IV.10.3 Other Reactions Involving
Palladacyclopropanes and
Palladacyclopropenes
ARMIN DE MEIJERE and OLIVER REISER
Although palladacyclopropanes and palladacyclopropenes can be postulated as intermediates when palladium in the oxidation state 0 or 2 interacts with an alkene or an alkyne, there is very little evidence that Pd-catalyzed reactions of alkenes and alkynes actually proceed this way.
In principle, a palladacyclopropane might be generated starting from palladium(0) or palladium(II) 1 and an alkene 2 (Scheme 1): after coordination to form a -complex 3, oxidative addition would yield a -complex 4, a pallada(II)- or pallada(IV)cyclopropane. Likewise, the analogous reaction with an alkyne 5 would lead to a palladacyclopropene 7.
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Scheme 1 |
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Conclusive evidence for palladacyclopentanes[1] and palladacyclopentadienes[2],[3] as potentially stable intermediates has been put forward, and they have a close analogy to such metallacycles with other metals.[4],[5] They frequently are intermediates in metalcatalyzed or -mediated formal [2 2 2] cyclotrimerizations of alkynes and strained alkenes. It is plausible to assume that palladacyclopropanes or -cyclopropenes also play a role in such formal [2 2 2] processes catalyzed by palladium(0), for example, in the type of Pd-catalyzed cyclotrimerization of 3,3-dimethylcyclopropene reported by Binger et al.[6] (see also Sect. IV.2.4), for which a palladacyclopentane as intermediate was already isolated.[7] Moreover, dimethyl 3,3-dimethylcyclopropenedicarboxylate 8 has recently been shown to form the tricyclic palladacyclopentane derivative 10, the structure
Handbook of Organopalladium Chemistry for Organic Synthesis, Edited by Ei-ichi Negishi ISBN 0-471-31506-0 © 2002 John Wiley & Sons, Inc.
1647
1648 |
IV Pd-CATALYZED REACTIONS INVOLVING CARBOPALLADATIONS |
of which was proved by X-ray analysis of its 1,1 -bis(diphenylphosphinyl)ferrocene adduct 11 (Scheme 2).[1] It is most reasonable that 10 is formed via carbopalladation of 8 by the palladacyclopropane derivative 9.
Pd(dba)2 reacts rapidly with dimethyl acetylenedicarboxylate (DMAD) to form the polymeric palladacyclopentadiene TCPC(13),[2] which could be fully characterized as diazadiene complexes 14 (Scheme 3).[3]
The palladacyclopropene 12 would most probably be an intermediate en route to 13, and, indeed, 1:1 complexes of palladium and DMAD have been obtained and characterized by 1H NMR and IR spectroscopy with stabilizing ligands such as triphenylphosphine[2] and N,N -di-tert-butyl-1,4-diaza-1,3-diene[3] present (Scheme 4). Unfortunately, however, it was
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E = COMe
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Pd
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PPh2 PPh2
Fe
Scheme 2
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E Pd E
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Scheme 3 |
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IV.10.3 REACTIONS OF PALLADACYCLOPROPANES AND PALLADACYCLOPROPENES |
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Scheme 4 |
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not possible to distinguish whether their structures are best described as -complexes 15 and 17 or as palladacyclopropenes 16 and 18, respectively, but the latter seem plausible by analogy to platinacyclopropenes being well established via similar pathways.[8]
In enyne cycloisomerizations developed by Trost (see Sect. IV.2.5 and IV.3.1), palladacycles are also likely intermediates (Scheme 5).[9] In the palladium(II)-initiated cyclization of an enyne 19, the palladacyclopentene 21 has been postulated, requiring palladium to adopt the formal oxidation state 4. Catalytic cycles involving Pd(II) – Pd(IV) are not well precedented; however, by now several reactions have been reported in which Pd(IV) intermediates are most likely,[10] – [16] and one palladium(IV)
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4+
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Scheme 5
1650 IV Pd-CATALYZED REACTIONS INVOLVING CARBOPALLADATIONS
complex has been fully characterized by an X-ray crystal structure analysis.[17] Most probably, 21 would be formed from the -complex 22 via the palladacyclopropene 20 by intramolecular carbopalladation of the double bond.
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