Alkenes fragmentation

Intramolecular thermal [4+2] cycloaddition occurs smoothly, though at a high temperature, upon heating of the appropriately substituted electron-poor oxadiazole containing electron-rich alkene fragment (Equation 1) <2002JOC7361>. 

In contrast to reactions with vinyl epoxides and palladium catalysts, the reactions with rhodium retain the stereochemistry of the alkene fragment during the reaction [20]. This is illustrated by the reactions of trans-37a/h and cis-37a/b, which give only one product possessing the same olefin geometry as the starting epoxides (Eqs. 4 and 5). The retention of olefin stereochemisty has also been documented in allylic functionalizations with iridium catalysts, indicating that similar modes of action may be present [21, 22]. 

Fig. 1 Qualitative molecular orbital diagrams for the alkene fragment of benzoquinone and dioxygen highlighting the key differences in their respective frontier orbitals...

Benzoquinone is widely recognized as a useful oxidant in organic and inorganic chemistry [79,80]. It often reacts with transition metals by coordination of the electron-deficient alkene fragment to the metal center, forming... 

In contrast, when the mode of substitution is reversed, that is when the radical is fluorinated and the alkene fragment is not, as is the case with radicals 19, 21-23, a much greater impact on reactivity is observed. 

In mass spectrometry, the loss of an alkene fragment by a cyclic rearrangement of a carbonyl compound having y hydrogens, (p. 826)... 

The substitution of functional groups at the electrophilic double bond or catalytic replacement of the hydrogen atom of alkene fragment by pyrroles or indoles has found wide use in the synthesis of U-vinylpyrroles or -indoles. Thus, indole was reacted with ethyl 2-nitro-3-ethoxyacrylate 662 to give ethyl 2-nitro-3-(3-indolyl)acrylate 663 as a 1 1 mixture of the (E)- and (Z)-isomers (Equation 159) <1996TL3309>. 

Bicyclo[3.3.0]oct-l(5)-ene 178 (Scheme 4.55) is a stable compound with a flattened alkene fragment and exhibits a regular pattern of reactivity. Computational studies revealed, however, that installation of a short 3,7-bridge should lead to noticeable pyramidalization of the double bond. Compounds like 179-181 were synthesized to check this prediction. Tricyclic hydrocarbon 179, with the smallest possible bridge, was generated as a transient species from diiodide 182. The formation of 179 is implicated by the isolation of its cyclodimer 183 (or respective Diels-Alder adduct if the reaction is carried out in the presence of a 1,3-diene). The next member of this series, 180, is more stable. In fact, the formation of 180 was ascertained not only from the structure of the final products (as was done for 179), but also by its matrix isolation and analysis of spectral data. The selenium derivative 181 was found to be stable at ambient temperature in the absence of oxygen. X-ray data confirmed a noticeable pyramidalization of the double bond in 181 but the distortion was different [Pg.372]

The origin of the olefin product requires some interesting speculation. The only silicon-containing intermediates that are known to lose an alkene fragment thermally with the specific transfer of a (3 hydrogen are the alkyl silylenes (activation energy [E ] 30 kcal/mol) (74-77). 

The first splitting of a phosphirane derivative into an alkene and a phosphinidene fragment was described by Quast and Heuschmann <82CB90i>, and involved an oxide (29) (Scheme 11). The retention of the stereochemistry of the alkenic fragment suggests a concerted mechanism. A clean thermal splitting has also been observed for several phosphirane complexes (Schemes 12 and 13)... 

Coke deposits were studied using mass spectra obtmed from the probe El and Cl analyses of the deactivated catalysts arising from the various feed streams. Alkane and alkene fragments were observed to dominate the individual mass spectra (particularly, m/z 57, 71 and 55, 69, respectively, in the El mode). Although alkylaromatics were evident for the catalyst from the tests with n-hexadecane and the n-hexadecane/phenanthrene mixture PACs are only present in trace quantities. Quinoline addition gave rise to much less intense ions from the deactivated catalyst due to its lower carbon content and the reduced sensitivity made it difBcult to observe the aromatic fragments. Indeed, the most intense peak was from quinoline itself (m/z 129 El, 130 Cl). 

Equation 12.97 illustrates use of catalyst 104 in the synthesis of a bicyclic ring system.207 Note that the presence of a catalyst also allows the transformation to occur under a CO pressure of 1 bar. Equation 12.98 demonstrates a clever application of the so-called traceless tether method for running a Ru-catalyzed P-K reaction.208 Although this is an intermolecular P-K reaction, the alkene is tethered to a pyridylsilyl group (compound 105), which directs regioselective reaction of the alkene fragment with the alkyne. The presence of residual H20 in the reaction mixture removes the silyl group, which can be recycled. 

The ene reaction involves an alkene fragment (the ene) that removes a hydrogen from an allylic fragment, with formation of a new carbon bond, as in 666.. s with the other reactions in this chapter, reactivity and stereochemistry in the ene reaction can be explained by frontier orbital theory. The reaction proceeds via interaction of the HOMO of the alkene (ene) and the LUMO of the allylic partner (enophile), illustrated by 667 in Figure 11.23." The intermolecular reaction usually requires very high temperatures (typically 250-... 

A similar strategy was used for the final coupling in a synthesis of okadaic acid, which involves an olefination reaction (eq 39). More recently, the same reaction conditions have been used for the preparation of another cyclopropyl alkene fragment in the total synthesis of ambruticin S (eq 40)7 ... 

The Michael addition mechanism offers an easy explanation for by-product formation if zwitterion 21 attacks a second molecule of alkene instead of the proton transfer in Scheme 6.10. In this case (Scheme 6.11), the newly generated zwitterion 21 can yield a by-product with two alkene fragments or attack another alkene to eventually produce by-products derived from the insertion of three or more alkenes (23). 

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