodiborane, although comparatively stable for a short time at room temperature, slowly reequilibrates, exchanging chlorine and hydrogen atoms as shown in Fig. 9. During the disproportionation according to Eq. (61)... 

Most monoalkylboranes are not stable for prolonged periods. They tend to redistribute to give mixtures containing dialkylborane, trialkylborane and borane itself. Methylborane appears to be more stable than many, but the others can be stabilized and stored as TMEDA or other trialkylamine com-plexes. 59-6i xhe free borane is liberated when required by addition of trifluoroborane. 

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. 

Less hindered dialkylboranes cannot generally be prepared by direct stoichiometric hydroboration of 2 equiv. of an alkene with borane (Section 3.10.2.1), although 3,5-dimethylborinane and 3,6-dimethylbore-pane are exceptions. Borinane itself is obtained by a sequence involving hydroboration (dieneiBHs = 3 2), thermal isomerization, and then redistribution with further borane-THF, but there is as yet no similar direct route to the parent borepane. ° ... 

Borabicyclo[3.3.1]nonane (9-BBN) has been prepared by the thermal redistribution of 9-n-propyl-9-BBN, and the hydroboration of 1,5-cyclooctadlene with borane-tetrahydrofuran complex followed by thermal isomerization of the mixture of dialkylboranes at BS C. Solutions of 9-BBN have been prepared from the hydroboration of 1,5-cyclooctad ene with borane-methyl sulfide in solvents other than THF.6 The present procedure involves the cyclic hydroboration of 1,5-cyclooctadiene with borane-methyl sulfide in 1,2-dimethoxyethane.7 Distillative removal of the dimethyl sulfide in this special solvent system provides a medium that gives high purity, large needles of crystalline 9-BBN dimer in excellent yield. The material can be handled in air for brief periods without measurable decomposition. 

These difficulties can be circumvented by application of alkyl- or aryl-substituted boranes as hydroborating agents. The reagents are obtained either by partial hydroboration of hindered alkenes [Eqs. (ae), (af), (bj)-(bl)] or by other methods, e.g., redistribution or reduction - ... 

In redistributions of R3B incapable of forming boracyclanes, statistical mixtures of products can result. If the equilibrium can be shifted by selective removal of one or more components, however, the reaction may still be of synthetic use e.g., tricyclopropyl-borane can by synthesized by disproportionation-distillation of a dialkylcyclopropyl borane. ... 

Monoisopinocampheylborane (IpcBH, 1) is prepared from a-pinene by a direct78, or. for material of higher optical purity, by an indirect method79-S4. Thus, hydroboration of a-pinene in a 1 1 molar ratio with borane in tctrahydrofuran proceeds beyond the monoisopinocampheylborane stage. Triisopinocampheyldiborane, which initially predominates in the reaction mixture, undergoes redistribution with free borane present in the solution78. 

The calculations discussed so far are for reaction of monomeric BH3 with alkenes in the gas phase. In solution the borane is most likely to be a dimer or, in ether solvents such as THF, a borane-solvent complex. It is difficult to study the kinetics of borane addition in solution because the reaction is complicated by three addition steps (one for each B-H bond), three redistribution equilibria (in which borane and the alkyl boranes exchange substituents), and five different monomer-dimer equilibria involving all the species with at least one B—H bond. In the hydroboration of 2,3-dimethyl-2-butene with diborane in THF, the reacting species is most likely a borane-THF complex. The reaction was foxmd to be second order overall, first order in alkene and first order in BH3-THF. The Eg was foimd to be 9.2kcal/mol, while the activation entropy was —27 eu. These results stand in contrast to the value of 2 kcal/mol determined for AH for the reaction of BH3 with ethene in the gas phase. °... 

With only one center per boron, its overall reaction with an alkene involves only one dissociation step and one hydroboration step in contrast, borane (BHj) has three consecutive addition reactions, three redistribution equilibria, and five monomer-dimer equilibria [1]. 

Brown and coworkers [ 13] have conducted exhaustive studies of the hydrobo-ration of a,o)-dienes 1,3-butadiene, 1,4-pentadiene, 1,5-hexadiene, 1,7-heptadi-ene, 1,8-octadiene, 1,8-nonadiene, 1,9-decadiene, 1,11-dodecadiene, and 1,13-tetradecadiene with 2 molar equiv of 9-BBN, followed by the redistribution of the resulting dumbbell-shaped trialkylboranes with 1 molar equiv of borane-di-methylsulfide complex (BMS). 

---