This report details the effects of ligand variation on the mechanism and activity of ruthenium-based olefin metathesis catalysts. A series of ruthenium complexes of the general formula L(PR(3))(X)(2)Ru=CHR(1) have been prepared, and the influence of the substituents L, X, R, and R(1) on the rates of phosphine dissociation and initiation as well as overall activity for olefin metathesis reactions was examined. In all cases, initiation proceeds by dissociative substitution of a phosphine ligand (PR(3)) with an olefinic substrate. All of the ligands L, X, R, and R(1) have a significant impact on initiation rates and on catalyst activity. The origins of the observed substituent effects as well as the implications of these studies for the design and implementation of new olefin metathesis catalysts and substrates are discussed in detail.

The N-heterocyclic carbene-coordinated ruthenium benzylidene complex [(H 2 IMes)(PCy 3 )(Cl) 2 Ru¼CHPh] (1) is a highly active catalyst for a wide variety of olefin-metathesis reactions, [1] including those with sterically demanding [2] and electron-deficient olefins (Scheme 1). [3] In spite of recent advances, there are several processes that remain challenging, such as olefin cross metathesis (CM) with directly functionalized olefins. [4] For example, acrylonitrile CM has only been successful with Schrock©s arylimido molybdenum alkylidene catalyst [5] and the ether-tethered ruthenium alkylidene derivative [(H 2 IMes)(Cl) 2 Ru¼CH(o-iPrOC 6 H 4 )] (4). [6,7] Phosphane-ligated ruthenium catalysts have given poor results for this transformation, [3a, 5c, 6c, 8, 9] except for one report of efficient CM between purified acrylonitrile and 1-decene mediated by 1. [10] We have determined that dissociation rates of ligands are related to catalyst efficiency during CM with acrylonitrile. On this basis, we have developed a new, highly efficient ruthenium complex to perform acrylonitrile CM with unpurified acrylonitrile; this catalyst is the fastest initiator of any ruthenium-based catalyst reported to date. [11] Previous studies have shown that precatalysts of the type [L 2 X 2 Ru ¼ CHR] initiate by dissociating one L-type ligand before entering the catalytic cycle (Scheme 2); [12] in complexes 1±3, L is a phosphane (PR 3 ), and in complex 4, L is a tethered ether ligand (iPrO). Importantly, complexes 1±4 all provide the same propagating species (A and B) after a single turnover. [13] If either A or B is trapped by L, dissociation of L must occur before catalysis can continue. The relative affinity of A and/or B for the olefin in preference to L (i.e., favoring propagation) controls how long these species remain in the catalytic cycle. Consequently, the differences in activity between catalysts 1±4 depend on their rates of initiation and rebinding of L, both of which can be tuned by the nature of L. [12b, 14±16] Complexes 1±4 are all active for a variety of metathesis processes, such as CM, ring-closing metathesis (RCM), and ring-opening-metathesis polymerization (ROMP). However, the situation involving acrylonitrile CM is more complex; CM between acrylonitrile and allylbenzene proceeds efficiently with 4 (68 % yield) but not with 1±3 (21 %, 35 %, and 36 % yields, respectively; Table 1). [17] Clearly, this difference cannot simply be an issue of precatalyst initiation; 2 and 4 have roughly the same initiation rates and initiation is faster with 3 than 1, 2, or 4 Table 2. The difference is also not a result of COMMUNICATIONS

The ruthenium complex (IMesH2)(Cl)2(C5H5N)2RuCHPh [IMesH2 = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene] (3) was prepared by the reaction of (IMesH2)(PCy3)(Cl)2RuCHPh (2) with an excess of pyridine. Complex 3 contains substitutionally labile pyridine and chloride ligands and serves as a versatile starting material for the synthesis of new ruthenium benzylidenes.

Experimental Section(H 2 IMes)[(p-CF 3 C 6 H 4 ) 3 P](Cl) 2 Ru=CHPh (6). Complex 12 (200 mg, 0.275 mmol) and (p-CF 3 C 6 H 4 ) 3 P (140 mg, 0.300 mmol) were combined in benzene (2 mL), and the resulting solution was stirred for 10 minutes. The solution was frozen using a dry-ice/acetone bath and the solvent was removed under vacuum. Benzene (1 mL) was added to the resulting brown residue. The solution was again frozen and the solvent was removed under vacuum. This cycle was repeated a third time, then the resulting brown residue was washed with 3 x 3 mL pentane and dried under vacuum at room temperature. Complex 6 was obtained as a pink powder (145 mg, 51% yield). 31

