By What Mechanisms Are Metal Cyclobutadiene Complexes Formed from Alkynes?

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by (13)C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.

Section: Discussionmentioning

confidence: 55%

Molecular Heterogeneous Catalysis: A Single‐Site Zeolite‐Supported Rhodium Complex for Acetylene Cyclotrimerization

Kletnieks

Liang²,

Craciun

et al. 2007

“…[49] As a result, a one-step transformation featuring the replacement of one carbenic M=C bond by two MÀC single bonds has been suggested for the transition state ( Figure 4). This symmetry-forbidden pathway is energetically prohibitive, [50] unless the substituents on the acetylene are changed.…”

Section: Resultsmentioning

confidence: 98%

Ruthenium‐Catalyzed Synthesis of Alkylidenecyclobutenes via Head‐to‐Head Dimerization of Propargylic Alcohols and Cyclobutadiene–Ruthenium Intermediates

Paih

Dérien

Demerseman

et al. 2005

Self Cite

The reaction of propargylic alcohols with carboxylic acid, or phenol derivatives, in the presence of the precatalyst [RuCl(cod)(C5Me5)] leads selectively to a variety of alkylidenecyclobutenes through head-to-head dimerization of propargylic alcohol. The first step is the formation of a cyclobutadiene-ruthenium intermediate resulting from the head-to-head coupling of two molecules of propargylic alcohol. On protonation with strong acids (HPF6, HBF4) dehydration of the cyclobutadiene complex leads to formation of an alkylidenecyclobutenyl-ruthenium complex. The X-ray structure of one such complex, [RuCl(C5Me5)(eta4-R'CCH--CH--C=CR2)] (R'=cyclohexen-1-yl, CR2 = cyclohexylidene) has been determined. Carboxylate is added at the less substituted carbon of the cyclic allylic ligand. DFT/B3 LYP calculations confirm that the intermediate arising from head-to-head coupling of alkyne to the RuClCp* species yields the cyclobutadiene-ruthenium complex more easily with propargylic alcohol than with acetylene.

“…Substitution of PPh 3 by PH 3 is a common procedure in theoretical organometallic chemistry. [10,31,[43][44][45][46][47] In addition, we have checked that, despite the electronic and steric differences, substitution of PPh 3 by PH 3 does not introduce significant changes in the thermodynamics and kinetics of the cyclotrimerisation of three acetylene molecules. [48] A previous study found that solvent effects due to toluene and acetonitrile in [2+2+2] cyclotrimerisations are minor, likely due to the absence of charged or polarised intermediates and transition states in the reaction mechanism.…”

Section: Methodsmentioning

confidence: 99%

“…[30] -cyclobutadiene-like complexes as a result of a thermal cyclodimerisation from 15-A2, because this process was found to be kinetically quite unfavourable in similar species. [31] The photochemical process instead is more favourable.…”

mentioning

confidence: 98%

Rhodium(I)‐Catalysed Intramolecular [2+2+2] Cyclotrimerisations of 15‐, 20‐ and 25‐Membered Azamacrocycles: Experimental and Theoretical Mechanistic Studies

Dachs¹,

Torrent²,

Roglans³

et al. 2009

A new series of 20- and 25-membered polyacetylenic azamacrocycles have been satisfactorily prepared and completely characterised by spectroscopic methods. Various [2+2+2] cyclotrimerisation processes catalysed by the Wilkinson's catalyst, [RhCl(PPh(3))(3)], were tested in the above-mentioned macrocycles. The 25-membered azamacrocycle (like the previously synthesised 15-membered azamacrocyle) led to the expected cyclotrimerised compound in contrast to the 20-membered macrocycle, which is characterised by its lack of reactivity. The difference in reactivity of the 15-, 20- and 25-membered macrocycles has been rationalised through density functional theory calculations.