Alkylboranes preparation

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). 

Mercuration. Mercury(II) salts react with alkyl-, alkenyl-, and arylboranes to yield organomercurials, which are usehil synthetic intermediates (263). For example, dialkyhnercury and alkyhnercury acetates can be prepared from primary trialkylboranes by treatment with mercury(II) chloride in the presence of sodium hydroxide or with mercury(II) acetate in tetrahydrofuran (3,264). Mercuration of 3 -alkylboranes is sluggish and requires prolonged heating. Alkenyl groups are transferred from boron to mercury with retention of configuration (243,265). 

Carbanioas derived from the other above mentioned methane derivatives react with alkylboranes, bornic, and boronic esters, providing rich possibihties for the preparation of siagle-carbon iasertion products. Thus 2-aLkyl-l,3,2-dithiaborolanes are converted iato acids or thioacetals by trichloromethyllithium (335). 

Deuterioboration of 5a-cholest-2-ene (171), followed by oxidation of the alkylborane intermediate with hydrogen peroxide in the presence of sodium hydroxide, illustrates the application of this method for the preparation of c/5-deuterium labeled alcohols.(For the preparation of tra 5 -deuterium labeled alcohols see section VII-A.) The predominant reaction product is 2a-di-5a-cholestan-3a-ol (172, 1.03 D/mole) which is accompanied by 3a-di-5a-cholestan-2a-ol (173) and other minor products." ... 

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. 

Several alkylboranes are available in enantiomerically enriched or pure form and can be used to prepare enantiomerically enriched alcohols and other compounds available via organoborane intermediates.196 One route to enantiopure boranes is by hydroboration of readily available terpenes that occur naturally in enantiomerically enriched or pure form. The most thoroughly investigated of these is bis-(isopinocampheyl)borane (Ipc)2BH), which can be prepared in 100% enantiomeric purity from the readily available terpene a-pinene.197 Both enantiomers are available. 

Very recently, we reported liquid imidazole-borane complexes (scheme 5)57 that are air stable. Judging from their polarity and viscosity (Table 3), they are expected to be a new class of solvent or electrolyte. Preparation of polymer homo-logues of imidazole-alkylborane complexes will also be reported elsewhere in the near future. 

Alkylmercury reagents can also be prepared from alkylboranes. 

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). 

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 ... 

Methylborylene [62785-41-5] B Preparative hazard See Dibromomethylborane Sodium—potassium alloy (next below) See related ALKYLBORANES ch3b... 

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... 

The reaction of ate complexes (24), formed from trialkylboranes and trimethyl-silylpropargyl phenyl ether, with a mixture of acetic acid and hexamethylphosphoric triamide (HMPT) gives trimethylsilylacetylenes (25) selectively (Eq. 54) whereas the corresponding trimethylsilylallenes (26) are selectively prepared by the reaction with sodium methoxide instead of acetic acid and HMPT (Eq. 54) In the latter, when primary alkylboranes are used, the corresponding allene derivatives are obtain in high purity, but secondary alkylboranes give reverse ratios of the isomer distribution. 

Recently, the reactions of l-alkenyltrialkylborates possessing a functionality such as a methoxy group at the C=C bond have been explored. Thus Levy et al. reported that alkenyltrialkylborate salts (S2), prepared from a-methoxyvinyllithium and tri-alkylboranes, which are stable at —80 °C, upon warming to room temperature, affords alkenyldialkylmethoxyborates 33) via an alkyl group migration. These ate complexes (33) have been prov i to be versatile intermediates, as illustrated in Eq. 66 . 

A longer sequence is therefore necessary to convert the isopinocampheyl alkylboranes to the corresponding optically pure alkylamines, since the pinanyl group, which is used to induce chirality in the preliminary hydroboration step, must be removed. The optically active amines, prepared as shown in the scheme below, were obtained in 48-62% overall yield, based on the alkene or the hydrobora ting agent (Table 2)26. 

Thexylborane slowly isomerizes to a primary alkylborane at ambient temperature, so it is usual to prepare it freshly and to use it, at 0 °C, when it is required. However, addition of an equimolar quantity of NA -diethylaniline produces a reagent which appears significantly more stable yet retains the hydrobora-ting and reducing capabilities. ... 

Recently, methylborane has been prepared from lithium methyltrihydroborate (equation 19). Other alkylboranes have been obtained by similar methods.5 ... 

BBN-H is readily prepared (Section 3.10.2.1, equation 8) and is commercially available. It shows considerable stability, even in air for limited periods, and is therefore a very convenient hydroborating agent.Unlike di-primary-alkylboranes it is not prone to disproportionation, but it is substantially less hindered than other di-5-alkylboranes such as dicyclohexylborane and disiamylborane. Thus, it hydroborates hindered alkenes such as 2,3-dimethyl-2-butene slowly. It is less sensitive to steric factors and more sensitive to electronic factors than disiamylborane. Thus, it shows relatively little ability to discriminate between ( )/(Z) pairs but readily discriminates between 4-methoxystyrene and 4-(trifluoro-methyl)styrene. ... 

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