Diene complexes, electrophilic attack

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

This reaction has been studied computationally with Zn2+ as the metal cation.32 The calculations indicate that a stepwise reaction occurs, beginning with electrophilic attack of the complexed dienophile on the diene. 

One possible mechanism is electrophilic attack of the complexed carbene carbon atom at the terminal carbon of the diene. The resulting zwitterionic intermediate can now eliminate the metallic group (CO)5M directly, or, alternatively, the metallic group can migrate to yield a new, more stable zwitterion (stabilization of the allyl cation by the heteroatom X). 

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. 

Substitution of one carbonyl by PI13P in (diene)Fe(CO)3 complexes results in a change of regiospecificity of electrophilic attack and thus provides an easier access to [(allyl)FeL,4]X salts. Similar PI13P substitution in [(dienyl)Fe(CO)3]X complexes decreased reactivity towards nucleophiles495. 

In addition to protection, a change of diene reactivity is effected by coordination to carbonyl. Butadiene forms the very stable complex 56 and its reactions are different from those of free butadiene. Electrophiles attack C(l) or C(4) of the complexed dienes, and reactions that are impossible with uncomplexed dienes now become possible. 

These have previously been obtained by electrophilic attack on ene-yl complexes [equation (a) Y = CH(C02Me)2, OMe ch = 1J-C5H5 diene = 1,5-cyclooctadiene]1 or by reaction of the compounds (diene)MBr2 with 57-C5H6Fe(CO)2Br (diene = 1,5-cyclooctadiene or 1,2,3,4-tetraphenyl-l,3-cyclobutadiene).2 An example of the former method is given in which the methoxy-cyclooctenyl derivative is used as the substrate and tetrafluoro-boric acid as the electrophile. The substrate is conveniently prepared and used without isolation, and in this way the reaction takes only a few hours, starting with dichloro(l,5-cycloocta-diene)palladium, prepared as described above. 

Other reports of (jj -allyl)Fe(CO)3R complexes are much more scattered and much less systematic. At least three general types of reactions have been observed in more than one case. First, the reaction of (diene)Fe(CO)3 complexes with electrophilic alkenes gives aUyhron complexes in two different ways. If the diene in question is acylic, electrophilic attack at C-1 of the diene gives compounds of type (158) (equation 33). In the case of substituted rj -cycloheptatriene-or azepine-Fe(CO)3 complexes, reaction... 

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]. ... 

The diene-Br2 complex is again in equilibrium with the reagents, and nucleophilic attack at carbon can be carried out either by the bromide of the ammonium bromide ion pair, formed at the moment of the electrophilic attack, or by the less nucleophilic pyridine added in excess in the reaction medium. It is noteworthy that this mechanism is characterized by a rate- and product-limiting nucleophilic step which should be quite insensitive to steric hindrance around the double bond. In agreement with a weak influence of the steric effects, pyridinium perbromide reacts in chloroform and tetrahydrofuran with substituted conjugated and non-conjugated dienes to give selectively (>95%) bromine addition to the more alkylated double bond (equation 44). 

The regiospecificity of cyclohexadienyl complex formation via the oxidation of [Fe(CO)3 77" -substituted diene)] substrates by thallium tris(trifluoroacetate) has been examined.These reactions are considered to proceed via initial electrophilic attack of Tl(III) at the coordinated diene to give allyl intermediates such as (56) and (57). By using optically active starting diene complexes it was confirmed that... 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

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. 

The combination of Birch reduction and electrophilic abstraction, followed by nucleophilic attack on the resulting cation, generates a modified coordinated diene complex, as shown in Equation 12.72. Decomplexation and hydrolysis yields the enone product. Alternatively, additions of nucleophiles and electrophiles have been conducted in reverse, and one such sequence is shown in Equation 12,73. In this case, nucleophilic attack on a coordinated polyene leads to a polyenyl system that is susceptible to hydride abstraction. Abstraction of the hydride restores the polyene system and allows for a second nucleophilic attack. Flydrolysis, lactonization of the half acid, and oxidative decomplexation gives the final free lactone. The product of this sequence contains the two nucleophiles cis to each other because they are delivered to the dienyl fragment on the side anti to the metal center. 

Direct electrophilic attack on the exo side of a coordinated diene is also possible, even to complexes of modestly electron-rich metal centers. For example, Friedel-Crafts acetylation of (cyclohexa-l,3-diene)Fe(CO)3 gives principally the exo isomer. However, such direct exo attack occurs more commoiJy on the uncoordinated portion of a partially coordinated polyene or polyenyl ligand (recall that uncoordinated olefins and dienes are generally more nucleophilic than coordinated ones). An example of such exo attack on the uncoordinated portion of a pentadienyl ligand is shown in Equation 12.74. In ttiis case, coordination of a CO ligand is thought to displace one or more carbons of the dienyl ir-system from the iron center to trigger exo acylation. Similar results have been reported for electrophilic attack on the uncoordinated portion of the polyene system in [(C )Fe(CO)3]". ... 

Cyclohexadienyl complexes react with nucleophiles to give 1,3-diene complexes. An example is shown in Eq. 8.17 the arrow refers to the point of attachment of the nucleophile to the polyene ligand. The synthesis of the starting complex by an electrophilic abstraction is also shown this activates the ligand for nucleophilic attack. Once again, directing effects can be used to advantage a 2-OMe substituent directs attack to the C-5 position of the cyclohexadienyl, for example. ... 

The iron complexes show two-fold reactivity. They react with both strong electrophiles and with strong nucleophiles as the iron can stabilize both the cationic and anionic intermediates. While the electron-withdrawing iron moiety activates the diene to nucleophilic attack, it deactivates it towards electrophilic attack. Electrophilic attack is still useful - the iron stabilizes the diene to all the side reactions that could go along with electrophilic attack, and stabilizes the cationic product. 

As cations, cobalt tricarbonyl diene complexes 10.86 are also more electrophilic than the iron tricarbonyl complexes (Scheme 10.21). The product of nucleophilic attack is a T -allyl cobalt complex 10.87, which is still electrophilic and may undergo a second attack. 

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