Ethene unsymmetrical

Titanium (iv) chloride with zinc in pyridine has been found to couple ketones reductively to afford symmetrical tetrasubstituted ethenes. Unsymmetrical alkenes can be synthesized in useful yields by titanium-induced ketone coupling if the less reactive component is used in excess. Cycloalkenes (ring size 4—16) are prepared in good yield by intramolecular coupling of the corresponding alkanedione. ... 

Although the yields are variable and isomeric products are obtained using unsymmetrical ethenes, the reaction conditions are mild and show promise of stereoselectivity. 

Most Diels-Alder reactions, particularly the thermal ones and those involving apolar dienes and dienophiles, are described by a concerted mechanism [17]. The reaction between 1,3-butadiene and ethene is a prototype of concerted synchronous reactions that have been investigated both experimentally and theoretically [18]. A concerted unsymmetrical transition state has been invoked to justify the stereochemistry of AICI3-catalyzed cycloadditions of alkylcyclohexenones with methyl-butadienes [12]. The high syn stereospecificity of the reaction, the low solvent effect on the reaction rate, and the large negative values of both activation entropy and activation volume comprise the chemical evidence usually given in favor of a pericyclic Diels-Alder reaction. 

The n orbital amplitudes of ethene are identical on both carbons. Unsymmetrical substitutions polarize the n orbital. Electron acceptors or electrophiles attack the carbon with the larger r amplitude. The polarization of frontier orbitals is important for regioselectivities of reactions. Here, mechanism of the n orbital polarization of ethene by methyl substitution [4] is described (Scheme 5). 

It must be emphasised that only a slightly unsymmetrical distribution of electrons is required for a reaction s course to be dominated the presence of a full-blown charge on a reactant certainly helps, but is far from being essential. Indeed the requisite unsymmetrical charge distribution may be induced by the mutual polarisation of reagent and substrate on their close approach, as when bromine adds to ethene (p. 180). 

Formation of the oxazole ring is not always the last step in synthesis of the brightener. Unsymmetrical compounds that contain both a benzoxazole group and an ethene linkage can be prepared by the anil synthesis [51], in which a compound possessing an activated methyl group reacts with a Schiff base. The preparation of brightener 11.31 is an illustration of this method (Scheme 11.12). 

The Chemistry of Regiospecificity in Unsymmetrically Substituted Bromonium Ions Bromonium Ions of Ethene, Propene, and 2-Methylpropene... 

For unsymmetrical zirconacyclopentadienes, Cp2ZrEt2, which we developed as an equivalent to the zirconocene—ethene complex (3), is a very useful reagent [13]. Two different alkynes couple selectively via zirconacyclopentenes (4) (Eq. 2.3). 

In order to prepare very clean unsymmetrical zirconacyclopentadienes, the use of ethene is a prerequisite [14] (Eq. 2.4). An excess of ethene stabilizes the intermediates such as zirconacydopentane 5a and zirconacyclopentene 4. Such a transformation from a metallacyclopentane to a metallacyclopentene was first demonstrated by Erker in the case of the hafnium analogues [15]. 

Fig. 4.78 Majority rules principle as model for chirality amplification in (intermolecular) polymerisation of (unsymmetrically substituted) monomeric ethenes to form polymers, which can also convey the concept of (intramolecular) transmission of chiral perturbations [e.g. emanating from a stereogenic centre) in dendritic molecules The perturbation introduced into a system having a prior order (dance group a) in the form of an ex-...

The formation and cleavage of cyclobutane systems have been discussed in Sect. 3.1 and 4.4. The structure of the intermediates is of major interest. The cyclobutane radical cation has been calculated by several groups. Bauld and coworkers [342] modeled the cycloaddition of ethene radical cation to ethene by the MNDO method. At this level of theory an unsymmetrical structure with one long one-electron C—C o-bond is of lowest energy (Scheme 10, type Q. 

It was found that treatment of the corresponding dibromostannane with lithium naphthalenide afforded stannylene <1995OM3620>. The reaction of the latter with an excess of carbon disulfide resulted in the formation of the unsymmetrical ethene 24 (Scheme 53). Although the details of the mechanism are not clear, it is considered that the reaction proceeds via the formation of dithiocarbene 142 followed by dimerization to give a product that is thermally unstable and easily loses carbon disulfide. Thermolysis of 24 afforded symmetrically substituted ethene 143, in a quantitative yield via extrusion of carbon disulfide. 

