4.4.5.5- Tetramethyl-l,3,2-dioxaborolane

Methoxy-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane Typical Procedure23 ... 

A mixture of 1.97 g (10 mmol) of (Z)-2-(3-methoxy-2-propenyl)-4,4,5,5-tetramethyl-l, 3,2-dioxaborolane and 755 mg (13 mntol) of propanal is stirred without solvent for 2d at r.t., and then for 3 h at 50LC. Residual aldehyde is removed in vacuo then the residue is dissolved in 3 inL of CH,C1, and treated with 1.5 g (10 mmol) of triethanolamine for 12 h. The mixture is filtered, concentrated and bulb-to-bulb distilled from a bath at 50 CC/0.01 Torr, and the distillate is chromatographed over silica gel eluting with CI1,C12 yield 1.22 g (94%) d.r. synjanti) 92 8. 

Several detailed studies of reactions of achiral aiiylboronates and chiral aldehydes have been reported4,52 - 57. Diastereofacial selectivity in the reactions of 2-(2-propenyl)- or 2-(2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes with x-methyl branched chiral aldehydes are summarized in Table 252, 53, while results of reactions with a-heteroatom-substituted aldehydes are summarized in Table 34,52d 54- 57. 

Excellent double diastereoselection has also been realized in the reactions of (7 )-2,3-[isopro-pylidenebis(oxy)]propanal and chiral 2-butenylboron reagents (Table 8). The best selectivity for the (3R,4R)- and (SS /Q-diastereomers was obtained by using the tartrate ( )- and (Z)-2-butenylboronates. (S.S -D and (R,R)-D, respectively69,81, while (E)- and (Z)-2-butenyl-2,5-dimethylborolane reagents (R,R)-C and (S,S) C provided the greatest selectivity for the (3S, 45)- and (3y ,4S )-diastereomers< 9. Comparative diastereoselectivity data for reactions with the achiral (E)- and (Z)-2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes have also been provided in the table. 

Racemic l-methyl-2-butenylboronates (E)- and (Z)-3 may be prepared selectively via reactions of the l-methyl-2-butenyl Grignard reagent with the appropriate borate ester. Use of triisopropyl borate provides a 96 4 mixture of (E)-3l(Z)-3 on a 0.36 mol scale15. Use of a bulkier borylating agent, such as 2-isopropyloxy-4,4,5,5-tetramethyl-l,3,2-dioxaborolane, reverses the selectivity, enabling a 91 9 mixture of (Z)-3/( )-3 to be obtained on a 0.5 mol scale. The diastereomeric purity of this mixture may be enhanced to 95 5 by treatment with 0.15 equivalents of benzaldehyde, since ( )-l-mcthyl-2-butenylboronatc ( )-3 is more reactive than (Z)-3. Repetition of this process provides (Z)-3 that is 98% isomerically pure. 

Studies have established that the partition between transition states 3 and 4 depends on the nature of the diol unit bound to boron and on the steric and electronic effects of the a-sub-stituent X23. The data shown below demonstrate that the reactions of2-(l-methyl-2-propenyl)-4,4,5,5-tetramethyl-l,3,2-dioxaborolane proceed with a moderate preference for transition state 3 with the C2 methyl group in an axial position. Selectivity diminishes with 2-(l-methyl-2-propenyl)-l,3,2-dioxaborolane and reverses with dimethyl (l-methyl-2-propenyl)boronale, suggesting that steric interactions (gauche interactions in the case of the tetramethyl-1,3,2-diox-aborolane) between X and the diol unit on boron are capable of destabilizing transition state 4 relative to 3. 

