Ethers as catalysts

The patent literature indicates that polymerisation may be carried out in the range -80 to -i-25°C using boron trifluoride or its ethereate as catalyst. 

With the discovery of the crowns and related species, it was inevitable that a search would begin for simpler and simpler relatives which might be useful in similar applications. Perhaps these compounds would be easier and more economical to prepare and ultimately, of course, better in one respect or another than the molecules which inspired the research. In particular, the collateral developments of crown ether chemistry and phase transfer catalysis fostered an interest in utilizing the readily available polyethylene glycol mono- or dimethyl ethers as catalysts for such reactions. Although there is considerable literature in this area, much of it relates to the use of simple polyethylene glycols in phase transfer processes. Since our main concern in this monograph is with novel structures, we will discuss these simple examples further only briefly, below. 

A direct method for introduction of a C-21 acetoxyl group into a 20-keto-pregnane is by reaction with lead tetraacetate at room temperature. Although originally the reaction carried out in hot acetic acid gave low yields, a careful study by Henbest has defined conditions so that yields as high as 86 % can be obtained at room temperature. The preferred solvent is 5 % methanol in benzene, with boron trifluoride etherate as catalyst. With either methanol or benzene, the yield is less than 4%. 

In a similar reaction of ketene with diethyl phosphite in the presence of boron fluoride etherate as catalyst, diethyl 1-acetoxyvinylphosphonate is formed [113] see Eq. (69) ... 

There are two other approaches to enhancing reactivity in nucleophilic substitutions by exploiting solvation effects on reactivity the use of crown ethers as catalysts and the utilization of phase transfer conditions. The crown ethers are a family of cyclic polyethers, three examples of which are shown below. 

T Yamada, N Isono, A Inui, T Miyazawa, S Kuwata, H Watanabe. Esterification of iV-(benzyloxycarbonyl)amino acids and amino acids using BF3-etherate as catalyst. Bull Chem Soc Jpn 51, 1897, 1978. 

In 1988 Kunz and Pfrengle introduced the preparation of chiral amino acid derivatives by the U-4CR in the presence of 2,3,4,6-tetra-6)-pivaloyl- 3-D-galacto-pyranosylamine, 57, in the presence of ZnCl2-etherate as catalyst. They obtained excellent stereoselectivity and high yields of their products. One of the disadvantages of such U-4CRs is that only formic acid can be used as the acid component, and the auxiliary group of the products can only be removed by half-concentrated hot methanolic HCl. 

Use of BF3 etherate as catalyst was suggested by the report of Yamamoto and Maruyama4 that -C,H(iCu complexed with BF3 undergoes conjugate addition to a,/ -unsaturated acids and esters, a reaction that has not been observed with R CuLi, even in the presence of BFj etherate. 

Cyclic oligomers with x - 2-9 are found to be present in poly(1,3-dioxolane) samples prepared by monomer-polymer-equilibrations using boron trifluoride diethyl etherate as catalyst. The molecular cyclization equilibrium constants 7fx are measured and the values are in agreement with those calculated by the Jacobson-Stockmayer theory, using an RIS model to describe the statistical conformations of the corresponding chains and assuming that the chains obey Gaussian statistics. 

Section II, 2 b. Hydronopyl-vinyl-ether and 2-hydronopoxy-ethyl-vinyl-ether have been polymerized using BFS etherate as catalyst. The polymers have optical activity not very different from the monomers [C. S. Marvel, L. A. Malzaiin, I. L. Comp J. Polymer Sci., Part A, 3, 961 (1965)]. 

Several other high inductions have been reported by using crown ethers as catalysts (Scheme 10.8). The Toke group has used a chiral crown 11 (Chart 10.2), which incorporates a glucose unit, for the addition of 2-nitropropane to a chalcone (Scheme 10.8) [38], Several other effective chiral crowns (12-17, Chart 10.2 and Scheme 10.8) are noted [24e,39-42,48b]. An interesting study of the Michael addition under both solvent-free (0% ee) and liquid-liquid conditions (up to 70% ee) was reported by Diez-Barra and co-workers, who also addressed the question of free -OH quats (28, 58% ee) verses O-benzyl quats (30, 46% ee) [43]. 

