Alkyldichloroboranes

Most other organometallic compounds also react with oxygen. Aryllithium reagents have been converted to phenols by treatment with oxygen. Trialkylboranes and alkyldichloroboranes (RBCI2) can be conveniently converted to hydroperoxides by... 

Borazines with dialkylaminoalkyl [RgN(CH2)n ] substituents on the nitrogen sites have been described in the patent literature. 1,3,5-Tris-(p-dimethylaminophenyl)-2,4,6-trialkylborazines are prepared either by the reaction of p-dimethylaminoaniline with BCI3 followed by Grignard-alkylation 101>, or by reaction of alkyldichloroboranes with p-dimethylaminoaniline 102). The preparation of l,3,5-tris(aminoalkyl)borazines by the reaction 103>, is illustrated by Eq. (24). 

The preparation of optically active secondary amines is based on the conversion of alkenes to optically active alkyldichloroboranes and the reaction of these compounds with organic azides22. 

Di- and trisubstituted alkenes were converted to optically active alkylboronic esters (1) with 99% ee according to the procedure described in Section 1.23.3. The boronic esters were then treated sequentially with lithium aluminum hydride and hydrochloric acid to afford the required optically active alkyldichloroboranes 2 (>99% ee). The reaction of the alkyldichloroboranes with organic azides was best performed in 1,2-dichloromethane as the solvent to yield the required amines 3. Representative secondary amines containing one or two stereogenic centers, C2-sym-metrical and mew-amines have been synthesized by this route with >99% ee (Table 3)22. 

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 ... 

Hawkins et al. described a simple and efficient catalyst for the Diels-Alder reaction based on a chiral alkyldichloroborane (Eq. 15) [16]. A molecular complex between methyl crotonate and the chiral catalyst have been isolated for the first time. A study of the crystal structure of the complex enabled the authors to propose a model predicting the approach of the diene on one of the faces of the methyl crotonate, because the other face is protected by n-n donor-acceptor interactions. This secondary attractive substrate-catalyst interaction is the basis of the stereocontrol. 

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)." ... 

BF3 or with BCI3 and Me3SiH gives alkyldichloroboranes RBCI2. Extensions of this basic approach are possible with dihalo alkylboranes. The reaction of an alkene with allyl dibromoborane, incorporated an allyl group and the bom on adja-... 

Alkyldichloroboranes are oxidized by oxygen to alkyl hydroperoxides (equation 607) [51]. 

Alkoxy-1,2,4,6,8-pentaazanaph thalenes, 431 Alkoxysulfonium fluoroborates, 78 a-Alkylacrylic adds, 301-302 N-Alkylanilines, 439 Alkylaiylacetylenes, 494 Alkylation of ketones, 30,64 2-Alkyl-1,3,2-benzodioxaboroles, 25 Alkylbenzohydroxamates, 389 OAlkylbenzohydroximoyl chlorides, 389 Alkylboronic add esters, 130 4-Alkyl-2-butene-l-ols, 53 2-Alkylcyclopentenones, 484 Alkyldichloroboranes, 130 2-r- Alkyl-1,3,2-dioxaborolanes, 69 Alkylene carbonates, 41 a-Alkylhydracrylic adds, 301-302 2-Alkylidenecydobutanols, 470 Alkylidene sulfones, 61 N-Alkylindoles, 246 Alkyl iodides, 336 Alkylmagnesium fluorides, 260... 

It is evident that minimization of the degrees of freedom of the dienophile in the transition state is an important criterion for reaction selectivity. A unique catalyst system designed by Hawkins and coworkers takes advantage of two distinct binding interactions to rigidify the catalyst-substrate complex [65]. The aromatic alkyldichloroborane 13 is an effective cycloaddition catalyst for acrylate dienophiles (Scheme 15) [66] however, reports utilizing this catalyst are strictly confined to ester substrates with either cyclopentadiene or cyclohexadiene. 

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. ... 

