Diene complexes nucleophilic 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). 

Transition metals promote formation of w-allyl complexes from dienes by nucleophilic attack. Different nucleophiles and dienes participate in the reaction, which may take two routes where the nucleophile and the metal add either from the same (path a) or the opposite face of the diene (path b). Nucleophiles may be divided into two classes one, which includes alcohols, amines, carboxylates and malonates, reacts preferentially according to (a) ... 

K. F. McDaniel, Comp. Organomet. Chem. II, 1995,12, 601-622. Transition Metal Alkene, Diene, and Dienyl Complexes Nucleophilic Attack on Alkene Complexes. R. W. Bates, Comp. Organomet. Chem. II, 1995, 12, 349-386. Transition Metal Carhonyl Complexes. 

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. 

Non-conjugated dienes ate the source of many palladium enyl complexes. Nucleophilic attack at one of the double bonds of the coordinated diene has been used most, and many examples can be found in COMC (1982) and COMC (1995). The external attack of the nucleophile leads to an overall /rtf r-addition of Pd and the incoming species to the double bond. Oxidation followed by nucleophilic attack on COD affords <7,7 -enyl derivatives from Pd(l,5-COD)(benzoquinone). In the presence of acid, benzoquinone oxidizes Pd(0) to a Pd(n) diene complex with the concomitant formation of hydroquinone. If a suitable nucleophile is present, such as OAc when acetic acid is used, attack on the diene takes place, and a palladium complex is formed (Scheme 80). A weaker nucleophile like CH3S03 is not capable of adding to the double bond, and the reaction stops at the COD Pd(n) complex. The behavior of other quinones and acids was also studied. ... 

The steric crowding introduced in the latter by the four ethyl substituents inhibits nucleophilic attack at platinum, so that complexes of this type tend to undergo substitution by a dissociative mechanism [89]. The complex of the more rigid ligand, 2,2, 2"-terpyridyl, Pt(terpy)Cl+, is found to be about 103 to 104 times more reactive to substitution than the dien analogue this is ascribed to steric strain [90], which is reflected in the short Pt—N bond to the central nitrogen (Pt-N some 0.03 A shorter than the other two Pt-N bonds) and N—Pt—N bond angles of 80-82°). 

Comparison of results for complexes of tridentate amines R2N(CH2)2-NH(CH2)2NR2 show similar effects. With dien (R = H), rapid substitution of chloride in Pt(dien)Cl+ by bases occurs at room temperature however with Et4dien (R = Et) the reaction is considerably slowed, since the four ethyl groups crowd the metal above and below the plane of the molecule (Figure 3.82) making nucleophilic attack harder. Such a complex can be attacked more easily by a small nucleophile rather than a better nucleophile which happens to be larger [89],... 

Palladium-catalyzed oxidation of 1,4-dienes has also been reported. Thus, Brown and Davidson28 obtained the 1,3-diacetate 25 from oxidation of 1,4-cyclohexadiene by ben-zoquinone in acetic acid with palladium acetate as the catalyst (Scheme 3). Presumably the reaction proceeds via acetoxypalladation-isomerization to give a rr-allyl intermediate, which subsequently undergoes nucleophilic attack by acetate. This principle, i.e. rearrangement of a (allyl)palladium complex, has been applied in nonoxidative palladium-catalyzed reactions of 1,4-dienes by Larock and coworkers29. Akermark and coworkers have demonstrated the stereochemistry of this process by the transformation of 1,4-cyclohexadiene to the ( r-allyl)palladium complex 26 by treatment... 

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

Examination of the reactivity of acyclic (diene)Fe(CO)3 complexes indicates that this nucleophilic addition is reversible. The reaction of (C4H6)Fe(CO)3 with strong carbon nucleophiles, followed by protonation, gives olefinic products 195 and 196 (Scheme 49)187. The ratio of 195 and 196 depends upon the reaction temperature and time. Thus, for short reaction time and low temperature (0.5 h, —78 °C) the product from attack at C2 (i.e. 195) predominates while at higher temperature and longer reaction time (2 h, 0 °C) the product from attack at Cl (i.e. 196) predominates. This selectivity is rationalized by kinetically controlled attack at the more electron-poor carbon (C2) at low temperature. Nucleophilic attack is reversible and, under conditions where an equilibrium is established, the thermodynamically more stable (allyl)Fe(CO)3" is favored. The regioselectivity for nucleophilic attack on substituted (diene)Fe(CO)3 complexes has been reported187. The... 

Liebeskind and coworkers have examined the reactivity of (2//-pyran)Mo(CO)2Cp+ cations 210, which may be prepared in optically active form from carbohydrate precursors. Nucleophilic attack on cation 210 occurs at the diene terminus bonded to the ring oxygen to give jr-allyl complexes 51 (Scheme 53)85. Hydride abstraction from 51 gives the cation 54 addition of a second nucleophilie occurs regioselectively to give... 

Catalyst conditions very similar to those employed in Eq. 42 and Scheme 12 were used recently by Lloyd-Jones, Booker-Milburn, and coworkers to achieve Pd-catalyzed diamination of conjugated dienes with urea nucleophiles (Eq. 44) [181], Both dioxygen and BQ were evaluated as oxidants, and BQ proved to be significantly more effective (Eq. 44). The beneficial effect of BQ probably arises from its ability to promote nucleophihc attack on the intermediate Tt-allyl palladiiun complex (see below. Sect. 4.4). This hypothesis is supported by the observation that the oxidative amination of styrene with urea, which does not undergo the second nucleophilic attack, proceeds equally effectively with both O2 and BQ as the oxidant (Eq. 45). 

A diene system with unsymmetrical 1,4-disubstitution is converted to the iron carbonyl complex 1 which is resolved into its enantiomers. The aldehyde function is conformationally locked in the transoid position and is diastereofacially shielded from the bottom face. Nucleophiles attack from the top face with high selectivity. Alternatively, chain elongation leads to the triene 2 which is reacted with diazomethane. Cerium(IV) oxidation removes the metal and furnishes the substituted cyclopropane 3. 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

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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