ChemInform Abstract: ISOMERIZATION OF THE ALKYL GROUP IN (η5‐C5H5)FE(CO)(PPH3)(ALKYL) COMPLEXES

Reger, Daniel L.; Culbertson, E. C.

doi:10.1002/chin.197811288

Cited by 2 publications

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“…In contrast to related reactions of metal–oxo complexes that typically occur at the weaker C–H bonds (and also transfer a ligand to a hydrocarbon), the borylation process selectively functionalizes the strong primary C–H bond. This observation implies that the reaction occurs through an organometallic intermediate because linear alkyl complexes are typically more stable than branched alkyl complexes …”

Section: Initial Discovery Of C–h Borylation In High Yieldsmentioning

confidence: 99%

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

2011

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Methods that functionalize C-H bonds can lead to new approaches for the synthesis of organic molecules, but to achieve this goal, researchers must develop site-selective reactions that override the inherent reactivity of the substrates. Moreover, reactions are needed that occur with high turnover numbers and with high tolerance for functional groups if the C-H bond functionalization is to be applied to the synthesis of medicines or materials. This Account describes the discovery and development of the C-H bond functionalization of aliphatic and aromatic C-H bonds with borane and silane reagents. The fundamental principles that govern the reactivity of intermediates containing metal-boron bonds are emphasized and how an understanding of the effects of the ligands on this reactivity led us to broaden the scope of main group reagents that react under mild conditions to generate synthetically useful organosilanes is described. Complexes containing a covalent bond between a transition metal and a three-coordinate boron atom (boryl complexes) are unusually reactive toward the cleavage of typically unreactive C-H bonds. Moreover, this C-H bond cleavage leads to the formation of free, functionalized product by rapid coupling of the hydrocarbyl and boryl ligands. The initial observation of the borylation of arenes and alkanes in stoichiometric processes led to catalytic systems for the borylation of arenes and alkanes with diboron compounds (diborane(4) reagents) and boranes. In particular, complexes based on the Cp*Rh (in which Cp is the cyclopentadienyl anion) fragment catalyze the borylation of alkanes, arenes, amines, ethers, ketals, and haloalkanes. Although less reactive toward alkyl C-H bonds than the Cp*Rh systems, catalysts generated from the combination of bipyridines and iridium(I)-olefin complexes have proven to be the most reactive catalysts for the borylation of arenes. The reactions catalyzed by these complexes form arylboronates from arenes with site-selectivity for C-H bond cleavage that depends on the steric accessibility of the C-H bonds. These complexes also catalyze the borylation of heteroarenes, and the selectivity for these substrates is more dependent on electronic effects than the borylation of arenes. The products from the borylation of arenes and heteroarenes are suitable for a wide range of subsequent conversions to phenols, arylamines, aryl ethers, aryl nitriles, aryl halides, arylboronic acids, and aryl trifluoroborates. Studies of the electronic properties of the ancillary ligand on the rate of the reaction show that the flat structure and the strong electron-donating property of the bipyridine ligands, along with the strong electron-donating property of the boryl group and the presence of a p-orbital on the metal-bound atom, lead to the increased reactivity of the iridium catalysts. Based on this hypothesis, we studied catalysts containing substituted phenanthroline ligands for a series of additional transformations, including the silylation of C-H bonds. A sequence involving the silylat...

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Section: Initial Discovery Of C–h Borylation In High Yieldsmentioning

confidence: 99%

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

2011

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“…To the best of our knowledge, thus far only few tertiary alkyl-metal structures have been described. Some of them contain a tert -butyl group linked to the metal, − while others are supposed to be formed by isomerization of the primary alkyl-metal intermediate via reversible hydrogen 2,3-migration. , We studied the competition between secondary and tertiary Rh-alkyl intermediates in the Rh-catalyzed hydroformylation of various substituted cyclohexenes and related linear olefins under the hypothesis of nonreversibility with a heuristic purpose (i.e., the evaluation of the steric hindrance by comparing selectivity ratios in a systematic way: the relevant article, containing a wealth of results, is still in preparation) . In the meantime, deuterioformylation experiments carried out on ketal-masked β-isophorone brought out evidence for the formation of t (a tertiary Rh-alkyl intermediate) that evolved to c (a primary Rh-alkyl intermediate) .…”

Section: Introductionmentioning

confidence: 99%

Rhodium-Catalyzed Hydroformylation of Ketal-Masked β-Isophorone: Computational Explanation for the Unexpected Reaction Evolution of the Tertiary Rh-Alkyl via an Exocyclic β-Elimination Derivative

Alagona

Ghio

2014

J. Phys. Chem. A

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Ketal-masked β-isophorone (7,9,9-trimethyl-1,4-dioxaspiro[4.5]dec-7-ene) is an interesting case study of Rh-catalyzed hydroformylations not only for the competition between secondary and tertiary rhodium alkyls but also for the unexpected isomerization of the tertiary Rh-alkyl to the exocyclic olefin which undergoes hydroformylation, yielding the acetaldehyde derivative (2) of 7,9,9-trimethyl-1,4-dioxaspiro[4.5]decane. Although experimental results at 100 °C pointed to reaction reversibility, the reason for this kind of behavior was however obscure. A thorough density functional theory (DFT) computational investigation of the various transition states (TS) and intermediates along the reaction pathways making use of B3P86 hybrid functionals and the 6-31G* basis set, coupled to effective core potentials for Rh in the LanL2DZ valence basis set, has been carried out to shed some light on the reaction mechanism. The TS barrier heights, based on alkyl-Rh TS free energies, computed under the hypothesis of nonreversibility were in favor of a normal hydroformylation reaction (III:II = 70:30). While the endocyclic olefins produced skew or twisted arrangements of the six-membered ring similarly to the CO insertion TS that can be even higher than the alkyl-Rh ones, grid-point calculations during the potential energy surface (PES) scan produced the much more stable chair conformation for the exocyclic olefin complex. The relevant TS were found to be very favorable as well, thus explaining the preference for the exocyclic arrangement of the tertiary intermediate, for which the reaction is therefore entirely reversible and invariably proceeds to the acetaldehyde derivative (2). Conversely for the secondary isomers, the reaction is only partially reversible, thus enriching the tertiary fraction and producing the secondary aldehyde (3) in a very limited amount.

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ChemInform Abstract: ISOMERIZATION OF THE ALKYL GROUP IN (η5‐C5H5)FE(CO)(PPH3)(ALKYL) COMPLEXES

Abstract: Die sek.‐Butylgruppe im Fe‐Komplex (I) isomerisiert sich bei vierstündigem Erhitzen in Xylol zur koordinierten n‐Butylgruppe in (II).

Cited by 2 publications

References 1 publication

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

Borylation and Silylation of C–H Bonds: A Platform for Diverse C–H Bond Functionalizations

Rhodium-Catalyzed Hydroformylation of Ketal-Masked β-Isophorone: Computational Explanation for the Unexpected Reaction Evolution of the Tertiary Rh-Alkyl via an Exocyclic β-Elimination Derivative

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