Diene complexes nucleophiles

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). 

Cyclopentadiene(diene)cobalt complexes, the largest catagory of diene complexes of Co, may be prepared by direct complexation, by preparation of the dienes within the coordination sphere of Co and by nucleophilic addition to ( j5-dienyl)CoCp cations. In comparison to (diene)CoCp complexes, there are considerably fewer examples of (diene)RhCp and (diene)IrCp complexes known. 

Numerous synthetically useful carbon-carbon bond-forming reactions are based on the fact that unsaturated hydrocarbon ligands bound to electrophilic transition metal moieties are activated toward addition of nucleophiles. Normally the metal moiety in such complexes is a neutral or cationic metal carbonyl group. Prominent and well-studied examples include [Cr(arene)(CO)3] complexes (covered in Chapter 2.4, this volume),1 [Fe(dienyl)(CO)3]+ complexes (covered in Chapter 3.4, this volume),2 [FeCp(CO)2(alkene)]+ complexes3 and [M(CO) (diene)] complexes.4... 

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. 

Intramolecular nucleophilic attack has been demonstrated in several cases, particularly with amine and (rj -allyl)Fp complexes an example of the former is depicted (equation 20). In such cases, the driving force for formation of five- or six-membered rings overrides the preference for attack at the more substituted alkene center. The ability of trialky-lamines to convert (> -aIkene)Fp+ complexes into (rj -allyl)Fp complexes via deprotonation allows conversion of alkadiene complexes (see Diene Complexes) (96) into carbocycles (97,98). The reasons for the stereochemical outcome of these cyclization reactions have not been fully delineated. Finally, ( ) -butadiene)Fp+ cations react in two sequential ( ) -allyl)Fp-( -alkene)Fp+ pairs to give a mixture of cyclic... 

Because of the stability of iron tricarbonyl diene complexes, conjugated dienals are protected from polymerization when complexed, while other reactions can be carried out at the aldehyde functionaUty. A number of synthetically attractive nucleophilic transformations of the aldehyde can be performed on these complexes. These include, aldol reactions, Michael additions, reactions with organozinc, -silicon, -boron, and -tin... 

This process takes advantage of the fact that coordination of a 1,3-diene with Pd, presumably in the ) -mode, can activate the diene toward nucleophile addition. The product is an aUyl Pd complex, which, as described in the previous section, is also reactive toward nucleophiles, also at the less-substituted position, and eventually leads to a 1,4-disubstituted 2-alkene. The Pd ends up as Pd and must be reoxidized in order to operate a smooth catalytic cycle (equation 79). The optimum conditions for the process have been worked out in fine detaU. ... 

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. 

The geometry of alkene coordination appears to affect the reactivity toward nucleophiles. 7-Methylenenorbomene reacts with PdCl2 to give the product from Pd-Cl addition across the exo-methylene double bond (equation 37). In the intermediate PdCl2-diene complex, which could not be isolated, the 7-exo double bond would be coordinated so as to lie in the ring plane, while the other aUcenic group would lie perpendicular to the palladium square plane, as is normal. From the structure of the product, trans addition of Pd and Cl occurs. 

Both 1,4- and 1,5-dienes form stable complexes with Pd. For most 1,3-dienes, such as 1,3-butadiene, reaction with Pd° compounds leads to 7r-allyl formation. These reactions are described in Section 7. The coordinated double bonds in palladium diene complexes are reactive toward attack by many nucleophiles, and the resulting chelating alkene palladium alkyls are easily isolated. Many useful reactions of dienes were discovered by Jiro Tsuji in the 1960s and 1970s. These have been recently reviewed in a historical memoir. ... 

Many nncleophiles add to one of the double bonds of chelating palladium(diene) complexes to give a chelating Pd alkyl(alkene) derivative, as exemphfied by the reaction of PdCl2(l,5-cod) with methoxide (equation 41). In most cases, the direction of attack is exo. If the nucleophile is in a form that can undergo transmetalation with the Pd l bond, such as Ph2Hg, the nucleophihc group can be delivered endo. In this case, prior formation of a Pd nucleophile bond accounts for the direction of attack (equation 42). 

Similar kinetic rate laws have recently been found for the reaction between [PdCl(dien)]+ and inosine, i, k-i, and ks all being of similar magnitude (32). With adenosine instead of Ino, A3 is diminished to the extent that Ai and A 1 can be treated as a pre-equilibrium, and with the unreactive nucleobase uridine, even the reverse anation step, A 3, can again make a contribution. This type of complication has been found in reactions of palladium-dien complexes with the common buffering agent tris(hydroxymethyl)aminomethane (33), which acts as a poor nucleophile. 

Interestingly, this 1,4-carbochlorination occurs syn, which constrasts with that via the vinylpalladation in Eq.(49), which occurs anti. An explanation for this difference is that the allylsilane attacks the palladium-diene complex anti, leading to a rrans-carbopalladation of the double bond. This is the first example of nucleophilic attack by an allylsilane on an olefin coordinated to a metal. Direct evidence for a frans-carbopalladation was provided by the isolation of the proposed 7r-allyl intermediate of Eq.(51) as its chlorodimer 98a from reaction of 97 with Li2PdCl4 in the absence of benzoquinone [Eq.(52)] [119b]. The trans relationship between palladium and the carbon that has attacked the diene was established by the reporter ligand technique used for 41 in Section 8.3.1.1 under Intramolecular 1,4-diacyloxylation . 

In this light the results with diolefins do not appear surprising. In fact with the Pd(II)-carbon systems, also addition has recently been shown to be cis (Section IV, A). When Pd(II) is not 7r-complexed to the double bonds, the addition is cis-exo, and when Pd(II) is complexed in the endo position the addition is cis-endo. Another factor in the diene complexes is the fact two cis coordination positions are taken up. Since chloride may not be as readily displaced to give the nucleophile, attack may tend to be trans. Moreover these are neutral complexes and thus may be more susceptible to nucleophilic attack. 

---