Oxirane complex with

The latter method has the drawback that it can only be applied successfully to a limited number of compounds, such as [(RC02)Pd(00H)2] (R = CF3, CH3)77 and [(CH3C02)Cu2(0H)2(00H)] .78 Alternative synthetic procedures leading to group VIII metal hydroperoxy complexes involve either oxygen insertion into a metal hydride bond79 or protonation of easily accessible metal oxirane complexes with strong acids.80... 

Polymerization occurs by cleavage of the oxygen bond attached to the less-substituted carbon atom of the oxirane ring with inversion of the configuration of the secondary carbon atom and formation of a secondary alkoxide polymer molecule while the approaching monomer oxirane complexes with the now vacant aluminum atom site. Stereospecificity of the resulting polymer depends on whether the coordination of monomer is enantiomorphic selectic (97). 

Oxirane is an important Lewis base in the present context. The O atom carries two equivalent n-pairs of electrons, as it does in H20, but oxirane has the advantage over water in that it is possible to determine both angles 0 and 9 for its complexes with HC1 and ClF because the non-zero off-diagonal element Xab(Cl) of the Cl nuclear quadrupole coupling tensor is available. The corresponding Lewis base in which an S atom carries two equivalent n-pairs is thiirane. Each of the pair of complexes (CL S- -HC1 and (CL S- -ClF has Cs symmetry and here it is the off-diagonal element Xac(Cl) that is non-zero... 

Fig. 8 The n-pair models of 2,5-dihydrofuran, oxetane and oxirane (first column) and the experimental geometries of their complexes with HC1 (second column) and C1F (third column), each drawn to scale. The angle 0 is almost identical in B- HC1 and B- ClF for a given B but increases from 2,5-dihydrofuran, through oxetane, to oxirane, as expected from the model (see text). The non-linearity of the hydrogen bond increases monotoni-cally from 2,5-dihydrofuran to oxirane. See Fig. 1 for key to the colour coding of atoms...

The patterns of regio- and stereoselectivities become more complex in disubstituted oxiranes. Beginning with 2,2-disubstituted oxiranes, attack is always at the accessible C-atom. In terms of substrate enantioselectivity, it was found that 2-butyl-2-methyloxirane (2-methyl-1,2-epoxyhexane, 10.43, R = Bu) was hydrolyzed with a preference for the (5)-enantiomer. This substrate enantioselectivity was lost for branched analogues, namely 2-(/er/-bu-tyl)-2-methyloxirane (10.43, R = /-Bu) and 2-(2,2-dimethylpropyl)-2-meth-yloxirane (10.43, R = (CH3)3CCH2) [124], Thus, it appears that the introduction of a geminal Me group suppresses the enantioselectivity seen with branched monoalkyloxiranes, and reverses it for straight-chain alkyloxiranes. 

In general, addition of lithiated dialkoxydihydropyrazines occurs at the less hindered site of the oxirane. However, with phenyloxirane, ring opening does not take place regioselectively, and a complex product mixture is formed29. 

The cleavage of an oxirane ring with boron trifluoride-diethyl ether complex has been broadly investigated in the field of steroid chemistry. Both 5a,6a- and 5j6,6j6-epoxycholestane (10) rearranged when treated with boron trifluoride-diethyl ether complex in benzene.35-36... 

Oxirane and thiirane also form complexes with proton donors, but the hydrogen bond is oriented towards one of the lone pairs of the heteroatom52. 

On the other hand, the situation is more complex with flexible oxirane derivatives. This is clearly demonstrated by comparison of the equilibrium mixtures resulting after epoxide migration of different types of anhydro hexoses,278,286,304 their 6-deoxy derivatives 175, 176,520 branched-chain hexoses 177, 178,520,521 and anhydro hex-2-uloses.304... 

Z)-Phenol-substituted alkenes (67) can be produced by the palladium(0)-catalysed reaction of propargylic oxiranes (66) with phenols. This regio- and stereo-selective (g) addition is believed to occur via the formation of 7t-propargyl- and Tr-allylpalladium complexes. The phenoxy-substituted enones were obtained as by-products and their proportion depended on the reaction conditions.75... 

Ring opening occurs in the presence of iodine in an SnI process, the oxirane oxygen being initially complexed with the iodine. The two oxygen atoms bound to C-l serve to destabilize positive charge formation at C-2, rendering nucleophilic attack to C-3 more favorable. This accounts for the diequatorial product observed. 

