Double bonds, formation

Classic procedures for C—C double bond formation involve yS-elimination of two vicinal substituents X and Y from a C—C single bond. 

Regioselectivity of C—C double bond formation can also be achieved in the reductiv or oxidative elimination of two functional groups from adjacent carbon atoms. Well estab llshed methods in synthesis include the reductive cleavage of cyclic thionocarbonates derivec from glycols (E.J. Corey, 1968 C W. Hartmann, 1972), the reduction of epoxides with Zn/Nal or of dihalides with metals, organometallic compounds, or Nal/acetone (seep.lS6f), and the oxidative decarboxylation of 1,2-dicarboxylic acids (C.A. Grob, 1958 S. Masamune, 1966 R.A. Sheldon, 1972) or their r-butyl peresters (E.N. Cain, 1969). 

Ketones may be prepared from 5a- and 5/5-saturated-3-ketones, A -3-ketones, A -3-ketones and 5a-A -3-ketones. At one time it was the only direct chemical alternative to microbiological methods for the preparation of the prednisone type of corticoids. A -Double bond formation is not observed, although A -trienones can be prepared from A -3-ketones. In common with DDQ, selenium dioxide is not normally useful for the preparation of A -3-ketones from 5a-3-ketones or A -3-ketones from 5j5-3-ketones. 

Synthesis of hetarylethylenes using novel methods for C=C double-bond formation 98ACR584. 

Takeshita H., Mori A., Tian G. R. Carbon-Carbon Double Bond Formation by Means of High-Pressure Cycloaddition-Retro-Diels-Alder Reaction Between 2,3-Bis(Methoxycarbonyl)-7-Oxanorbornadiene and Dienes Yuki Gosei Kagaku Kyo-kaishi 1990 48 132-143... 

Ring-Closure Reactions by C=C Double Bond Formation. 144... 

Dehydrohalogenation of the 314 proceeded in excellent yield under the action of morpholine or piperidine at rt, during double bond formation between the C-l and C-2 atoms <2003CHE640>. The active methylene group of 3,4-dihydro-1 ///>//-[ 1,4 oxazino[3,4- quinazolin-6-onc 315 readily condensed with aromatic aldehydes at 160 °C in a melt to give the 1-benzylidenes, and coupled with aryldiazonium chlorides to give the arylhydrazono derivatives <1996BMC547>. 

The mechanism proposed involves hydrogenation of the C2 C3 double bond, formation of 2-vinylthiophenol by an E2 elimination, and hydrocarbon elimination by homolysis of the S—Caryi bond. This pathway rationalizes the primary formation of (104) observed in some HDS reactions of (102) over Co/Mo/S catalysts, as well as the kinetic evidence that the rate-determining step on real catalysts is the removal of surface sulfur.158-160... 

This reaction is very exothermic (A// —180 to —200kJ mol-1) and, therefore, seems to be very probable from the thermochemical point of estimation. The pre-exponential factor is expected to be low due to the concentration of the energy on three bonds at the moment of TS formation (see Chapter 3). To demonstrate that this reaction is responsible for the oxidative destruction of polymers, PP and PE were oxidized in chlorobenzene with an initiator and analyzed for the rates of oxidation, destruction (viscosimetrically), and double bond formation (by the reaction with ozone) [131]. It was found that (i) polymer degradation and formation of double bonds occur concurrently with oxidation (ii) the rates of all three processes are proportional to v 1/2, (iii) independent of p02, and (iv) vs = vdbf in PE and vs = 1.6vdbf in PP (vdbf is the rate of double bond formation). Thus, the rates of destruction and formation of double bonds, as well as the kinetic parameters of these reactions, are close, which corroborates with the proposed mechanism of polymer destruction. Therefore, the rate of peroxyl macromolecules degradation obeys the kinetic equation ... 

One could plunge into the steric problems posed by the mechanism of protein synthesis on the ribosome 25 26)> or consider the steric fit of the hormone insulin to its acceptor in the cell membrane 27>. Or one could delve into the beautiful intricacy of terpenoid, squalene and steroid metabolism, or get lost in double bond formation, or in the steric problems posed by the branched chain fatty acids and their derivatives 28-34). 

Fig. 3.3. Possible mechanisms of carbon-metal double-bond formation by a-abstraction.

Some degradation reactions occur without an increase in cross-linking or a lessening in chain length. Thus, with minute amounts of HCl, water, ester, etc. elimination can occur with vinyl polymers giving localized sites of double bond formation. Because such sites are less flexible and more susceptible to further degradation, these reactions are generally considered as unwanted. 

In conclusion, the anti selectivity for hydrogen abstraction of the ene reaction of trisubstituted olefins is related (a) to the degree of crowdedness of the more substituted side of the olefin (b) to the non-bonded interactions during the new double bond formation and (c) to the lack of interaction of oxygen with two allylic hydrogens. 

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