1, 2-Disubstituted ethanes

Due to these procedures as well as to some other transformations, the a-azido oximes can be considered as efficient basic reagents in the synthesis of various derivatives of 1,2-disubstituted ethanes using readily available AN (431) as starting compounds. 

Turning now to disubstituted ethanes, we will consider the case where both substituents are identical and bear lone pair electrons. We shall compare the typical conformations shown below. 

The importance of hydrogen bonding in determining the preferred conformation of 1,2-disubstituted ethane can be appreciated by reference to the calculations of Pople and his collaborators68. Representative systems were examined and in all cases the most stable conformer was calculated to be the one involving hydrogen bonding between the two vicinal substituents of the ethane molecule. Two typical examples of such structures are shown below. 

The approach used in this section can be used to treat any 1,2-disubstituted ethane. In general, sigma nonbonded interactions may favor the syn geometry, the... 

A slightly more complicated case than ethane is that of a 1,2-disubstituted ethane (YCH2—CH2Y or YCHj—CH2X),197 such as n-butane, for which there are four extremes a fully staggered conformation, called anti, trans, or antiperiplanar another staggered con-... 

Glycidic esters can easily be converted to aldehydes (2-40). The reaction has been extended to the formation of analogous aziridines by treatment of an imine with an a-halo ester or an a-halo N,N-disubstituted amide and f-BuOK in the solvent 1,2-dimethoxy-ethane.636 However, yields were not high. Acid-catalyzed Darzens reactions have also been reported.637 See also 6-61. 

Oxathiazole S-oxides are prepared by the reaction of thionyl chloride with suitable 1,2-disubstituted ethanes the 2-aminoethanol (382) gives (383) (CHEC 4.34). 

A series of molecules of great interest in principle and also well suited for electron diffraction studies are the 1,2-disubstituted ethanes. Gauche- and anri-conformers coexist in the gas phase in all known cases, sometimes with the anri-conformer as the prevailing conformer as in 1,2-dichloroethane136), sometimes with gauche prevalence as in 2-chloroethanol7,8). 

Becker et al. prepared a series of 10-substituted and 10,10 -disubstituted l,2-di(9-anthryl)ethanes 349 and their photochemical properties have been studied by determining the quantum yields for intramolecular (4tt + 4 77) photocyclomerization and the quantum yields of fluorescence in both cyclohexane solutions [347] (Scheme 96). The quantum yields for the intramolecular cyclization of monosubstituted 1,2-dianthrylethanes are higher than those for disubstituted analogs (see Table 8). Bulky substituents increase the fluorescence efficiencies and decrease the quantum yields for cycloaddition. The rate of the cycloreversion is enhanced by the addition of trifluoroacetic acid. 

Primary alkyl halides can also be alkynylated by silver acetylides. Isabelle and coworkers reported the reaction of methyl iodide, ethyl iodide and <7rmethyl iodide with several silver acetylides to give disubstituted alkynes.116 The authors preferred a non-radical-mediated mechanism for this reaction, as neither methane nor ethane, expected byproducts of a radical reaction, were observed. 

In the presence of high concentrations of Cu(II), 1,2-disubstituted ethanes are formed even in the absence of nitrate.523,S29 535a b These reactions are described by the general equation (X = AcO, Cl, etc.) ... 

Thus, in the absence of nitrate, it is possible to obtain a chloroalkyl acetate as the main product.5353 b The most reasonable rationalization for these results is to include the formation of vinyl acetate and saturated ester as proceeding through a common /J-acetoxyalkylpalladium(II) intermediate. Depending on the conditions, the latter can decompose via /3-hydrogen elimination to vinyl acetate or via nucleophilic displacement at the a-carbon giving 1,2-disubstituted ethanes ... 

