Michał Barbasiewicz

Mechanistic Insights into the cis–trans Isomerization of Ruthenium Complexes Relevant to Catalysis of Olefin Metathesis

The mechanism of the trans to cis isomerization in Ru complexes with a chelating alkylidene group has been investigated by using a combined theoretical and experimental approach. Static DFT calculations suggest that a concerted single-step mechanism is slightly favored over a multistep mechanism, which would require dissociation of one of the ligands from the Ru center. This hypothesis is supported by analysis of the experimental kinetics of isomerization, as followed by (1)H NMR spectroscopy. DFT molecular dynamics simulations revealed that the variation of geometrical parameters around the Ru center in the concerted mechanism is highly uncorrelated; the mechanism actually begins with the transformation of the square-pyramidal trans isomer, with the Ru==CHR bond in the apical position, into a transition state that resembles a metastable square pyramidal complex with a Cl atom in the apical position. This high-energy structure collapses into the cis isomer. Then, the influence of the N-heterocyclic carbene ligand, the halogen, and the chelating alkylidene group on the relative stability of the cis and trans isomers, as well as on the energy barrier separating them, was investigated with static calculations. Finally, we investigated the interconversion between cis and trans isomers of the species involved in the catalytic cycle of olefin metathesis; we characterized an unprecedented square-pyramidal metallacycle with the N-heterocyclic carbene ligand in the apical position. Our analysis, which is relevant to the exchange of equatorial ligands in other square pyramidal complexes, presents evidence for a remarkable flexibility well beyond the simple cis-trans isomerization of these Ru complexes.

Is the Hoveyda–Grubbs Complex a Vinylogous Fischer‐Type Carbene? Aromaticity‐Controlled Activity of Ruthenium Metathesis Catalysts

Three naphthalene-based analogues (4 a-c) of the Hoveyda-Grubbs metathesis catalyst exhibited immense differences in reactivity. Systematic structural and spectroscopic studies revealed that the ruthenafurane ring present in all 2-isopropoxyarylidene chelates possesses some aromatic character, which inhibits catalyst activity. This aromatic stabilization within the chelate ring may be controlled by variation of the polycyclic core topology as was demonstrated for tetraline and phenanthrene derivatives (4 d, e). General conclusions about a new mode of ligand-structure tuning in catalytic systems are presented.

A new family of halogen-chelated Hoveyda-Grubbs-type metathesis catalysts.

Coordination, not insertion: New ruthenium benzylidenes with a chelating halogen atom were easily prepared and showed excellent stability and activity as metathesis catalysts (see figure). Structure-activity studies reveal that strength of the ruthenium-halogen interaction can be tuned across a wide range to set up a family of latent to active catalysts.

Dibridgehead Diphosphines that Turn Themselves Inside Out

Molecules and macromolecules that undergo topologically complex dynamic processes—such as knot-forming and multistep folding sequences—have attracted considerable attention from numerous standpoints. However, there is much less awareness that certain types of molecules, including but not limited to macrocyclic bicyclic (macrobicyclic) compounds, are able to turn themselves inside out. This has been termed “homeomorphic isomerization”, even though the process can be degenerate. At the time of a 1996 review, only four (degenerate) cases had been rigorously established by spectroscopic means. Although we are unaware of additional confirmed examples since, such equilibria have been invoked to rationalize the NMR spectroscopic properties of other macrobicyclic compounds. Herein we demonstrate this type of dynamic behavior—in both degenerate and nondegenerate manifestations—with three stereoisomers of a macrobicyclic aliphatic dibridgehead diphosphine (in,in, out,out, and in,out, referring to the orientations of the lone pairs of electrons on the phosphorus atoms). In the nondegenerate case, the lone pairs of electrons are alternately directed in a convergent manner towards an interior domain (in,in) or directed externally (out,out). Thus, such processes can potentially mediate the sequesterization, transport, and delivery of Lewis acid guests. Analogous dynamic behavior has recently been proposed for a hexaaryl dibridgehead diphosphine and other types of aromatic dibridgehead diphosphorus compounds. Our story begins with the platinum dichloride complex trans-1 (Scheme 1), in which three (CH2)14 chains connect the trans-arranged phosphorus atoms. This complex exemplifies a class of compounds termed “gyroscope like”, because of the rapid rotation of the caged MLn moieties in suitably sized systems on the NMR time scale, and their structural similarities to common toy gyroscopes. Treatment of trans-1 with an excess of the nucleophiles NaC CH, LiC CPh, or KC N afforded the macrobicyclic dibridgehead diphosphine 2 as an analytically pure, moderately air-sensitive white powder in 66–91% yield. The dianionic platinum tetrakis(acetylide) complex 3 could also be isolated (35%) when LiC CPh was employed. The three in/out stereoisomers of 2 are depicted in Scheme 1 (middle). In the PtCl2 adduct 1, both lone pairs of electrons of the dibridgehead diphosphine ligand are directed inwards. Treatment of 2 with PtCl2 in C6D6 regenerated 1, which constitutes an overall retention of configuration at the phosphorus atoms. For this reason, it was originally thought that only in,in-2 was produced. For small bicycles, out,out isomers are energetically much more favorable, but computational data for analogous hydrocarbons indicate that in,out isomers become most stable at medium ring sizes, and that in,in isomers become most stable at larger ring sizes. DFT calculations (see the Supporting Information) indicated (as did preliminary molecular mechanics calculations) that in,in-2 was considerably more stable than out,out-2 (6.98 kcalmol ), and somewhat more stable than in,out-2 (1.59 kcalmol ). The longest aliphatic dibridgehead diphosphine reported previously features one (CH2)3 and two (CH2)4 chains, which are much shorter than the (CH2)14 linkers in 2. [11] The isomers of 2 can be regarded as configurational diastereomers that are interrelated by pyramidal inversions at Scheme 1. Synthesis and complexation of the dibridgehead diphosphine 2.

