Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst

Camp, Andrew M.; Kita, Matthew R.; Blackburn, P. Thomas; Dodge, Henry M.; Chen, Chun-Hsing; Miller, Alexander J. M.

doi:10.1021/jacs.0c11601

Cited by 44 publications

(33 citation statements)

References 80 publications

(182 reference statements)

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“…Pincer ligands and metal–ligand cooperativity (MLC) represent two of the most active themes in the development of transition metal and main group metal chemistry and catalysis, and there has been a substantial overlap between these areas of investigation. Numerous examples of pincer ligands engaging in novel catalytic and stoichiometric MLC-related reactions have been and continue to be reported. − Yet, despite the large number of pincer ligands for which MLC is based on a coordinating N atom, − there are very few examples involving the next congener, phosphorus. In particular, in spite of the great value that has been proven for addition of H 2 across M–N bonds, there are relatively few examples of H 2 addition across a phosphorus metal bond − and even fewer examples of the reverse elimination of H 2 . , …”

Section: Introductionmentioning

confidence: 99%

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

et al. 2022

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The selective functionalization of alkanes and alkyl groups is a major goal of chemical catalysis. Toward this end, a bulky triphosphine with a central secondary phosphino group, bis(2-di-t-butyl-phosphinophenyl)phosphine (tBuPHPP), has been synthesized. When complexed to iridium, it adopts a meridional (“pincer”) configuration. The secondary phosphino H atom can undergo migration to iridium to give an anionic phosphido-based–pincer (tBuPPP) complex. Stoichiometric reactions of the (tBuPPP)Ir complexes reflect a distribution of steric bulk around the iridium center in which the coordination site trans to the phosphido group is quite crowded; one coordination site cis to the phosphido is even more crowded; and the remaining site is particularly open. The (tBuPPP)Ir precursors are the most active catalysts reported to date for dehydrogenation of n-alkanes, by about 2 orders of magnitude. The electronic properties of the iridium center are similar to that of well-known analogous (RPCP)Ir catalysts. Accordingly, DFT calculations predict that (tBuPPP)Ir and (tBuPCP)Ir are, intrinsically, comparably active for alkane dehydrogenation. While dehydrogenation by (RPCP)Ir proceeds through an intermediate trans-(PCP)IrH2(alkene), (tBuPPP)Ir follows a pathway proceeding via cis-(PPP)IrH2(alkene), thereby circumventing unfavorable placement of the alkene at the bulky site trans to phosphorus. (tBuPPP)Ir and (tBuPCP)Ir, however, have analogous resting states: square planar (pincer)Ir(alkene). Alkene coordination at the crowded trans site is therefore unavoidable in the resting states. Thus, the resting state of the (tBuPPP)Ir catalyst is destabilized by the architecture of the ligand, and this is largely responsible for its unusually high catalytic activity.

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Section: Introductionmentioning

confidence: 99%

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

et al. 2022

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“…As mentioned above, our Ru-1 complex is also a highly efficient catalyst for the isomerization of CC double bonds. Thus, when 4-phenyl-1-butene 1-2t was employed as the substrate under the alkene isomerization conditions, in the absence of H 2 gas (eq ), 3-2t was obtained selectively after 48 h, following the CC bond shift to the 3-position (92% yield, >20:1 for 3-2t/2-2t ) . Interestingly, NACET was found to be too poisonous to allow for efficient isomerization in this case, but the addition of a catalytic amount of HexNH 2 to Ru-1 enabled it to selectively catalyze the isomerization of the CC bond to the 2-position, affording product 2-2t in 89% yield (>20:1 for 2-2t/3-2t ). …”