Hydroboration and hydrosilylation. A mixture of EtjSiH and BCI3 is used for hydroboration of alkenes, leading to alkyldichloroboranes. Conjugated dienes give (Zj-alkenyltriethylsilanes on reaction with EtjSiH photochemically in the presence of Cr(CO),. 

Molecular nitrogen then acts as the leaving group in the migration step. The best results are achieved with dialkylchloroboranes or alkyldichloroboranes. " ... 

Alkyldichloroboranes (RBCI2) are easier to convert than BCI3 and will give up to 50% yield of B2H0 at 320 to 340°C with H2 under pressure over activated carbon (125). The reaction proceeds via HBCI2. 

DCBS is highly regioselective, exhibiting the directive effects similar to those observed for monochloroborane diethyl etherate. Alkyldichloroboranes are readily isolated by distillation under vacuum. Methanolysis provides the corresponding alkyl-boronic esters (eq 2). Alkenylboronic esters can be obtained from alkynes (eq 3). ... 

An intramolecular version of the reaction of alkyldichloroboranes with organic azides has been employed in a highly... 

Alkyldichloroboranes can be converted into a host of valuable organoborane intermediates via the hydridation-stepwise hydroboration procedure. The procedure enables a general synthesis of unsymmetrical dialkylboranes and totally mixed trialkylborane intermediates for the synthesis of ketones and tertiary alcohols, respectively. 

Preparation of 2°-Amines Using Alkyldichloroboranes, RBCI2. Organyl azides undergo facile reaction with alkyldichloroboranes (readily prepared via the hydroboration of alkenes with dichloroborane) to provide the corresponding secondary amines (eq 12).21... 

Cyclobutane synthesis allows introduction of substituents on the cyclobutane ring in various patterns (Scheme 8.24) [55], Allyl bromide with boron trichloride and tri-ethylsilane yields the alkyldichloroborane 103, which is converted into pinacol (3-bro-mopropyl)boronate (104) and on to the cyano derivative 105 by standard methods. Transesterification of 105 and reaction with LiCHClj was used to make 100. However, 105 can be deprotonated and monoalkylated efficiently, and transesterification then yields 106. Transesterification with DICHED and asymmetric insertion of the CHCl group furnishes 107, which cyclizes to 108 or 109 with about the same 20 1 di-astereoselection as seen with the unsubstituted intermediate 100. The pattern of substitution shown by 111 was achieved via reaction of pinacol (bromomethyl)boronate (63) with lithioacetonitrile to form 110, which underwent chain extension and substitution in the usual manner. It was necessary to construct 110 in this way because substitution of a (p-haloalkyl)boronic acid is not possible. With R = H or CH3, substituents included Me, Bu, and OBn [55]. 

The first application of the alkyltrifluoroborate salts was the conversion into alkyldihaloboranes by silyl hahdes and subsequent reaction with alkyl azides [77]. An example of a usefid synthesis was the preparation of (S)-2-phenylpyrrolidine (141) (Scheme 8.32). (S)-DICHED (3-bromopropyl)boronate (13S) was converted into the 3-azido derivative 136 at reflux temperature under phase-transfer conditions. The usual reaction with (dichloromethyl)lithium followed by phenylmagnesium bromide to form DICHED ester 137 was followed by treatment with potassium bifluoride in aqueous methanol to provide the alkyltrifluoroborate salt 138. Neither boronic esters nor alkyltrifluoroborate salts react with alkyl azides. Reaction of 138 with trimethylsi-lyl chloride yielded (S)-2-phenylpyrrolidine (141), but reaction with silicon tetrachloride proved much faster and more efficient. At first it was thought that the intermediates 139 and 140 were probably difluoroboranes in accord with literature precedent [76], but careftil reinvestigation has revealed that reaction of alkyltrifluoroborate salts with silicon tetrachloride in coordinating solvents yields alkyldichloroboranes [78]. 

Hawkins et al. have reported a simple, efficient catalyst for the Diels-Alder reaction based on a chiral alkyldichloroborane (4, Equation 21) [21]. A molecular complex between methyl crotonate and the chiral catalyst has been isolated for the first time. 

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