Retention of Stereochemistry in the Formation and Fragmentation of Diosmacyclobutanes, and Other Evidence against a Diradical Mechanism

Hembre, Robert T.; Ramage, David L.; Scott, Christopher A.; Norton, Jack R.

doi:10.1021/om00020a015

Cited by 13 publications

(17 citation statements)

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“…In none of these cases was a concerted dinuclear elimination process demonstrated. Stereospecific alkene extrusion from a 1,2-diosmacyclobutane complex has been observed, and evidence against a diradical intermediate was presented . It is clear, however, that the factors that affect the course of elimination reactions in bimetallic systems, as well as their intimate mechanisms, are not yet well understood.…”

mentioning

confidence: 99%

Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

1999

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Whereas the majority of homogeneously catalyzed reactions are believed to involve reactions at a single metal center, there are many instances in which the possibility of a cooperative effect between two or more metals exists, and there are a growing number of cases where such interactions are clear. [1][2][3][4][5][6][7][8][9] Mechanistic studies of monomeric complexes have established that dinuclear reductive eliminations occur in a few cases, 10 but there have been relatively few studies involving discrete, bimetallic systems. Elimination of H 2 or CH 4 from dppm-bridged diplatinum species has been reported, 11,12 and ethane was formed when solutions of [Pt 2 Me 3 (µ-dppm) 2 ] + were photolyzed (both intra-and intermolecular mechanisms were involved). 13,14 Both methane and ethane were reported 15 to be eliminated when a solution of [Pd 2 Me 2 (µ-H)(µ-dppm) 2 ]BPh 4 was warmed above ambient temperature. Carbonylation of [Rh 2 Me 2 (µ-CO)(µ-dppm) 2 ] produced acetone and/or butanedione, depending on the CO pressure employed. The former was shown to be formed by an intramolecular pathway, whereas butanedione was produced by a radical mechanism. 16 In none of these cases was a concerted dinuclear elimination process demonstrated. Stereospecific alkene extrusion from a 1,2-diosmacyclobutane complex has been observed, 17 and evidence against a diradical intermediate was presented. 18 It is clear, however, that the factors that affect the course of elimination reactions in bimetallic systems, as well as their intimate mechanisms, are not yet well understood.We have prepared a series of halide-and hydride-bridged dipalladium A-frame cations of the type [Pd 2 R 2 (µ-X)(µ-dppm) 2 ] + , and these serve as excellent bimetallic substrates from which to study reductive elimination reactions. We have prepared the former by one of three routes, namely, (i) addition of dppm to [PdClR(cod)] or [Pd 2 R 2 (µ-Cl) 2 (AsPh 3 ) 2 ], 19 (ii) reaction of [PdBr 2 -(dppm)] with RMgBr, 20 or (iii) treatment of the palladium(I) complex [Pd 2 Cl 2 (µ-dppm) 2 ] with RMgBr, followed by CBr 4 . 21 The halide-bridged species may be isolated as their halide or PF 6 -salts; subsequent treatment with NaBH 4 or NaBH 3 CN gave their hydride-bridged analogues (Scheme 1). 19,20 In contrast to an earlier report, 16 we have found complexes of the type [Pd 2 R 2 (µ-H)(µ-dppm) 2 ]PF 6 to be thermolytically inert at ambient temperature, at least as their PF 6 -salts. We generated the mixed metal complex [PdPt(4-tolyl) 2 (µ-H)(µ-dppm) 2 ]PF 6 by method iii using [PdPtCl 2 -(µ-dppm) 2 ] as the precursor. 22 We had succeeded previously in preparing unsymmetrical platinum-containing A-frames starting from [PtR(dppm-PP′)(dppm-P)] + , 23 but this approach is not generally applicable to palladium. With a bulky organic group on palladium, however, we were able to produce [Pd(Mes)(dppm-PP′)(dppm-P)] + quantitatively, and further treatment with [PdClMe(cod)], TlPF 6 , and NaBH 4 gave the unsymmetrical dipalladium complex [Pd 2 (Mes)Me(µ-H)(µ-dppm) 2 ]PF 6 .The 1...

