Sigmatropic rearrangements of hydrogen

Note that a [ 1,5] sigmatropic rearrangement of hydrogen is also an internal ene synthesis (15-20). 

Note that a [l,5]-sigmatropic rearrangement of hydrogen is also an internal ene synthesis (15-20). Since we are using the arbitrary designations U, V, Y, and Z, we have been arbitrary in which isomer to call (R,Z) and which to call (S,E). 

Fig. 4-13 Frontier orbitals for antarafacial [1,3] sigmatropic rearrangement of hydrogen. The reaction is not observed...

Sigmatropic rearrangements of hydrogen are suprafaeial if the total number of electrons is a (4n+2) number and antarafacial if the total number of electrons is a (An) number. 

Sc(OTf)3 was proved to be an effective catalyst for Diels-Alder reactions of a new type of 1-hydrazinodiene with many dienophiles (Scheme 12.16) [32]. The obtained Diels-Alder adducts could be converted to six-member ring alkenes via suprafacial 1,5-sigmatropic rearrangements of hydrogens after the loss of dinitrogens. This process enabled a 1,3-transfer of stereochemistry to a new position on the six-member ring system. 

Sigmatropic 1, 5-shift This method of analysis can be illustrated by the example of suprafacial [1, 5] sigmatropic rearrangement of hydrogen in which homolytic cleavage produces H-atom and pentadienyl radical ... 

Though the PECH decomposes to indefinite fragments with n-butyl lithium or sodium hydride in THF at room temperature, it reacts with sodium methoxide with liberation of Cl in which the -elimination of hydrogen chloride predominates instead of nucleophilic substitution. For instance, PECH in DMSO was reacted with double the molar quantity of sodium methoxide at room temperature for 24 h to give the unsaturated polyether (DS 92.3%,v(C=C) 1630,5 (=CH2) 795 cm" ) after purification by dissolution(DMF)-precipitation (H20) technique. A similar unsaturated polyther was obtained by the pyrolysis of the sulfilimine 13 (110-130°C) but not of sulfoxide 12 (100-150°C). When the polymer 26, was heated to 90°C, the absorption of C=C and =CH2 decreased and a new absorption at 1720 cm appeared and increased. This is explained as a result of [3.3] sigmatropic rearrangement of to afford including C=CH2 and C=0 structure as shown in equation 7. 

Studying [3.3] sigmatropic rearrangements of difluoroallylic alcohols 4 (R = H, Et) under Daubcn-Dictsch conditions [reflux with ethyl vinyl ether in presence of mercury(II) acetate], has shown that in the case of R = Et, besides the rearrangement in situ, elimination of hydrogen fluoride occurs. It has been claimed that the dehydrofluorination is probably catalyzed by acetic acid which is formed in the course of the reaction.206... 

The process shown in Equation 12.112, in the forward direction termed the ene reaction and in the reverse the retro-ene reaction, bears the same relationship to the [3,3]-sigmatropic reactions as does the [2,3]-elimination (p. 668) to the [2,3]-sigmatropic shift. It is also closely related to the [l,5]-rearrangement of hydrogen. The reaction can occur either intermolecularly, as in, for example, Equations 12.113 and 12.114, or intramolecularly, as in Equations 12.115 and 12.116. 

Selenium dioxide oxidation of alkenes with a hydrogen in an a-position involves the formation of the allyl selenic ester (X = OH) by an ene reaction. [2,3] Sigmatropic rearrangement of the allyl selenic ester to the selenium(II) ester and its hydrolysis also resulted in the formation of allylic alcohols. The oxidation of alkenes with selenium dioxide is covered in Section D.4.10. 

Similarly, 1-halo-l-sulfonylallenes (139) have been prepared by heating in toluene at 80 °C of propargyl esters (138) via [2,3]sigmatropic rearrangement of the latter (Scheme 32) [51]. 1-Bromo-l-sulfonylallenes 139, when treated with bromine, undergo attack on central allenic carbon with formation of intermediate carbenium bromide followed by hydrogen bromide eUmination, and afford stereospecifically the 2,3-dibromo-l-sulfonyl-l,3-dienes 140. 

Rearrangement to the aromatic toluene requires a [1,3] sigmatropic shift of hydrogen. 

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