The objective study of self‐recognition, with a mirror and a mark applied to the face, was conducted independently by Gallup (1970) for use with chimpanzees and monkeys, and by Amsterdam (1972) for use with infant humans. Comparative psychologists have followed the model (and assumptions) set by Gallup, whereas developmental psychologists have followed a different model (e.g., Lewis & Brooks‐Gunn, 1979). This article explores the assumptions in the definitions and methods of self‐recognition assessments in the 30 years since these initial studies, and charts the divergence between the developmental mark test and the comparative mark test. Two new studies, 1 with infant chimpanzees and 1 with infant humans, illustrate a reconciliation of the 2 approaches. Overt application of the mark, or other procedures related to how the mark is discovered, did not enhance mirror self‐recognition. In contrast, maternal scaffolding appears to enhance performance, perhaps by eliciting well‐rehearsed verbal responses (i.e., naming self). When comparable testing procedures and assessment criteria are used, chimpanzee and human infants perform comparably. A combined developmental comparative approach allows us to suggest that mirror self‐recognition may be based on a specific aspect of mental representation, the cognitive ability to symbolize.

As a rapidly growing field across all areas of chemistry, C-H activation/functionalisation is being used to access a wide range of important molecular targets. Of particular interest is the development of a sustainable methodology for alkane functionalisation as a means for reducing hydrocarbon emissions. This Perspective aims to give an outline to the community with respect to commonly used terminology in C-H activation, as well as the mechanisms that are currently understood to operate for (cyclo)alkane activation/functionalisation.

Alkyne hydrothiolation is a potentially attractive method for the formation of vinyl sulfides, which are valuable synthetic intermediates. Known methods for hydrothiolation using alkyl thiols are quite limited. We report herein that Tp*Rh(PPh3)2 (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) is a highly active catalyst for alkyne hydrothiolation with alkyl and aryl thiols. Hydrothiolation using alkyl thiols proceeds with excellent regioselectivity, providing convenient access to branched alkyl vinyl sulfides, which are difficult to synthesize by other means. A mixture of regioisomers is obtained when using aryl thiols, with the branched isomer as the major product, opposite that reported for other Rh complexes.

As part of our continuing studies of transition metal-catalyzed [m+n] cycloadditions, 1 we recently reported the first examples of rhodium(I)-catalyzed [5+2] cycloadditions between vinylcyclopropanes and alkynes (afd). 2 This new class of reactions has been shown to provide convenient access to 5,7-fused ring systems, 3 the commonly encountered bicyclic core of a variety of designed and natural molecules. 4 This reaction was formulated around the expectation that various metal catalysts (ML n ) could be used to mediate the formation of a metallacyclopentene (b) from vinylcyclopropanes and various π-systems (a). 5 Driven by the strain of the attached cyclopropane ring, 6 this metallacyclopentene intermediate (b) would ring expand to a metallacyclooctadiene (c) which, upon reductive elimination, would provide the seven-membered cycloadduct (d) and regenerate the metal catalyst. An alternative mechanism can be formulated around metallacycle b′. This reaction has been shown thus far to work with a wide range of alkynes, including terminal, internal, electron poor, electron rich, and conjugated systems. Prompted by the synthetic and mechanistic importance associated with the use of alkenes in this reaction and the attendant unexplored issues of stereochemistry, we report herein the first examples of rhodium(I)catalyzed [5+2] cycloadditions between vinylcyclopropanes and alkenes and the first studies of the scope and stereochemistry of these remarkably efficient and selective processes.Our investigation began with substrate 1 (Table 1). 7 In the presence of a catalyst system derived from 0.1 mol % tris-(triphenylphosphine)rhodium(I) chloride and 0.1 mol % silver trifluoromethanesulfonate, ene-vinylcyclopropane 1 (1 g, 4 mmol, 1.0 M in toluene) gave after 17 h at 110°C cycloadduct 2 in 86% isolated yield as a single diastereomer (entry 2). 8 The stereochemistry of cycloadduct 2 was assigned on the basis of the planar symmetry 9 of its hydrogenation product as determined by 13 C NMR spectroscopy. The remarkable stereoselectivity of this cycloaddition is consistent with the preferential formation and reaction of a cis-fused metallacyclopentane intermediate b.Additional studies of the reaction variables revealed that the cycloaddition of 1 proceeds efficiently with substrate concentra-(1) For representative examples, see: [4+4] cycloadditions: Wender, P. A.; Ihle, N. C. The mechanistic pathway presented is intended only to facilitate synthetic application and guide mechanistic analysis. Other mechanisms are possible and await experimental investigation. (6) For a review on metal-mediated cleavage of cyclopropanes, see: Khusnutidinov, R. I.; Dzhemilev, U. M. J. Organomet. Chem. 1994, 1 and references therein.(7) Substrate 1 was prepared in 41% yield by alkylation of allyl dimethylmalonate with 3-bromo-1-cyclopropyl-1-propene, which was prepared by the method of Fleming: Ward, S. C.; Fleming, S. A. J. Org. Chem. 1994, 59, 6476. Details are provided in the Supporting Information.(8) In a representative procedure, tris(tr...