Scott A. Laneman

A bimetallic hydroformylation catalyst: high regioselectivity and reactivity through homobimetallic cooperativity.

The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.

STRUCTURAL CHARACTERIZATION OF 1,2,4,5-TETRAPHENYLCYCLO-3,6-DICARBA-1,2,4,5-TETRAPHOSPHINE: A HIGHLY FOLDED CHAIR CONFORMATION

Abstract The title compound crystallized in the monoclinic space group P21/c, with the following unit cell parameters: a = 11.235(8), b = 8.473(2), c = 26.136(10) A, β = 102.07(5)°, V = 2432(3) A3, Z = 4.1657 unique data (Fo 2 > 3o(Fo 2)) were used to solve (direct methods) and refine the structure to give R = 0.126 and Rw = 0.118. The six-membered ring system is in a highly folded chair conformation with the phenyl group substituents on the phosphorus atoms oriented in the equatorial positions. The dihedral angles of the P-CH2-P plane with the P4 basal plane is a remarkable 101.8° (average). This dihedral angle is the closest to perpendicular ever observed for a monocyclic ring system.

Asymmetric hydrogenations of diketopiperazines

The dehydrodiketopiperazines 6 undergo face selective hydrogenation with a 5% palladium on carbon catalyst to give the diketopiperazines 7 in high yield with excellent diastereoisomeric ratios (>97%).

The synthesis of heterobimetallic systems based on a hexatertiaryphosphine ligand system: cobalt-nickel and nickel-platinum complexes

Abstract Heterobimetallic complexes can be prepared in an extremely straightforward fashion by the use of the binucleating hexatertiaryphosphine ligand (Et 2

William Standish Knowles (1917–2012)

Bill Knowles passed away on June 13, 2012 at his home in Chesterfield, Missouri at the age of 95. His Nobel Prize brought the importance of industrial and applied chemistry to the forefront. William was born on June 1, 1917 in Taunton, Massachusetts and grew up in nearby New Bedford. His father was the owner of a cotton mill and he could have gone into the family business but didn t want to do that. He was educated at private schools and thinking that he was too young to go to college, spent a year at Phillips Academy in Andover, Massachusetts, where he won the Boylston prize (US

Di-μ-carbonyl-bis­[bis­(triphenyl­phos­phane)rhodium(0)](Rh—Rh) acetone disolvate

50) in chemistry. After this, Knowles went to Harvard where he specialized in organic chemistry, even though his ability to do mathematics had led his advisors to suggest a physical chemistry pathway. Louis Fieser was the main influence in the move to organic chemistry. He obtained his AB degree in 1939. Bill obtained his PhD from Columbia University in 1942, working on steroid chemistry with Robert Elderfield. While performing one experiment, which required the distillation of diazomethane, he had an explosion that destroyed a hard-fought intermediate and he had to remake it. The war aided the short time to get his degree. He then joined the Monsanto Company at their facility in Dayton, Ohio, moving to St. Louis in 1944. The reason he chose Monsanto was because it was the farthest west of the companies that offered him a job. Chemists were in short supply during the war. The work at Monsanto covered many topics, starting with the manufacture of hexamethylenetetramine for the explosive cyclonite and mite repellents, and benzyl benzoate for soldier s clothing. Another project involved the synthesis of vanillin from catechol, and this was to have an influence later in Bill s career, even though this chemical route was superseded by an isolation of lignin from paper waste. He also studied a manufacturing route to the antibiotic chloramphenicol, which was taken off the market because of aplastic anaemia developing in a few patients. However, this did not stop him from mixing some of this antibiotic with an ointment of other drugs that were being administered to his dog suffering from a fungal infection. The dog was better in two days and lived to be 17. Monsanto also looked at the synthesis of cortisone, and Bill was nominated to join this effort from his PhD work on steroids. This allowed him to return to Harvard and work in the labs of Professor R. B. Woodward, whom he grew to admire for his encyclopedic knowledge of chemistry and ability to look at problems from new angles. The first manufacturing project Bill had that involved a hydrogenation was the reduction of para-nitrophenetol. Nickel was the catalyst being used but the reaction was capricious owing to the hydrogen supply. The use of the more expensive palladium on carbon gave a much more reliable reaction, which was cheaper in the long term. The research area that led to the Nobel Prize was asymmetric hydrogenation. Monsanto became interested in the manufacture of the Parkinson s drug l-Dopa, and to make it as a single enantiomer. Even in the 1960s, it was realized that the d-isomer was inactive and just baggage for the patient to take. As noted in his Nobel Lecture, naivety can be useful and lead to inventions. In a rather academic investigation, a new PhD, Jerry Sabacky, was given the task of reducing a-phenylacrylic acid in a homogeneous hydrogenation reaction with rhodium and a chiral phosphine. The resultant enantiomeric excess was 15 %. At this time, Monsanto was selling vanillin to Hoffman–LaRoche, who then made racemic Dopa and resolved it. Knowles found out that the method being used involved the Erlenmeyer azlactone procedure, something in the public domain, but not simple to find out through “official” channels. As he put it, “the less you have to hide, the more secretive you become.” Use of the rhodium catalyst with the CAMP ligand gave an 88% ee of the amino acid derivative, and this was good enough to use in a commercial process, as the unwanted enantiomer was rejected in downstream purifications. Kagan s work with DIOP led to an investigation where the phosphines were dimerized, and this led to DIPAMP. As this was easier to make than CAMP, gave a higher ee in the Dopa synthesis (95 %), and was a crystalline air-stable solid, the manufacturing process moved to the use of [Rh(DIPAMP)], which is still used today on a commercial scale. The reduction of enamides to make a variety of a-amino acids and esters was continued by NSC Technologies, part of Monsanto, to make a wide variety of intermediates used in the pharmaceutical industry. Bill regretted the fact that he did not push harder to get Monsanto to make l-phenylalanine by an asymmetric hydrogenation, as this is a key component of the sweetener aspartame. NutraSweet was the parent company of NSC Technologies, and some of Bill s colleagues, especially Billy Vineyard, were key in working out a crystallization for the isolation of the correct isomer of aspartame at commercial scale. After retiring in 1986, Bill continued to act as a consultant and interacted with many of us who were looking to apply asymmetric hydrogenations on an industrial scale to a variety of compounds. These included a new route to the NSAID, naproxen, and the synthesis of unnatural amino acids using, what was then affectionately known as Knowles catalyst. John Talley was very impressed W. S. Knowles . Angewandte Obituary

