Ligands dienes, with electrophiles

Iron tricarbonyl forms exceptionally stable complexes with 1,3-dienes. The complexes are uncharged, readily soluble species, chromatographable and, for the simpler versions, distillable. They are formed by direct reaction of the 1,3-diene with Fe(CO)5, Fc2(CO)9, or Fe3(CO)i2. These iron diene complexes are known to be reactive toward electrophiles, undergoing the analogous reaction to electrophilic aromatic substitution under Friedel-Crafts conditions. However, it is clear that the metal-ligand unit increases the polarizibility of the diene unit, and, with a sufficiently reactive nucleophile, can provide a sink for electron density. How reactive does the nucleophile need to be The other important selectivity question for 1,3-dienes concerns the regioselectivity. 

Few examples of functionalization on the benzene ring of benzisothiazole have been reported (see Section 4.05.7.2). Studies on the reactivity of unsaturated chains in cycloaddition reactions have been reported (see Section 4.05.7.3). The high reactivity of 4-vinylisothiazolin-3-one A-oxides in Diels-Alder cycloadditions, both as diene and dienophile, is illustrated by their tendency to dimerize. 5-Vinylisothiazole A,A-dioxides react at the vinyl function with different 1,3-dipoles. Isothiazolo-3-sulfolenes 265 give an o-quinodimethane which can be trapped with a dienophile. Different isothiazole derivatives substituted with a carbon chain functionalized with heteroatoms have been prepared as ligands for the formation of complexes. 3-Oxocamphorsulfonimide reacts with the anion of alkynes and several studies on the reactivity of the products with electrophiles are reported. 

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

Addition of carbon nucleophiles to an internal carbon atom of a diene ligand, even with highly reactive nucleophiles such as diphenylmethyllithium, is reversible at higher temperatures, around 0°C. Equilibration allows slower but more favorable addition at a terminal position, to give (irreversibly) theallyl-Fe(CO)3 anionic complex. The process is illustrated in Scheme 26. Intermediate (9) can be trapped with electrophiles at - 78 °C and undergoes transformation to the thermodynamically stable rf -d y structure, (10), at elevated temperatures. 

As discussed in Chapter 3, olefins and dienes bind to electron-poor metal centers by a flow of electrons from the olefin iT-system to the metal and from the metal to the olefin t -system. Thus, olefins bound to electron-rich and strongly backbonding metal centers react with protons and electrophiles directly at the metal-carbon bond. However, olefins and dienes coordinated to electron-poor metal centers are less reactive toward electrophiles than those bound to electron-rich metal centers or even free olefins and dienes. However, electron-poor olefin and diene complexes do imdergo reactions with electrophiles at the coordinated ligand by an indirect pathway. This indirect pathway occurs by insertion of the olefin or diene into the bond formed by attack of the electrophile at the metal. 

This chapter illustrates that electron-rich transition metal-diene complexes can couple with carbon electrophiles and, thereby, provide unusual methods for carbon-carbon bond formation. These procedures are of interest from a synthetic viewpoint since normally uncomplexed dienes or polyenes are not reactive toward weak carbon electrophiles or, with strong electrophiles, undesirable reactions such as polymerization occur. Furthermore, the metal-mediated route often results in desirable regio- and/or stereo-selectivity. Important to the utility of these methods is the ability to free the organic ligand from the metal. In most instances efficient oxidative procedures have been developed for such cleavage reactions. 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

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