Diborane reaction with alkenes

Methylarsine, trifluoromethylarsine, and bis(trifluoromethyl)arsine [371-74-4] C2HAsF, are gases at room temperature all other primary and secondary arsines are liquids or solids. These compounds are extremely sensitive to oxygen, and ia some cases are spontaneously inflammable ia air (45). They readily undergo addition reactions with alkenes (51), alkynes (52), aldehydes (qv) (53), ketones (qv) (54), isocyanates (55), and a2o compounds (56). They also react with diborane (43) and a variety of other Lewis acids. Alkyl haUdes react with primary and secondary arsiaes to yield quaternary arsenic compounds (57). 

Diborane as well as mono- and dialkylboranes give cis addition in reactions with alkenes, as noted above, which is a result of the four-center transition state in which the boron and hydrogen are delivered from the same face, illustrated by hydroboration of 1-methylcyclopentene with R2BH to give 19. The methyl-bearing... 

Dialkylboranes can disproportionate, although hindered alkylboranes are relatively stable, and this limits their utility in reactions with alkenes. One solution to this problem was the discovery that boranes containing heteroatoms stabilize the borane and are easy to prepare. Diborane reacts with alcohols such as methanol to... 

The sluggish reaction of disiamylborane with tri- and tetrasubstituted alkenes as compared to terminal and most internal disubstituted alkenes, makes it one of the most versatile hydroborating agents. The relative reactivity of disiamylborane with alkenes, dienes, and alkynes varies over a range of 104 (Table 2)26), whereas that of diborane varies in the range of 20-30. 

However, there is another way in which the overall conversion from propene to propan-l-ol may be accomplished. This proceeds by the indirect route of hydroboration, using diborane, followed by the oxidation of the resultant trialkylboron with alkaline hydrogen peroxide. For the present purposes, we are only concerned with the reaction between diborane and the alkene. Diborane is generated in situ, but reacts as if it were the monomer BH3, which is a Lewis acid. Write down the... 

The reaction of diborane, B2H6 (which reacts as BH3), with alkenes gives adducts that can be used further in synthetic schemes, for example to give alcohols this is important synthetically. Reaction (7.20) shows that the overall effect is to give addition in the opposite sense to that given by direct hydration of alkenes. 

The reaction between alkenes and boranes forms the basis of hydroboration (see Chap. 5). Borane (B2H6, formally known as diborane) is a soft Lewis base and will complex with soft bases such as alkenes. The boron... 

An excellent in situ preparation of diborane results from the reaction of boron trichloride (BCI3) or boron trifluoride (BF3) with sodium borohydride in the presence of alkenes, as shown above. This reagent gave virtually instantaneous conversion of an alkene to a trialkylborane at ambient temperatures, when done in a ether solvent. When diborane was isolated and purified it also reacted with alkenes, rapidly and quantitatively in ether solvents. 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

Alcohols can be generated from alkenes by reaction with diborane (B H or BH ), followed by treatment with hydrogen peroxide (Following fig.). The first part of the reaction involves the splitting of a B-H bond in BH with the hydrogen joining one end of the alkene and the boron joining the other. Each of the... 

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. °... 

In the reaction of 1-hexene to give 56 as the major product, an essential component is easy to overlook. The ether solvent is important, and the solvent glytne is commonly used. The formal name of glyme is 1,2-dimethoxyethane and it has the structure CH3OCH2CH2OCH3. If borane is simply mixed with 1-hexene with no solvent, no reaction occru-s unless the mixture is heated to about 180-200°C. In ether solvents, however, the reaction occurs rapidly at ambient temperatures (i.e., room temperatru-e). In fact, ethers catalyze the reaction of borane with an alkene and, although the ether structure does not appear in the product, the ether is not simply a solvent. This is probably because the oxygen atom of an ether is a Lewis base (remember the ate complex formed from borane and diethyl ether) and reacts with diborane to form a highly reactive complex that reacts with the alkene. All reactions of alkenes with borane in this chapter will use an ether solvent. 

Although the reaction between diborane and an alkene proceeds only very slowly in the gas phase, in the presence of weak Lewis bases such as diethyl ether or dimethyl sulphide it occurs rapidly at room temperature. In early work it was customary to generate diborane in situ with the alkene substrate from Et OBFj and NaBH dissolved in an ether such as diglyme. Diborane is not evolved until the second reaction is complete. Nowadays a solution of diborane in THF (approx. 

Hydroboration of alkenes with diborane (e.g. BH3 THF or Me S BH ) gives mono-, di- or trialkylboranes, depending on the steric bulk of the alkene. Mono-and disubstituted alkenes generally yield R3B, trisubstituted (R BH) and tetrasubstituted (RBHJ. Some useful bulky hydroborating agents can be prepared from substituted alkenes (Table 3.6). These show greater selectivity than diborane itself (Table 3.7). Their reactions with alkynes can stop at the alkenylboranes, whereas diborane itself gives dihydroboration. 

The diborane is generated (in situ, or separately, from NaBH4 and Et2Offi—BF3e), and probably complexes, as the monomeric BH3, with the ethereal solvent used for the reaction. BH3 is a Lewis acid and adds to the least substituted carbon atom of the alkene (Markownikov addition), overall addition is completed by hydride transfer to the adjacent, positively polarised carbon atom ... 

Hydroboration of alkenes in non-ethereal solvent has been reported using diborane generated in situ from a quaternary ammonium borohydride and bromoethane (see Section 11.5). Almost quantitative yields of the alcohols are reported [e.g. 1 ]. As an alternative to the haloalkane, trimethylsilyl chloride has also been used in conjunction with the ammonium borohydride [2]. Reduction of the alkene to the alkane also occurs as a side reaction (<20%) and diphenylethyne is converted into 1,2-diphenylethanol (70%), via the intermediate /ra 5-stilbene. 

Any of these BH3 compounds adds readily to most alkenes at room temperature or lower temperatures. The reactions usually are carried out in ether solvents, although hydrocarbon solvents can be used with the borane-dimethyl sulfide complex. When diborane is the reagent, it can be generated either in situ or externally through the reaction of boron trifluoride with sodium borohydride ... 

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