John W. Faller

Co-reporter:John R. Miecznikowski, Stephan Gründemann, Martin Albrecht, Claire Mégret, Eric Clot, Jack W. Faller, Odile Eisenstein and Robert H. Crabtree

Dalton Transactions 2003 (Issue 5) pp:831-838

Publication Date(Web):29 Jan 2003

DOI:10.1039/B210784H

Interconversion of the two chiral conformations of the square planar Pd(II) CCC pincer carbene complex, 1 (η3-C,C′,C″) (2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}phenyl)bromopalladium(II), and the CNC cation, 2, (η3-C,C′,N)(2,6-bis{[N-methyl-N′-methylene]imidazol-2-ylidene}pyridine)bromopalladium(II)(1+), is characterized by VT NMR spectroscopy. Combined DFT/experimental work indicates two alternative mechanisms. In the case of 1, having no counterion, and several derivatives of 2 with weakly nucleophilic counterions, the fluxional process goes in two steps via an unsymmetrical cationic 4-coordinate intermediate. In this case one carbene ring moves through the square plane before the other. In some cases for 2 with more nucleophilic counterions, such as [{CNC}PdI]I, a second lower-barrier process takes over that depends on the nature of the counterion. We propose that the outer sphere anion reversibly displaces the central N (pyridine) unit of the pincer in a rate limiting step to form a neutral dihalo intermediate that undergoes rapid conformer interconversion. This accounts for the counterion dependence and constitutes an unusual type of fluxionality that couples anion substitution at the metal with the conformational change of the ligand. A pyridine, even when present as the central element of a CNC pincer ligand, can therefore be labile even under mild conditions and reaction mechanisms involving decoordination of such group are therefore possible.

Reversible α-elimination in the conversion of N–CH3 to N–CHIr by double C–H activation

Co-reporter:Dong-Heon Lee, Junyi Chen, Jack W. Faller and Robert H. Crabtree

Chemical Communications 2001 (Issue 2) pp:213-214

Publication Date(Web):09 Jan 2001

DOI:10.1039/B007679L

2-Dimethylaminopyridine (pyNMe2; py = 2-pyridyl) reacts with [H2Ir(OCMe2)2L2]+ (L = PPh3) to give a cyclic carbene complex [H2Ir(CHN(Me)py)L2]+via an oxidative addition, reversible α-elimination sequence.

Highly enantioselective Diels–Alder catalysis with a chiral ruthenium bisoxazoline complex

Co-reporter:J.W Faller, Adrien Lavoie

Journal of Organometallic Chemistry 2001 Volume 630(Issue 1) pp:17-22

Publication Date(Web):2 July 2001

DOI:10.1016/S0022-328X(01)00871-3

When treated with AgSbF6, the cation (−)-[(η6-cymene)RuCl(L)]SbF6, where L=(+)-INDABOX=[3aR-[2(3′aR*,8′aS*),3′aβ,8′aβ]]-(+)-2,2′-methylenebis[3a,8a-dihydro-8H-indeno[1,2-d]-oxazol], yields a dication which is an efficient catalyst for the condensation of either methacrolein or ethylacrolein with cyclopentadiene in high ee. For example, the reaction of methacrolein with cyclopentadiene at −24 °C produced (S)-(+)-exo-2-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde in 95% conversion with de=96% and ee=91%. The monocationic catalyst precursor was characterized spectroscopically and by single crystal X-ray diffraction.When treated with AgSbF6, the cation (−)-[η6-cymene)RuCl(L)SbF6, where L=(+)-INDABOX, yields a dication which is an efficient catalyst for the condensation of either methacrolein or ethylacrolein with cyclopentadiene in high enantiomeric excess. For example, the reaction of methacrolein with cyclopentadiene at −24 °C produced (S)-(+)-exo-2-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde in 95% conversion with de=96% and ee=91%.

Rearrangements in Allylpalladium Complexes with Hemilabile Chelating Ligands

Co-reporter:Heather L. Stokes-Huby;Mauricio A. Albrizzio;John W. Faller

Helvetica Chimica Acta 2001 Volume 84(Issue 10) pp:3031-3042

Publication Date(Web):16 NOV 2001

DOI:10.1002/1522-2675(20011017)84:10<3031::AID-HLCA3031>3.0.CO;2-8

In addition to η³- to η¹-allyl rearrangement, η²- to η¹-chelating-ligand rearrangements can affect the dynamic properties in allylpalladium complexes containing hemilabile ligands. These rearrangements have the potential of altering stereochemistry, which can ultimately affect the stereochemistry and regiochemistry of reactions upon the allyl group, e.g., allylic alkylation. Apparent rotation of an η³-allyl in four-coordinate Pd^II-complexes can be the result of ligand exchange or intramolecular processes. NMR Studies indicate that the effects of η²- to η¹-chelating-ligand stereochemistry and rearrangements may be hidden or subtle, and both intra- and intermolecular processes may be important.

Diastereoselectivity an chiral ruthenium complexes of chiraphos monoxide ligands: Epimerization, quasiracemates, and lewis acid catalysis

Co-reporter:J.W. Faller;Jonathan Parr;Xiang Liu

Chirality 2000 Volume 12(Issue 5‐6) pp:325-337

Publication Date(Web):19 MAY 2000

DOI:10.1002/(SICI)1520-636X(2000)12:5/6<325::AID-CHIR5>3.0.CO;2-6

Heterobidentate chelating ligands comprising P,O-donor sets produce chiral metal centers when bound to arene-ruthenium complexes. The cymene complex [CyRu(η²-chelate-P,O)Cl]⁺ cations are precursors to 16-electron dicationic strong Lewis acids which have potential use in asymmetric catalysis. Sixteen-electron complexes of this type, however, also provide a pathway with a low energy barrier to epimerization of the metal center in intermediates. With the chiral ligand (R,R)-chiraphos monoxide, the metal center in [CyRu(η²-(R,R)-Ph₂PCHMeCHMe Ph₂PO-P,O)Cl]⁺ epimerizes to minimize steric interactions in the two diastereomers yielding a 8.2:1 dr in solution. Nevertheless, upon crystallization a new equilibrium is obtained in the solid and the crystals obtained have a 1:1 dr. These crystals are quasiracemates, which contain equal amounts of the (R_Ru) and (S_Ru)-CyRu-P-O-Cl stereocenters within the same crystal. The doubly charged Lewis acid, [CyRu(η²-(R,R)-Ph₂PCHMeCHMe Ph₂PO-P,O)(solvate)]²⁺ derived from the chloro complex by chloride abstraction with AgSbF₆ gave modest ee's (30%) in the Diels-Alder reaction of methacrolein with cyclopentadiene. Chirality 12:325–337, 2000. © 2000 Wiley-Liss, Inc.