Dimethyl Boronate Esters

Cu salt and/or generates a dimethyl boronic ester. The latter explanation is consistent with Chan s observation with boronic acid esters (vide infra) [10]. 

Dimethyl Boronate Esters. Dimethyl esters of arylboronic acids have been prepared by heating a mixture of the acid, 2,2-dimethoxypropane, and Zinc Chloride (catalyst). ... 

The oxazaborolidines are easily prepared by heating ephedrine with borane dimethyl sulfide or the appropriate boronate ester. The aluminum reagent C is obtained by mixing ephedrine and trimethylaluminum. Borolidinc A is superior to its methyl derivative B and to the aluminum analog C. The diastereomeric borolidine obtained from borane and (S,S)-pseu-doephedrine failed to show any cnantioselectivity25. A variety of aromatic aldehydes can be enantioselectively alkylated in the presence of A, however, with heptanal the enantioselectivity is poor25. 

If a diastereomeric mixture of (S,S)-2,5-dimethyl-3.4-hexanediol esters 2 and 3 reacts with methylmagnesium bromide, the result is kinetic resolution, as verified for R1 = Bn. The diastereomeric mixture was prepared from the racemic ethylene glycol a-chloro boronic ester via transesterification with chiral diol. The products isolated were (S,S)-2,5-dimethyl-3,4-hexanediol [(/ )-2-phcnyl-l-methylethyl]boronate, diastereomeric ratio >95 5 as indicated by the rotation of the (/ )- -phenyl-2-propanol derived by deboronation with hydrogen peroxide, and (S,S)-2,5-dimethyl-3.4-hexanediol methylboronate. Phenylacetaldehyde was identified by 1H NMR4. 

Treatment of 2-lithiofurans with ethyl borate and hydrolysis of the resultant boronic ester yields 2(3//)-furanones (69AK(29)229). 2,5-Dimethyl-3(2i/)-furanone (358) has been prepared by a Curtius reaction on the ester (359) and acid hydrolysis of the intermediate urethane (360) (Scheme 97). This type of reaction has been used to synthesize muscarine and its stereoisomers (61QR153). 

Benzyloxyisothiazole 316 (R = H) is regioselectively lithiated at C-5 with LDA and the lithiated species reacts with MeOD and various C-electrophiles <2002JOC2375>. Treatment with I2 or B(0/-Pr)3/2,2-dimethyl-l,3-propanediol affords the iodo compound 316 (R = I) and the boronic ester 315, respectively (see Section 4.05.7.7) <2004JOC1401 4he Grignard reagent of 3 iodoisothiazole reacts rvith a series of electrophiles 1995SC1383. ... 

A )-and (Z)-alkenylboranes, or the corresponding boronic esters or potassium tri-fluoroborates, are conveniently available via hydroboration of terminal alkynes or 1-halo-1-alkynes using either disiamylborane, dicyclohexylborane, dibromoborane dimethyl sulfide, or catecholborane as hydroborating agents. ... 

A problem with this methodology is that only one of the three alkyl groups is transferred to the unsaturated carbonyl compound. A solution to this uses the radical generated from the boronic ester, itself derived from hydroboration with catecholb-orane 8. Treatment of the boronic ester with oxygen and 1,3-dimethyl-hexahydro-2-pyrimidinone (DMPU) in the presence of the a,3-unsaturated aldehyde or ketone gives the desired radical addition product, with transfer of the S-alkyl group. Thus, cyclohexene was converted to l-cyclohexyl-3-pentanone 24 using this chemistry (5.38). 

Alkenyldibromoborane as a Dienophile. (l )-Hex-l-en-l-yldibromoborane dimethyl sulfide, obtained by the hydroboration of hex-l-yne with DBBS, reacts with 2,3-dimethylbuta-1,3-diene to provide Diels-Alder adduct that upon methanolysis affords boronic ester in high 3ueld (eq 18). ... 

A related system 70 containing two boronic esters has been developed by Aldridge et al., who observed that the bis-fluoride adduct undergoes a color change from orange to green as the ferrocene is aerobically oxidized to ferrocenium. The same group has also developed a boronic acid with dimethyl amine 71 to act as a ditopic receptor for HF. ... 

