[121758-17-6]  · C28H26BBrN2O4S2  · (MW 609.36)

(chiral controller group for asymmetric carbonyl allylations,1-8 allenations,9 and propargylations,9 enantioselective Claisen rearrangements,10,11 and enantioselective enolborinations12)

Alternate Name: 2-bromo-4,5-diphenyl-1,3-bis-(toluene-4-sulfonyl)-[1,3,2]diazaborolidine.

Solubility: soluble in dichloromethane.

Form Supplied in: not commercially available.

Preparative Methods: prepared from the corresponding bissulfonamide, (R,R)-1,2-diphenyl-1,2-diaminoethane-N,N´-bis(4-methylbenzenesulfonamide)13-16 and BBr3 in dichloromethane. After drying under high vacuum (0.1 mmHg) overnight (8-16 h) at 80-100 °C in a Schlenk flask, the sulfonamide (1.4 equiv) is dissolved in dichloromethane (0.1 M), and cooled to 0 °C. Care should be taken during drying, as temperatures above 100-110 °C produce a brownish-colored material which is insoluble in dichloromethane at 0.1 M and ineffective for chemical transformations. BBr3 (1.0 M in dichloromethane; 1.4 equiv) is added, the mixture is stirred at 0 °C for 10 min, warmed to room temperature, and stirred for 1 h. The solvent and HBr are then carefully removed under high vacuum, kept at room temperature under high vacuum for 15 min after all solvent has been removed, and the white to tan residue is then redissolved in dichloromethane (0.1 M). This evaporation-redissolution procedure is repeated two additional times, giving a 0.1 M solution of 1 in CH2Cl2 dichloromethane.7

Handling, Storage, and Precautions: highly moisture-sensitive; should be prepared immediately prior to use under inert atmosphere, preferably using standard Schlenk techniques (use of a glove box not required). Best results are obtained when fresh solutions of BBr3 are used.

Carbonyl Allylations

Reagent 1 was originally introduced by Corey in 1989 for the asymmetric allylation of aldehydes with tri-n-butylallylstannane,2 as previously reviewed.1 Subsequent studies have demonstrated the utility of this reagent for the stereocontrolled generation of complex homoallylic alcohols via the convergent coupling of various functionalized, C2-symmetric allylstannanes and substituted aldehydes.4-8 The absolute stereochemistry of the newly formed alcohol stereocenter is predictable using a Zimmerman-Traxler model, and product formation generally is governed by the absolute stereochemistry of 1 (1).

In situ transmetallation of the starting allylstannane to an intermediate allylic borane is rationalized via a 1,3-transposition pathway. In reactions with chiral aldehydes, matched and mismatched diastereotopic pathways are possible based upon the asymmetry of 1, and the intrinsic face selectivity exhibited for the carbonyl addition process. Yields are generally high (85-99%) with good to excellent stereoselectivity. Numerous functional groups are tolerated in the starting allylstannane, including esters, silyl and benzyl or para-methoxybenzyl ethers, dithioketals, and vinylstannanes. Lewis acid sensitive functionalities (acetals, ketals, tetrahydropyranyl ethers) are not compatible. The aldehyde component may contain a wide variety of common protecting groups and additional functionality, including basic heteroaromatic systems such as pyridines and oxazoles.

Reactions of achiral aldehydes and homochiral stannanes exhibit stereoselectivity which is predominantly dictated by the chiral auxiliary 1 if the pre-existing asymmetry of the stannane is located at least two carbons or more (b) from the reactive allyl unit (eqs 2-4).4,5

Achiral stannanes undergo reactions with aldehydes bearing a-asymmetry, and provide examples of matched diastereoselectivity with respect to 1 (5),7 as well as cases of mismatched diastereoselection of these controlling factors (6).4

In a similar fashion, asymmetric allylations with 1 and chiral aldehydes bearing b-substitution also display the expected behavior of diastereotopic transition states (eqs 7 and 8).4

The presence of a-asymmetry in the stannane component can have a dramatic impact on diastereoselection (9).4 The minimization of A1,3 strain in the allylic component is a factor that influences the face selectivity enforced by the auxiliary 1.