Section: Resultsmentioning

confidence: 95%

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

et al. 2022

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Catalytic semihydrogenation of internal alkynes using H2 is an attractive atom-economical route to various alkenes, and its stereocontrol has received widespread attention, both in homogeneous and heterogeneous catalyses. Herein, a novel strategy is introduced, whereby a poisoning catalytic thiol is employed as a reversible inhibitor of a ruthenium catalyst, resulting in a controllable H2-based semihydrogenation of internal alkynes. Both (E)- and (Z)-alkenes were obtained efficiently and highly selectively, under very mild conditions, using a single homogeneous acridine-based ruthenium pincer catalyst. Mechanistic studies indicate that the (Z)-alkene is the reaction intermediate leading to the (E)-alkene and that the addition of a catalytic amount of bidentate thiol impedes the Z/E isomerization step by forming stable ruthenium thiol(ate) complexes, while still allowing the main hydrogenation reaction to proceed. Thus, the absence or presence of catalytic thiol controls the stereoselectivity of this alkyne semihydrogenation, affording either the (E)-isomer as the final product or halting the reaction at the (Z)-intermediate. The developed system, which is also applied to the controllable isomerization of a terminal alkene, demonstrates how metal catalysis with switchable selectivity can be achieved by reversible inhibition of the catalyst with a simple auxiliary additive.

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“…Monitoring the catalysis at short reaction times (15 min) and assuming zero-order reaction kinetics led to an estimated TOF of 2004 h –1 with a t 1/2 = 0.016 h. These results are only estimates because the amount of isomerized products was greater than 10% where zero-order kinetics can no longer be assumed. However, these results seem to align well with similarly reported isomerization catalysts …”

Section: Results and Discussionmentioning

confidence: 97%

Configurational Flexibility of a Triaryl-Supported SBS Ligand with Rh and Ir: Structural Investigations and Olefin Isomerization Catalysis

2022

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Here, we report Rh and Ir complexes containing a triaryl SBS pincer ligand flanked by neutral thiomethyl donor groups. Oxidative addition of the B−H bond in the diazaborole proligand H( Me SBS Me ) to [Ir(COD)Cl] 2 or [Rh(COE)Cl] 2 yielded the chloride-bridged dimers [( Me SBS Me )MH(μ-Cl)] 2 , where M = Ir (1) or Rh (2). Addition of CO to 1 and 2 yielded unstable products that were difficult to isolate, but crystals of monomeric ( Me SBS Me )IrH(CO)Cl (3) recovered from the reactions with 1 revealed a change in the Me SBS Me coordination mode from mer to fac. Treating 1 and 2 with LiN(SiMe 3 ) 2 yielded mer-( Me SBS Me )IrH[N(SiMe 3 ) 2 ] (4) and the unusual Rh I −Rh III dimer ( Me SBS Me )Rh(μ-H)[( Me S(μ-B)(μ-S Me )]Rh[N(SiMe 3 ) 2 ] ( 5) with both mer and fac Me SBS Me . 4 and 5 did not exhibit any alkane transfer hydrogenation reactivity when tested with tert-butylethylene and cyclooctane, but they are highly active for alkene isomerization with 1-hexene. Optimized isomerization reactions showed the highest turnover number (TON) with 4 at 60 °C after 16 h (TON = 10 000), and both catalysts are effective even when tested at room temperature with similar loadings (TON = 600). Collectively, these data highlight the reactivity and inherent coordinative flexibility of the Me SBS Me ligand for comparison to more well-established PBP complexes.

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Selecting Double Bond Positions with a Single Cation-Responsive Iridium Olefin Isomerization Catalyst

Cited by 44 publications

References 80 publications

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

Reactivity of Iridium Complexes of a Triphosphorus-Pincer Ligand Based on a Secondary Phosphine. Catalytic Alkane Dehydrogenation and the Origin of Extremely High Activity

Controlled Selectivity through Reversible Inhibition of the Catalyst: Stereodivergent Semihydrogenation of Alkynes

Configurational Flexibility of a Triaryl-Supported SBS Ligand with Rh and Ir: Structural Investigations and Olefin Isomerization Catalysis

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