Diene complexes attack

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the rr-diene complex and the second on the intermediate jr-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. 

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). 

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. 

Friedel Crafts acetylation of butadiene complex 56 proceeds smoothly to give a mixture of 1-acetyldienes 58 and 59 via the cationic 7r-allyl complex 57 [16]. Intramolecular Friedel-Crafts acylation with the acid chloride of the diene complex 60, promoted by deactivated AICI3 at 0 °C, gave the cyclopentanones. The (Z)-dienone complex 61 was the major product and the ( )-dienone 62 the minor one [17]. Acetylation of the 1,3-cyclohexadiene phosphine complex 63 proceeded easily at —78°C to give the rearranged complex 65 in 85% yield. Without phosphine coordination, poor results were obtained [18,19], In this reaction, the acetyl group at first coordinates to Fe, and attacks at the terminal carbon of the diene from the same... 

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... 

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). 

Secondary or primary amines Dienes Primaiy attack resulting in a Tr-allyl complex [3]... 

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. 

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the r-diene complex and the second on the intermediate 7r-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. Also, the 1,4-dialkoxylation of acyclic 1,3-dienes was stereoselective. For example, the reaction of ( , )-2,4-hexadiene gave the d,l products 38 by a 1,4-syn addition. The double bond was exclusively of E configuration (equation 18),... 

In selected cases, l-/ -alkyl-3,4,5-t -allyl complexes can be obtained via nucleophilic attack of carbanions at an internal carbon of pentadienyl ligands. Thus, in contrast to a previous communication, nucleophilic attack of the malonate carbanion does not occur at the terminal position of a pentadienyltricarbonyliron cation complex 1 to give a diene complex product 2 as the major product. Instead, the internal position is attacked to give an alkyl-allyl complex 3, which upon oxidative decomposition aifords a vinylcyclopropane product X ... 

Typical examples of the half sandwich type Cp complex are anionic zerovalent metal complexes of the type [CpM(CO)3]" and their binuclear derivatives [Cp2M2(CO)e] [12, 19]. Starting from the anion [CpM(CO)3] , a variety of complexes have been derived, including the neutral hydrides, CpMo(H)(CO)3, alkyls, CpM(R)(CO)3, [12] and the cationic allyl or diene complexes, [CpMo( n -C3H5)(NO)(CO)] [20, 21] or [CpMo(r -C4H5)(CO)2] [21, 22]. The stereochemistries of nucleophilic attack on the last two complexes have been extensively studied [23]. Permethylcyclopentadienyl of these complexes, e.g. (Ti -C5Me5)Mo(H)(CO)3 [24], have also been extensively reported. 

S.8.2.4.2. Diene Complexes by Nucleophilic Attack on Metal Cationic Complexes. 

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