9-Borabicyclo

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

Borabicyclo [3.3.1] nonane [280-64-8], 9-BBN (13) is the most versatile hydroborating agent among dialkylboranes. It is commercially available or can be conveniendy prepared by the hydroboration of 1,5-cyclooctadiene with borane, followed by thermal isomerization of the mixture of isomeric bicychc boranes initially formed (57,109). 

Bromo-9-borabicyclo[3.3.0]nonane (9-Br-BBN), CH2CI2, reflux, 87-100% yield.9-Br-BBN also cleaves dialkyl ethers, allyl aryl ethers, and methylenedioxy groups. 

Simple one-part acrylic compositions containing 9-borabicyclo-[3,3,l]-nonane (often called 9-BBN) cure and give good adhesion to wood [90], It has also been used to cure a reactive acrylic hot-melt [91] (Scheme 11). 

Alkyl-9-borabicyclo[3.3. IJnonanes in oxidation reactions leading to polyolefins 99JOM(581)176. 

Since borane BH3 reacts with only one or two equivalents of a sterically hindered alkene, it is possible to prepare less reactive and more selective borane reagents R2BH and RBH2 respectively. In addition to disiamylborane 8 and thexylbo-rane 10, the 9-borabicyclo[3.3.1]nonane (9-BBN) 14 is an important reagent for hydroboration, since it is stable to air it is prepared by addition of borane 2 to cycloocta-1,5-diene 13 ... 

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%). 

Allyldialkylboranes, such as 9-(2-propenyl)-9-borabicyclo[3.3.1]nonane, are generally prepared by treating a methoxydialkylborane with allylmagncsium bromide or an allylaluminum reagent89. 

Bulcnyl)-9-borabicyclo[3.3.1]nonanc is configurationally unstable and consists of a rapidly interconverting mixture of E- and Z-olefin isomers. 

The silicon- and sulfur-substituted 9-allyl-9-borabicyclo[3.3.1]nonane 2 is similarly prepared via the hydroboration of l-phenylthio-l-trimethylsilyl-l,2-propadiene with 9-borabicy-clo[3.3.1]nonane36. The stereochemistry indicated for the allylborane is most likely the result of thermodynamic control, since this reagent should be unstable with respect to reversible 1,3-borotropic shifts. Products of the reactions of 2 and aldehydes are easily converted inlo 2-phenylthio-l,3-butadienes via acid- or base-catalyzed Peterson eliminations. 

The stereochemistry of this reaction is consistent with transition state 2 in which the ethoxycar-bonyl unit adopts an equatorial position. The same result could occur, however, via boat-like transition state 3 with an axial ethoxycarbonyl group47. The reactions of 2-oxopropanoate esters and 9-(2-butenyl)-9-borabicyclo[3.3.1]nonane occur at — 78°C, reflecting the greater reactivity, but stereoselectivity is generally poor except in cases where a very hindered ester is employed3811. 

Substituted 2-butenylboranes 1 can be prepared via the metalation of 9-[(E)-2-butenyl]-9-borabicyclo[3.3.1]nonane with lithium tetramethylpiperidide (LTMP) in tetrahydrofuran at 23 "C followed by treating the resulting anion with chlorotrimethylsilane or chlorotrimethyl-slannanels. 

Finally, it has been reported that allylboranes 4 and 5 may be prepared by the hydroboration of 4-trimethylsilyl-2,3-alkadienes by using either 9-borabicyclo[3.3.1]nonane or dicyclo-... 

E)-1-me thyl-3-trimethylsily l-2-alkeny -9-borabicyclo[3 3. l]nonane 5 dicyclohexyl [( .)-1-methyi-3-trimethylsilyl-2-alkeny[ borane... 

In a second set of examples, it was shown that the stereoselectivity of the aldehyde allylborations of 9-[( T)-l-trimethylsilyl- or l-trimethylstannyl-2-butenyl]-9-borabicyclo[3.3.1]nonane is controlled to a significant extent by conversion to an ate complex by treatment with butyllithium, MT-butyllithium or pyridine19. 

The reactions of 1-trimethylsilyl- or l-trimethylstannyl-2-butenyl-9-borabicyclo[3.3.1]-nonancs in the presence of pyridine are adequately explained by the usual transition structure 20 since the aldehyde should be capable of displacing pyridine as a ligand on boron. 

To 0.89 g (4.0 mmol) of (45>,55>)-2-ethyl-4,5-dihydro-4-melhoxymelhyl-5-phenyloxazole and 0.52 g (4.0 mmol) of cthyldiisopropylaminc in 10 rnL of Et20 arc added 1.08 g (4.0 mmol) of 9-borabicyclo[3.3.1]nonyl trifluo-romethanesulfonate30 in a dropwise fashion at — 78 C under nitrogen. The mixture is stirred for 2 h at — 78 °C and 4.0 mmol of the aldehyde are added dropwise at — 78 °C. Stirring is continued at this temperature for 3 h then 20 mL of CH,OH, 9 raL of pH 7 phosphate buffer and 9 mL of 30% H202 are added sequentially. After 45 min at 0 C, workup as described for the anti-esters furnishes 4,5-dihydro-2-(2-hydroxy-l-methylalkyl)-4-methoxymethyl-5-phenyloxazoles 18 as oily products. Without further purification, the crude adducts are heated to reflux in 4.5 N sulfuric acid for 12-14 h. Isolation of the 3-hydroxy esters follows the procedure for an -3-hydroxy esters (vide supra). 

To a solution of 0.18 g (1 mmol) of /V-benzylideneaniline in F.t,0 is added 0.194 g (1.1 mmol) of 9-(2-butenyl)-9-borabicyclo[3.3.1]nonane (crotyl-9-BBN) at — 78 °C. The reaction is quenched at O C with several drops of coned [ICl. The mixture is stirred overnight at r.t.. and a 3 N aq soln of NaOH is added at 0°C to make the solution basic. The mixture is extracted twice with ht20. dried, condensed, and filtered through a short column of silica gel (hexane/Et20 10 1) to remove the 9-BBN residue. 

In contrast to ordinary chiral aldehydes (having no ability to be chelated), the reaction of 9-allyl-9-borabicyclo[3.3.1]nonane(allyl-9-BBN) with the corresponding chiral imines 4 produces the isomer syn-6 either exclusively or predominantly (Cram selectivity Table 8)5,6. The very high 1,2-asymmetric induction is explained by a six-membered. chair-like transition state, in which the inline R group occupies an axial position due to the stereoelectronic effect of imines (R CH = NR). 

The results show that modest chelation control can be obtained using organomctallics like allylmagnesium chloride, chloromagnesium allyltriethylaluminate and allylzinc bromide, which are capable of /1-chelation. In contrast to this, 9-allyl-9-borabicyclo[3.3.1]nonane (9-allyl-BBN)... 

A solution of 1 mmol of butyl (-)-(S)-2-(1-phenylethylimino)acetate in THF is cooled under N2 to —78 CC. 1.1 mmol of 9-allyl-9-borabicyclo[3 3 1 Jnonane or 1.1 mmol of allylzinc bromide, respectively, is added and the resulting mixture is warmed to r.t. over a period of 12 h. A few drops of coned HC1 are added at 0°C... 

Although dichloroborane has been successfully used to reduce dialkyl sulphoxides, it was less useful for the reduction of diaryl sulphoxides and recent work26 has found that fully substituted bromoboranes can fill this gap. Three reagents were introduced, viz. bromodimethylborane, 9-bromo-9-borabicyclo[3.3.1]nonane (9-BBN-Br) and tribromo-borane, the reactions being shown in equation (5) ... 

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