6-C6H3-i-Prz; OR = OCMe(CF3),, OCMe(CFJ) form fivecoordinate adducts upon addition of PMe3 or quinuclidme. PMe3 attacks the C/N/O face of the pseudotetrahedral complexes to give chiral TBP species in which the phosphine is bound in an axial position and the imido and alkylidene ligands lie in the equatorial plane. Two isomers containing syn and anti rotamers of the alkylidene ligand are observed. The syn rotamer forms first; the anti rotamer is the final product. PMe3 binds weakly when OR = 0-t-Bu and is lost readily in vacuo. Quinuclidine adds to either the C 0/0 face or N/O/O face to give an achiral syn isomer and to the C/N/O face to of syn and anti forms is observed with time. An X-ray structure of syn-Mo(CH-t-Bu)(NAr)[OCMe-(CF,),I2(PMe3) shows that the t-Bu group points toward the imido ligand and the phenyl ring of the imido ligand lies approximately in the equatorial plane in a relatively crowded coordination environment (a = 10.979 (4) A, b = 17.945 (7) A, c = 18.375 (8) A, 19 = 106.34 (3)O, 2 = 4, V = 3474 (4) AS, p = 1.490 g/cm3, R = 0.037, R, = 0.045). Pyridine adducts of Mo complexes containing the 2,6-dichlorophenoxide ligand also have been characterized. Three isomers of fivecoordinate molybdenum or tungsten complexes containing a cis-or trans-2-butenylidene ligand and quinuclidine are found at equilibrium, syn and anti rotamers of the chiral core previously described and a syn rotamer with an achiral core. An X-ray structure of anti-W(tram-CHCH=CHMe)(NAr)[OCMe(CF3)z]2(quin) showed the expected trigonal-bipyramidal core with alkylidene and imido ligands occupying equatorial sites and OCMe(CF3)2 ligands occupying one axial and one e uatorial site (a = 12.972 (9) A, b = 18.049 (7) A, c = 15.038 (9) A, p = 92.07 (3)O, 2 = 4, V = 3518 (6) A?, p = 1.673 g cm3, R1 = 0.038, R, = 0.040). The only significant difference between the structure in the equatorial plane. Syn and anti rotamers in five-coordinate adducts have been shown to interconvert after losing the base in several cases. The barrier to rotation of the alkylidene ligand has been measured in several four-coordinate species and shown to lie in the range AG*= = 15-18 kcal mol-'. These findings are discussed in relation to the proposed mechanism of olefin metathesis by pseudotetrahedral complexes of the type M(CHR')(NAr)(OR),.give an anti chiral T 6 P species analogous to that formed for the PMe3 adduct. An equilibrium mixture of this anti adduct an d the syn adduct described above is that the anti adduct is markedly less crowded

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Functionalized olefin metathesis has been a goal in transition metal catalysis for some time now. This goal has driven the search for milder, more selective catalysts and the move toward the more electronegative transition metals lying further to the right in the periodic table. Acylonitrile is a functional olefin which has eluded all previous attempts at productive olefin metathesis. Herein we report the first examples of this useful reaction.In 1985, Mol and co-workers systematically studied the metathesis reactions of unsaturated nitriles in the presence of WCla-SnMe4 or Re207-Al203-SnMe4 catalyst and found that acrylonitrile would not undergo either self-metathesis or crossmetathesis reactions.' Likewise, we have found that no identifiable new products are obtained when acrylonitrile is treated with either of the two well-defined catalysts Mo(CHCMezPh)(NAr)-[oCMe(CF3)21~~ (Ar = 2,6-i-Pr&H3) or Ru(CHCHCPhz)(PQ3)2-c12.3 Indeed, the sole reported example of an olefin metathesis reaction involving acylonitrile seems to be the phosphoranecatalyzed methylene exchange reaction depicted in eq 1.4 R COrEtPh,P=/ .=/"N + CO&t(1)The now well-established Chauvin mechanism5 for olefin metathesis requires that two roles be played by terminal olefins in a productive metathetical coupling reaction. Olefins must donate alkylidene ligands, placing the olefin substituent a to the transition metal center (eq 2). They must also react as nucleophiles with electrophilic (at the metal) alkylidene intermediates, placing the olefin substituent @ on the metallacycle precursor to the coupled product (eq 3). These two roles place different steric and electronic demands on the olefin substituent, and one olefin need not (and in some cases cannot) fulfill both functions. In fact, we recently showed that selective crossmetathesis can result when different olefins from a reacting pair fulfill the two functions.6 We reasoned that acrylonitrile's failure to undergo productive self-metathesis may be due to its (1) Bosma, R. H. A,; Van Den Aardweg, G. C. N.; Mol, J. C. J. Organomet. Chem. 1985, 280, 115-22. (2) (a) Murdzek, J. S.; Schrock, R. R. Organometallics 1987, 6, 1373. (b) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.; O'Regan, M.low nucleophilicity and that selective cross-metathesis might result if a suitable catalyst were treated with a mixture of acrylonitrile and a more nucleophilic olefin. This indeed turned out to be the case, as illustrated in Table 1.Cross-metathesis reactions were run by treating a mixture of acrylonitrile and a second olefin with Schrock's molybdenum catalyst M~(CHCM~ZP~)(NA~)[OCM~(CF~)~]Z (5 mol %).297Initial attempts to optimize reaction conditions showed that the reaction is slowed down considerably by coordinating solvents but does not proceed well in pentane or toluene. Dichloromethane seems to represent the best compromise of coordinating and solvating ability, leading to complete conversion of starting material within 2-3 h with yields as reported in Table 1. Reactions run in Et20...

Conformational preferences of two classes of organometal complexes have been surveyed by inspecting the Cambridge Structural Database (CSD). Lewis acid carbonyl complexes demonstrate a variety of coordination geometries, depending on the electronic and steric requirements of the carbonyl ligands and the nature of the Lewis acid. SimilarIy, the solid-state conformation of various x-bonding ligands in metal acyl, metal nitrosyl, metal acetylene, and metal imido complexes is revealed. These insights have stimulated the development of a conformational model that is based on considerations of n-bond hybridization and frontier molecular orbital theory. The analyses are relevant to the mechanism and transition structures of many synthetically important transformations. A deeper understanding of the conformational properties of organometal complexes, based on accurate structural information, will likely expedite the design and improvement of metal-mediated processes.