In complex examples, high levels of stereodifferentiation require the consideration of the conjoined influences of a-asymmetry in the allylstannane, and chirality of the starting aldehyde, in addition to the choice of auxiliary 1 (10).4,8

Claisen Rearrangements

Claisen rearrangements of catechol allylic ethers, which avoid production of the ‘abnormal’ Claisen product, have been achieved using 1.5 equiv 1 and 1.5 equiv Et3N at low temperature in dichloromethane with excellent (80-97%) yields and high (86-95%) enantioselectivities (eqs 11 and 12). The absolute configuration of the newly created benzylic stereocenter is dependent upon both the olefin geometry and the configuration of the controller. Lewis acid catalysis with (S,S)-1 and E-olefins led to vinylic substituents bearing the S configuration (11), whereas (S,S)-1 and Z-olefins yielded products with R stereochemistry (12).10

Similarly, Claisen rearrangements of difluorovinyl allyl ethers occurred with moderate to excellent yields (39-90%) and moderate enantioselectivities (13). Simple alkyl-substituted olefins rearrange at -15 °C with modest stereocontrol (41-56% ee) whereas vinylsilanes rearrange at -78 °C with good (85% ee) selectivity. The absolute configuration of the newly formed benzylic stereocenter appears to depend upon both the geometry (E or Z) of the starting olefin as well as the configuration of 1, although the absolute stereochemistry of the product was proven only in the case cited below.11

Other Uses

Reagent 1 has been used for enantioselective enolborination, albeit with poor (1.1:1) selectivity.12 Similar bis-sulfonamide-derived boron Lewis acids have been used for aldol additions,17-23 ester-Mannich reactions,24 Diels-Alder reactions,13,25,26 Ireland-Claisen reactions,27,28 and [2,3]-Wittig rearrangements.29,30 Similar bis-sulfonamide-derived aluminum Lewis acids have been used for aldol additions,13 Diels-Alder reactions,13,31-34 [2 + 2] ketene-aldehyde cycloadditions,35,36 cyclopropanation of allylic alcohols,37-39 and polymerization.40,41

Related Reagents.

