Dichloroboranes

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Although dichloroborane reacts direcdy with alkenes in the gas phase (118), its complexes with diethyl ether and dimethyl sulfide are so strong that direct hydroboration does not proceed (119,120). The addition of a decomplexing agent, eg, boron trichloride, is necessary for hydroboration. 

A/-Trimethoxybora2ines are available from reaction of dichloroboranes and 0-methyl-X,X-his(trimethylsilyl)hydroxylamine (eq. 31). The B-trichloro-bora2iQes undergo substitution reactions at the B atoms to give B-tri(/ f -butoxy)- or B-tri(/ f2 -but5i)-A/-trimethoxybora2iaes (101)... 

Chiral imines derived from 1-phenylethanone and (I. Sj-exo-l, 7,7-trimethyIbicyclo-[2.2.1]heptan-2-amine [(S)-isobornylamine], (.S>1-phenylethanamine or (R)-l-(1-naphthyl) ethanamine are transformed into the corresponding (vinylamino)dichloroboranes (e.g., 3) by treatment with trichloroborane and triethylamine in dichloromethane. Reaction of the chiral boron azaenolates with aromatic aldehydes at 25 "C, and subsequent acidic hydrolysis, furnishes aldol adducts with enantiomeric excesses in the range of 2.5 to 47.7%. Significantly lower asymmetric inductions are obtained from additions of the corresponding lithium and magnesium azaenolates. Best results arc achieved using (.S )-isobornylamine as the chiral auxiliary 3. 

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) ... 

The observation that l,3,5-trimethyl-2,4,6-trichloroborazine yields some dimeric (dichloromethyleneamino)dichloroborane upon photochemical chlorination 30> (c.f. Eq. (7)) prompted the synthesis of this compound by the reaction of BC13 with C1SNCC12 2I> (c.f. Eq. (7)) and also by the 1,2-addition of trichloroborane to chlorocyanide 23> as depicted in Eq. (15). 

In some cases monomerization of dimeric iminoboranes (e.g., trichloro-methylchloromethyleneamino)dichloroborane and (trichloromethylbromo-methyleneamino)dibromoborane) can be observed on dissolving the materials in chlorinated saturated hydrocarbons. These compounds are monomeric in the gas phase and consequently have an unexpectedly low boiling point 24>. 

Preparations of (dichloromethyleneamino)dichloroborane have been mentioned earlier (c.f. Eq. (7)). 

Replacement of halogen in dimeric iminoboranes containing tetracoordin-ated boron is not so readily accomplished. For example, bis[(dichloromethyl-eneamino)dichloroborane]reacts with C4H9MgCl in nonetheric solvents to yield dimeric (dichloromethyleneamino)butylchloroborane, whereas C4H9Ii replaces both boron-bonded chlorine atoms by butyl groups 25). 

Monomeric and dimeric species obtained from the reaction of alkylthio-cyanates with tris(phenylthio)borane have not been isolated in a pure state 32). The reaction of (organothio)dichloroboranes with nitriles as illustrated by... 

Dichloroborane, 4 141 Dichloroborane- 1,4-dioxane, 23 638 Dichloroborane complexes, 23 638 Dichlorobutadiene, 19 838... 

The procedure has been extended to the synthesis of 7V,7V -substituted diamines 166 from the diazides 164 and dichloroboranes 165177. Primary amines protected by the t-butoxycarbonyl group are obtained by the action of trialkylboranes R3B (R = Bu, s-Bu, CsHn or cyclohexyl) on the lithium or potassium salt of t-butyl 7V-(tosyloxy)carbamate (equation 62)178. 

Reilly and Oh explored the asymmetric induction of chiral catalysts derived from bis(dichloroborane) 397 in the cycloaddition of cyclopentadiene with a-bromoacrolein and methacrolein. /V-Tosyltryplophan (394) and chiral diols 395 and 396 were employed as chiral ligands246,247. The application of chiral iV-tosyltryptophan afforded the best results (equation 118, Table 22). 

After some painstaking experimentation. Song was finally successful in a re-giospecific p o-displacement of one of the silyl groups with boron trichloride, furnishing presumably a dichloroborane (Scheme 29). The dichloroborane was not isolated and was hydrolyzed directly with either a dilute acid or a dilute... 

A-Ethoxycarbony 1-1H-azepine is converted rapidly (5 min) and quantitatively into ethyl N -phenylcarbamate by boron trifluoride (81JCS(P1)447). The dichloroborane (15 R1 = H, R2 = BCl2) results from the action of boron trichloride on 10,11-dihydro-5//-dibenz[f>,/]azepine (74IC2783). 

Apparently chlorine and trifluoromethanesulfonate groups attached to boron and silicon can be replaced by fluorine in lithium fluoride at lower temperatures than that reported for C-X (X = halogen) exchange. Dichloroborane 5 is converted to the difluoro derivative 6 with lithium fluoride in diethyl ether at 0-22°C.7... 

Novel bidentate chiral Lewis acids derived from 1.8-naphthalenediylbis(dichloroborane) and modified amino acids as chiral auxiliary have been successfully utilized as effective catalysts for the asymmetric Diels-Alder reaction of a,[ -unsaturated aldehydes. The enantioselectivity is highly sensitive to the kind of chiral amino acids. Moderate enantioselectivity was obtained with the tryptophan-derived ligand for the endo adduct, but amino acids without aromatic groups... 

The system B2H6-BC14 has also been studied in ethers as solvent by infrared and1 B NMR spectroscopy (32,227). Only dichloroborane etherate has been detected as sole redistribution product. 

Cowley and coworkers have reported two other reactions. One is the reaction of the silylphosphide 11a with dichloroborane, which afforded the diphosphadiboratane, resulting in the formation of the phosphaborene by flash-vacuum pyrolysis (equation 101)135. The other is the reaction of the silylphosphide 11a with ( j5-C5H4Me)2WCl2, which gave compounds 14 and 15 (equation 102)136. A phosphinidene tungsten complex was assumed to be a reactive intermediate. 

The synthesis of secondary amines from azides is efficient in terms of chemos-electivity [57] and has found valuable applications in the preparation of diamines [58,59], m-alkylaminoboronic esters [60], and in Diels-Alder-based amination reactions [61]. A convenient general route to open-chain polyamines, which play major roles in cellular differentiation and proliferation, has also been developed using the reductive alkylation of aliphatic aminoazides by (co-halogenoalk-yi)dichloroboranes as a key step [62] (Scheme 21). 

Gabriel and Wenker type syntheses are thoroughly viable for IV-alkylaziridines, and diere are examples of aziridination of alkenes, via vic-dihalides etc., where by use of a chiral amine a kinetic induction of chirality in the aziridine nucleus has been achieved. 2-Iodoalkyl aades react with alkyl and aryl dichloroboranes, forming aziridines (Scheme 17). Sequences resembling the Wittig and Wittig-Hor-... 

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