Enantioselective preparation of nonfunctionalized oxiranes 92CRV873. Epoxidation induced by transition metal complexes with N-heterocycles as ligands 92YGK997. 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

Vinyl epoxides can be cross-coupled with vinylstannanes <2001JOC589>. This reaction proceeds through an (if-allyOpalladium complex (Equation 57) <2001JOC589>. Similar reactivity can be observed using bismuth reagents (Equation 58) <2001SC2365>. Vinyl oxiranes react with substituted allenes to form functionalized allyl alcohols <2004JOC4686>. 

Synthesis of 1,3-oxazolidines 285 has been described by a palladium-catalyzed cycloaddition of vinylic oxiranes 284 with Af-tosyl imines 283 (Scheme 81) <2000H(52)885>. This reaction proceeded with high yields and good regioisomeric selectivity and the palladium catalyst of choice was a 1 2 Pd(dba)2 DPPE complex. 

Molybdenum -peroxo complexes give oxiranes in high yields. " For anhydrous hydrogen peroxide, a three-step mechanism is assumed, with an a-hydroxyhydroperoxide intermediate. Detailed studies have been made on the mechanism of the reaction of the MoO(02)2-HMPT complex with olefins (Eq. 27). 2 ... 

Sodium cyclopentadienyl-dicarbonylferrate 96 and tetrafluoroboric acid react with oxiranes to give olefins with the same stereostructure, after decomposition of the iron-alkene complex. With 96, diaryl and dialkyloxiranes are transformed to the olefins with inversion, by thermal decomposition of the intermediate alkoxide. Oxiranes interact with CF3C(0)I to yield olefins with the same geometry via a 3-iodotrifluoroacetate. ... 

Organoaluminium compounds are stronger Lewis acids than organomagnesium compounds and form complexes with basic solvents. In ethers, for instance, R3AI either does not react at all with oxiranes or gives a very low yield of product and, accordingly, hydrocarbons are preferred as solvents. Important roles are played in the reaction by the molar ratio of R3AI to oxirane and the nature of the R substituents. 

Cyclic amines react with oxirane to introduce 2-hydroxyethyl TV-substituents. The TV-hydroxyethyl-substituted cyclic amines form complexes with a wide range of cations, the formation constants being smaller than for the unsubstituted amine for the later (/-cations, larger for hard and large cations. Structures usually have some OH groups coordinated. Tetrakis(2-hydroxyethyl)-substituted cyclic tetraamines insert cations much more rapidly than the unsubstituted amines, and the complexes are also more rapidly demetallated in add. Studies with differing patterns of TV-hydroxyethyl substitution showed that at least two civ-hydroxyethyl groups were required for this rate enhancement, and no enhancement was observed for mono- or trans-bis-substituted tetraamines.122... 

With silyl-substituted oxiranes, dibal-H favors the primary alcohol and Bu 3A1H favors the secondary alcohol. These observations have been interpreted in terms of the timing of the hydride transfer to one of the oxirane carbons. dibal-H, which exists as a Lewis complex in donor media (R3N-A1H(Bu )2, or R20-A1H(Bu )2) acts as a nucleophilic hydride source, which preferentially attacks the least-hindered carbon. With Bu 3A1, complexation with the oxirane oxygen precedes isobutene elimination and the generation of the Al—H bond. A considerable carbocation character is acquired in the transition state, hence formation of the primary alcohol is favored. It is worthy of note that trialkylstannyl-substituted oxiranes are reduced with Red-Al invariably at the oxirane... 

Note The experimental rotational constants of the fluoroform-oxirane complex measured in Ref [89a] are consistent with the two Cj stationary structures. One of them is 2 whereas the other has a bifurcated C-H- - -F- - -C-H bond. 2 is the global minimum at the MP2/6-311++G(2(/f,2/ ) level [89a]. The latter is the transition structure, distinguished by the energy offset of only 0.22 kcal - moF [89a]. The topological analysis of the electron density in 1 is investigated in Ref [103]... 

The cationic catalysts used for THF polymerisation, with the general formula R+X" are trialkyloxonium salts of superacids, esters of superacids, oxocarbenium salts, Lewis acid - oxirane complexes, superacid anhydrides, (e.g., triflic anhydride), and others [1-38] (Figure 7.2) ... 

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