62. Reaction in boron trichloride–cyclic ether systems

Abstract

Reactions of boron tichloride with ethylene, propene, and trimethylene oxides, epichlorohydrin (1-chloro-2 : 3-epoxypropane), tetrahydrofuran, and tetrahydropyran have been investigated ; the results supplement our earlier work' with ethylene oxide and tetrahydrofuran. Only the ethers having five-or six-membered rings formed 1 : 1-complexes with the trichloride; these were solids stable at 20" and the stoicheiometries of pyrolysis, hydrolysis, and reaction with pyridine were established. The other cyclic ethers were cleaved by boron trichloride to give chloroalkoxyboron esters, the ChloroaJkoxy-groups of which contained either the same number of carbon atoms as the ether or a small integral multiple. Corresponding boron esters were obtained either when the boron trichloride complex of tetrahydrofuran or tetrahydropyran was heated with the ether, or from boron tricbloride and tetrahydrofuran or tetrahydropyran, the ether being present in excess. These boron esters were identified by conversion into the appropriate alcohol by " methanolysis." The results are discussed.EARLIER we described reactions of boron trichloride with ethylene oxide and with tetrahydrofuran. These have now been examined in greater detail and the investigation has been extended to the corresponding propene oxide, epichlorohydrin, trimethylene oxide, and t e t rah ydrop yran systems.In many of the reactions to be described, products included chloroalkoxyboron chlorides, RO*BCl, and (RO),B*Cl, and trischloroalkyl borates, (RO),B. For identification, we used our earlier characterisation of some of these,2 but identification was difficult, particularly if the esters were present in mixtures, and for this pu.pse a new technique was devised, which we term "methanolysis." The boron compound is treated with a large excess of methanol, so th'at all the boron in the esters is converted into methyl borate and chlorine attached to boron into hydrogen chloride. Fractional distillation affords initially the methanol-methyl borate azeotrope (b. p. 66') and methanol (b. p. 65') and subsequently the alcohols, ROH, as higher-boiling fractions. This method, by identifying the alcohol derived from the ester, is superior to the alternative of hydrolysis because of the h g h water-solubility of many of the alcohols and the losses inevitably incurred in extraction and drying.The first series of experiments concerned the interaction in equimolecular proportions of the cyclic ether with boron trichloride. It had previously been shown that tetrahydrofuran afforded a solid 1 : 1-complex, whereas ethylene oxide underwent ring fission even at -80", to produce 2-chloroethyl dich1oroboronite.l It is now shown that tetrahydropyran, like tetrahydrofuran, forms a solid 1 : 1 complex, both having a finite stability at 20". With trimethylene oxide there was no evidence for complex formation; evidence for ring fission was that the product had only approximately two-thirds of the total chlorine easily hydrolysable, suggesting that a dichloroboronite, or a mixture of these, had bee...

Reactions Of Boron Trichloride With Organic Compounds

“…Reactions of boron trichloride have been studied with ethers having one oxygen atom in the molecule [ethylene oxide (46,47,85), propylene oxide (47, 85), epichlorohydrin (l-chloro-2,3-epoxypropane) (47), trimethylene oxide, tetrahydrofuran (46, 47, 85), and tetrahvdropyran (47, 85)] and with two in the molecule [1,3-dioxalane (54), 1,4-dioxan (54, 91, 98), and catechol methylene, dimethylene, propylene, and trimethylene ethers (54)]. Perfluoro-l-butyltetrahydrofuran, CsFieO, did not react with the trichloride even at 350°C.…”

“…With propylene oxide (equation 39, Y = CH3), the mixture of IX and X was richer in IX (55-70 per cent of the total) and there was almost complete absence of polymeric (similar to scheme 38, with m -1) material, whereas epichlorohydrin (Y = Cl) afforded X and no IX and a substantial quantity of polymeric products (ca. 30 per cent of the total), consisting mainly of l-chloro-3-(2-chloro-l-chloromethylethoxy)propan-2-yl dichloroboronite (XI) (47).…”

“…In view of the formation of polymeric compounds in some of the 1:1 reactions, experiments were carried out in which a large excess (5 to 6 moles) of cyclic ether was used (47). The results (scheme 41) with tetrahydrofuran and tetrahydropyran were identical, irrespective of whether their complexes or the free trichloride was used.…”

“…The relative stabilities of the tetrahydropyran and tetrahydrofuran complexes and the nonformation (or lack of stability) of complexes in the other cyclic monoether systems have been interpreted in terms of I steric strain (47).…”

“…A mechanism for the carbon-oxygen fission in cyclic monoether systems must satisfy the condition that -chloroalkyl dichloroboronites, and also polymeric products, are obtained by equimolecular interaction; a mechanism has been proposed and is illustrated for the case of ethylene oxide in scheme 42 (47).…”