Hydroboration of 1-Alkynes

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne. This avoids the complications which occur with borane that lead to polymeric structures. Catecholborane is a particularly usefiil reagent for hydroboration of alkynes. Protonolysis of the adduct with acetic acid 

SECTION 4.9. ADDITION AT DOUBLE BONDS VIA ORGANOBORANE INTERMEDIATES 

Treatment of the vinylborane with bromine and base leads to vinyl bromides. The reaction occurs with net anti addition. The stereoselectivity is explained on the basis of anti addition of bromine followed by a second anti elimination of bromide and boron  

Exceptions to this stereoselectivity have been noted. 

The adducts derived from catecholborane are hydrolysed to vinylboronic acids. These materials are useful intermediates for preparation of terminal vinyl iodides. Because the hydroboration is a syn addition and the iodinolysis occurs with retention of the alkene geometry, the iodides have the -configuration. ° 

Alkynes are reactive toward hydroboration reagents. The most useful procedures involve addition of a disubstituted borane to the alkyne. This avoids complications that can occur with borane itself which lead to cross-linked structures. Catechol-borane (1,3,2-benzodioxaborole), which is prepared from equimolar amounts of catechol (1,2-dihydroxybenzene) and borane, is a particularly useful reagent for hydrocarboration of alkynes.Protonolysis with acetic acid of the adduct of an alkyne with this reagent results in reduction of the alkyne to the corresponding Z-alkene. Oxidative workup with hydrogen peroxide gives ketones via enol intermediates. 

Other disubstituted boranes have also been used for selective hydroboration of alkynes. 9-BBN can be used to hydroborate internal alkynes. Protonolysis can be carried out with methanol, and this forms a convenient method for formation of a disubstituted Z-alkene.  

Miyaura and co-workers also reported the Pt(dba)2 atalyzed pinacolboration of terminal aUenes the regioselectivity was highly dependent on the nature of the added phosphine ligand [267]. For example, whereas the bulky tris(2,4,6-trimethoxy-phenyl) phosphine often led to substantial amounts of the external Markovnikov prod- 

Like alkenes, alkynes can be hydroborated by boranes.The mechanism is very similar to that for alkenes (p. 390) the stereochemistry of addition of HBH2 is syn, and it is the boron that becomes attached to the less substituted end of a terminal alkyne (Fig. 10.73). 

FIGURE 10.73 Hydroboration of alkynes gives vinylboranes that can react further (R = alkyl). 

The initial products of hydroboration are prone to further hydroboration reactions. This complication can be avoided by using a sterically hindered borane so that further reaction is slowed. A favorite suitably hindered reagent is the borane formed by reaction of two equivalents of 2-methyl-2-butene with BH3 (Fig. 10.74). 

FIGURE 10.74 This sterically hindered dialkylborane reacts only once with alkynes to give an isolable vinylborane (R = aUgrl). 

Like alkylboranes, dnylboranes can be converted into alcohols by treatment with basic peroxide (Fig. 10.75).The products are ends in this case, and they are in equilibrium with the corresponding carbonyl compounds. Here s an important point The regiochemistry of this reaction is the opposite of the regiochemistry of the hydration reaction of alkynes. This situation is exactly the same as the one that exists with the alkenes Hydration of an alkene gives overall Markovnikov addition, whereas the hydroboration/oxidation sequence gives the anti-Markovnikov product. 

1- alkynes (1) (Table 5.20) based on the NMR studies conducted on vinylbo-ranes (2) and diboryl adduct (3) in THE It is discovered that because 2 but not 3 is partially complexed by THE and thus, its B signal in NMR is significantly shifted upheld (Table 5.21) in THE and product distribution has been assessed directly, avoiding extra steps associated with GC analyses of protonolysis and oxidation products. The data (Table 5.20) clearly reveals for the first time that the relative amount of 2 versus 3 generally increases with size of the R group in 1, with SiMej exhibiting its expected exceptional behavior [4h, 9]. 