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confidence: 99%

Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

1999

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“…Further investigation of the possible radical nature of olefin addition to hydride 3 utilized the well-studied rearrangements of certain unsaturated organic radicals. These rearrangements have been used to “clock” competitive reaction pathways for both organic and organometallic systems. − For olefin addition to 3 by the radical chain pathway of Scheme , the species subject to rearrangement is produced by the addition of the metalloradical A to the olefin. Subsequent H atom transfer to B then competes with the rearrangement so that the ratio of rearranged to unrearranged product indicates the relative rates of hydrogen atom transfer and radical rearrangement as shown in Scheme for bis(allyl) ether.…”

Section: Resultsmentioning

confidence: 99%

Reactions of the Schiff Base Rhodium Hydride Complex RhH(Bu₄salophen) with Olefins. Observations and Mechanistic Studies

and

Eisenberg

1996

Organometallics

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The tetradentate dianionic Schiff base ligand Bu 4 salophenH 2 reacts with [RhCl(C 2 H 4 ) 2 ] 2 and NR 4 OH (R ) n-Bu, Et) to produce the complexes RhR(Bu 4 salophen) (1, R ) n-Bu; 2, R ) Et), which undergo photolysis (λ > 475 nm) under a hydrogen atmosphere to generate RhH(Bu 4 salophen) (3) and the corresponding alkanes. In benzene at room temperature, olefins insert into the metal-hydride bond ofIn a competition experiment, insertion of styrene is only slightly favored over that of ethylene. For olefinic substrates capable of radical rearrangement (CH 2 dCHR where R ) CH 2 OCH 2 CHdCH 2 and c-C 3 H 5 ), insertion products are formed that do not correspond to radical rearrangements. Specifically, Rh(CH 2 CH 2 OCH 2 CHdCH 2 )(Bu 4salophen) ( 6) is observed as the only product of the reaction of 3 with bis(allyl) ether while Rh(CH 2 CH 2 -c-C 3 H 5 )(Bu 4 salophen) ( 7) is seen as the major product in the reaction with vinylcyclopropane. For the vinylcyclopropane reaction, additional products are also identified by 2D NMR methods as Rh(CH 2 CH 2 CHdCHCH 3 )(Bu 4 salophen) ( 8) and (Bu 4 salophen)Rh-(µ-CH 2 CH 2 CHdCHCH 2 )Rh(Bu 4 salophen) (9). Both the poor selectivity for styrene versus ethylene insertion and the absence of radical rearranged product with bis(allyl) ether and vinylcyclopropane indicate that, in the reaction of 3 with olefins, an intermediate containing a pure carbon-based radical is not involved.

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“…A f u l l paper has appeared that reports our work on the stereochemistry of olefin exchange into diosmacyclobutanes: (ref 3) "Retention of Stereochemistry in the Formation and Fragmentation of Diosmacyclobutanes, and Other Evidence Against a Diradical Mechanism," R.T. Hembre, D.L. Ramage, C.P. Scott andJ.R.…”

Section: B Mechanism Of the Formation And Fragmentation Of Diosmacycmentioning

confidence: 99%

Catalytic applications of mono- and dinuclear complexes containing metal-carbon sigma bonds. Final report, November 1, 1993--October 31, 1996

Norton¹

1996

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This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, m o mmendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Retention of Stereochemistry in the Formation and Fragmentation of Diosmacyclobutanes, and Other Evidence against a Diradical Mechanism

Cited by 13 publications

References 1 publication

Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

Reactions of the Schiff Base Rhodium Hydride Complex RhH(Bu₄salophen) with Olefins. Observations and Mechanistic Studies

Catalytic applications of mono- and dinuclear complexes containing metal-carbon sigma bonds. Final report, November 1, 1993--October 31, 1996

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Retention of Stereochemistry in the Formation and Fragmentation of Diosmacyclobutanes, and Other Evidence against a Diradical Mechanism

Cited by 13 publications

References 1 publication

Reductive Elimination of Alkanes and Arenes from [Pd2R2(μ-H)(μ-dppm)2]PF6 Complexes

Reductive Elimination of Alkanes and Arenes from [Pd2R2(μ-H)(μ-dppm)2]PF6 Complexes

Reactions of the Schiff Base Rhodium Hydride Complex RhH(Bu4salophen) with Olefins. Observations and Mechanistic Studies

Catalytic applications of mono- and dinuclear complexes containing metal-carbon sigma bonds. Final report, November 1, 1993--October 31, 1996

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Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

Reductive Elimination of Alkanes and Arenes from [Pd₂R₂(μ-H)(μ-dppm)₂]PF₆ Complexes

Reactions of the Schiff Base Rhodium Hydride Complex RhH(Bu₄salophen) with Olefins. Observations and Mechanistic Studies