Hydrolysis of 1,3,2-dioxaborolanes is thermodynamically disfavored, no doubt as a result of the same kinds of entropic factors that favor formation of cyclic acetals in preference to acyclic acetals. Hydrolysis of a 1,3,2-dioxaborolane (1) converts three molecules to two, but hydrolysis of a boronic acid dimethyl ester keeps the total number of molecules at three (Scheme 5). (It m be noted that hydrolysis of a cyclic acetal with one molecule of water keeps the number of reactant and product molecules equal at two, and hydrolysis of an acyclic acetal converts two molecules to three.) Adding base does almost nothing to the equilibrium, since hydroxide ion coordinates to the boronic ester 1 as well as to the boronic acid product. Furthermore, it spears that the trans R° substituents in 1 further stabilize the stmcture. Chiral boronic esters of this series are harder to hydrolyze than pinacol boronic esters, and treatment of pinacol boronic esters with DICHED results in liberation of the pinacol and formation of the DICHED boronic ester. 

The chemistry of boronic esters that have cyano substituents closely resembles that of boronic esters with carboxylic ester substituents. The same rules apply to the requirements for separation between the boron atom and the substituent. Lithioace-tonitrile reacts with (a-haloalkyl)boronic esters in the same maimer as does tert-butyl lithioacetate, but the less bulky nitrile function encounters fewer steric obstacles. Some examples of sterically hindered compounds that can be made in this way include pinanediol ester 97 [46], (J )-DICHED ester 98 [54], and the (a,a-dimethyl-P-cyanoethyl)boronic ester 99 (Scheme 8.22) [37]. 

Thus, the pinane diol derivative 78 was reacted with dimethyl-4-bromobutylboronate 79 to afford the boronic ester 80 in good yield. A one-carbon homologation was then achieved by first treating the boronic ester 80 with lithium methylene dichloride, which affords an intermediate boronate 81, that undergoes a diastereoselective rearrangement to afford the a-chloroboronic ester 82. 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

Dimethyl-3-hydroxyselenophene has been prepared by hydrogen peroxide oxidation of the corresponding boronic acids and esters.122 The... 

Furstner has also used his methodology [58] as presented in Scheme 24 for the preparation of lycogalic acid dimethyl ester (135) and this is presented in Scheme 27. The ability to access indole boronic acids is an important consideration for this particular route. 

Electrolytic reduction, apparatus, 52, 23 Enol acetates, acylation of, 52,1 Enol esters, preparation, 52, 39 Epichlorohydrin, with boron trifluoride diethyl therate and dimethyl ether to give trimethyloxonium tetra-fluoroborate, 51,142 ESTERIFICATION OF HINDERED ALCOHOLS f-BUTYL p-TOLUATE,... 

Product 117 is a convenient starting compound for the subsequent modification of photochromes. Publication (09TL1614) gives an efficient synthetic route to both symmetrical 118 and unsymmetrical 119 phenyl-substituted dihetarylethenes bearing amino, hydroxy, or carboxy groups based on a Suzuki reaction of dichloride 117 with commercially available substituted boronic acids (or their pinacol esters) in a dimethyl ether (DME)-H20 mixture (4 1). For the symmetrical products, the yields are 85-95% for the unsymmetrical products, they are 60%. 

ANTI-SELECTIVE BORON-MEDIATED ASYMMETRIC ALDOL REACTION OF CARBOXYLIC ESTERS SYNTHESIS OF (2S, 3R)-2,4-DIMETHYL-1,3-PENTANEDIOL... 

C Chemical shift data for the simple dimethyl ester and the diastereomeric boronates of a 1,2-, 1,3- and 1,4-diol are shown in Figure 11. 

Methyl esters may be prepared by reaction of the aromatic carboxylic acid with diazomethane (cf. Section 4.2.25, p. 433) or, more conveniently, by reaction with a boron trifluoride-methanol reagent. The latter procedure is illustrated by the preparation of methyl m-chlorobenzoate and dimethyl terephthalate (Expt 6.164). t-Butyl esters may be prepared by conversion of the acid into an N-acylimidazole by reaction with N,N -carbonyldiimidazole, followed by reaction with t-butyl alcohol in the presence of DBU62 (Expt 6.165). 

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