Full Picture of the Catalytic Cycle Underlying Palladium-Catalyzed Metal−Carbon Bond Formation

Abstract

A catalytic cycle of reaction between [η5-1-(diphenylphosphino)-2,4-diphenylcyclopentadienyl]tricarbonylmetal iodides, [η5-1-Ph2P-2,4-Ph2C5H2]M(CO)3I (1a, M = Mo; 1b, M = W), with zerovalent palladium and tributyltin acetylides affords the final acetylide products [η5-1-Ph2P-2,4-Ph2C5H2]M(CO)3C⋮CPh (5a, M = Mo; 5b, M = W). This communication describes the main processes and intermediates involved in this palladium-catalyzed metal−carbon bond formation.

Mechanism of the Palladium-Catalyzed Metal−Carbon Bond Formation. A Dual Pathway for the Transmetalation Step

“…With the use of properly designed model substrates [η 5 -(1-Ph 2 P-2,4-Ph 2 )C 5 H 2 ](CO) 3 MI ( 1 , M = Mo; 2 , M = W), bearing a chelating diphenylphosphine sidearm on a cyclopentadienyl ring, it has been possible to isolate and characterize the products of the stoichiometric reaction between the metal−iodide complexes 1 and 2 and zerovalent palladium (Figure ). The complexes ( 3 , M = Mo; 4 , M = W) are the product of oxidative addition of the M−I moiety of 1 or 2 to the palladium center.…”

“…The subsequent reaction of 3 and 4 with trialkyltinacetylides Bu 3 Sn−C⋮C−R affords the corresponding transmetalated complexes ( 5 , M = Mo; 6 , M = W), which are further converted into complexes 7 and 8 , respectively, by trans-to-cis isomerization . The final products of the catalytic metal−carbon coupling process [η 5 -(1-Ph 2 P-2,4-Ph 2 )C 5 H 2 ](CO) 3 M(C⋮CR) ( 9 , M = Mo; 10 , M = W) are then generated by reductive elimination on palladium (Figure ) 9c. Therefore, an entire catalytic cycle, from metal halides 1 and 2 to metal acetylides 9 and 10 , has been examined by analysis of each of the consecutive reaction steps, which are closely related to the typical sequence of the Pd-catalyzed C−C bond-formation process. ,

2 Catalytic cycle of the Pd-promoted metal−carbon (M−C) bond-formation process.

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The Wonder of Palladium Catalysis: From Carbon-Carbon to Metal-Carbon Bond Formation. An Opportunity of Getting Astonishment from Reality

“…35 The processes described in Scheme 22 strongly suggested very close analogies between the Pd-catalyzed carboncarbon and metal-carbon bond formation mechanisms. This parallelism might give helpful hints to understand the mechanism of the former process that, despite its widespread use, is still under much debate.…”

“…Besides isolation and identification of these key intermediates, the overall catalytic cycle was completely mapped by a beautiful 31 P NMR experiment which showed the existence of transient species of critical importance to reveal the intimate dynamics of the phenomenon. 35 The processes described in Scheme 22 strongly suggested very close analogies between the Pd-catalyzed carboncarbon and metal-carbon bond formation mechanisms. This parallelism might give helpful hints to understand the mechanism of the former process that, despite its widespread use, is still under much debate.…”

Catalyzed M–C coupling reactions in the synthesis of σ-(pyridylethynyl)dicarbonylcyclopentadienyliron complexes

“…[52][53][54][55] The role of palladium catalysts in promoting these transformations was also investigated. 52,[56][57][58][59] Although, the Lo Sterzo approach was shown to be a valuable route to alkynyl complexes, the need for preparation of tin reagents and removal of tin impurities limits the appeal of this method. To overcome these disadvantages, one can use an organometallic analogue of the Sonogashira protocol, 60,61 where transition metal halides react with terminal alkynes to form metal alkynyls via the Pd/Cu-or Pd-catalyzed dehydrohalogenation route, which provides milder reaction conditions and facilitates purication of products.…”