Alcohol-boronate exchange

The most common catalysts in order of decreasing reactivity are haUdes of aluminum, boron, zinc, and kon (76). Alkali metals and thek alcoholates, amines, nitriles, and tetraalkylureas have been used (77—80). The largest commercial processes use a resin—catalyst system (81). Trichlorosilane refluxes in a bed of anion-exchange resin containing tertiary amino or quaternary ammonium groups. Contact time can be used to control disproportionation to dichlorosilane, monochlorosilane, or silane. 

Organozinc reagents have been used in conjunction with a-bromovinylboranes in a tandem route to Z-trisubstituted allylic alcohols. After preparation of the vinylborane, reaction with diethylzinc effects migration of a boron substituent with inversion of configuration and exchange of zinc for boron.176 Addition of an aldehyde then gives the allylic alcohol. The reaction is applicable to formaldehyde alkyl and aryl aldehydes and to methyl, primary, and secondary boranes. 

The reaction product XVII on decomposition with water exchanges the boron atom for hydrogen and gives a saturated hydrocarbon XVIII and on oxidation by H202 in alkaline medium forms an alcohol XIX. 

The addition of allylic boron reagents to carbonyl compounds first leads to homoallylic alcohol derivatives 36 or 37 that contain a covalent B-O bond (Eqs. 46 and 47). These adducts must be cleaved at the end of the reaction to isolate the free alcohol product from the reaction mixture. To cleave the covalent B-0 bond in these intermediates, a hydrolytic or oxidative work-up is required. For additions of allylic boranes, an oxidative work-up of the borinic ester intermediate 36 (R = alkyl) with basic hydrogen peroxide is preferred. For additions of allylic boronate derivatives, a simpler hydrolysis (acidic or basic) or triethanolamine exchange is generally performed as a means to cleave the borate intermediate 37 (Y = O-alkyl). The facility with which the borate ester is hydrolyzed depends primarily on the size of the substituents, but this operation is usually straightforward. For sensitive carbonyl substrates, the choice of allylic derivative, borane or boronate, may thus be dictated by the particular work-up conditions required. 

The addition of alcohols to alkenes is also catalyzed by boron trifluoride-hydrogen fluoride, but the reaction proceeds only under vigorous conditions and the yields are generally low.394 Recently, the etherification of alkenes has also been achieved industrially by using acidic montmorillonite395 and macroporous sulfuric acid resins (ion-exchange resins).396... 

A catalytic enantioselective arylation of arylaldehydes employs a chiral /3-amino alcohol and a boronic acid-diethylzinc exchange reaction to generate the reactive arylzinc species.247... 

Termination also occurs if ligand exchange produces a weaker Lewis acid which is unable to reactivate the covalent adduct. In this case, addition of another aliquot of the stronger acid may reactivate the system. For example, hydroxy or alkoxy groups are exchanged in polymerization systems based on boron trifluoride and adventitious water or alcohol, respectively. Termination occurs infrequently by irreversible cleavage of B—F bond from the BF3OH anion, with irreversible C—F bond formation [Eq. (127)]. 

An unusual feature of the Beat ligand in metal-Bcat complexes is the resistance of the the B-O(catecholale) bonds to exchange with other alcohols. For example, whereas Ru(Bcat)Cl(CO)(PPh3)2 may be recovered unchanged from EtOH solutions, compound (15), where the Beat function is no longer directly attached to the metal, undergoes rapid ethanolysis.11 The observation of substitution reactions at the boron centre therefore... 

Ester formation occurs with complete stoichiometric conversion at relatively low reaction rates (t./, 100-600 s). In the absence of water no back reaction takes place. There is a very slow ester-ester exchange reaction ty, k 5000 s). The boronic ester moieties can be readily cleaved in water/alcohol. This reaction is, in principle, fast and complete (t./, 100 s), so that the template can be released from an imprinted macroporous polymer in 85-95% yield. 

Tin-lithium exchanges. A method for the preparation of a-hydroxy boronic esters takes advantage of the facile Sn/Li exchange of a-stannylalkyl MOM ethers. Subsequent reaction of the lithioalkyl ethers with boronic esters displaces one of the alkoxy groups of the latter." The reaction of chiral A-tributylstannylmethyl-2-methoxymethylpyrrolidine with BuLi and then aldehydes gives diastereomeric mixtures of P-amino alcohols. ... 

The high regioselectivity in the formation of a-allenic alcohols from boron reagents at low temperature is markedly different from that seen with the titanium reagent derived from 1-trimethylsilyl-l-bu-tyne, in which exclusive formation of 3-alkynic alcohols is observed. This result was explained by a rapid exchange between the allenic and alkynic structures, as shown in equation (2). The alkynic structure is thermodynamically less stable and kinetically more reactive than that of the allenic form. At lower temperature, the rate of equilibrum becomes faster than the subsequent reaction with aldehydes and thus the alkynic species becomes the major reaction form. 

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