Borinate esters

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

In view of the highly strained o- B—C bonds of 74-75, there is experimental evidence that such ir-bonding is actually responsible for the stability of the ring to cleavage. For example, addition of pyridine to colorless 74d generates an intense yellow color and many new resonance signals in the l3C-NMR spectrum consistent with isomeric zwitterions 76a,b. Addition of r-BuOH (which does not itself react with 74d) yields borinate ester 77 (Scheme 3). These results are consistent with the interpretation... 

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. 

Ethanolamine is used to displace the adduct from the borinate ester. The facility with which the transfer of acetylenic groups occurs is associated with the relative stability of the. v/ -hybridizcd carbon. This reaction is an alternative to the more common addition of magnesium or lithium salts of acetylides to aldehydes. 

Lithium triethylcarboxide (1) is the base of choice for these conversions. The use of less hindered alkoxide or amide bases results in poorer yields. In this procedure 1.5-2.0 equivalents of base are required, although with more bulky alkyl groups attached to boron only 1 equivalent is necessary. The use of the more hindered 2,6-diisopropylphenol to form the borinic ester gives a 96% yield of the bicyclic ketone with only 1 equivalent of base however, in the work-up procedure this phenol is more difficult to separate from the ketone. 

The boronic esters (Chart 9) are easily hydrolyzed to the corresponding homoallylic alcohols using triethanolamine 98). Consequently, the allylboration sequence provides a synthesically useful alternative to the familiar Grignard synthesis of homoallylic alcohols. However, the protonolysis by triethanolamine causes a problem in the isolation of homoallylic alcohol from the thick, sticky, air sensitive boron-containing mixture. Fortunately, treatment of a pentane solution of borinate esters of 9-BBN with 1-equivalent of ethanolamine results in the rapid formation of a fluffy white... 

The reaction undergone by alcohols with trialkyl- and triaryl-boranes in the presence of pivalic acid, to give borinic esters, and the thermal cyclization of bis(dialkylborinates) to boronates, are discussed briefly in Section II. Many borinates have been prepared in quantitative yield from mono-, di-, oligo-, and poly-saccharides,338,108 and mixed... 

Preparative Methods 9-BBN, by reaction with the chiral alcohol l,2 5,6-di-0-isopropylidene-a-D-glucofuranose (DPGF) (both commercially available), is transformed into the borinic ester 9-0-DIPGF-9-BBN which, by treatment with a modest excess (1.1-1.5 equiv) of potassium hydride, is completely transformed within 2 h into K-glucoride. ... 

Methyl and 9-[(trimethylsilylmethyl)]-9-BBN derivatives are easily synthesized by the reaction of the corresponding lithium reagents with 9-methoxy-9-BBN. Unfortunately, such derivatives are spontaneously flammable in air, making them particularly hazardous to handle for the purpose of isolation. However, selective oxidation with anhydrous triethylamine A -oxide converts them to air-stable borinate esters which are efficient reagents for the methylation [149, 150] of haloalkenes or the syntheses of allylic and propargylic silanes (Scheme 2-52) [151]. 

Wang, B. B., Smith, P. J. Synthesis of a terbenzimidazole topoisomerase I poison via iterative borinate ester couplings. Tetrahedron Lett. 2003, 44, 8967-8969. 

Alkylbromoalkenylthexylboranes are available from thexylborane by sequential hydroboration of al-kenes and bromoalkynes, and they rearrange to borinic esters when treated with methoxide. Protonation yields ( )-alkenes of high purity (Scheme 34). Rearrangement takes place with inversion of configuration at the halogen center, while protonation proceeds with retention of configuration. 

Chiral boronic esters react with organolithium reagents to form diorganylalkoxyboranes (borinic esters). Subsequent reaction with the anion of dichloromethyl methyl ether then yields chiral ketones by rearrangement of both of the groups on boron (Scheme 42). No racemization is observed in this sequence and alkyl-, aryl- or alkynyl-lithium reagents can be used. 

More efficiently, Grignard reagents or lithium compounds react with boronic esters to give borinic esters which can be oxidized with hydrogen peroxide or f-butyl hydroperoxide to give phenols in good yields (equation 52) . The mechanism has been formulated as... 

Reaction with trialkylboranes. A trialkylborane (1), readily available by hydro-boration of an alkene, undergoes fast 1,4-addition to methyl vinyl ketone to give an enol borinate ester (2), which on hydrolysis affords a methyl ketone (3).24... 

Boronic Ester Borinic Ester or Dialkyl-(1,1,2-trimethylpropyl-propyljborane ... 

Hydroboration of alkenes (185) with dibromoborane gives alkyldibromoboranes (186) which are easily converted into dioxaborinanes (187) (Scheme 25). These heterocycles also provide a convenient entry to aldehydes (189) through the addition of lithiated a-methoxy thioethers <83JA6285>. Unsym-metrical borinate esters (191) can be prepared from these boronates through the addition of organolithium compounds (cf. (190)) followed by treatment with acetyl chloride <850M1788,... 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

As a useful alternative to carbonylation, the Brown dichloro-methyl methyl ether (DCME) process has been effectively used for the synthesis of 9-alkylbicyclo[3.3.1]nonan-9-ols. 2 The ketone bicyclo[3.3.1]nonan-9-one (eq 29) has also been prepared from a hindered B-aryloxy-9-BBN derivative, with simple B-alkoxy-9-BBN derivatives failing to undergo this process. However, most borinate esters are smoothly converted to ketones through this process, including germa- and silaborinanes (eq 30). In these cases, 9-BBN-H provides the essential 1,5-... 

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