Boranes, reaction with alkenes

Strategy What is an immediate precursor of a primary alcohol " Perhaps a terminal alkene, which could be hydrated with non-Markovnikov regiochemistiy by reaction with borane Followed by oxidation with H2O2-... 

Telluroaldehydes have been generated and trapped for the first time by the reaction of benzylidenetriphenylphosphorane with "activated" tellurium (a method analogous to that previously used to prepare selenoaldehydes) (Scheme 22).71 A wide range of reactive ylides have been converted into the adducts (118) by reaction with borane.72 On heating, (118) rearrange to triphenylphosphine-monoalkylborane adducts (119) which undergo the expected hydroboration reactions with alkenes. A new route to phosphaalkenes (121) is available from the reaction of phosphinomethylenetriphenylphosphoranes (120) with Lewis acids.73 In the case of (120, R2=NPr 2) the compounds (121) can be isolated and in one case an X-ray structure was obtained. However, similar reactions of (120, r2=Bu ) lead to the dimers (122). 

An atom or a molecule does not have to be positively charged to be an electrophile. Borane (BH3), a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet. Thus, alkenes undergo electrophilic addition reactions with borane serving as the electrophile. When the addition reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide is added to the reaction mixture, and the resulting product is an alcohol. The addition of borane to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide, is called hydroboration-oxidation. The overall reaction was first reported by H. C. Brown in 1959. 

The boron adds stereoselectively to the less sterically hindered face of the alkene. Reaction with norbornene, for example, gave 99.6% of the exo-borane (5) with only 0.4% of the endo- borane. l This is consistent with the normal addition of reagents to the exo face of norbornyl derivatives (sec. 1.5.E). 

With 1,2-disubstituted alkenes, however, there is little discrimination in reactions with borane itself. In addition, the regioselectivity in the hydroboration of terminal alkenes, although high, is not complete. Further, there is little difference in the rate... 

There is another way to present this multistep reaction. The reactions are stacked on an arrow, and a number is provided to indicate each separate reaction. For methylcyclopentene, the first reaction with borane (reaction 1) is followed by reaction with NaOH and H2O2 (reaction 2). When presented this way, the alkene starting material reacts with borane to give 69, which is not shown, and in a second reaction 69 is converted to 70. Only the starting material and the final product are shown, however, so the focus is on the isolated product rather than the mechanism. This method of stacking reactions is used throughout this book. 

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. 

In addition to the oxymercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H. C. Brown and cailed hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

Catalytic hydrogenation of triple bonds and the reaction with DIBAL-H usually give the eis alkene (15-11). Most of the other methods of triple-bond reduction lead to the more thermodynamically stable trans alkene. However, this is not the case with the method involving hydrolysis of boranes or with the reductions with activated zinc, hydrazine, or NH2OSO3H, which also give the cis products. 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

Both alkynes and alkenes can be obtained from adducts of terminal alkynes and boranes. Reaction with iodine induces migration and results in the formation of the alkylated alkyne.32... 

The utilization of phosphine-boranes as stable forms of the phosphine has been exploited for addition reactions with alkenes. With several types of a,P-unsaturated compounds, Michael addition of primary phosphine-boranes has been accomplished in excellent yield for the preparation of secondary phosphine-boranes (Equation 3.30).443... 

---