Patai S., Rappoport Z. 1997 The chemistry of functional groups. The chemistry of dienes and polyenes. V. 1 / 0653 681

A. Principle for the Oxidation of Unsaturated Hydrocarbons

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled -orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin Ł -orbital (back donation), leads to an electrophilic activation of the alkene9.

PdII

(1)

If the unsaturated hydrocarbon is a diene, both double bonds may coordinate to palladium(II). (Diene)palladium(II) complexes have been isolated and characterized. For example, 2 and 3 are stable complexes in which both double bonds are coordinated to the metal10. Conjugated dienes constitute a special case and although 4-diene complexes, e.g. 4, are postulated as intermediates, they have not yet been isolated. The butadiene complex 4 is in equilibrium with the -allyl complex 5 in solution, and attempts to isolate the diene complex from this mixture lead to formation of a yellow crystalline complex 511.

A common pathway in palladium-catalyzed oxidation reactions is that the -olefin complex formed reacts with a nucleophile, either external or coordinated, and the new organometallic intermediate may then undergo a number of different reactions (Scheme 1):

(i) an intramolecular hydride shift leads to ketone formation; (ii) a ˇ-elimination results in the formation of a vinyl functionalized olefin; (iii) an oxidative cleavage of the palladium carbon bond produces a 1,2-functionalized olefin7; and (iv) an insertion reaction, exemplified by insertion of an olefin, leads to formation of a new palladium carbon bond, which may be cleaved according to one of the previous processes (ˇ-elimination or oxidative cleavage). In all cases palladium has removed 2 electrons from the organic molecule, which becomes oxidized. These electrons, which end up on Pd(0), are in turn transferred to the oxidant and Pd(II) is regenerated. In this way a palladium(II)-catalyzed oxidation is realized.

In this reaction the first addition product was isolated. In catalytic reactions this is not the case and in these reactions the first ( -alkyl)palladium complex formed from the nucleophilic addition reacts further. For example, in the palladium-catalyzed oxidation of 1,5-cyclooctadiene with Pb(OAc)4 in acetic acid the corresponding diacetate 6 was obtained in 76% yield together with some chloroacetate (equation 3)13,14. Adduct 7 is the suggested intermediate. An additional number of such palladium-catalyzed cyclizations of nonconjugated dienes have been reported15 21. In this system the two nucleophiles incorporated are either two acetates or one acetate and one chloride. For example, norbornadiene gives a mixture of three products, 8, 9a and 9b, where the chloroacetate 8 is the main product (equation 4)15. Another example is the reaction of vinylnorbornene 10, which gives a substituted brendane system 11 in good selectivity and yield16,20. The latter compound was transformed to the important synthetic intermediate 12 containing 5 stereogenic centers (equation 5). This transformation was done via hydrolysis, oxidation to ketone and subsequent Beyer-Villiger oxidation.

(3)

OAc

PdCl

H

(7)

(4)

658 Jan-E. Backvall¨

CO2 Me

H

Cl

Cl

Isolated intermediate:

(5)

(13)

The palladium-catalyzed oxidations of the nonconjugated dienes to give products such as 6, 8, 9 and 11 involve three fundamental organometallic reactions: nucleophilic addition insertion oxidative cleavage of the Pd C bond. It is interesting to note that in the formation of 6 and 11 all three reactions are highly stereoselective. It is generally assumed that the first two reactions are always stereospecific, while the oxidative cleavage may occur with either retention or inversion. The organopalladium intermediate was trapped by the addition of AgBF4 and bipyridyl and in this way the cationic complex 13 was isolated.

When the PdCl2 CuCl2 system was applied to 1,2-divinylcyclohexane 14, only one nucleophile is added to the diene system (equation 6)16. After nucleophilic addition and insertion the -palladium complex 15 formed undergoes a ˇ-elimination. The primary product 16 generated undergoes a double bond isomerization to give 17, which is the product observed. The latter reaction has been improved and developed into a synthetically useful reaction3,4,22 25. By changing the oxidation system to MnO2/p- benzoquinone(BQ)26 in acetic acid with Pd(OAc)2 as catalyst, the product 16 with the exo methylene compound was obtained (equation 7)22. In the copper-catalyzed reaction the latter compound was postulated as the primary product, which under the reaction conditions undergoes an isomerization to the observed product. With MnO2/BQ as the oxidation system, 16 does not isomerize.

SCHEME 3

with palladium chloride in methanol (equation 12)30. The ( -allyl)palladium complex, in which the methoxy group is trans to palladium, was isolated and used for further organic reactions.

PdCl2

ClPd

H Pd Cl

re-addition

MeO

PdCl/2

(26)

(12)

III.CONJUGATED DIENES

A.Intermolecular Reactions

As mentioned in the introduction, -complexes of conjugated dienes with palladium(II) are not stable enough to be isolated. However, reaction of a conjugated diene with PdCl2 in alcoholic solvents or acetic acid gives a ( -allyl)palladium complex 27 in which the