Diazomethane is an electron-rich 1,3-dipole, and it therefore engages in Sustmann type I 1,3-dipolar cycloadditions. In other words, diazomethane reacts with acceptor-substituted alkenes or alkynes (e. g., acrylic acid esters and their derivatives) much faster than with ethene or acetylene (Figure 15.36). Diazomethane often reacts with unsymmetrical electron-deficient... 

In contrast to the great number of calculations concerning the all-carbon Diels-Alder reaction [39], there are only a few theoretical studies on the hetero Diels-Alder reaction [41,42,45 - 53 ]. The general mechanism of the Diels-Alder reaction is still in discussion however, in most cases a concerted reaction is assumed,but there is also evidence for a two-step path. The ab initio calculations carried out for the butadiene/ethene system by Houk, Ortega, Bernardi und Gajewski gave a symmetrical transition structure only using the semiempirical AM1/CI method (half electron approximation) an unsymmetrical diradicaloid intermediate was found [40]. 

It would be synthetically interesting to cross-couple two different unsym-metrical alkynes with complete regiocontrol. However, one limitation of this chemistry is that neither 1-hexyne nor 3-hexyne couples cleanly with a second terminal alkyne, which would give the largest steric difference between the ends of the alkyne [39]. To have very clean unsymmetrical zirconacyclopenta-diene derivatives, the use of ethene is primordial. Indeed, an excess of ethylene leads first to a zirconacyclopentane 23, which can react successfully with two different alkynes to give 20 (Scheme 8). 

An easier route to the same unsymmetric zirconacyclopentadiene 20 is the use of Cp2ZrEt2 as reagent (easily prepared from Cp2ZrCl2 with two equivalents of EtMgBr). This reagent (Scheme 9), equivalent to a zirconocene-ethene complex 24, reacts similarly to the Negishi-type reagent, which was prepared in the... 

Ethylene and Ethylidene Compounds.—The fact that the symmetrical di-chlor ethane is readily prepared from ethylene, has given to it the name of ethylene chloride. To distinguish the two isomers by name the other, the unsymmetrical di-chlor ethane, has been called ethylidene chloride. In connection with our discussion of the constitution of the ethene series of unsaturated hydrocarbons (p. 154), we have used the constitution of ethylene chloride as proving the constitution of ethylene or ethene, as H2C = CH2. Isomerism of the character shown in these two di-chlor ethanes, as above explained, is found in all classes of di-substitution products of ethane, so that we may express the compounds by general formulas as follows ... 

We will use the addition of hydrogen bromide to ethene as an example of the addition of an unsymmetrical molecule to a symmetrical substrate. 

When monosubstituted 1,2,4,5-tetrazines and unsymmetrically substituted dienes are used, two isomeric products can be formed, depending on the orientation of the two reactants. In most reactions only one regioisomer can be isolated. In most cases the sterically more hindered transition state is passed almost exclusively only traces of the second isomer can be detected by HPLC. The regiochemistry of the reaction of 3-phenyl-l,2,4,5-tetrazine (8) with l,l-bis(di-methylamino)ethene exhibits strong solvent dependency. 

There have been several computational studies of the peroxy acid-alkene reaction. The proposed spiro TS has been supported in these studies for alkenes that do not present insurmountable steric barriers. The spiro TS has been found for ethene (B3LYP/6-31G ), propene and 2-methylpropene (QCISD/6-31G ), and 2,3-dimethylbutene and norbornene (B3LYP/6-311- -G(c , These computational studies also correctly predict the effect of substiments on the and account for these effects in terms of less synchronous bond formation. This is illustrated by the calculated geometries and a(B3LYP/6-31G ) of the TS for ethene, propene, methoxyethene, 1,3-butadiene, and cyanoethene, as shown in Eigure 5.3. Note that the TSs become somewhat unsymmetrical with ERG substituents, as in propene, methoxyethene, and butadiene. The TS for acrylonitrile with an EWG substituent is even more unsymmetrical and has a considerably shorter C(3)- O bond, which reflects the electronic influence of the cyano group. In this asynchronous TS, the nucleophilic character of the peroxidic oxygen toward the (3-carbon is important. Note also that the is increased considerably by the EWG. 

If the dipolarophile is assymmetric (d e in d=e. Sect. 6.2, Scheme 6-5), there are two alternatives for the diazoalkane and all other unsymmetrical 1,3-dipoles in their cycloadditions with that dipolarophile. For example, diazomethane and an ethene derivative with an alkyl substituent R may yield the 3- or the 4-alkyl-l-pyrazolines (6-19), or a mixture of both. These primary products rearrange at higher temperature or in the presence of base to give the corresponding alkyl-2-pyrazolines. 

---