Alkenes alkylboranes

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Hydrolioration (Section 7.5) Addition of borane (BH i) or an alkylborane to an alkene. The resultant trialkyliiorane products are useful synthetic intermediates that can be oxidized to yield alcohols. 

Alkylboranes can be coupled by treatment with silver nitrate and base." Since alkylboranes are easily prepared from alkenes (15-16), this is essentially a way of coupling and reducing alkenes in fact, alkenes can be hydroborated and coupled in the same flask. For symmetrical coupling (R = R ) yields range from 60 to 80% for terminal alkenes and from 35 to 50% for internal ones. Unsymmetrical coupling has also been carried out, but with lower yields. Arylboranes react similarly, yielding biaryls. The mechanism is probably of the free-radical type. 

Table 4.3. Regioselectivity of Diborane and Alkylboranes toward Some Alkenes...

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

An indirect method for the hydroformylation of olefins involves formation of the tri-alkylborane (5-12) and treatment of this with carbon monoxide and a reducing agent (see 8-26). Hydroacylation of alkenes has been accomplished, in variable yields, by treatment with an acyl halide and a rhodium complex catalyst, e.g.,587... 

The syn steroselectivity was based on the assumption that the oxidation step—the transformation of the carbon-boron bond to carbon-oxygen bond—took place with retention of configuration. More recent NMR studies of alkylboranes formed in hydroborating labeled alkenes indeed confirmed the validity of the earlier conclusion.351... 

Alternatively, boranes can be prepared in solution and then coupled with support-bound carbon electrophiles. The Suzuki coupling of alkylboranes, generated in situ from 9-BBN and alkenes, with brominated cross-linked polystyrene has been used to link substituted alkyl chains directly to the polymer (Entry 4, Table 5.3). Alkylboranes have also been used to alkylate polystyrene-bound aryl iodides (Entries 3 and 5, Table 5.3). 

Alkylboranes formed in the hydroboration of alkenes and alkynes seldom are isolated for the most part they are used as reactive intermediates for the synthesis of other substances. In the reactions of alkylboranes, the B-C bond... 

The second reaction (Equation 11 -3) achieves the synthesis of a primary alcohol by the oxidation of the alkylborane with hydrogen peroxide in basic solution. Starting with a 1-alkene, one can prepare a primary alcohol in two steps ... 

Diborane or alkylboranes are used for reduction of alkenes and alkynes via hydrobora-tion (see pp. 37f., 47f., 130f.) followed by hydrolysis of the borane with acetic acid (H.C. Brown, 1975). 

These two alkylboranes are important reagents for the hydroboration of other alkenes, since they frequently show in this further reaction a regioselectivity which is greater than that of borane itself. 

Primary alkylboranes derived by hydroboration of terminal alkenes with 9-BBN-H are coupled with aryl and alkenyl triflates and halides under properly selected conditions. The reaction proceeds smoothly without elimination of /1-hydrogen using PdCTklppf) or Pd(Ph3P)4 and K3PO4 in dioxane or DMF [132]. The intramolecular cross-coupling of the alkenyl triflate with the alkylborane in 292, prepared by in situ hydroboration of the double bond in 291 with 9-BBN-H, is applied to the annulation to... 

Aqueous chromic acid has been used to oxidize alkylboranes derived from cyclic alkenes to ketones. For example, hydroboration and oxidation of 1-methylcyclohexene converts it into 2-methylcyclohexanone (Equation B2.10). 

After addition of the alkyne anion to the alkylborane, iodination facilitates alkyl group migration from boron to carbon in a transfer that resembles the one seen in the synthesis of (Z)-alkenes described in Section B4.1. Elimination to give the product alkyne occurs under the iodination reaction conditions (Figure B4.4). 

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