Bimetallic Hydroformylation Catalysis

The dirhodium complex, [Rh2(C18H15P)4(CO)2]·2(CH3)2CO, has crystallographic twofold symmetry and the Rh—Rh distance is 2.6266 (8) Å. The four atoms proximate to each Rh atom [Rh—P = 2.3222 (7) and 2.3283 (8) Å, and Rh—C = 1.961 (3) and 2.045 (3) Å] form a distorted tetrahedron with large deviations from the putative tetrahedral angles [r.m.s. deviation = 23 (1)°]. The six angles more closely approximate those of a trigonal bipyramid [r.m.s. deviation = 14 (1)°] with one missing equatorial ligand. The two bridging carbonyl ligands are much more linearly coordinated to one Rh [Rh—C O = 151.0 (2)°] than to the other [127.0 (2)°], and the two Rh2CO planes form a dihedral angle of 45.43 (5)°. The two acetone solvent molecules are disordered, and their estimated scattering contribution was subtracted from the observed diffraction data using the SQUEEZE routine in PLATON [Spek (2009 ▶). Acta Cryst. D65, 148–155].

A new class of binucleating tetratertiaryphosphine ligands. The synthesis and crystallographic characterization of the chiral diastereomer of a rhodium(I) dimer: Rh2Cl2(CO)2(eLTTP) (eLTTP = (Et2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PEt2))

Bimetallic transition metal complexes based on a new binucleating tetratertiary-phosphine ligand system (Et2PCH2CH2)(Ph)PCH2P(Ph)(CH2CH2PEt2), eLTTP, have been prepared and characterized. The Rh(I) bimetallic complexes meso- and racemic-Rh2Cl2(CO)2(eLTTP) have been synthesized and characterized and the racemic structure characterized. The hydroformylation activity of the norbornadiene derivative, Rh2(norb)2(CO)2(eLTTP)2+(norb = norbornadiene), with respect to simple 1-alkenes and functionalized alkenes such as vinyl acetate are reported. The Rh2(eLTTP) catalyst system is unusual for several reasons: 1) excess phosphine ligand is not required to maintain catalyst stability; 2) unusually high normal/branched product aldehyde ratios are seen in the hydroformylation of 1-alkenes (18:1 normal to branched, at 120 psi and 80°C); 3) the hydroformylation catalysis is markedly faster than that seen for monometallic model system Rh(norb)(depe)+ (depe = Et2PCH2CH2PEt2) and 4) the vinyl acetate hydroformylation reaction does not sufrer from the extensive product, reactant and catalyst decomposition reactions typically seen for Rh(I)/PR3 catalyzed systems. The enhanced rate for the bimetallic verses monometallic system is believed to be due to bimetallic cooperativity in the form of intramolecular hydride transfer and elimination of the product aldehyde.