J. W. Faller

Organometallic conformational equilbria : XVI. Steric effects on π-allyl and π-indenyl orientation in molybdenum and tungsten complexes

Abstract Steric effects on π-allyl and π-indenyl orientation in molybdenum and tungsten complexes have been studied. Magnetic anistropies associated with indenyl derivatives have provided a definitive technique for determination of stereochemistry in these complexes. The steric factors which determine the stability of the orientations of the allyl moiety have been discussed. Generally π-cycloentadienyl or π-indenyl ligands have been considered to be essentially freely rotating. Analysis of the magnitude of the magnetic anisotropy arising from the benzene ring has suggested that there is relatively free rotation of the indenyl ligands, but there is a preferred conformation with the six-membered ring oriented over the allyl. Appropriate substitution on the allyl offers sufficient steric hindrance to make other conformations more probable.

Distortions in trihapto-allyls induced by electronic asymmetry: A comparison of the structures of (η5-C5H5)Mo(CO)2(η3-C3H5) and (η5-C5H5)Mo(NO(I)(η3-C3H5)

Abstract The electronic asymmetry induced by replacing two carbonyls by a nitrosyl and iodide ligand causes severe distortion in the allyl moiety. The allyl group in the nitrosyl complex is bound in a sigma-pi mode rather than the symmetrical mode found in the dicarbonyl. This change in ground state structure alters the exo—endo conformer interconversion mechanism from a rotation of the allyl in the dicarbonyl to a sigma-pi interconversion in the nitrosyl iodide.

The application of ESCA to the study of bonding in nitrosyls

A study of a series of nitrosyl complexes by ESCA has provided a complementary method to infrared spectroscopy for distinguishing bent and linear nitrosyls. The relative shifts of the binding energies of O 1s and N 1s electrons tend to be found in the range of 132 ± 1 eV for linear nitrosyls and 128 ± 2 eV for bent nitrosyls. This approach leads to reversal of previous assignments of N 1s binding energies observed in certain complexes. Evaluation of N 1s binding energy shifts illustrates that the “NO−” in a bent nitrosyl may actually have less electron density associated with the nitrogen atom than a “NO+” in a linear nitrosyl.

Use of organometallic complexes of ruthenium in the Lewis acid catalyzed hetero Diels-Alder reaction

Abstract The hetero Diels-Alder reaction between benzaldehyde and a functionalized diene can be catalyzed by the Lewis acid transition metal complexes, [CpRuLL′(CH2 CH2)]PF6 (L L′ PPh3, or LL′ = 1,2-bis-diphenylphosphinoethane, (−)-DIOP or (S,S)-CHIRAPHOS).

High oxidation state organometallics. Pentamethylcyclopentadienyloxo-molybdenum(VI) and -tungsten(VI) complexes

Abstract The preparation, isolation, and characterization of [(η 5 -C 5 Me 5 )MoO 2 ] 2 O ( 1 ) and (η 5 -C 5 Me 5 )MoO 2 Cl ( 2 ) and their tungsten analogues ( 1 ′ and 2 ′) are described. A substantial improvement in stability, ease of preparation, and ease of separation from other reaction products are observed for these Cp * complexes compared to their cyclopentadienyl counterparts. The binuclear pentaoxo compound, 1 , crystallizes in the monoclinic space group P 2 1 c with one and a half molecules in the asymmetric unit, Z = 6, a 21.265(7), b 9.237(3), c 17.669(5) A, β 101.78(3)°, and V 3398(4) A 3 . Anisotropic refinement of the molybdenum and isotropic refinement of the non-metal atoms with no hydrogen atoms included converged to the residuals R 1 = 0.075, R 2 = 0.088. Two independent μ-oxo complexes are represented in the cell: one is centrosymmetric having the bridging oxygen on a special position: and the other nearly centrosymmetric having a MoOMo angle of 177.9(5)°. Grignard addition to 2 ′ yields the air-stable alkyl, (η 5 -C 5 Me 5 )WO 2 CH 2 SiMe 3 .

Organometallic conformational equilibria : XVIII. Preferred orientations and rotational barriers of π-olefins in cyclopentadienyl and indenyl complexes of iron and ruthenium

Abstract The synthesis and spectral analysis of a series of some η5-cyclopentadienyl-and η5-indenyl-ironolefin complexes have allowed the elucidation of the orientational preferences and dynamic properties of the olefin ligand. The olefins rotate rapidly about the metalolefin bond with the barrier to rotation on the order of 8 kcal as determined by the observation of signal broadening in several of the ethylene complexes. Dissociation of the olefin and rotation about the carbon double bond are excluded as possible mechanisms on the basis of spectral evidence. The mode of rotation is consistent with the behavior of the olefin ligands in η5-C5H5Rh(C2H4)2 first noted by R. Cramer [J. Amer. Chem. Soc., 86 (1964) 217]. Thetermodynamically preferred orientations were defermined for each of the olefin complexes. Chemical shift differences resulting from the substitution of an indenyl ligand for a cyclopentadienyl ligand allow determination of the preferred orientations in the ethylene and propylene complexes. For ethylene, the orientation in which the CC bond is parallel to the plane of the cyclopentadienyl ring is preferred. Methyl substitution of the olefin produces a deviation of preferred orientation. A dihedral angle of about 10° is estimated for the propylene complex.