Boron-bissulfonamide Lewis acids: (R,R)-1,3-bis{[3,5-bis(trifluoromethyl)phenyl]sulfonyl}-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis(methylsulfonyl)-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis[(trifluoromethyl)sulfonyl]-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis(phenylsulfonyl)-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis[(4-fluorophenyl)sulfonyl]-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis[(4-nitrophenyl)sulfonyl]-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis(2-naphthalenylsulfonyl)-2-bromo-4,5-diphenyl-1,3,2-diazaborolidine; (R,R)-1,3-bis(phenylsulfonyl)-2-bromooctahydro-1H-1,3,2-benzodiazaborole; (R,R)-1,3-bis[(4-methylphenyl)sulfonyl]-2-bromooctahydro-1H-1,3,2-benzodiazaborole. Aluminum-bissulfonamide Lewis acids: (R,R)-[N,N´-(1,2-diphenyl-1,2-ethanediyl)bis(1,1,1-trifluoromethanesulfonamidato)](2-)-N,N´methylaluminum; (R,R)-{[N,N´-(1,2-diphenyl-1,2-ethanediyl)bis(1,1,1-trifluoromethanesulfonamidato)](2-)-N,N´}(2-methylpropyl)aluminum; (R,R)-{[N,N´-(1,2-diphenyl-1,2-ethanediyl) bis(4-methylbenzenesulfonamidato)](2-)-N,N´}chloroaluminum; (R,R)-{[N,N´-(1,2-diphenyl-1,2-ethanediyl)bis[3,5-bis(trifluoromethyl)benzenesulfonamidato]](2-)-N,N´}ethylaluminum; (S,S)- [N,N´-(1,2-diphenyl-1,2-ethanediyl)bis[2,4,6-trimethylbenzenesulfonamidato](2-)-N,N´]methylaluminum; (S,S)-[N,N´-(1,2-diphenyl-1,2-ethanediyl)(2,4,6-trimethylbenzenesulfonamidato)-2,4,6-tris(1-methylethyl)benzenesulfonamidato(2-)-N,N´]methylaluminum; (S,S)-{[N,N´-(1,2-diphenyl-1,2-ethanediyl)bis[4-(1,1-dimethylethyl)-2,6-dimethylbenzenesulfonamidato]](2-)-N,N´}methylaluminum; (S,S)-{N,N´-(1,2-diphenyl-1,2-ethanediyl)[(4-(1,1-dimethylethyl)-2,6-dimethylbenzenesulfonamidato]-2,4,6-tris(1-methylethyl)benzenesulfonamidato(2-)-N,N´}methylaluminum; (R,R)-{(N,N´-(1,2-diphenyl-1,2-ethanediyl)bis[2,4,6-tris(1-methylethyl)benzenesulfonamidato])(2-)-N,N´}ethylaluminum; (S,S)-{[N,N´-[1,2-bis(3,5-dimethylphenyl)-1,2-ethanediyl]bis(1,1,1- trifluoromethanesulfonamidato)](2-)-N,N´}methylaluminum; (R, R)-{[N,N´-1,2-cyclohexanediylbis(1,1,1-trifluoromethanesulfonamidato)](2-)-N,N´}methylaluminum; (R,R)-{[N,N´-1,2-cyclohexanediylbis(benzenesulfonamidato)](2-)-N,N´}(2-methylpropyl) aluminum; (R,R)-{[N,N´-1,2-cyclohexanediylbis(4-nitrobenzenesulfonamidato)](2-)-N,N´}methylaluminum; (R,R)-{[N,N´-1,2- cyclohexanediylbis(4-nitrobenzenesulfonamidato)](2-)-N,N´} ethylaluminum; (R,R)-{[N,N´-1,2-cyclohexanediylbis(4-nitrobenzenesulfonamidato)](2-)-N,N´}(2-methylpropyl)aluminum; (R,R)-{(N,N´-1,2-cyclohexanediylbis[4-(trifluoromethyl)benzenesulfonamidato])(2-)-N,N´}(2-methylpropyl)aluminum; (R,R)-{(N,N´-1,2-cyclohexanediylbis[3,5-bis(trifluoromethyl)benzenesulfonamidato])(2-)-N,N´}(2-methylpropyl)aluminum. Other chiral controllers for allylation: (R)-[(1,1´-binaphthalene)-2,2´-diolato(2-)-κO, κO´]dichlorotitanium; (R)-[(1,1´-binaphthalene)-2,2´-diolato(2-)-κO, κO´]bis(2-propanolato)titanium; (R)- [(1,1´-binaphthalene)-2,2´-diolato(2-)-κO, κO´]bis(2-propanolato)zirconium; (R)-[(1,1´-binaphthalene)-2,2´-diylbis(diphenylphosphine-κP)]trifluoromethanesulfonato-κO-silver; chloro(h5-cyclopentadienyl)[(4R, trans)-2,2-dimethyl-a,a,a´,a´-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2-)-Oa,Oa´]titanium; 2,2-dimethyl-a,a,a´,a´´-tetraphenyl-1,3-dioxolane-4,5-dimethanolatotitanium diisopropoxide; chloro(cyclopentadienyl)bis[3-O-(1,2:5,6-di-O- isopropylidene-a-D-glucofuranosyl)]titanium; {2,2´-methylenebis[(4S,5R)-4,5-dihydro-4,5-diphenyloxazole-κN3]}bis(trifluoromethanesulfonato-κO-zinc; aqua{2,6-bis[(4S)-4,5-dihydro-4-(1- methylethyl)-2-oxazolyl-κN3]phenyl-κC}dichlororhodium; (S,S)-[2,6-bis(1-methylethoxy)benzoyl]oxy-5-oxo-,3,2-dioxaborolane-4-acetic acid; B-methoxydiisopinocampheylborane; 1,3,2-benzodioxastannol-2-ylidene complex with diisopropyl tartrate; 2,2,2-trifluoro-N-[(1R, 2R)-1-methyl-2-phenyl-2-[(trimethylsilyl)oxy] ethylacetamide; (R,R)-octahydro-1,3-dimethyl-2-(1-piperidinyl)-1H-1,3,2-benzodiazaphosphole-2-oxide.

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David R. Williams & David C. Kammler

Indiana University, Bloomington, Indiana, USA

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