Boranes with alkenes

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. 

Disiamylborane (an abbreviation for di-s-isoamyl borane—not a name we should use now, but the abbreviation has stuck) is also easily made by hydroboration of a simple trialkyl alkene with borane. Two hydroborations occur easily, in contrast to the tetrasubstituted alkene above, but the third is very slow. Disiamylborane is exceptionally regioselective because of its very hindered structure. The structures of these reagents are cumbersome to draw in full and they are often abbreviated. 

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. ° ... 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Brown, H. C., Chandrasekharan, J. Mechanism of hydroboration of alkenes with borane-Lewis base complexes. Evidence that the mechanism of the hydroboration reaction proceeds through a prior dissociation of such complexes. J. Am. Chem. Soc. 1984,106,1863-1865. 

Hydroboration of certain functionalized alkenes with borane also stops at the monoalkylborane stage. Thus, in the reaction of borane with unsaturated amines and sulfides, the initial addition product may undergo intramolecular complexation to form a cyclic product ... 

The reaction of a chiral alkene with borane in the proper stoichiometry may afford alkyl boranes R BH2 or dialkyl boranes R BH, where R is a chiral ligand. Attempts to achieve highly selective reductions of ketones using such reagents have met with little success, however. Trialkyl boranes R3B were first reported to reduce aldehydes and ketones (under forcing conditions) in 1966 by Mikhailov [50]. Mechanistic studies (summarized in ref. [46]) showed that there are two limiting mechanisms for the reduction of a carbonyl compound by a trialkylborane, as shown in Scheme 7.4 a pericyclic process reminiscent of the Meerwein-Pondorf-Verley reaction (Scheme 7.4a), and a two step process that involves dehydro-... 

Table 5.1. Directive Effects in Reactions of Alkenes with Borane...

If boron of an alkylborane could be replaced with a halogen, the product would be an alkyl halide. However, reaction of alkylboranes (neat) with chlorine, bromine, or iodine is very difficult. a when halogenation is done with bromine or iodine dissolved in dichloromethane, however, the reaction is fast and is synthetically useful.A simple example is the reaction of alkenes with boranes followed by addition of bromine, which leads to the alkyl bromide. An example is taken from the synthesis of 2-bromobutane (70) from 2-butene in 88% yield. 0 jhe bromination occurs by a free radical mechanism. Initial reaction with bromine generates a... 

Hydroboration of mono- and disubstituted alkenes with borane gives rise typically to a trialkylborane product. However, trisubstituted alkenes normally give a dialkylborane and tetrasubstituted alkenes form only the monoalkylboranes (5.4). The extent of hydroboration may also be controlled by the stoichiometry of alkene and borane. This has been exploited in the preparation of a number of mono-and dialkylboranes that are less reactive and more selective than borane itself. Important in this respect are the so-called disiamylborane 1 (name derived from... 

Draw a structural formula for the alcohol formed by treating each alkene with borane in tetrahydrofuran (THF), followed by hydrogen peroxide in aqueous sodium hydroxide, and specify the stereochemistry where appropriate. (See Example 5.9)... 

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. 

The regioselectivity and syn stereochemistry of hydroboration-oxidation coupled with a knowledge of the chemical properties of alkenes and boranes contribute to our under standing of the reaction mechanism... 

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). 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

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-... 

Trialkylboranes react rapidly and in high yields with a-halo ketones,a-halo esters, a-halo nitriles, and a-halo sulfonyl derivatives (sulfones, sulfonic esters, sulfonamides) in the presence of a base to give, respectively, alkylated ketones, esters, nitriles, and sulfonyl derivatives. Potassium tert-butoxide is often a suitable base, but potassium 2,6-di-tert-butylphenoxide at 0°C in THF gives better results in most cases, possibly because the large bulk of the two tert-buXy groups prevents the base from coordinating with the R3B. The trialkylboranes are prepared by treatment of 3 mol of an alkene with 1 mol of BH3 (15-16). With appropriate boranes, the R group transferred to a-halo ketones, nitriles, and esters can be vinylic, or (for a-halo ketones and esters) aryl. " °... 

Secondary amines are formed by reaction of trisubstituted boranes with alkyl or aryl azides. The most efficient borane intermediates are monoalkyldichloroboranes, which are generated by reaction of an alkene with BHCl2 Et20.190 The entire sequence of steps and the mechanism of the final stages are summarized by the equation below. 

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