Controlling stereochemistry in C-C and C-H bond formation with electronically asymmetric organometallics and chiral poisons

Air stable (cyclopentadienyl)Mo(NO)(halide)(q3-allyl) complexes add to aldehydes to yield homoallylic alcohols in high enantioselectivity and diastereoselectivity. A new strategy, chiral poisoning, is applied to asymmetric hydrogenation and the kinetic resolution of allylic alcohols using transition metal catalysts prepared from racemic bis-phosphine ligands. The activation afforded an organic moiety by complexation led to the extensive development of transition metal reagents for organic synthesis. The increased reactivity is influenced by differences in the steric and electronic nature of the metal and its ligands. Ultimately the environment at the metal can induce high stereoselectivity in reactions involving the coordinated ligands. Our aim has been the improvement of our understanding of the origins of selectivity in these reactions and the application of these principles to the rational design of reagents and catalysts. Our approach modifies the conventional emphasis on steric effects in catalyst design and focuses attention on electronically controlled selectivity. Our initial work emphasized the control which could be obtained with nucleophilic additions to chiral (CpMo(allyl)(NO)(CO))* cations, wherein the difference in electronic influences of the carbonyl and

Controlling stereochemistry in crotyl additions to aldehydes with crotylmolybdenum complexes

Abstract Air stable (Cyclopentadienyl)Mo(NO) (Cl) (π-crotyl) complexes add to aldehydes in the presence of methanol to yield homoallylic alcohols with high regioselectivity and diastereoselectivity. Reaction of (−)-(S)-(Neomenthylcyclopentadienyl)Mo(NO)(Cl)(π-crotyl) with benzaldehyde yields (+)-(R,R)-2-methyl-l-phenyl-3-buten-1-ol in > 98% ee.

Reactions and properties of oxo and peroxo derivatives of (pentamethylcyclopentadienyl)-molybdenum and -tungsten

The characterization and reactivity of complexes (η5-C5Me5)M(O)(η2-O2)Cl (M = Mo, W) and (η5-C5Me5)M(O)Cl3 are described. In addition, the preparation and reactivity of some molybdenum-oxo alkyl complexes are discussed in this paper. The oxo-H2O and oxo-peroxo exchange behavior of these systems was studied with infrared spectroscopy using 18O isotope labeling techniques.

Controlling the regiochemistry of nucleophilic attack on unsymmetrical allyl complexes

Abstract The NO and CO ligands in a [CpMo(NO)(CO)(η 3 -allyl)] + complex exert differential electronic effects on the allyl moiety and control the regiochemistry of nucleophilic attack. These directing influences are sufficiently strong that they overcome the normal directing influences of substituents of the allyl moiety. An essential task, therefore, is arranging for the appropriate terminus of the allyl to have the relationship to the nitrosyl group that will result in attack at the desired location. Replacement of a single CO ligand in a neutral monosubstituted allyl complex of CpMo(CO) 2 with NO + yields a product for which addition occurs at the unsubstituted end of the allyl. Hence, an E -olefin with no newly created chiral centers is formed upon nucleophilic attack. A new synthetic strategy has been developed which allows us to build a chiral center in the alylic position of a terminal olefin. Sequential addition of two nucleophiles to a CpMo(NO)(CO)(allyl) + cation creates these chiral centers, which were not accessible by addition of NO + to the dicarbonyl. Addition of nucleophiles to homochiral (Neomenthylcyclopentadienyl)Mo(NO)(CO)(phenylallyl) + complexes yields chiral olefins in high enantiomeric purity. The cationic phenylallyl complexes prepared by different routes were identified by single crystal X-ray diffraction determinations. endo-cis-syn -[CpMo(NO)(CO)(η 3 -CH(Ph)CHCH 2 )]BF 4 crystallizes in the monoclinic space group P 2 1 / n with a 8.226(2), b 12.273(2), c 16.051(2) A, β 100.02(2)°, V 1595.7(9) A 3 , Z = 4. exo -cis-syn-[CpMo(NO)(CO)(η 3 -CH(Ph)CHCH 2 )]BF 4 crystallizes in the monoclinic space group P 2 1 / c with a 10.706(3), b 14.700(4), c 10.491(3) A, β 96.48(2)°, V 1640.5 A 3 , Z = 4.