On the mechanism of oxygen-atom or nitrene-group transfer in reactions of epoxides and aziridines with tungsten(II) compounds

Abstract

The tungsten(I1) complexes WCI2(PMePh2), (1) and WC12(CH2=CH2)2(PMePh2)2 (2) react with epoxides and aziridines to form tungsten(IV)-oxo and -imido complexes. The relative reactivities of epoxides with 2 have been determined from competition experiments. More substituted epoxides are harder to deoxygenate: the reactivities of ethylene, isobutylene, and tetramethylethylene oxides fall in the geometric progression 100: IO: I . cis-2-Butene oxide is deoxygenated faster than its trans isomer. Reaction occurs with predominant (285%) retention of configuration (e.g. cis epoxides to cis olefins). The reaction of 2 with ethylene-d4 oxide yields W(0)C12(CH2=CH2)( PMePh,), (4) and uncoordinated CD2=CD2. The data suggest that de-epoxidation occurs via oxygen atom abstraction, and not via an oxametallacyclobutane which rearranges to an oxo-ethylene complex. Similarly, the tungsten center is suggested to attack the nitrogen atom of the aziridines, rather than react by initial oxidative addition of a C-N bond. This is indicated by the observation of an N-bound complex of aziridine and by the much slower rates of reaction for aziridines with bulky substituents on the nitrogen. The reactivities of para-substituted styrene epoxides are not strongly affected by the nature of the substituent (a p + value of -0.5 is calculated with Hammett u' parameters) indicating that the transition state is not very polar, although there appears to be some conjugation between the phenyl ring and the epoxide. In sum, the data are most consistent with either concerted oxygen or nitrene transfer to tungsten or a mechanism involving a short-lived radical intermediate.The mechanism(s) of olefin epoxidation by metal-oxo compounds and the reverse reaction, t h e formation of metal-oxo complexes by deoxygenation of epoxides, have attracted a great deal of interest in recent yeaw3-I4 Epoxides a r e valuable in-( I ) PRF Summer Faculty Fellow. Present address: North Seattle Community College, (8) (a) Collman, J. P.: Kodadek, T.; Raybuck, S. A,; Brauman, J. 1.; Papazian, L. M. J . Am. Cfiem. SOC. 1985, 107,4343-5. (b) Collman, J. P.; Brauman, J. 1.; Meunier. B.; Hayashi, T.; Kodadek, T.; Raybuck, S. A. Other examples of deoxygenation of epoxides to olefins by transition-metal complexes: (a) Sharpless, K. B.; Umbreit, M. A.; Nieh, M. T.; Flood, T. C. J . Am. Cfiem. SOC. 1972, 94, 6538-40. (b) Berry, M.; Davies, S. G.; Green, M. L. H. Chem. Commun. 1978, 99-100. (c) Togashi, S.; Fulcher, J. G.; Cho, B. R.; Hasegawa, M.; Gladysz, J. A.

Three‐Membered Heterocycles. Structure and Reactivity

“…The net reversal of the epoxidation reaction, namely the eliminative deoxygenation of epoxides, has been carried out in various ways. For example, tungsten reagents react with epoxides to form tungsten(IV)-oxo complexes that ultimately lead to the corresponding olefins with predominant retention of configuration [656]. Glycidyl acetates 626 (Scheme 2.90) undergo deoxygenation and concomitant deacetylation upon treatment with lithium telluride, which is conveniently prepared in situ from tellurium metal and lithium triethylborohydride [657].…”

“…Hodgson and Lee [677] have devised a clever method for accessing enantiopure bicyclic alcohols from meso-epoxides by such a reaction. For example, treatment of cyclononene oxide (656) with isopropyllithium in the presence of an excess of (À)-sparteine leads to an enantioselective a-deprotona- Thiiranes are produced in nature. For example, 3,4-epithiobutanenitrile, also known as 4ETN, is found in many cruciferous vegetables and may function as a weak biological alkylating agent [679], and humulene-4,5-episulfide is one of the components in the essential oil of hops [680].…”

Rhodium complexes of 1,3-diaryltriazenes: Usual coordination, N–H bond activation and, N–N and C–N bond cleavage

“…Examples of such g 1 -bound aryl complexes are relatively less common in the literature [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73]. It may be added in this context that metal mediated C-N bond cleavage, though not very uncommon, is mostly limited to strained amines and amidines [74][75][76][77][78][79], and hence there is significant interest in transition metal mediated C-N bond cleavage [80][81][82][83][84][85][86][87][88][89]. While the exact mechanism behind the observed C-N bond cleavage during formation of the 1-R complexes is not completely clear to us, some speculated sequences, that seem probable, are illustrated in Scheme 1.…”

Synthesis, structural characterization, antimicrobial and cytotoxic effects of aziridine, 2-aminoethylaziridine and azirine complexes of copper(ii) and palladium(ii)

“…With regard to coordination chemistry, however, the reactivity of nucleophilic aziridines towards electrophilic transition metal centers differs strongly from those of oxiranes and thiiranes with a only few exceptions. [19][20][21] While the latter act as oxidizing agents towards organometallic complexes resulting in the formation of oxo-or thiocomplexes via ethene elimination, [22][23][24][25][26] aziridines usually remain intact and prefer metal coordination via the nitrogen atom. Therefore, only one example of two-fold breakage of C-N bonds, elimination of ethene and formation of an imido complex is known in the literature.…”

“…Therefore, only one example of two-fold breakage of C-N bonds, elimination of ethene and formation of an imido complex is known in the literature. 26 However, many aziridine complexes of a variety of transition metals with up to four aziridine ligands are currently known, most of which have been published by us. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] Ring opening reactions especially with functionalized metal carbonyls have also been reported, e.g.…”