Dimethylborolanes

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

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. 

The reactions of disulfane with PhBCl2 or MeBBr2 in CS2 at 20 °C give the corresponding 3,5-diphenyl- and 3,5-dimethylborolanes in yields of 50% and 73% respectively [82]. 

Dimethylborolane has been shown to be effective in asymmetric reduction of ketones (Imai, T. Tamura, T. Yamamuro, A. Sato, T. Wollmann, T. A. Kennedy, R. M. Masamune, S., J. Am. Chem. Soc., 1986, 108, 7402-7404). Using the frontier orbitals of a borolane and a ketone, show the probable course of the initial interaction between the two. 

A number of optically active allylic boron compounds have been used, including380 B-allylbis(2-isocaranyl)borane (28),381 E- and Z-crotyl-(/ ,/ )-2,5-dimethylborolanes (29),382... 

Oxaborolidine, 9-borabicyclo[3.3.1]nonane, and 2,5-dimethylborolane derivatives as catalysts for the asymmetric reduction of ketones 92S605. 

As we saw in Chapter 7, one of the goals of synthetic organic chemistry is to develop methods that produce only a single enantiomer of a desired chiral compound rather than the usual racemic mixture that is the result of most reactions. Recently, a method has been developed that employs one enantiomer of a chiral borane to prepare a single enantiomer of a chiral alcohol from an achiral alkene. The chiral borane that is used is /rans-2,5-dimethylborolane. 

The stereoisomer of trans-2,5-dimethylborolane shown here has the R configuration at both stereocenters. Reaction of this chiral borane with rrans-2-butene produces 2-butanol in 71% yield. The 2-butanol is almost enantiomerically pure. It consists of 99.5% of the ( -enantiomer and only 0.5% of the (R)-enantiomer. This occurs because the transition state leading to the (50-enantiomer is much more favorable on steric grounds than is the transition state leading to the (R)-enantiomer. 

Related Reagents. (+)-S-Chlorodiisopinocampheylborane Diisopinocampheylboron Trifluoromethanesulfonate Dilongi-folylborane (/ ,/J)-2,5-Dimethylborolane B-Methoxydiiso-pinocampheylborane Monoisopinocampheylborane. 

Related Reagents. fi-Allyl-9-borabicyclo[3.3.1-nonane B-Allyldiisocaranylborane fi-Allyldiisopinocampheylborane B-Crotyldiisopinocampheylborane (f ,f )-2,5-Dimethylborolane. 

Preparative Methods a ca. 1 1 mixture of cis- and trans-B-methoxy-2,5-dimethylborolanes is first obtained by reaction of the Grignard reagent derived from 2,5-dibromohexane with diethylaminodichloroborane, followed by acidic methanolysis (eq 1). ... 

Handling, Storage, and Precautions (/ ,/ )-2,5-dimethylborolane is stored as an air- and moisture-stable complex (1) with (5)-(+)-prolinol. The (5, 5)-isomer is stored as a stable complex with either (R)-(—)-prolinol or as a complex (2) with (5)-(+)-valinol, which is more readily available. The reagent and many of its derivatives are extremely air- and moisture-sensitive, and may ignite when exposed to air. Precautions for the handling of such materials should be rigorously followed. ... 

Asymmetric Reduction of Ketones. A reagent system consisting of (/ ,/ )-2,5-dimethylborolane (1.0 equiv) and the corresponding borolanyl mesylate (0.2 equiv) reduces a variety of prochiral ketones with asymmetric induction in the range of 80-100% ee. The reagent system is prepared in situ by addition of 1.4 equiv of Methanesulfonic Acid to a solution of the lithium dihydridoborate, prepared as in eq 5 above (eq 7). 

Related Reagents. 10,2-Camphorsultam (/ )-2-t-Butyl-6-methyl-4//-l,3-dioxin-4-one (/ )-(+)-(t)-Butyl 2-(/ -Tolysulfin-yl)propionate Chloro(cyclopentadienyl)bis[3-0-( 1,2 5,6-do-0-isopropylidene-a-D-glucofuranosyl)]titanium lO-Dicyclohexyl-sulfonamidoisobomeol Diisopinocampheylboron Trifluoro-methanesulfonate (R,/ )-2,5-Dimethylborolane 2-Hydroxy-... 

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