Alkenes from boranes

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). 

Formation of Alkynes, Alkenes, and Ketones from Boranes and Acetylides... 

Entries 5 to 7 are examples of oxidation of boranes to the carbonyl level. In Entry 5, chromic acid was used to obtain a ketone. Entry 6 shows 5 mol % tetrapropylam-monium perruthenate with Af-methylmorpholine-lV-oxide as the stoichiometric oxidant converting the borane directly to a ketone. Aldehydes were obtained from terminal alkenes using this reagent combination. Pyridinium chlorochromate (Entry 7) can also be used to obtain aldehydes. Entries 8 and 9 illustrate methods for amination of alkenes via boranes. Entries 10 and 11 illustrate the preparation of halides. 

Addition of alane and borane to alkenes affords a host of alkylated alanes and boranes with various reducing properties (and sometimes bizarre names) diisobutylalane (Dibal-H ) [104], 9-borabicyclo[3.3.1]nonane (9-BBN) (prepared from borane and 1,5-cyclooctadiene) [705], mono- [106,107] and diiso-pinocampheylborane (B-di-3-pinanylborane) (both prepared from borane and optically active a-pinene) [108], isopinocampheyl-9-borabicyclo[3.3.1 Jnonane alias B-3-pinanyl-9-borabicyclo[3.3.1]nonane (3-pinanyl-9-BBN) (prepared from 9-borabicyclo [3.3.1]nonane and a-pinene) [709], NB-Enanthrane prepared from 9-borabicyclo[3.3.1]nonane and nopol benzyl ether) [770] and others. ... 

The best procedure to get the desired product is to generate the 1-alkene from the borane with 1-decene (Section 11-6C) and then add hydrogen bromide by a polar mechanism (Section 10-4). Incursion of radical-chain addition must... 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

Reaction 8.14 was carried out by Singleton in an attempt to minimize the contamination of the contribution of second and third additions of alkene to borane. The observed ratio of anti-Markovnikov to Markovinkov product is 90 10. Assuming that this ratio derives from the difference in the TS energies leading to the two products, TST gives an estimate of the energy difference of the two activation barriers of 1.1-1.3 kcal mor ... 

Secondary boranes (dialkylboranes) react with alkenes in a 1 1 ratio. Disiamylborane, which is prepared in situ from borane-dimethyl sulfide with 2 mol of 2-methyl-2-butene, converts 1-octene into 1-octyldisiamyl-borane in ether at 0 °C in 2 h [611] (vide infra). 

In order to obtain the enol as the product of the addition reaction, only one equivalent of BH3 can be allowed to add to the alkyne. In other words, the reaction must stop at the alkene stage. In the case of internal alkynes, the substituents on the boron-substituted alkene prevent the second addition from occurring. However, there is less steric hindrance in a terminal alkyne, so it is harder to stop the addition reaction at the alkene stage. A special reagent called disiamylborane has been developed for use with terminal alkynes ( siamyl stands for secondary iso amyl amyl is a common name for a five-carbon fragment). The bulky alkyl groups of disiamylborane prevent a second addition to the boron-substituted alkene. So borane can be used to hydrate internal alkynes, but disiamylborane is preferred for the hydration of terminal alkynes. 

In Fig. 4, reaction A is a highly stereoselective reduction of 1-aryl alkanones with (-)-chlo-ro diisopinocampheylborane [21]. Upon co-ordination of the ketone oxygen with the Lewis acidic chirotopic and non-stereogenic [22] boron atom of the chiral reagent, two diastereo-isomeric complexes arise. The sterically less hindered one is preferentially formed and leads the major (,S)-enantiomer, which is isolated after a work-up that allows recovery of a-pinene, the chiral alkene from which the borane is prepared. 

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... 

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. 

Again, as seen with alkenes, both borane (B2H6) (Equation 6.22) and diimide (H-N=N-H) (Scheme 6.15) can be used to reduce alkynes. Indeed, the reaction of internal alkynes with borane is apparently more facile than that with the alkene that results from consummation of the reduction. Further, as would be expected, suprafacial addition of hydrogen and boron obtains and Z- (or cis-) alkene is the only product. The use of deuterated boron compounds (e.g., the hindered 9- H-9-borabicyclo[3.3.1]nonane [9- H-9-BBN]), commercially viable since it contains only one deuterium ( H), followed by use of deuterated acetic acid (CH3C02, i.e., reductive workup) produces Z- (or cw-)-dideuteroalkene of high stereospecificity and in high yield (Scheme 6.65). 

This general procedure for the synthesis of alkenes from the enamines, when applied to the acyclic enamines derived from the acyclic ketones by modification of hydroboration-elimination procedure, permits a facile, diastereospecific conversion to either (Z)- or (E)-alkene at will (A) The hydroboration of enamine with 9-BBN, followed by treatment with methanol gives (Z)-alkenes of 99% isomeric purity. (B) The hydroboration of the same enamine with borane-methylsulfide, followed by methanolysis and oxidation with neutral hydrogen peroxide gives ( )-alkenes of 99% isomeric purity (Scheme 24.2) [13a]. 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Step 1 A molecule of borane (BH3) attacks the alkene Electrons flow from the 7C orbital of the alkene to the 2p orbital of boron A 7C complex is formed... 

Mono-, di-, and trialkylboranes may be obtained from olefins and the trifunctional borane molecule. Simple unhindered alkenes yield trialkylboranes and it is not possible to halt the reaction at the mono- or dialkylborane stage. With more hindered and trisubstituted alkenes the reaction can be controlled to stop at the dialkylborane stage. 

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... 

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