Mechanistic Studies of Hoveyda–Grubbs Metathesis Catalysts Bearing S‐, Br‐, I‐, and N‐coordinating Naphthalene Ligands

Derivatives of the Hoveyda-Grubbs complex bearing S-, Br-, I-, and N-coordinating naphthalene ligands were synthesized and characterized with NMR and X-ray studies. Depending on the arrangement of the coordinating sites on the naphthalene core, the isomeric catalysts differ in activity in model metathesis reactions. In particular, complexes with the RuCH bond adjacent to the second aromatic ring of the ligand suffer from difficulties experienced on their preparation and initiation. The behavior most probably derives from steric hindrance around the double bond and repulsive intraligand interactions, which result in abnormal chemical shifts of benzylidene protons observed with (1) H NMR. Furthermore EXSY studies revealed that the halogen-chelated ruthenium complexes display an equilibrium, in which major cis-Cl2 structures are accompanied with small amounts of isomeric forms. In general, contents of the minor forms, measured at 80 °C, correlate with the observed activity trends of the catalysts, although some exceptions complicate the mechanistic picture. We assume that for the family of halogen-chelated metathesis catalysts the initiation mechanism starts with the cis-Cl2 ⇌trans-Cl2 isomerization, although further steps may become rate-limiting for selected systems.

Syntheses and Palladium, Platinum, and Borane Adducts of Symmetrical Trialkylphosphines with Three Terminal Vinyl Groups, P((CH2)mCH=CH2)3

Reactions of Br(CH2)mCH=CH2 with Mg powder and then PCl3 (0.33 equiv.) afford P((CH2)m- CH=CH2)3 (1; m = a, 4; b, 5; c, 6; d, 7; e, 8; f, 9; 52 - 87%). Reactions of 1a - c, e with PdX2(COD) (X = Cl, Br) give trans-PdX2(P((CH2)mCH=CH2)3)2 (35 - 92%). Reactions of 1b - e with PtCl2 in benzene give mainly trans-PtCl2(P((CH2)mCH=CH2)3)2 (trans-5b-e; 52 - 75%), whereas those with K2PtCl4 in water give mainly cis-5b-e (33 - 70%). The reaction of equimolar quantities of 1c and H3B・S(CH3)2 gives the 1 : 1 adduct H3B・P((CH2)6CH=CH2)3 (85%). In none of these transformations are by-products derived from the C=C linkages observed Graphical Abstract Syntheses and Palladium, Platinum, and Borane Adducts of Symmetrical Trialkylphosphines with Three Terminal Vinyl Groups, P((CH2)mCH=CH2)3

The Key Role of the Nonchelating Conformation of the Benzylidene Ligand on the Formation and Initiation of Hoveyda–Grubbs Metathesis Catalysts

Experimental studies of Hoveyda-Grubbs metathesis catalysts reveal important consequences of substitution at the 6-position of the chelating benzylidene ligand. The structural modification varies conformational preferences of the ligand that affects its exchange due to the interaction of the coordinating site with the ruthenium center. As a consequence, when typical S-chelated systems are formed as kinetic trans-Cl2 products, for 6-substituted benzylidenes the preference is altered toward direct formation of thermodynamic cis-Cl2 isomers. Activity data and reactions with tricyclohexylphosphine (PCy3 ) support also a similar scenario for O-chelated complexes, which display fast trans-Cl2 ⇄cis-Cl2 equilibrium observed by NMR EXSY studies. The presented conformational model reveals that catalysts, which cannot adopt the optimal nonchelating conformation of benzylidene ligand, initiate through a high-energy associative mechanism.