Overcoming a Longstanding Challenge: X-Ray Structure of a [Co2(CO)6]-Complexed Propargyl Cation

Melikyan, G. G.; Bright, Stephen; Monroe, Todd; Hardcastle, Kenneth I.; Ciurash, Joana

doi:10.1002/(sici)1521-3773(19980202)37:1/2<161::aid-anie161>3.0.co;2-8

Cited by 48 publications

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“…A wide range of cationic dicobalt propargyl complexes or similar heterobimetallic complexes have been synthesized and characterized ( 37 - 44 , Figure 13B; Gruselle et al, 1993; Osella et al, 1993; Melikyan et al, 1998; Chetcuti and McDonald, 2002). The propargyl cation always preferentially coordinates to one of the metal atoms in each cluster due to accumulation of positive charge.…”

Section: π-Complexesmentioning

confidence: 99%

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations

2019

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Transition metal-stabilized carbocations are characterized by synthetically valuable interactions, yet, to date there are no comprehensive reports of the many bonding modes that can exist between a metal and carbocation. This review summarizes developments in these complexes to provide a clear picture of their properties and reactivities. In order to strategically exploit them, we propose this summary of the different bonding modes for transition metal-carbocation complexes. These models will help chemists understand the orbital interactions involved in these compounds so that they can approach their synthetic goals most effectively. Multiple transition metals and carbocations will be discussed.

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Section: π-Complexesmentioning

confidence: 99%

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations

2019

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“…The cobalt-complexed propargyl cation was first isolated in 1977 by the treatment of a propargyl alcohol with a strong acid in the presence of propionic anhydride. 2 An X-ray crystallographic study 13 revealed the intimate structural details of the propargyl triad with the cationic center being ideally sp 2hydridized and through-space coordination causing a structural asymmetry and noticeable shift toward one of the cobalt atoms. In intermolecular coupling reactions, an optimized experimental protocol 14 involves the treatment of propargyl alcohol 15 with a 6-fold excess of tetrafluoroboric acid at 0 °C (Scheme 2).…”

Section: Intermolecular Radical Dimerization: Homo-and Cross-coupling...mentioning

confidence: 99%

Propargyl Radical Chemistry: Renaissance Instigated by Metal Coordination

Melikyan

2015

Acc. Chem. Res.

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Over the last two decades, radical chemistry of propargyl systems was developed into a potent synthetic field providing access to classes of organic compounds that are otherwise hardly accessible. The levels of diastereoselection thus achieved (up to 100%) are unprecedented for free propargyl radicals, as well as for organic radicals π-bonded to transition metals. These advances were enabled by the coordination of the triple bond to a Co2(CO)6 core that precluded an acetylene-allene rearrangement, stabilized requisite propargyl cations, created conformational constraints at the carbon-carbon bond formation site, configurationally altered the acetylenic moiety allowing for 1,3-steric induction upon the newly formed stereocenters, increased bulkiness of propargyl triads thus controlling the spatial orientation of converging radicals, and allowed for α-to-γ projection of the reaction site and alteration of the transiency of radical intermediates. In the course of these studies, a number of popular "beliefs" were proven to be untrue. First, cobalt-complexed propargyl cations, which have long been considered to be thermally labile species, were engaged in synthetically meaningful transformation at temperatures as high as 147 °C. Second, in radical dimerization reactions, higher reaction temperatures did not adversely impact the yields and levels of d,l-diastereoselectivity. Third, π-bonded organometallic radicals, deemed unruly, were effectively controlled with complementary mechanistic tools, thus achieving the highest levels of stereoselectivity (up to 100%) in inter- and intramolecular reactions. Fourth, meso stereoisomers, being thermally labile and kinetically disfavored, were discovered to be major products in intramolecular cyclizations induced by cobaltocene. Fifth, propargyl cations were synthesized in the absence of strong acids, thus increasing the functional tolerance and achieving a long sought after compatibility with acid-sensitive functionalities. A concept of sequestered propargyl radicals was introduced to explain disparity in diastereoselectivity data: heterogeneous reducing agents allegedly produce "free" radicals, while homogeneous reductants generate "sequestered" radicals associated with reductant-derived oxidized species. Among mechanistic tools, a 1,3-steric induction was found to be most efficient for controlling the stereoselectivity of radical reactions (up to 100% d,l). In intramolecular reactions, a d,l-to-meso reversal of stereoselectivity was discovered with zinc being replaced with cobaltocene as a reductant. Among efficient tools for controlling the stereoselectivity in intramolecular cyclizations is a rigidity of the carbon tether that provides for an exclusive formation of d,l-diastereomers. Two novel reactions that belong to a new field of unorthodox organometallic radical chemistry were discovered: the spontaneous conversion of cobalt-complexed propargyl cations to radicals and the THF-mediated process wherein a THF molecule assumes a new role of an initiator in radical reactions....

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“…For dicobalthexacarbonyl stabilized propargyl cations a fair number of isolated and well characterized examples are known [12,13]. For the silicon case, silylium ions are likely to be involved in reactions of the respective complexes, as was demonstrated by Corriu and co-workers, who reacted their dicobalthexacarbonyl coordinated hydrosilylalkyne with Ph 3 C[BF 4 ] to obtain the respective dicobalthexacarbonyl complexed fluorosilane [11].…”

Section: Introductionmentioning

confidence: 99%

Spectroscopic and Structural Study of Some Oligosilanylalkyne Complexes of Cobalt, Molybdenum and Nickel

2019

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Metal induced stabilization of α-carbocations is well known for cobalt- and molybdenum complexed propargyl cations. The same principle also allows access to reactivity enhancement of metal coordinated halo- and hydrosilylalkynes. In a previous study, we have shown that coordination of oligosilanylalkynes to the dicobalthexacarbonyl fragment induces striking reactivity to the oligosilanyl part. The current paper extends this set of oligosilanylalkyne complexes to a number of new dicobalthexacarbonyl complexes but also to 1,2-bis(cyclopentadienyl)tetracarbonyldimolybdenum and (dippe)Ni complexes. NMR-Spectroscopic and crystallographic analysis of the obtained complexes clearly show that the dimetallic cobalt and molybdenum complexes cause rehybridization of the alkyne carbon atoms to sp3, while in the nickel complexes one π-bond of the alkyne is retained. For the dicobalt and dimolybdenum complexes, strongly deshielded 29Si NMR resonances of the attached silicon atoms indicate enhanced reactivity, whereas the 29Si NMR shifts of the respective nickel complexes are similar to that of respective vinylsilanes.

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Overcoming a Longstanding Challenge: X-Ray Structure of a [Co2(CO)6]-Complexed Propargyl Cation

Cited by 48 publications

References 0 publications

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations

Molecular Orbital Insights of Transition Metal-Stabilized Carbocations

Propargyl Radical Chemistry: Renaissance Instigated by Metal Coordination

Spectroscopic and Structural Study of Some Oligosilanylalkyne Complexes of Cobalt, Molybdenum and Nickel

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