Alkyl halides using organoboranes

Several papers have appeared describing the synthesis of alkyl halides from organoboranes. Alkyl chlorides are obtained by the reaction of trialkylboranes with dichloramine-T, and in somewhat lower yield with iV,N-dichlorourethane/ whereas alkyl bromides are produced in excellent yield when the boranes derived from terminal alkenes and dicyclohexylborane are treated with either Bt2 or BrCl. Alkyl iodides are obtained from the same boranes by the use of ICl-NaQAc " or iodide ion-chloramine-T/ and this latter method has been applied to the radioiodination of olefins (with Vinyl iodides result from... 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of boron by iodine is rapid in the presence of base.150 The best yields are obtained with sodium methoxide in methanol.151 If less basic conditions are desirable, the use of iodine monochloride and sodium acetate gives good yields.152 As is the case in hydroboration-oxidation, the regioselectivity of hydroboration-halogenation is opposite to that observed for direct ionic addition of hydrogen halides to alkenes. Terminal alkenes give primary halides. 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

The in situ synthesis of organoboranes via reaction of alkyl halides with magnesium in the presence of diborane can also be used to prepare coupled products (equations 20 and 21). Oxidation of the reaction mixture with alkaline silver nitrate leads to good yields of dimeric products. The reaction is successful for primary and secondary halides. A related reaction is the coupling of secondary alkyl halides in the presence of catalytic quantities of thallium salts. This procedure fails for primary alkyl halides and gives modest yields for secondary alkyl halides (equation 22). 

What alkyl halides would you utilize to synthesize the following compounds, using the organoborane shown ... 

Some interesting developments concerning C-C bond formation using organoborane coupling reactions have been made. 9-Alkyl-9-BBN derivatives (144) react with aryl and benzyl halides in the presence of transition metal catalysts and carbon monoxide to give... 

A second means of effecting alkylations of organoboranes involves reactions with highly reactive alkyl halides, especially a-halocarbonyl compounds. For example, ethyl bromoacetate has been found to alkylate a number of trialkylboranes in excellent yield. This synthetic transformation is more efficiently carried out using a... 

Organoborane compounds have also been successfully coupled to alkyl halides. This can be done for alkyl iodides using a simple catalyst system consisting of (Ph3P)4Pd (Scheme 2.97), or, for alkyl bromides, with a nickel catalyst with a bidentate nitrogen ligand (Scheme 2.98). ... 

An improved procedure for the synthesis of alkyl iodides from organoboranes (and thus ultimately from alkenes), involving the use of iodine and sodium meth-oxide, has been shown to entail inversion at the C-B bond, and is employed in a new route to optically active iodides from alkenes e.g. Scheme 30). A similar sequence for conversion of alkenes to primary halides via hydrozirconation (Scheme 31) has been reviewed. Zr also catalyses addition of lithium aluminium hydride to alkenes under mild conditions, and the alanes so formed can likewise be converted into alkyl halides (Scheme 32). 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

The reagent is used in a new sequence for the preparation of alkylmercuric halides via hydroboration-mercuration.3 A terminal olefin is converted into an organoborane by reaction with dicyclohexylborane, and the organobqrane is treated with mercuric acetate. An alkyl mercuric acetate is formed, which is then converted into the more easily handled alkylmercuric halide by reaction with a sodium halide and water. 

In the presence of palladium(O), organoboranes undergo coupling reactions with unsaturated halides (or inflates). This type of S-alkyl Suzuki reaction is a useful method for the synthesis of substituted alkenes (see Section 1.2.4, Scheme 1.205). For example, hydroboration of the alkene 22 and coupling with the alkenyl iodide 23 was used in a synthesis of the alkaloid halichlorine (5.36). 

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