Reactions of Organoboranes

The organoboranes have proven to be very useful intermediates in organic synthesis. In this section, we will discuss methods by which the boron atom can be 

CHAPTER 4 ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS 

The most widely used reaction of organoboranes is the oxidation to alcohols. Alkaline hydrogen peroxide is the reagent usually employed to effect the oxidation. The mechanism is outlined below. 

The R—O—B bonds are hydrolyzed in the alkaline aqueous solution, generating the alcohol. The oxidation mechanism involves a series of boron-to-oxygen migrations of the alkyl groups. The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with the stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the less substituted carbon of a double bond results in the alcohol being formed with regiochemistry that is complementary to that observed for direct hydration or oxymercuration. 

Conditions that permit oxidation of organoboranes to alcohols using molecular oxygen  

The usefulness of the hydroboration reaction in synthesis arises from the fact that the alkylboranes formed can be converted by further reaction into a variety of other products. On hydrolysis (protonolysis), for example, the boron atom is replaced by a hydrogen atom. Particularly important is the oxidahon to alcohols. Some of the transformations are described below. 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

The oxidation reaction involves migration of an alkyl group from boron to oxygen, in an intermediate borate species. All three alkyl groups on the boron atom can undergo this reaction. Hydrolysis of the resulting B(OR)3 derivative releases the desired alcohol product (5.21). 

The alkyl group migrates with retention of stereochemistry at the migrating carbon centre. Since the hydroboration reaction occurs by a syn addition pathway, subsequent oxidation results in syn addition of the elements of water across the 

Hydroboration of 15 is thought to occur wathe conformation with the allylic hydrogen atom eclipsing the alkene (compare with reactions of allylic derivatives in Section 1.1.8)  

The organoboranes have proven to be very useful intermediates in organic synthesis. In this section we discuss methods by which the boron atom can be replaced by hydroxy, carbonyl, amino, or halogen groups. There are also important processes that use alkylboranes in the formation of new carbon-carbon bonds. These reactions are discussed in Section 9.1. 

Several other oxidants can be used to effect the borane to alcohol conversion. Oxone (2K2S05 KHS04 K2S04) has been recommended for oxidations done on a 

More vigorous oxidants such as Cr(VI) reagents effect replacement of boron and oxidation to the carbonyl level.185 

Conditions that permit oxidation of organoboranes to alcohols using molecular oxygen,139 sodium peroxycarbonate,140 or amine oxides141 as oxidants have also been developed. The reaction with molecular oxygen is particularly effective in perfluoroalkane solvents.142 

The oxidation by amine oxides provides a basis for selection among non-equivalent groups on boron. In acyclic organoboranes, the order of reaction is tertiary secondary primary. In cyclic boranes, stereoelectronic factors dominate. With 9-BBN derivatives, for example, preferential migration of a C—B bond which is part of the bicylic ring structure occurs. 

SECTION 4.9. ADDITION AT DOUBLE BONDS VIA ORGANOBORANE INTERMEDIATES 

An alternative procedure for oxidation to ketones involves treatment of the alkyl-borane with a quaternary ammonium perruthenate salt and an amine oxide. Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.  

The boron atom can also be replaced by an NH2 group. The reagents that effect this conversion are chloramine or hydroxylamine-O-sulfonic acid. The 

Organoborane intermediates can also be used to synthesize alkyl halides. Replacement of the boron by iodine is rapid in the presence of base. The best yields are obtained using sodium methoxide in methanol.If less basic conditions are required, the use of iodine monochloride and sodium acetate as the base gives good yields.A similar process using bromine and sodium hydroxide affords 

---

Organoboranes react with propargylic carbonates. Usually, addition of a base is essential for the Pd-catalyzed reactions of organoboranes, but the reaction with propargylic carbonates proceeds without addition of the base, because methoxide is generated in situ from carbonates. For example, the 1,2,4-triene 80 is prepared by the reaction of alkenylborane under neutral conditions[36]. 

Oxidation. The oxidation reactions of organoboranes have been reviewed (5,7,215). Hydroboration—oxidation is an anti-Markovnikov cis-hydration of carbon—carbon multiple bonds. The standard oxidation procedure employs 30% hydrogen peroxide and 3 M sodium hydroxide. The reaction proceeds with retention of configuration (216). 

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). 

Only a few years have passed since we initiated our systematic survey of the reactions of organoboranes. Optimistic as I was at the time this exploration was initiated, I must admit my own surprise at the immense amount of promising new chemistry that our studies have uncovered. 

Examples of these transformations are discussed in Chapter 9, where carbon-carbon bond-forming reactions of organoboranes are covered. 

C-Sn bond then, complete transfer of the tin to the /3-position is accompanied by a 1,2-nucleophilic shift of an alkyl group from boron to carbon, as occurs in so many reactions of organoboranes.212... 

Palladium-catalyzed cross-coupling reaction of organoboranes with organic halides, triflates, etc. in the presence of a base (transmetalation is reluctant to occur without the activating effect of a base). For the catalytic cycle, see Kumada coupling on page 345. 

Carbon-Carbon Bond-Forming Reactions of Organoboranes... 

In Scheme 9.1 there are described reactions of organoboranes with cyanide ion, lithiodichloromethane, and dichloromethyl methyl ether. Compare the structures of these reagents and the final reaction products from each of these reagents. Develop a general mechanistic outline with encompasses these reactions, and discuss the structural features which these reagents have in common with one another and with carbon monoxide. 

Our entry into the field of fine chemicals and catalysis owes much to chance. Some years ago, during studies of the reaction of organoboranes with dichloramine-T (DCT, Fig. I) we utilized column chromatography over silica gel to purify the chloroalkane product. During the course of the chromatography additional minor products were formed, indicating that the silica had in some way brought about a reaction of one or more of the components of the reaction product mixture. 

Synthesis of Primary Amines via the Reaction of Organoboranes with Trimethyl silyl Azide in Neutral Medium 2-exo-Norbornyl Amine. 

The syntheses of carbon-13 labelled trans-tricyclic ketones 93 and 94 have been carried out79 via the cyanidation reaction of organoborane (equation 43). The isomerically pure tram -tricyclic ketones 93 and 94 were needed in connection with the undertaken synthesis... 

Complexation is believed to be involved in the first step of the hydrobo-ration of alkenes and alkynes rearrangement reactions of organoboranes most likely involve intermediates of a rc-complex type. However, the stability of such complexes is generally too low to allow their isolation (112). However, evidence for 7t-complex formation has been obtained by the device of anchoring the alkene function to the metal atom in question by means of a... 

IV/V Synthetic aspects of free radical reactions of organoboranes 23... 

The normal synthetic pathway for hydroboration is reaction with an ambiphilic nucleophile of which the simplest example is hydroperoxide ion. This elicits a 1,2-migration of an alkyl group from boron to oxygen with concurrent loss of hydroxide ion. The step occurs with essentially complete retention of configuration. In similar vein, ambiphilic species with the structure NH2X may be used in amination, so that the overall reaction is an addition of ammonia to the alkene with the regio- and chemoselectivity driven by the hydroboration step. A majority of reactions of organoboranes can be rationalized in terms of these ionic mechanistic pathways, or closely related protocols (Scheme 2). 

---