Glycosides of unprotected D-glycuronic acids have been formed directly in the appropriate alcohol with boron trifluoride etherate as catalyst, affording D-glycosiduronates.246 D-Glucosyluronic acid trichloroacetimidate has also been used for glycosylation of uronic acids.247... 

Synthesis of standards. Friedel-Crafts acylation of 2,3-dimethylthiophene with hexadecanoic acid and a subsequent reduction of the ketone with LiAlD4 yielded the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene (II, Table II). Experimental details of these reactions have been reported elsewhere (10). Ionic hydrogenation of the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene in trifluoroacetic acid using triethylsilane and BF3.etherate as catalyst yielded 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiolane (IV, Table II). For... 

Preparation of Etoposide employing 2,3-di-0-dichloroacetyl-(4,6-0-ethylidene)-p-D-glucopyranose and boron trifluoride etherate as catalyst... 

Scheme 7. Use of crown ether as catalyst in the Michael reaction.

The Friedel-Crafts acetylation of 3-phenylsydnone was accomplished with boron trifluoride etherate as catalyst. Formylation at C-4 by the Vilsmeier procedure occurred with 3-phenylsydnone. Mercuration is easily afforded with mercury(II) acetate or mer-cury(13) chloride and thioethers can be made directly with DMSO in acetyl chloride (74T409). At least one fused ring as in compound (35) has been made by a coupling reaction on the sydnone (34) at C-4 (79JCS(P2)175l). 

When the reactions of benzenetellurinyl acetate or trifluoroacetate to olefins were carried out with nitriles as solvents and boron trifluoride diethyl etherate as catalyst, 2-acylamino-1-alkyl phenyl telluroxides were obtained. Telluroxide elimination produces 4,5-dihydrooxazoles2. 

When the reaction was carried out in chloroform in the presence of ethyl carbamate with boron trifluoride-etherate as catalyst, /J-ethoxycarbonylaminoalkyl phenyl tellurium oxides were formed. These tellurium oxides were not purified but reduced with hydrazine to the corresponding tellurium compounds. ... 

Reactions between benzenetellurinyl trifluoroacetate and cycloalkenes, terminal olefins, or internal olefins in acetonitrile with boron trifluoride-etherate as catalyst produced fi-acetamidoalkyl or -cycloalkyl phenyl tellurium oxides. These oxides were reduced with hydrazine hydrate in ethanol2. 

Chloro-l-lithio- -pentene, CH2=C(Li)CH2CH2CH2Cl (I)." Both the Grignard and the cuprate reagents derived from 2 undergo conjugate addition to enones in moderate yield. In some cases use of BF, etherate as catalyst improves yields significantly. The products are cyclized by base to annelated methylenecyclohexanes. Overall yields vary from 40 to 70%. 

Steroidal O-tetrahydropyran-2-yl derivatives, Ott et al. prepared the 0-tetra-hydropyran-2-yl derivative of testosterone by the reaction of testosterone with 2,3-dihydropyrane catalyzed by p-toluenesulfonic acid monohydrate. The reaction required three weeks and the yield was 59 %. Use of boron trifluoride etherate as catalyst raises the yield to 67% and lowers the reaction time to 10 hr. ... 

When 2-crotyloxypyridine (63) was heated under similar conditions, however, the results were catalyst dependent (Scheme 5). With boron trifluoride-diethyl ether as catalyst, an 82 18 mixture of the pyridones 64 and 65 was obtained, whereas the H2PtCl6-catalyzed reaction gave a [3,3]-rearrangement product, the pyridone 66 (68JOC4560). Interestingly, when Pt(PPhj)4 was used as catalyst, 63 and 2-(l-methylallyloxy)pyridine (67) both gave the same 14 86 mixture of the pyridones 64 and 66, suggest-... 

A different route to 3 also involved a conjugate addition of a cuprate to a hindered enone, 4 -> 5, which was possible with BF3 etherate as catalyst.3... 

A simpler procedure is to use boron trifluoride etherate as catalyst, either alone or in acetic acid solution. Thus a solution of 300 mg. of androstenolone... 

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