Note, however, that in a molecule with an asymmetric center, rapid internal rotation does not guarantee magnetic equivalence even when nuclei are rendered chemically equivalent by the rotation. For example, in a 1,2-disubstituted ethane, the two protons attached to a given carbon atom are enantiotopic, hence chemically equivalent in an ordinary achiral solvent, as we saw in Section 4.9. Of the three conformations III—V, III and IV are mirror images, the average of which must have vA = vh and vx = vY, while V contains a plane of symmetry, so that the same equalities hold here. On the other hand, HA and HB are not magnetically equivalent, because the vicinal coupling constants are not necessarily... 

The offer made by program is diverse mechanisms leading to experimentally proved synthons are preferred. Those in the solved example [56] are ethylene, 1,2-disubstituted ethane, carbon dioxide, 3-hydroxypropanenitrile 28, isocyanic acid 17, hydrogen cyanide 9, acrylonitrile 29, and cyanoformic acid 12. 

The 1,4-dinucleophilic building blocks used most are 1,2-disubstituted ethanes of the type HXCHiCHjYH, semicarbazides, thiosemicarbazides, hydroxamic acid amides, amidrazones, and 1,2-disubstituted aromatic and heteroaromatic compounds. 1,1-Dielectrophilic building blocks preferentially used are perfluorinated carboxylic acids and their derivatives, such as acid halides, anhydrides, imidoesters, nitriles, perfluoroalkyl chlorosul-... 

Either of two systems can be studied in this experiment (1) o- and m-dichlorobenzene, or (2) succinonitrile and propionitrile. hi the first case, the results can be compared with the vector sum of C—Cl bond moments obtained from the known dipole moment of monochlorobenzene. In the second case, one obtains direct information about internal rotation in a simple 1,2-disubstituted ethane... 

Polyhaloarylcopper and trichloro-2-thienylcopper reagents react with a number of polybromo and polychloro ethanes and ethylenes to give disubstituted acetylenes in yields which can be as high as 87% (see Table IX) (144, 145, 264). The halogen compounds were C2HX5,... 

Many different routes are available for the synthesis of vinylboranes and several of them are shown in Scheme 7. Hydroboration and diboration reactions of alkynes and borylated alkynes provide access to the frill series of mono-, di-, tri-, and tetraborylated olefins. 1,2-Diborylated olefins (33) are obtained via diboration of alkynes and 1,1-diborylated olefins (34) are accessible through hydroboration of borylalkynes. An alternative route to 1,1-disubstituted products involves the diboration of carbenoids formed in situ from vinylhalides and butyl hthium. In certain cases, metal-catalyzed dehydrogenative borylation of olefins may be used. Borylalkynes serve as precursors to triborylated (35) and tetraborylated (36) olefins. Thus, the sparingly soluble tetraborylethylene derivative (36) forms in good yield through platinum-catalyzed diboration of diborylacetylene in toluene at 40 °C if the base-free catalyst [Pt(cod)2] is used. If the reaction, however, is performed at higher temperature, ftnther diboration of (36) leads directly to the hexaborylated ethane (23) shown above. Intramolecular B-O interactions were postulated for (36) based on HF-SCF calculations. ... 

Electrostatic interactions have been revealed as important in influencing the "gauche effect , whereby, in 1,2-disubstituted ethanes (4.9bA), the gauche conformer (4.9bB) is populated to a larger extent than the anti conformer (4.9bC) [31, 31a] (Figure 4.9b). 

Let US first examine current practice in nomenclature of the phenom-menon of internal rotation. Standard textbooks on conformational analysis on the one hand and those on spectroscopy on the other hand deal differently with the basic definitions. Internal rotation is measured by one or more torsion angles (dihedral angles, azimuthal angles). In the simple cases of e.g. a 1,2-disubstituted ethane the potential energy associated with the internal rotation may be written in a formal sense as a truncated Fourier expansion ... 

First, we moition the prqmration and use of the functional diazoic initiators, functional peresters, hydrogen peroxide, hybrid initiators (whidi contain two kinds of labile groups) and macromolecular polyinitiators. Then, new initi ors disubstituted (t aphenyl ethanes or thiurams functional or not) which are real precursms of living tdigomos are described. In each case, the Idnetical data (decomposition rate constants and efficiency) ate provided... 

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