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.

Structural modifications of the HoveydaGrubbs ruthenium metathesis complex via electronic and structural withdrawing of the chelating alkoxy ligand were investigated. By decreasing the donor properties of the oxygen atom, an acceleration in catalytic activity was achieved based on facilitation of the initiation step. Conformational constraints of the chelating ether linkage led to the unexpected disturbance of the complex geometry and a vast improvement of the activity.

Novel metathesis catalysts that are derivatives of 8-vinylquinoline, (H 2 IMes)(Cl) 2 Ru(CH-κ 2 (C,N)-8-C 9 H 6 N) (1a; H 2 IMes ) 1,3-dimesityl-4,5-dihydroimidzol-2-ylidene), and 5-vinylquinoxaline, (H 2 IMes)-(Cl) 2 Ru(CH-κ 2 (C,N)-5-C 8 H 5 N 2 ) (2a), which possess five-membered chelate rings, were synthesized in ligand exchange reactions with (H 2 IMes)(PCy 3 )(Cl) 2 RudCHPh. Both 1a and 2a are formed as complexes with the trans-dichloro geometry common for Grubbs-type complexes, while on prolonged storage in dichloromethane they isomerize to the thermodynamically favored cis-dichloro isomers (1b and 2b, respectively). The structures of compounds 1a,b were confirmed by X-ray analysis. The kinetics of trans to cis isomerization were measured by 1 H NMR to give the rate constants k 1af1b ) 3.2 × 10 -2 h -1 and k 2af2b ) 5.5 × 10 -3 h -1 in dichloromethane-d 2 at 23 °C. Investigations of the relative activities of these catalysts in model RCM and enyne reactions showed that catalyst 1a was faster than 1b (and 2a faster than 2b) but that 2a was faster than 1a.

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.

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Threefold intramolecular ring-closing metatheses of trans-[MCl2(P{(CH2)(m)CH=CH2}3)2] are effected with Grubbs' catalyst. Following hydrogenation catalyzed by [RhCl(PPh3)3], the title complexes trans-[MCl2(P((CH2)n)3P)] (n=2m+2; M/n=Pt/14, 4 c; Pt/16, 4 d; Pt/18, 4 e; Pd/14, 5 c; Pd/18, 5 e) and sometimes isomers partly derived from intraligand metathesis, trans-[MCl2{P(CH2)n(CH2)n}P(CH2)n)] (4'c-e, 5'e), are isolated. These react with LiBr, NaI, and KCN to give the corresponding MBr2, MI2, and M(CN)2 species (58-99%). (13)C NMR data show that the MX2 moieties rapidly rotate within the diphosphine cage on the NMR timescale, even at -120 °C. The reaction of 4 c and KSCN gives separable Pt(NCS)2 and Pt(NCS)(SCN) adducts (13 c, 28%; 14 c, 20%), and those of 4 c,e and Ph2Zn give PtPh2 species (15 c, 61%; 15 e, 90%). (13)C NMR spectra of 13 c-15 c show two sets of CH2 signals (ca. 2:1 intensity ratios), indicating that MX2 rotation is no longer rapid. Reactions of 4 c or 4'c and excess NaC≡CH afford the free diphosphines P{(CH2)14}3P (91%) and (CH2)14P(CH2)14P(CH2)14 (90%). The latter has been crystallographically characterized as a bis(BH3) adduct. The crystal structures of eight complexes with P(CH2)14P linkages (PtCl2, PtBr2, PtI2, Pt(NCS)2, PtPh2, PdCl2, PdBr2, PdI2) and 15 e have been determined, and intramolecular distances analyzed with respect to MX2 rotation. The conformations of the (CH2)14 moieties and features of the crystal lattices are also discussed.

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.

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