Bonds hydroboration

ANSWER In this case, H and OH are added across the pi bond in an anti-Markovnikov addition. No stereocenters were formed, so the stereochemical outcome is not relevant. We have only seen one way to achieve an anti-Markovnikov addition of water across a pi bond hydroboration-oxidation. Therefore, our answer is ... 

M. Zaidlewicz, Formation of C—H Bonds by Reduction of Olefinic Double Bonds—Hydroboration and Hydroalumination, in Methoden Org. Chem. (Houben-Weyl) 4th ed. 1952-, Stere-... 

Disiamylborane and dicyclohexylborane, in contrast to 9-BBN-H, reduce the triple bond of enynes in preference to the double bond. Hydroboration of functional alkynes is also possible e.g. equations 38 and 39 see also ref. 97). 

In the presence of propene, and with heating, the monomer is going to add across C=C bond (hydroboration... 

The els addition leads to exo/endo diastereoseleetion in the hydroboration of suitably substituted double bonds. Thus, depending on the substitution of the double bond, hydroboration of a prostcrcogcnic olefin can create one or two stereogenic centers. 

Addition to multiple bonds. Hydroboration of 1-alkynes with catecholborane (when /-Pr3P and EtjN are also present) gives (Z)-l-alkenylboronates. On the other hand, dehydrogenative coupling between styrenes and pinacolborane is observed. 

Several synthetically very useful processes involve the 1,2 insertion of unsaturated substrates into the M-SiRs bond (hydrosilation [193], dehydrogenative silation [193a,194], and disilation [176]) or the M-BR2 bond (hydroboration [195], dehydrogenative boration [196], and diboration [191,195,197]) Catalytic reactions have been developed lately that comprise the 1,2 addition of two different ER groups to an alkene or alkyne (silylstannation [198], borylsilation... 

The anfi-Markownikoff-hydration of carbon-carbon double bonds hydroboration-oxidation has been supplemented by a remarkably simple and likewise stereospecific Markownikoff-hydration achieved by a one-step oxymercuration-demercuration p-Nitrophenyl chloroformate has been recommended for the protection of hydroxyl groups in nucleosides... 

The addition is thus stereospecific, taking place across a given face of a double bond. Hydroboration is also remarkably regioselective. Where there is a choice, the boron adds preferentially to the less sterically hindered end of a double bond. 

Organoboranes are of special importance because they are easily formed in borane addition to C=C bonds (hydroboration). The high electronegativity of B (2.0) means that the B—C bond is not very polar and BR3 species are usually water, although not air stable but sufficiently reactive to be useful. In contrast to most other reagents, a B—H bond adds in an anti-Markownikov manner to an alkene to... 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

Methods of producing B —C bonds include hydroboration, nucleophilic displacement at a boron atom in BX., (X = halogens or B(0R>3) by e.g. a Grignard reagent, and a psewiio-Friedel-Crafts reaction with an aromatic hydrocarbon, BX3, and AICI3. 

Another possibility for asymmetric reduction is the use of chiral complex hydrides derived from LiAlH. and chiral alcohols, e.g. N-methylephedrine (I. Jacquet, 1974), or 1,4-bis(dimethylamino)butanediol (D. Seebach, 1974). But stereoselectivities are mostly below 50%. At the present time attempts to form chiral alcohols from ketones are less successful than the asymmetric reduction of C = C double bonds via hydroboration or hydrogenation with Wilkinson type catalysts (G. Zweifel, 1963 H.B. Kagan, 1978 see p. 102f.). 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Hydroboration is a reaction m which a boron hydride a compound of the type R2BH adds to a carbon-carbon bond A carbon-hydrogen bond and a carbon-boron bond result... 

Carbocation intermediates are not involved m hydroboration-oxidation Hydration of double bonds takes place without rearrangement even m alkenes as highly branched as the following... 

A second aspect of hydroboration-oxidation concerns its stereochemistry As illustrated for the case of 1 methylcyclopentene H and OH add to the same face of the double bond... 

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... 

Hydroboration-oxidation of alkenes (Section 6 11) H and OF add to the double bond with a regioselectivity opposite to that of Markovnikov s rule This is a very good synthetic method addition is syn and no rearrangements are observed... 

Hydroboration is the addition of a boron—hydrogenbond across a double or triple carbon—carbon bond to give an organoborane ... 

Tetrasubstituted and some hindered trisubstituted alkenes react rapidly only to the monoalkylborane stage. Rarely, when the tetrasubstituted double bond is incorporated in a cycHc stmcture, does hydroboration under normal conditions fail (25—27). However, such double bonds may react under conditions of greater force (25,28—31). Generally, trialkylboranes are stable at normal temperatures, undergoing thermal dissociation at temperatures above 100°C (32—34). In the presence of B—H bonds, trialkylboranes undergo a redistribution reaction (35—38). 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

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... 

Many other examples ia the Hterature illustrate the possibiUties of chemoselective hydroborations (124,186—189). For example, selectivity between double and triple bonds has been shown (124). 

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