Iron-catalyzed alkenylation

IV. IRON-CATALYZED CROSS-COUPLING REACTIONS A. Iron-catalyzed Alkenylation of Grignard Reagents... 

Iron-catalyzed alkenylation of Grignard reagents was used by Cahiez and Avedissian to prepare the pheromone of Argyroplace Leucotetra in three steps from l,2-( )-dichloro-ethene (Scheme 63). Two successive alkenylation reactions, the first involving a cobalt catalysis, the second an iron catalysis, allow one to obtain the desired product in 45% overall yield. 

Alternatively, iron-catalyzed alkenylation of organomagnesium compounds provides a highly stereo- and chemoselective synthesis of substituted alkenes. ... 

Cahiez and Knochel demonstrated the generality of iron-catalyzed alkenylation and recognized the advantageous use of cosolvents, such as NMP [188,189]. For example, the reaction of bromide 344 with BuMgCl furnishes in the presence of only 1 mol% Fe(acac)3 product 345 in 79% yield (Scheme 4.78) [188]. 

The first example of iron-catalyzed cross-conpling reaction between Grignard reagents and alkenyl bromides was reported by Kharasch and Fnchs in 1945 (Scheme 15). 

TABLE 4. Iron-catalyzed coupling of Grignard reagents with alkenyl bromides... 

The iron-catalyzed cross-coupling between Grignard reagents and alkenyl sulfides was also studied (Scheme 26) . Unfortunately, this reaction is very sensitive to steric and electronic effects and only two products were synthesized in modest yields. In fact, the scope of the reaction is very limited since only vinyl sulfides can be used. 

Table 5.3 Iron catalyzed cross-coupling reactions of alkenyl sulfonates.

Scheme 5.4 Iron-catalyzed cross-coupling reaction of alkenyl sulfones developed by Julia and coworkers.

Transition metal-catalyzed cross-coupling reactions between vinyl organometallic compounds and unactivated alkyl halides that can be usually performed with palladium, nickel and cobalt are of particular synthetic interest [37-39]. Recently, the groups of Cahiez [48] and Cossy [49] concurrently reported the first iron-catalyzed reaction of alkenyl Grignard compounds with primary and secondary alkyl halides (X=Br, I) (Scheme 5.15). The two protocols basically differ in the iron source... 

Cross coii ing of Grignard reagents with 1-alkenyl halides, in marked contrast to alkyl halides, occurs readily with the reduced iron catalyst, as described above. The iron-catalyzed reaction of Grignard reagents with 1-alkenyl halides can, however, be differentiated from the reaction with alkyl halides. Thus, a mixture of propenyl bromide and ethyl bromide on reaction with methylmagnesium bromide afforded butene-2 but no cross-over products such as pentene-2 or propylene. The latter certainly would have resulted if a propenyliron species per se were involved in the catalytic process. Cross coupling under tEese circumstances clearly merits further study. 

Scheme 1.14 Iron-catalyzed cross-coupling with alkenyl halide 39 (Kochi, 1971).

This method has been exploited in a synthetic sequence leading to salvinorin A that shows potent K opioid agonist activity. Alkenyl sulfrdes can also be employed as electrophiles in iron-catalyzed cross-coupling reactions with Grignard reagents (Scheme 4-215). The reaction is chemoselective for alkenyl sulfides, whereas aryl sulfides show only very low reactivity. ... 

Another feasible combination for iron-catalyzed cross-coupling reactions is those of functionalized arylcopper reagents as nucleophiles with alkenyl sulfonates as electrophiles. Tris(acetylacetonato)iron (10 mol%) functions as a precatalyst in this system that tolerates ester or cyano groups. The required arylcopper reagents can be prepared from the corresponding Grignard reagents by treatment with copper(I) cyanide di(lithium chloride) (Scheme 4—217). ... 

In addition to the cross-coupling reactions of alkyl halides or sulfonates with organometallic nucleophiles, the former can also directly undergo C-C bond formation with nonactivated C-H bonds of alkenes or arenes in a Heck-type reaction. Iron-catalyzed intramolecular dehydrohalogenation of 2-iodoethanal alkenyl acetals with phenyl-magnesium bromide in the presence of a catalytic amount of iron(II) chloride provides tetrahydrofliran derivatives in moderate yields (Scheme 4-240). Allyl 2-halophenyl ethers provide dihydrobenzofiiran derivatives under these conditions. ... 

Dry air can be employed as an oxidant in iron-catalyzed homocoupling of aryl and alkenyl Grignard reagents. The latter can be coupled in a stereoselective fashion. ... 

Scheme 4.81 Iron-catalyzed cross-coupling of alkenyl triflates.

Allylation of perfluoroalkyl halides with allylsilanes is catalyzed by iron or ruthenium carbonyl complexes [77S] (equation 119) Alkenyl-, allyl-, and alkynyl-stannanes react with perfluoroalkyl iodides 111 the presence ot a palladium complex to give alkenes and alkynes bearing perfluoroalkyl groups [139] (equation 120)... 

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