Trimethyl borate, reaction with

Trimethyl borate, reaction with Grignard reagents, 49, 91... 

Hexafluoro-2,5-dihydrofuran [24849-02-3] is distilled into sulfur trioxide [7446-11-9] at 25°C. Addition of trimethyl borate [121-43-7] initiates a reaction which upon heating and distillation leads to a 53% yield of difluoromaleic anhydride. Dichloromaleic anhydride [1122-17-4] can be prepared with 92% selectivity by oxidation of hexachloro-1,3-butadiene with SO in the presence of iodine-containing molecules (65). Passing vaporized... 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

Several nonreductive methods for cleavage of benzyl groups have also been developed. Treatment with s-butyllithium, followed by reaction with trimethyl borate and then hydrogen peroxide, liberates the alcohol.24 The lithiated ether forms an alkyl boronate, which is oxidized as discussed in Section 4.9.2. 

A convenient route to difluoromaleic anhydride is based on the reaction of F-2,5-dihydrofuran with S03 [183], Oxodefluorination proceeds at elevated temperature and is catalyzed by trimethyl borate. F-2,5-Dihydrothiophene undergoes a similar conversion when excess of S03 is employed, the primary thioanhydride 96 is oxidized to give 97 in high yield ... 

A subsequent reaction with trimethyl borate followed by hydrogen peroxide (see Section 15.1) gave the alcohol (1) (72%), with over 95% with the relative stereochemistry shown [16]. 

Phenylselenoetherification. This reaction provides a short synthesis of the C-nucleosides showdomycin (10) and epishowdomycin (11) from D-ribosc (7). Thus the stabilized ylide 8, derived from maleimide, couples with 7 in acetic acid to form 9. Cyclization with QH SeCl in refluxing trimethyl borate gives an unstable product, which on oxidation gives a mixture of 10 and 11. 

It is only fair when reviewing syntheses to mention unsuccessful reactions so as to outline any shortcomings of a particular reagent, and this has been done throughout this review. Sah et al. have recently reported that reaction of the metallated lithium phenolate (31) with either nitrobenzene or bis(trimethyl-silyl) peroxide gave only starting material, while reaction with trimethyl borate followed by oxidation furnished a complex mixture. 

Ti e borcn compounds used in iuzuk coupling reactions may b> prepared by reaction of the appropriate aryllithium compound with trimethyl borate, followed by reaction with hydrochloric acid. 

The in situ generated catalyst from ATBH and trimethyl borate has also been used in the stereoselective reduction of a-oxoketoxime ethers to prepare the corresponding chiral 1,2-amino alcohols. Thus the asymmetric borane reduction of buta-2,3-dione monoxime ether followed by acidic work-up and subsequent reaction with benzyloxycarbonyl chloride affords a 90% yield of 7V-(Z)-3-aminobutan-2-ol with excellent enantioselectivities (eq 5). A trityl group in the oxime ether is required for high enantioselectivity. This method has been successively applied to both cyclic and acyclic a-oxoketoxime ethers. 

Reaction of ATBH with trimethyl borate in THF presumably affords the B-methoxy oxazaborolidine, which effectively catalyzes asymmetric borane reduction of prochi-ral ketones. Thus the borane reduction of acetophenone with the reagent prepared in situ from 0.1 equiv of ATBH and 0.12 equiv of trimethyl borate provides... 

Arylboranes, which have a boron atom bonded to a benzene ring, are prepared from organo-lithium reagents by reaction with trimethyl borate [8(0013)3]. 

The above mentioned polymer-supported oxazaborolidines are prepared from polymeric amino alcohols and borane. Another preparation of polymer-supported oxazaborolidines is based on the reaction of polymeric boronic acid with chiral amino alcohol. This type of polymer can be prepared only by chemical modification. Lithiation of the polymeric bromide then successive treatment with trimethyl borate and hydrochloric acid furnished polymer beads containing arylboronic acid residues 31. Treatment of this polymer with (li ,2S)-(-)-norephedrine and removal of the water produced gave the polymer-supported oxazaborolidine 32 (Eq. 14) [41 3]. If a,a-diphenyl-2-pyrrolidinemetha-nol was used instead of norephedrine the oxazaborolidine polymer 33 was obtained. The 2-vinylthiophene-styrene-divinylbenzene copolymer, 34, has been used as an alternative to the polystyrene support, because the thiophene moiety is easily lithiated with n-butyl-lithium and can be further functionalized. The oxazaborolidinone polymer 37 was then obtained as shown in Sch. 2. Enantioselectivities obtained by use of these polymeric oxazaborolidines were similar to those obtained by use of the low-molecular-weight counterpart in solution. For instance, acetophenone was reduced enantioselectively to 1-phe-nylethanol with 98 % ee in the presence of 0.6 equiv. polymer 33. Partial elimination of... 

The synthesis of alkyl-, aryl-, and 1-alkenylboronic acids or their esters from Grignard or lithium reagents and trialkylborates is a classical and efficient method for making relatively simple boron compounds in large quantities (Scheme 2-6) [25]. The stereocontrolled synthesis of alkenylboronic acids and esters involves the reaction of a (Z)- or ( )-2-buten-2-ylmagnesium bromide with trimethyl borate [26]. 

Dihydropyridines generated from pyridinium salts carrying electron-withdrawing substituents at the 3 position by borohydride reduction are generally resistant to further reduction. The dihydro derivatives of 1-methyl-3-cyanopyridine, 68, 69, and 70, were recovered unchanged when treated with borohydride in water. Only 1,2-dihydro-l-methyl-3-cyano-pyridine (68) was converted to the tetrahydropyridine 71 when trimethyl borate was added to the reaction medium. Diborane/water achieved the same conversion, while added boric acid returned the starting materials. ... 

The reduction of pyrimidin-2(l//)-ones (285) or their sulfur analogs occurs readily with NBH to give mixtures of products (286-288). The ratio of these products varies markedly, depending on the presence of sodium hydroxide. The use of methanol/NBH favored 286 and 287 over 288 possibly as a result of the in situ formation of trimethyl borate. The presence of sodium hydroxide retards formation of trimethyl borate, and a large increase in the proportion of 286 and 287 is now observed. In some cases, products 286/287 or 288 could be obtained exclusively by judicious choice of reaction conditions. 

Alkylations. The effect of subjoined Lewis acid (e.g., trimethyl borate) on the catalytic ally lation of aldehydes with allylstannanes promoted by a BINOL-Ti complex has been examined. With allenyltributylstannane the products are homopropargyl alcohols.. Allylation in a Sn(II)-mediated Barbier reaction exhibits much lower ee, although jllenylation with (terminally substituted) propargyltributylstannanes shows good results. 4-Trimethylsilylbut-2-ynyl)tributylstannane undergoes destannylative addition to... 

Probably the best modem method for introduction of OF by electrophilic aromatic substitution is lithiation, reaction with a boronate ester, and oxidation.4 These are the same boron compounds that are used in Suzuki coupling (chapter 18) and are made the same way. In this example, selective mono-lithiation by Br/Li exchange on available tribromoanisole 39 (easily prepared by bromination of anisole or phenol) occurs ortho to the MeO group and reaction of aryl-lithium 39 with trimethyl borate gives the boronic ester 40. Peroxyacids such as peracetic acid are usually used for the final oxidation. 

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