π Bonds are not as strong as σ binding and practically always occur together with σ bonds, which administer the higher share (50–75%) that the force holding the atoms together.

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From: Chemistry: With inorganic Qualitative Analysis, 1980

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5.3 π-Bond Activation


π-Bonds are much more versatile than σ-bonds since they room prone come activation by transition metals.66 For circumstances the π-bond of one alkene have the right to donate electrons right into an north orbital the a transition metal. Subsequently an populated metal d orbital has the correct symmetry come overlap with a π* orbit at the olefin enabling the formation of a metallacycle. These organometallic types are very reactive and have a an excellent potential as man-made intermediates in organic synthesis. The following examples report the π-bond activation v iron catalysts in different ways but always with the typical feature that an stole metallacycle formation in that mechanistic proposal (Fig. 5.1).



Benzofurans have actually been synthesized native 2-alkynylanisoles 76 via a twin system consisting of iron(III) chloride and diselenyl dichalcogenides 77.67 A more facility outcome has actually been speculated by the authors with the following steps: (1) Fe(III) might be an initial reduced come Fe(II); (2) Fe(III) tetracoordinated square-planar selenoate is formed from diphenyl diselenide 77 and also Fe(II) via one oxidative addition; (3) alkyne coordination come the metal core come afford the cationic organoiron complicated XXIX; (4) strike of the oxygen atom native the methoxy group on the caused alkynyl moiety come afford intermediary XXX; (5) a coupling reaction to produce the salt XXXI and; (6) nucleophilic attack of selenoate anion top top the methyl team to offer rise come the final benzo-<b>-furan 78 (Scheme 5.21).


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Furan derivatives via domino procedure involving an intramolecular rearrangement complied with by cyclization and oxidation as a final step.68 In this particular case the reaction pathway illustrated in scheme 5.22 is composed of the activation the enyne adduct 81 to form an iron(III)–alkyne facility XXXII the undergoes a 6-endo-dig enhancement of the enol ether causing the development of a six-membered intermediate XXXIII. This intermediate XXXIII collapses into the β-allenic ketone XXXIV, which undergoes sequential cyclization, carbine oxidation, and also dehydration-oxidation to form furan 82 (Scheme 5.22).


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Scheme 5.22. Sequential intramolecular rearrangement, cyclization and also oxidation in the synthetic of furan derivatives.


Pyrrolidines and tetrahydrofuran derivatives have been also synthesized via reductive cyclization of 1,6-enynes 83.69 The process is thought to take location via an initial preactivation of the steel catalyst with diethy zinc and also magnesium bromide to kind a extremely reactive Fe(0) species. The formation of this reactive species forms the five-membered metallacycle XXXVI. Then, transmetallation with diethylzinc cleaves the metallacycle XXXVI providing intermediate XXXVII, which deserve to undergo β-hydride elimination followed by reductive removed of the iron hydride intermediary XXXVIII come afford pyrrolidine 84 (Scheme 5.23).


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Isochromenones 86 and 87 and also 2H-coumarins 89.70,71 The usage of selenium reagents has been reported for the synthesis of organoselenyl isochromenones and also coumarins. Because the selenium atom can act together a hydrogen-bond agree or electron donor, it transforms the biological properties of together molecules, opened a wide field of research in medicine chemistry, that is, the selenyl derivatives of biologically active products. Organoselenyl isochromenones 86 and also 87 have actually been synthesized native the equivalent alkynylaryl esters 85. Utilizing a comparable procedure the synthesis of 2H-coumarins 89 have actually been lugged out starting from arylpropiolates or arylpropiolamides 88. In this case the proposed device is based upon the assumption that the reaction proceeds v initial activation the the alkyne moiety by the complex Ln to type intermediate XXXIX. Nucleophilic attack from the fragrant ring to XXXIX produce the cyclized species XL, which is followed by reductive elimination to afford the stabilized carbocation XLI. Final aromatization step leads to final product 89 (Scheme 5.24).


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Organoselenylisoxazoles 91 starting from alkynone O-methyloximes 90.72 In this reaction system the writer proposed the formation of a reactive species by the association of iron(III) chloride with the diorganyl diselenide come furnish the reactive species R3SeCl or/and R3SeFeCl2. At this stage, two plausible pathways have the right to be envisaged: (1) the reaction the the alkynyl moiety with R3SeCl leads to selenonium intermediate XLIV, i m sorry is followed by nucleophilic anti-attack from the oxygen atom right into the caused C-sp atom affording the cationic varieties XLV, which offers the preferred product 91 by eliminating methyl chloride; (2) the coordination of the alkynyl moiety through R3SeFeCl2 affords intermediary XLII followed by a nucleophilic anti-attack that the oxygen lone pair right into the caused triple shortcut furnishing the isoxazolyliron types XLIII which then suffers an elimination procedure to provide rise come 91 (Scheme 5.25).

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Scheme 5.25. Synthetic of organoselenylisoxazoles by the association of iron(III) chloride through the diorganyl diselenide.