Dichloroboranes, reactions

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). 

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... 

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... 

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). 

Synthesis of alkyldichloroboranes. Dichloroborane diethyl etherate reacts with olehns very slowly in diethyl ether or TH F. However, addition of boron trichloride (Lewis acid) results in a rapid reaction at 0° to give alkyldichloroboranes in 80-90% yield ... 

Dichloroborane and monochloroborane etherates or their methyl sulfide complexes have been prepared by the reaction of borane and boron trichloride [44]. The hydroboration of alkenes with these borane reagents is, however, usually very slow because of the slow dissociation of the complex. Dichloroborane prepared in pentane from BCI3 and trimethylsilane is unusually highly reactive with alkenes and alkynes hydroboration is instantaneous at -78 °C (Eq. 20) [45]. 

Asymmetric catalysis by bimetallic catalysts is currently a research field of great interest. There have been few studies of the interaction between bidentate Lewis acids and carbonyl groups [22a]. Reilly and Oh [22c] have reported the only example of such an asymmetric Diels-Alder reaction catalyzed by a 1 1 complex of N-tosyltrypto-phan [17-19] with l,8-naphthalenediylbis(dichloroborane), in which the two Lewis-acidic sites work in a cooperative manner (Eq. 22). 

The reaction between SiH, and BClj, which should yield diborane(6) according to bond energy calculations, occurs at 150°C to afford dichloroborane ... 

Kotz and Post have reported an interesting reaction of B2CI4 with ferrocene, in which the principal products are BCI3, hydrogen, and dichloro-ferrocenylborane 61, 62). Traces of dichloroborane were obtained, and this compound was found to react also with ferrocene to yield hydrogen, dichloroferrocenylborane, and some BjH, . These results were interpreted in terms of an initial reaction of B2CI4 with ferrocene. 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

Bayer and Siebert reported the reaction of 3,3-dimethyl-l-butyne with -BuLi in pentane, followed by the addition of boron trichloride to afford the intermediate dichloro(3,3-dimethylbut-l-ynyl)borane that was hydroborated with dichloroborane formed in situ to give l,l-bis(dichloroboryl)-3,3-dimethylbutene 51 (Scheme 6) <2002ZN1125>. When the latter was subjected to halogen exchange with boron triiodide, it was converted into the corresponding tetraiodide 52, which underwent a redox reaction with hex-3-yne to give the 2-(2,2-dimethylpropylidene)-l,3-diborole 22 in 73% yield. 

Ground-state oxygen alone rarely oxidizes organic compounds. A classical example is the autoxidation of benzaldehyde to benzoic acid, a usually undesirable reaction that takes place even in the absence of light. Other examples of autoxidation without illumination are oxidations at the a positions with respect to aromatic rings or at tertiary carbons [47, 48, 49, 50] and the formation of alkyl hydroperoxides from alkyl dichloroboranes [57]. Some oxidations take place when a compound is treated with oxygen in the presence of bases [9, 52, 53]. 

Sulfoxides (1) may be oxidised to sulfones by treatment with hydrogen peroxide (in acetic acid) or peroxycarboxylic acids (see Chapter 10, p. 195). Sulfoxides can also be reduced to sulfides by powerful reducing agents, e.g. lithium aluminium hydride. The reduction can also be achieved by reaction with dichloroborane (13) in THF at 0°C without affecting other functional groups like C02R, COCI, CN or N02, and even side reactions with aldehyde and ketone groups are relatively slow (Scheme 8). 

Organoboranes possessing one B—C bond in the molecule can also be obtained by hydroborating alkenes and alkynes with heterosubstituted borane derivatives, especially dihalogenoboranes, which can be stabilized by complexation with amines, phosphines, ethers and sulfides. The last two are useful complexing agents for both the preparation and the reactions of dihalogenoboranes . The most convenient synthesis of dichloroborane etherate is the reaction of LiBH. with BCl, in Et,0 ... 

The attachment of one alkenyl group to boron can be achieved by the hydroboration of alkynes with dihalogenoboranes or 1,3,2-benzodioxaborole. Dichloroborane reacts sluggishly with alkynes in EtjO or THF and the reaction is accompanied by disproportionation. However, in pentane the addition of BCI3 generates the nascent HBCI2 and the reaction proceeds to completion in 1 h at RT. Alkenyldichloroboranes are isolated by distillation in high yields ... 

Secondary amines from azides. The reaction of alkyldichloroboranes with azides leads to secondary amines directly. This process has been incorporated into a polyamine synthesis. Note that the dichloroboranes can be prepared from organobis (diisopropylamino)boranes. ... 

Chloroboranes are another class of heteroatom-substituted boranes. Dichloroborane (CI2BH) and mono-chloroborane (CIBH2) are formed by the reaction of diborane with varying proportions of boron trichloride etherate (BCl3 OEt2). In ether solvents, both reagents are formed as the etherate.23 An alternative synthesis was the reaction of HCl with borane THF,24 but reduction of boron trichloride with lithium borohydride in ether proved to be the most useful route.23... 

Prenylation. Prenyltributylstannane is activated by conversion to the tertiary dimethyIallyl)dichloroborane on reaction with BCl,. Prenylation of ketones and indole (at C-2) is readily achieved. The method is crucial for the synthesis of tryprostatin-B. It is mportant that the dichloroborane be trapped immediately. This prevents it from attaining equilibrium, and, therefore, the generation of regioisomers (with respect to the allyl residue) - avoided. 

Reaction of (diethylamino)dichloroborane with the Grignard reagent (93) gave a mixture of cis and tram 2,5-dimethylborolanes (94) which were converted with ethereal HCl and methanol into the corresponding methoxyl derivatives (95) in 82-86% yield cis tram ratio 47 53) (Scheme 15) <85JA4549, 87USP4644075>. 

There are many other enantiomericaUy pure boron-based Lewis acid catalysts used in the Diels-Alder reaction, Amongst these the dichloroborane (8.40) used by Hawkins is noteworthy because the substrates used were enoate esters, such as methyl acrylate (8.41) ° a, 3-unsaturated ketones and acid chlorides such as (8.42) rather than the enals generally tested in Diels-Alder reactions. 

---