Midland reduction [10] of a carbonyl compound with trialkylborane, involves dehydroborylation of trialkylborane in a concerted manner to produce an alkene. However, s-Bu-9-BBN reacts sluggishly with aldehydes to produce 

Consequently, a new and completely stereoselective route to tram 2 from 3 is available from terminal alkynes by the dehydroborylation procedure [8]. 

The earlier concept of syntheses of vinylborane, with excess of alkyne is completely unworkable for an a,a)-diyne if both the carbon-carbon triple bonds are to be monohydroborated. However, a 4 1 ratio of 9-BBN and 1,8-nonadiyne give exclusively the corresponding l,l,9,9-fefra-9-BBN adduct, which on treatment with 2 equiv of PhCHO results in clean formation of tram,tram-l,9-di-9-BBN-1,8-nonadiene (Eq. 5.22) [8]. 

The new dehydroborylation route to trans-vinyl-9-BBN demonstrates its remarkable versatility as it undergoes deuterolysis, thermal isomerization, aldehyde insertion, oxidation, and Pd-catalyzed Suzuki s cross-coupling reactions vide infra). 

Electrophilic Additions to Carbon-Carbon Multiple Bonds 

---

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. 

An efficient general synthesis of a-chiral (Z)- and (H)-a1kenes ia high enantiomeric purity is based on the hydroboration of alkynes and 1-bromoaIkynes, respectively, with enantiomericaHy pure IpcR BH readily available by the hydroboration of prochiral alkenes with monoisopiaocampheylborane, followed by crystallization (519). 

Scheme 23. Synthesis of vinyl boranes by hydroboration of alkynes.

Schwartz s reagent (1) is an excellent catalyst for pinacolborane hydroboration of alkynes (Scheme 8-32) [234, 235]. 

Hydroboration of alkynes is easier to achieve than B-H addition to alkenes. A mixture of alcohol, ketone, and diol products is produced upon [Rh(PPh3)3Cl] catalyzed hydroboration of... 

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

The hydroboration of alkynes gives predominantly cis isomer, as in the following example. Diborane adds as BH3 to alkynes in a cis isomer and forms vinyl boranes which react with acetic acid at low temperature to give cis-alkenes. 

Scheme 3.2-14. Combined hydroboration of alkynes and diethyl(propyn-l-yl)borane in the hydride bath (24 and 26), followed by Et2BH-catalyzed condensation and self-...

Alkynes. The selectivity of hydroboration of alkynes depends on the hydroborating reagents. Diborane usually forms complex mixtures of organoboranes, including polymers.372,373 Other reagents, however, may be applied to perform selective transformations. 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

Hayashi et al. and Miyaura et al. have reported that far less nucleophilic aryl- and alkenyl-boronic acids can react with a variety of enones in the presence of a BINAP-rhodium catalyst to give adducts with high enantiopurity in general (Scheme 8D.5) [13], The one pot procedure, involving the hydroboration of alkynes as the first step (R = alkenyl), was achieved in the presence of amines without affecting the enantioselectivity [13]. 

The required vinyl boranes and vinyl iodides can both be easily made by tlie hydroboration of alkynes witli disiamyl borane (Sia). Thus the Suzuki reaction is an important methodology for the synthesis of conjugated polyene natural products. 

Dicarbonyltitanocene [Cp2Ti(CO)2] is also active for the hydroboration of alkynes at room temperature. Dimethyltitanocene [Cp2TiMe2] catalyses the hydroboration of alkene with CBH at room temperature [203],... 

Due to its boron-oxgen bonds it is a less reactive hydroborating reagent than H3B, H2BR, or HBR2. It is often used for hydroboration of alkynes. 

Alkenylboronic acids and esters have been prepared by thermal or catalyzed hydroboration of 1-alkynes with catecholborane (HBcat), pinacolborane (HBpin), or dihaloboranes 41-43, followed by hydrolysis to boronic acids or alcoholysis to boronic esters. A convenient alternative to improve chemo- and regioselectivity is the hydroboration of alkynes with dialkylboranes. For selective removal of dummy groups, the oxidation of two cyclohexyl groups was conduced by treatment of l-alkenyl(dicyclohexyl)borane intermediates with Me3N-0 (Equation (7)).116 The... 

---