Iron-Catalyzed Cycloaddition

R = Ph, 4-MeC6H4,4-MeOCfiH4, 4-Me2NC6H4, 4-FC6H4, 4-lCfiH4, 4-ACC6H4, 4- Me02CCfiH4, 

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Iron-catalyzed Cycloadditions and Ring Expansion Reactions... 

Scheme 4-311. Iron-catalyzed cycloaddition of alkynenitriles with...

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

This chapter will focus on preparative applications of iron catalysts in the synthesis of cyclic ring systems by means of intra- and intermolecular cycloadditions, Alder-ene reactions and ring expansion reactions. Previous reviews concerning iron-catalyzed chemistry have either focused their attention on different aspects involving iron-containing compounds as active catalysts [1] or have concentrated on certain reactions [2] and on the synthesis of specific substance classes [3]. A more recent general review concerning all aspects of modern applications in iron-catalyzed reactions has been summarized by Bolm et al. [4]. 

Scheme 9.10 Formal iron-catalyzed [4+ 4]-cycloaddition to a polycyclic product.

Recently, Chirik s group reported an iron-catalyzed [2 + 2]-cycloaddition process with a,co-dienes (Scheme 9.23) [50]. The tridentate pyridine-diimine complex 31 gave excellent conversions with a short reaction time (TOF>240h-1) and a broad substrate scope is accepted by the catalyst. Esters, amides, amines and even 1,6-heptadiene can be used as substrates without requiring the Thorpe-Ingold effect. 

Scheme 9.23 Intramolecular iron-catalyzed [2 + 2]-cycloaddition of a,co-dienes.

The iron-catalyzed [3 + 2]-cycloaddition (Huisgen reaction) of nitriles and carbonyl compounds as reported by Itoh et al. is one of the rare examples reported where an iron reagent can be utilized for the synthesis of 1,2,4-oxadiazoles (Scheme 9.35) [93]. In this reaction, methyl ketones are nitrated at the a-position by Fe(N03)3 to generate an a-nitro ketone. This intermediate rearranges to an acyl cyanate, which reacts further with the nitrile to give the heterocyclic product 48 in good to excellent yields (R1 = Ph, R2 = CH3 95% yield). 

A formal iron-catalyzed [3 + 2]-cycloaddition of styrene derivatives with benzoqui-none was reported by Itoh s group [96]. The process is believed to proceed via electron-transfer reactions mediated by a proposed Fe3+/Fe2+ couple, which generates a styrene radical cation and a semiquinone. These intermediates undergo stepwise addition to yield the benzofuran product 51 (Scheme 9.38). The reaction seems to be limited to electron-rich alkoxy-functionalized styrenes, as the Fe3+/Fe2+ redox couple is otherwise unable to transfer the electrons from the styrene to the quinone. 

Along with several other Lewis acids, FeCl3 was also tested by Hsung and coworkers in the formal [3 + 3]-cycloaddition of enolized 1,3-diketones with a,(3-unsaturated carbonyl compounds (Scheme 9.39) [97]. The iron-catalyzed reaction gave the desired bicyclic compound 52 in good yields better results were obtained utilizing BF3OEt2 or In(OTf)3 as Lewis acid. 

Fig. 21 Iron-catalyzed [2+2] and [3+2] cycloadditions of electron-rich alkenes...

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

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