Alkyl borane reagents

Alkylmercury reagents can also be prepared from alkyl boranes. 

Many transition metal alkyls react with carbon monoxide to give acyl compounds. In all these cases the acyl derivatives can be detected at least by infrared methods and in most cases isolated. Molybdenum, manganese, rhenium, iron, cobalt, rhodium, nickel, palladium, and platinum alkyls, Grignard reagents, and boranes, all react with carbon monoxide, and one can explain the products from these on the basis of carbon monoxide inserting into the metal alkyl. 

For a review of anions a lo a selenium atom on small rings, see Krief Top. Curr. Chem. 1987, 135. 1-75. For alkylation a to boron, see Pelter Smith Brown Borane Reagents, Academic Press New York, 1988, pp. 336-341. 

Two dialkyl boranes arc in common use. The bicyclic 9-borabicyclo[3.3.1] nonane (9-BBN), introduced in Chapter 34 as a reagent for diastereoselcctive aldol reactions, is a stable crystalline solid. This is very unusual for an alkyl borane and makes it a popular reagent. It is made by hydroboration of cyclo-octa-1,5-diene. The second hydroboration is fast because it is intramolecular but the third would be very slow. The regioselectivity of the second hydroboration is under thermodynamic control. 

Pyridinium chiorochromate (PCC) is a very useful reagent for the oxidation of oiganoboranes to carbonyl compounds in mildly kaline and anhydrous conditions. As well as oxidizing (secondary alkyl)boranes to ketones, PCC oxidizes (primary alkyl)boranes to aldehydes in excellent yields (equation 47). The latter transformation cannot be accomplished with chromic acid. The reagent tolerates the presence of alkene, ester and acetal groups. ... 

In a related reaction sulfurated boranes (R2B-SSLR 2) react with Grignard reagents, such as methyhnagneisum bromide to give the B-alkyl borane (e.g., R2B-Me) upon heating in vacuo ... 

The reaction of a chiral alkene with borane in the proper stoichiometry may afford alkyl boranes R BH2 or dialkyl boranes R BH, where R is a chiral ligand. Attempts to achieve highly selective reductions of ketones using such reagents have met with little success, however. Trialkyl boranes R3B were first reported to reduce aldehydes and ketones (under forcing conditions) in 1966 by Mikhailov [50]. Mechanistic studies (summarized in ref. [46]) showed that there are two limiting mechanisms for the reduction of a carbonyl compound by a trialkylborane, as shown in Scheme 7.4 a pericyclic process reminiscent of the Meerwein-Pondorf-Verley reaction (Scheme 7.4a), and a two step process that involves dehydro-... 

Secondary amines are formed by reaction of boranes with alkyl or aryl azides. The most efficient borane reagents are monoalkyldichloroboranes, which are generated by... 

Undoubtedly, the synthetic value of the Rh-catalyzed alkyl borylation reaction follows from the fact that it can selectively deliver a functional group, such as an alcohol, an amine or an alkene, at the terminus of an alkyl chain during the synthesis of more complex structures. With the proper substituent at boron, this process can extrude the same borane reagent that was used in the alkane functionalization step to allow recycling of the main group reagent for large-scale synthesis. 

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