Oxirane Lewis acid catalyzed

Polymerization is frequently observed as a side reaction in acid-catalyzed reactions, and under appropriate conditions many oxetanes can be quantitatively transformed into high molecular weight linear polyethers with useful properties. Polymerization is very general for oxetanes and is closely related to Lewis acid-catalyzed polymerization of oxiranes, THF and oxepane, but oxetanes generally polymerize much more rapidly than THF and oxepane and at a rate similar to oxiranes (72MI51300). 

Fig. 58.—Formation of a trans-( 1 — 2)-linked glycoside by a Lewis acid-catalyzed reaction of a glycosyl oxirane with a tributyltin either.343...

Unlike an A, A -disubstituted imidazoline-2-thione, the reaction of imidazolidine-2-thione 665 itself with 2-phenyloxirane in the presence of BF3 leads to the formation of l,3-bis[(i4)-2-phenylethen-l-yl]im>dazolidine-2-thione 666 (Scheme 161) <2005HCA3253>. This unusual mode of reaction is presumably due to a Lewis-acid-catalyzed rearrangement of oxirane to aldehyde followed by an ene-like reaction with the iminothiol. 

Lewis acid-catalyzed reactions of 1 with oxiranes lead to 1,3-oxathiolanes and/or 1,3-dithiolanes. Whereas cyclohexene oxide (63) yields a mixture of 64 and 65, only the corresponding 1,3-dithiolane is obtained in the case of cyclopentene oxide (eq 27). The formation of 1,3-dithiolanes is evidenced to involve... 

Alkyl halides are less investigated in this protocol. a-Halo ethers, thioethers, and lactones can be used in this protocol. a-Halocarbonyl compounds react abnormally with allyl- and acetonyltins to give oxiranes via the attack at the carbonyl followed by the cyclization (Scheme 34).p°]-P3] -pjjg reaction appears to be a variation of a simple Lewis-acid-catalyzed aldol-type condensation. On the other hand, if a catalyst without phosphine ligand is used, 1,4-diketone is formed (Scheme 35). 

More typically, mixtures of products are formed. Terpene epoxides that have been subjected to ZnBr2-catalyzed rearrangement include the oxiranes derived from limonene, carvomenthene, other pinenes, 2,3-carvene, pulegone and piperitone. ZnCh is rarely employed, but has been compared with AICI3 in a study involving sever epoxides. In most instances there appears to be no evidence for special advantages of zinc halides compared with other more commonly used Lewis acids. 

Ph4SbOTf is an effective Lewis acid catalyst of nucleophilic addition of primary and secondary amines to oxiranes (Scheme 14.70) [142]. Ph4SbOH catalyzes the azidation of oxiranes with MejSiNj [143], the regioselectivity being dependent on the substituents of fhe oxiranes (Scheme 14.71). Ph4SbOMe promotes intramolec-... 

The metal ion-catalyzed decomposition of alkyl hydroperoxides in the presence of olefins also leads to the formation of oxiranes. The effectiveness of Mo(VI), Ti(IV), and W(VI) complexes is related to their being hard Lewis acids (44, 45). 

Reactions with Oxiranes, Oxetanes, and Aziridines. Lewis acids, lanthanide salts, and titanium tetraisopropoxide or aluminum isopropoxide catalyze the reactions of cyanotri-methylsilane with oxiranes, oxetanes, and aziridines, yielding ring-opened products. The nature of the products and the regio-selectivity of the reaction are primarily dependent on the nature of the Lewis acid, the substitution pattern in the substrate, and the reaction conditions. Monosubstituted oxiranes undergo regiospe-cific cleavage to form 3-(trimethylsiloxy)nitriles when refluxed with a slight excess of cyanotrimethylsilane in the presence of a catalytic amount of potassium cyanide-18-crown-6 complex (eqs 8-10). The addition of cyanide occurs exclusively at the least-substituted carbon. 

The ambident nature of cyanotrimethylsilane can lead to the formation of nitriles or isocyanides, depending on the nature of the catalyst. For example, cyanotrimethylsilane reactions with epoxides and oxetanes catalyzed by soft Lewis acids give rise to 2-(trimethylsiloxy) isocyanides arising by attack on the more substituted carbon (eqs 15 and 16).Milder reaction conditions and better yields of isocyanides can be realized when the reaction of cyanotrimethylsilane with oxiranes is carried out in the presence of Pd(CN)2, SnCL, or MesGa (eq 17). Isocyanides are useful precursors for the synthesis of /3-amino alcohols and oxazolines. 

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

---