B. 3-(4,4,5,5-Tetramethyl-[l,3,2]dioxaborolan-2-yl)pyridine. A 250-mL, one-necked, round-bottomed flask equipped with a magnetic stirbar and a Dean-Stark trap fitted with a condenser capped with a nitrogen inlet adaptor is charged with tris(3-pyridyl)boroxin-0.85 H20 (3.0 g, 9.1 mmol), pinacol (4.07 g, 34.4 mmol) (Note 6), and 120 mL of toluene. The solution is heated at reflux for 2.5 hr in a 120°C oil bath. The reaction is complete when the mixture changes from cloudy-white to clear. The solution is then concentrated under reduced pressure on a rotary evaporator to afford a solid residue. This solid is suspended in 15 mL of cyclohexane (Note 7) and the slurry is heated to 85°C, stirred at this temperature for 30 min, and then allowed to cool slowly to room temperature. The slurry is filtered, rinsed twice using the mother liquors, washed with 3 mL of cyclohexane, and dried under vacuum to afford 4.59 g (82%) of 3-pyridylboronic acid pinacol ester as a white solid (Note 8). 

The direct Suzuki coupling of 2,6-dibromo-DTT 97 with (4,4,5,5-tetramethyl-l,3,2-dioxaborolan-2-yIIS - -hexy 1-2,2 -hi thiophene 150 and (4,4,5,5-tetramethyl-l,3,2-dioxaborolan-2-yl)-9,9-dimethyl-fluorene 152 gave... 

Preparation of 2- 5,-[6-(t-butyl-diniethyl-silanyloxy)-hexyl]-[2,2 ]bithiophe-nyl-5-yl -4,4,5,5-tetramethyl-[l,3,2]dioxaborolane... 

The step 3 product (52.3 mmol) was dissolved in 200 ml THF and then cooled to -78°C and treated with 22.0 ml of 2.5 M in hexane -butyl lithium. The mixture was slowly warmed to — 30° C and stirred for 15 minutes and then recooled to — 78°C and treated with 2-isopropoxy-4-4,4,5,5-tetramethyl-l,3,2-dioxaborolane (61.3 mmol). The solution was warmed to ambient temperature and stirred overnight after which a white precipitate formed. The mixture was then treated with 300 ml apiece of CH2C12 and water and the organic layer collected. The aqueous layer was washed three times with 100 ml CH2C12 and the combined organic layers washed twice with 150 ml brine, dried over MgS04, filtered, and concentrated. The crude white solid was triturated with cold methanol, filtered, washed with cold methanol, and 26.2 g product isolated. 

Ad = adamantyl Ar = aryl [BarF] = [B 3,5-(CF3)2C6 H3 4] bipy = 2,2 -dipyridyl COD = 1,5-cyclooctadiene CVD = chemical vapor deposition DAB =1,4 diazabuta-diene DBU = l,8-Diazabicyclo[5.4.0]undec-7-ene DIPP = 2,6-j-Pr2C6H3 DMF = dimethylformamide DMSO = dimethylsulphoxide,OSMe2 dppe = 1,2-diphenylphosphino-ethane dppf = l,l -bis(diphenylphosphino)ferrocene dppm = diphenylphos-phinomethane dppz = dipyrido[3,2-a 2, 3 -c]phenazine FTIR = Fourier transform infrared spectroscopy HBpin = pinacolborane, 4,4,5,5-tetramethyl-l,3,2-dioxaborolane HDS = hydrodesulfurization HMPA =... 

Furthermore, Masuda et al. have shown that dialkoxyhydroborane can replace diboron 55 in converting aryl halides and triflates into arylboronates [30]. In one example, treatment of 2-methoxyquinolinyl-8-triflate with 4,4,5,5-tetramethyl-[l,3,2]dioxaborolane in the presence of PdCl2 (dppf) and Et N in dioxane at 80°C for 4 h produced 2-methoxy-... 

Rh(Tp )(COD)] has been employed as catalyst in the dehydro-genative coupling of 4,4,5,5-tetramethyl-l,3,2-dioxaborolane with arene. Several pyrazolylborates have been employed and it has been found that the catalytic activity is affected by substituents on the pz... 

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