1.3.2- Dioxaborolane, 2- -4,4,5,5-tetramethyl

Methyl 4-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethyl)benzoate (7c) Colorless oil, reaction time 24 h yield 77%... 

Z)-2-(2-hulenyl)-4,4,5,5-tetramethyl-i,3,2-dioxaborolane-, yield 78% isomeric purity via fluorodimethoxyborane route4 97% isomeric purity via the triisopropyl borate route16 >99%. 

Tetramethyl-2-(3-trimethylsilyl-2-propenyl)-l,3,2-dioxaborolane Typical Procedure2 11 ... 

Z)-4,4,5,5-tetramethyl-2-[3- 2-trimethylsilylethoxy)-2-propenyl -1,3,2-dioxaboroUme yield 45% (Z)-2-[3-(tetralnclr -2H-pyran-2-y/oxy)-2-propenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane yield 52%. 

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

A second powerful route to functionalized allylboron compounds involves the reaction of an a-haloalkylboronatc and a vinyl organometallic reagent3 4-28-29, 50c-92 04. This method is especially useful for the preparation of allylboron compounds not accessible via the allylorganometal-lic route. Notable examples that fall into this category are ( )-4,4,5,5-tctramethyl-2-[4-(tetrahy-dro-2//-pyran-2-yloxy)-2-butenyl]-l,3,2-dioxaborolane (yield 41 %, 93% E) and (E)- or (Z)-2-(l,l-dimethyl-2-butenyl)-4,4,5,5-tetramethyl-1.3,2-dioxaborolane (yield 77-84%. 98% E or 93% Z). 

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. 

Relatively few studies of the reactions of allylboron compounds and ketones have appeared. Ketones are less reactive than aldehydes, and as a result these reactions tend to be much slower and often less diastereoselectivc. The reaction of (Z)-4,4,5,5-tetramethyl-2-[3-(tctrahy-dro-2/A-pyran-2-yloxy)-2-propenyl]-1,.3,2-dioxaborolane and ethyl 2-oxopropanoate, for example, was conducted under 6 kbar pressure at 45 C for 80 hours to give a 9 1 mixture of syn-and antz-diastereomers of 1 in 85% yield49. 

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. 

A — 55 C solution of [(trimethylsilyl)chloromethyl]lithium (theoretically 51 mmol prepared from 51 mmol of (chloromethyl)trimethylsilanc, 56 mmol of. vet-butyllithium and 56 mmol of TMF.DA in 68 mL of TIIF) is treated with 7.47 g (49 mmol) of 4,4.5,5-tetramethyl-2-(2-propeny))-l,3,2-dioxaborolane. The mixture is cooled to — 78 C and then allowed to warm to r.t. overnight. 50 mL of ice-cold 2 M hydrochloric acid are added and the mixture is extracted with three 60-mL portions of diethyl ether/CFI2CI2 (5 1). The extracts are concentrated and the residue distilled yield 6.36 g (54%) bp 52-55 rC/0.7 Torr. 

To a stirred solution of 5.70g (21.1 mmol) of 4,4.5,5-tetramethyl-2[(5)-(A)-3-(trimethylsilyloxy)-l-bulenyl]-l, 3,2-dioxaborolane in 130 mL of petroleum ether (bp 40-60 C) are added ca. 20 mg of cobalt(II) nitrate hexahydrale followed immediately by 2.75 g (23.1 mmol) of freshly distilled thionyl chloride. Slow evolution of sulfur dioxide ceases after 4h. The mixture is then filtered and concentrated in vacuo at r.t. to give crude 15, which is taken up in 50 mL of petroleum ether and washed with 30-mL portions of buffer (pH 5) until the pH is constant. 100 mL of brine are added to the organic phase and the pH is adjusted to 7 by addition of sat. aq NaHCO,. The pH should not exceed 7, otherwise decomposition ensues. The phases are separated and the organic phase is dried with MgS04 and concentrated at r.t. to give 15 yield 4.39 g (96%) Contact of 15 with metal surfaces should be avoided. 

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. 

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