9-Borabicyclo nonane, 9-methoxy

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

To a solution of 3.5 uiL (25 mmol) of trimethylsilylacetylcne in 40 mL of THF are added dropwise, at — 78 °C, 10.4 mL of 2.5 M BuLi (26 mmol) in hexane. After stirring for 15 min at —78 X, 25 mL of 1 M 9-methoxy-9-borabicyclo[3.3, l]nonane in hexane are added. The mixture is stirred at —78 "C for a further 1.5 h. Then, 4 mL (33 mmol) of BF3 OEt2 are added and the mixture is stirred for an additional 15 min at — 78 °C before being allowed to warm to r.t. The volatiles are evaporated under reduced pressure to afford a white solid. After the addition of 25 mL of pentane, the suspension is stirred for a few minutes and allowed to settle. The supernatant liquid is decanted into a second flask via a double-ended needle. This procedure is repeated twice, each time with 10 mL of pentane. The combined pentane solution is cooled to — 78 X to precipitate the product. The mother liquor is removed and the crystals are dried under vacuum to afford an extremely hygroscopic, white crystalline material yield 6.52 g (90%). 

Other reagents used for reduction are boranes and complex borohydrides. Lithium borohydride whose reducing power lies between that of lithium aluminum hydride and that of sodium borohydride reacts with esters sluggishly and requires refluxing for several hours in ether or tetrahydrofuran (in which it is more soluble) [750]. The reduction of esters with lithium borohydride is strongly catalyzed by boranes such as B-methoxy-9-bora-bicyclo[3.3.1]nonane and some other complex lithium borohydrides such as lithium triethylborohydride and lithium 9-borabicyclo[3.3.1]nonane. Addition of 10mol% of such hydrides shortens the time necessary for complete reduction of esters in ether or tetrahydrofuran from 8 hours to 0.5-1 hour [1060],... 

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)... 

Suzuki couplings. Pd-catalyzed reactions of B-allyl borate complex derived from 6-methoxy-9-borabicyclo[3.3.1]nonane with aryl triflates give allylarenes. This technique broadens the scope of the Suzuki coupling to allow transfer of Me, TMSCHj, and alkynyl groups which has eluded conventional manipulations. 

Methoxy-9-borabicyclo[3.3.1)nonane 9-Borabicyclo(3.3.1]nonane, 9-methoxy- (9) (38050-71-4)... 

Boraadamantane (5) does not react with iodine at room temperature. However, in the presence of bases, e.g., alcoholates, the reaction rate increases dramatically. Coordination of a base at the boron atom leads to increase in the polarity of the B—C bond, facilitating its heterolytic cleavage. For example, ate-complex (94), prepared by the treatment of (5a) with lithium methoxide, reacts with iodine at — 30°C giving 3-methoxy-7a-iodomethyl-3-borabicyclo[3.3.1]nonane (95) (Scheme 35) <83JOM(246)19>. 

A few studies have been carried out on coupling of alkynyl borones. The stable methoxy(alkynyl)borate complex 158 was prepared in situ by the treatment of B-methoxy-9-borabicyclo[3.3.1]nonane (157) with alkynyllithiums or potassium reagents, and used directly for the coupling with halides to afford a variety of the disubstituted alkynes sueh as 159 [72,73]. 

B-Alkenyl- and B-l-alkynyl-9-borabicyclo[3,3,l]nonanes undergo smooth 1,4-addition to acyclic -unsaturated ketones. Cycloalk-2-enones do not yield any conjugate addition product. Furthermore the alkynylboranes react with acyclic /8-methoxy-a/8-unsaturated ketones to give conjugated enynones. ... 

Reduction of Carbonyl Groups. Rapid reduction of salts of carboxylic acids to alcohols with 2 molar equivalents of borane in tetrahydrofuran (THF) has been reported. Full details have appeared of the improved procedure for reduction of esters to primary alcohols, using borane dimethyl sulphide (6,157) in THF at reflux, which allows the dimethyl sulphide that is liberated to distil off during the reaction. Further suggested procedures for the reduction of esters to primary alcohols use lithium borohydride in diethyl ether or THF, calcium borohydride in THF, and sodium borohydride in a t-butyl alcohol-methanol mixed solvent. The reactions using lithium borohydride-diethyl ether can be catalysed by lithium 9-boratobicyclo[3.3.1]nonane (1), lithium triethylboro-hydride, or the Lewis acid 9-methoxy-9-borabicyclo[3.3.1]nonane (2). ... 

Kagan reported in 1983 the synthesis of the whole series of H—T —H via organoboranes [26d]. The oligomers can be prepared separately in acceptable yields with organoborane reagents. To a solution of Li—T —H 9, 15, 16, 28, the 9-methoxy derivative of 9-borabicyclo[3.3.1]nonane is added and a corresponding boronated thiophene 29-32 is formed [Eq. (12)]. After neutralization with boron trifluoride etherate, a second Li—T -H, which may differ from the previous one, is added to boranes 33-36 and the resulting complex 37-41 is oxidized with iodine in order to... 

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