B-Alkyl-9-BBN

The iodination reaction can also be conducted with iodine monochloride in the presence of sodium acetate (240) or iodine in the presence of water or methanolic sodium acetate (241). Under these mild conditions functionalized alkenes can be transformed into the corresponding iodides. AppHcation of B-alkyl-9-BBN derivatives in the chlorination and dark bromination reactions allows better utilization of alkyl groups (235,242). An indirect stereoselective procedure for the conversion of alkynes into (H)-1-ha1o-1-alkenes is based on the mercuration reaction of boronic acids followed by in situ bromination or iodination of the intermediate mercuric salts (243). 

Coupling of aryl or alkenyl halides with trialkylboranes.1 This reaction can be carried out in refluxing THF with a base (NaOH or NaOCH3,1 equiv.) catalyzed by PdCl2(dppf). A B-alkyl-9-BBN is more useful than a trialkylborane, since only one alkyl group is utilized. 

Balance equations, for a mixture, 24 669-671 Balances 26 227 analytical, 26 245 precision, 26 245 Balke process, 17 139 B-Alkyl-9-BBN derivatives, 13 658 B-Alkylcatecholboranes, free-radical oxidations of, 13 648 Ball and chain structures, fullerene, 12 252... 

When the reaction between a trialkylborane and carbon monoxide (8-24) is carried out in the presence of a reducing agent such as lithium borohydride or potassium triisopropoxy-borohydride, the reduction agent intercepts the intermediate 73, so that only one boron-to-carbon migration takes place, and the product is hydrolyzed to a primary alcohol or oxidized to an aldehyde.333 This procedure wastes two of the three R groups, but this problem can be avoided by the use of B-alkyl-9-BBN derivatives (p. 785). Since only the 9-alkyl group... 

In general, yields are low unless a B-alkyl-9-BBN is used. Thus when "B-hexyl-9-BBN is used, 1-hexyl-l-methoxycyclopropane is formed in 81% yield. 

More recently, Brown and co-workers have demonstrated that six-membered boracyclanes are likewise capable of ring contraction. As an example, the light-induced reaction of bromine with 9-methoxy-9-borabicyclo[3.3.1]nonane (61) in the presence of water affords the boronic acid 62 which is in turn readily oxidized to the alcohol in 65 % overall yield.11S) The structurally related B-alkyl-9-BBN derivatives react under comparable conditions but in the dark to deliver analogous products.116)... 

Nitrogen trichloride converts all three alkyl groups of RsB into alkyl chlorides, but 3 equiv. of a hazardous and not readily available reagent must be used. Chlorodimethylamine or dichloramine-T (equation 55) may be used, but here too there is poor utilization of alkyl groups, a problem eased, but not completely overcome, by the use of B-alkyl-9-BBN derivatives. ... 

Yields are low when R is bulky however, they can be improved by use of B-alkyl-9-BBN derivatives instead of trialkylboranes. 

Catalytic coupling reactions of alkyl- [121,122] and 1-alkenyl- [123 —125] boronates usually proceed with a base. Cross-coupling between R3B or B-alkyl-9-BBN (BBN = borabicyclo[3.3.1]nonane) with aryl or 1-alkenyl halides, for example. 

B-alkyl-9-BBN derivatives (p. 1077). Since only the 9-alkyl group migrates, this method permits the conversion in high yield of an alkene to a primary alcohol or aldehyde containing one more carbon." When B-alkyl-9-BBN derivatives are treated with CO and lithium tri-ferf-butoxyaluminum hydride," other functional groups (e.g., CN and ester) can be present in the alkyl group without being reduced." Boranes can be directly converted to carboxylic acids by reaction with the dianion of phenoxyacetic acid." " ... 

Consistently high yields depend on the proper selection of reagents. In general, the best base is the bulky potassium 2,6-di-rerr-butylphenoxide. The best alkylating agent is B-alkyl-9-borabicyclo[3.3.1]nonane, or B-alkyl-9-BBN, available via successive hydroborations of alkenes ... 

Table 4. Methoxycyclopropanes via Ate Complexes Obtained from B-Alkyl-9-BBN and 1-Lithio-l-methoxyallene...

Highest regio- and stereoselectivity among all boranes is exhibited by 9-BBN. Terminal alkenes are transformed into the corresponding B-alkyl-9-BBN derivatives in purities approaching 100%. Unsymmetrically disubstituted alkynes and alkenes also react selectively . Addition to 3-methylcyclohexene and 3-methylcyclopentene gives no cis-1,2-isomer . 1,3-Dialkylcycloalkenes are transformed into single stereo isomers . 

Hydroboration of the byproduct alkene gives isomeric B-alkyl-9-BBN products. Generally, hydrocarbon solvents are preferable to ether or THF for this process because the greater stability of the intermediate methoxyborate complexes (i.e. Li[R(MeO)-9-BBN]) at —78°C in these solvents prevents the product from being formed and competing with B-MeO-9-BBN for the alkyl-lithium reagent prior to its complete consumption. The complex is stable for alkenyl and alkynyl derivatives which require... 

BF3 Et20 to remove the methoxy moiety. The procedure has also been used for the preparation of cA-vinyl-9-BBN derivatives since the normal route to such derivatives based upon the hydroboration of 1-haloalkynes, followed by hydride-induced rearrangement gives ring expansion products competitively with (Z)-l-halovinyl-9-BBNs. " Similar behavior has been observed for the reaction of a-methoxyvinyllithium with B-alkyl-9-BBNs (see 1-Methoxyvinyllithium) ... 

While the protonolysis of B-alkyl-9-BBNs, like other trialkylb-oranes, with carboxylic acids takes place only at temperatures above 100 °C, B-vinyl derivatives are readily cleaved by HOAc at 0 °C with complete retention of configuration. This can be combined with other 9-BBN processes (e.g. thermal isomerization or dehydroborylation) to give remarkable overall conversions (eq 23). The hydrolysis of allylic and alkynic 9-BBN derivatives is more facile, occurring even with water or alcohols. ... 

Part 1 (Formation and activation of B-alkyl-(9-BBN) reagents) To a suspension of 9-BBN dimer (4.6 g, 18.8 mmol, 1.0 equiv) in i-Pr20 (10 ml) was added allylbenzene (4.4 g, 5.0 ml, 1.0 equiv) in a N2-filled glovebox. The reaction mixture was then heated at 60 °C for 1.5 h outside of the glovebox, cooled down to room temperature, and diluted with i-Pr20 to produce a 1.5 M solution (total volume 25 ml) of the alkylborane reagent. 

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