Oxiranes complexes

The origin of the enantiodiscrimination appears to be strongly dependent on the structure of the HCLA employed. For HCLA bases of type A (53 to 56), stereoselectivity has been empirically deduced to arise [in the transition state (TS)] from the difference of energy between the two diastereoisomeric 1/1 HCLA/oxirane complexes TSl and TS2 (Scheme 27). Indeed, the steric repulsions between cyclohexene oxide and the pyrrolidinyl substituents in TS 1 favor TS 2, as proposed by Asami in 1990 for enantioselective rearrangement of cyclohexene oxide by HCLA 53 (Scheme 26) . ... 

Two examples using i-BuLi/(—)-sparteine mixture as base have been reported to promote effective allylic alcohols formation in moderate ee for the spiro epoxide 82 and cyclooctadiene oxide 83 ° (Scheme 33). In the last case, such regioselectivity could be explained by the relatively flat conformation of the medium-sized oxirane which favors the syn base-oxirane complex (see Section n.A). 

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... 

For the hydrogen-bonded phenol oxirane complex , the performance of the SCF and BLYP density functional methods was compared, using the Pople s 6-31G(d, p) and 6-311 -F- -G(d, p) basis sets. The MP2/6-31G(d, p) hydrogen-bond energy is Df. = 28.9 kJmoG and the dissociation energy is Do = 23.8 kJmoP. ... 

E. Cubero, M. Orozco, and F. J. Luque, Electron density topological analysis of the C—H O anti-hydrogen bond in the fluoroform-oxirane complex, Chem. Phys. Lett. 310, 445-450 (1990). 

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) ... 

Calcium initiators are much weaker than the aluminum or zinc systems in forming oxirane complexes. As a result, they are particularly effective in polymerizing EO, which forms complexes more readily in comparison with the substituted oxiranes. Calcium initiators form weaker complexes with the substituted oxiranes, PO in particular, and initiate a low-rate polymerization. 

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). 

A significant outlet for TBHP is the molybdenum-complex cataly2ed production of propylene oxide, a process developed by Oxirane (221—224). 

Uses. Magnesium iodide is used in the deoxygenation of oxiranes into olefins and iodine. This step is important to organic chemistry because it helps in the stmcture elucidation of complex organic molecules (110). Eor example. 

The addition of an oxygen atom to an olefin to generate an epoxide is often catalyzed by soluble molybdenum complexes. The use of alkyl hydroperoxides such as tert-huty hydroperoxide leads to the efficient production of propylene oxide (qv) from propylene in the so-called Oxirane (Halcon or ARCO) process (79). 

Oxirane and chlorine allegedly form complexes C2H4O Cl and C2H4O 3C1 (64HC(19-1)445) at -80 C, but in this and other cases explosions occurred at room temperature, perhaps caused by hypohalites. 

The bis(diene) (46) adds dienophiles preferentially on the side syn to the oxirane moiety (Scheme 35) (80X149). This may be due to formation of a charge-transfer complex by donation of electron density from oxygen into an antibonding orbital on the dienophile. 

Thiirane is more bactericidal than oxirane, and derivatives of 2-mei captomethylthiirane inhibit tuberculosis. The following pharmacological uses have been reported for compounds derived from thiirane derivatives gold complexes of the adducts of diethylphosphine and thiirane (antiarthritic), adducts of thiiranes and malononitrile (antibacterial, blood vessel dilators, muscle relaxants, sedatives), thermolysis products of thiirane 1-oxides and adducts of thiirane 1-oxides with sulfenyl chlorides (antibacterial), adducts of 2,3-diarylthiirene 1,1-dioxides with ynamines (antibacterial, parasiticidal), adducts of 2,3-diarylthiirene 1,1-dioxides with enamines (antifertility), adducts of p-aminophenylacetic esters with thiirane (immunosuppressants), adducts of amines and thiiranes (radioprotective drugs). 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

The aim of this contribution is to examine the most widely used methods of elaborating the oxirane functionality in the synthesis of complex molecules. In view of the extensive use of epoxides in simple transformation procedures in the early stages of total syntheses, only certain selected manipulations of epoxides representing the key steps of a total synthesis and/or their use at a late stage of the reaction sequence are considered. 

Fig. 6 The experimentally determined geometries of oxirane- -HC1 and oxirane- -ClF drawn to scale. The n-pair model of oxirane is shown for comparison. While the angle

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... 

Table 1 The angles and 9 (in degrees see Fig. 8 for definitions) in complexes B- HC1 and B- ClF, where B is one of the cyclic ethers 2,5-dihydrofuran, oxetane or oxirane...

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