The Application of Physical Organic Methods to the Investigation of Organometallic Reaction Mechanisms

or

A Nostalgia Trip Through Organometallic Chemistry of the late 20th Century

Robert G. Bergman

Department of Chemistry, University of California, Berkeley and Division of Chemical Sciences, Lawrence Berkeley National Laboratory

G. N. Lewis Lecture October 23, 2018 G. N. Lewis: Electrons and Chemical Bonding

Gilbert Newton Lewis’s memorandum of 1902 showing his speculations about the role of electrons in atomic structure. From Valence and the Structure of Atoms and Molecules (1923), p. 29 Lewis, Langmuir and the 18-electron Rule: a Cornerstone of Organotransition Metal chemistry

Irving Langmuir Gilbert Newton Lewis Lewis’s theory of chemical bonding continued to evolve and, in 1916, he published his seminal article suggesting that a chemical bond is a pair of electrons shared by two atoms…. Lewis in 1923 redefined an acid as any atom or molecule with an incomplete octet that was thus capable of accepting electrons from another atom; bases were, of course, electron donors.

Science History Institute; https://www.sciencehistory.org/historical-profile/gilbert-newton-lewis Me – around the time all this Brief professional history: started! B.A., Carleton College, 1963

Ph. D., University of Wisconsin, 1966 (Jerome A. Berson)

Postdoctoral Fellow, Columbia University, 1966-67 (Ronald Breslow)

Caltech, 1967-1978

UC Berkeley, 1978 – present (Professor Emeritus since 2016) Disclaimers!

1. There has been a lot of talk about “self plagiarism” in scientific articles, so…. 2. Full disclosure: the chemistry described in this talk has been “self- plagiarized” from earlier talks and papers.

Including: Blum, S. A.; Tan, K. L.; Bergman, R. G. J. Org. Chem. 2003, 68, 4127-4137

3. Many of the physical organic techniques described in this paper could represent the first time they were used to study organometallic reactions. However…

4. Stigler's law of eponymy (Plagiarized from Wikipedia, the free encyclopedia)

Stigler's law of eponymy is a process proposed by University of Chicago statistics professor Stephen Stigler in his 1980 publication "Stigler’s law of eponymy." In its simplest and strongest form it says: "No scientific discovery is named after its original discoverer."

Stigler named the sociologist Robert K. Merton as the discoverer of "Stigler's law", consciously making "Stigler's law" exemplify Stigler's law. Exothermicity of Carbon-Carbon σ – Bond Formation from Alkynes: A Novel Route to 1,4-Dehydrobenzene (“para-Benzyne”)

or

Jones, R.R.; Bergman, R.G. J. Am. Chem. Soc. 1972, 94, 660-661. Trapping Experiments (Including Isotope Tracers)

Only para- dideuteriobenzene (no detectable meta- or ortho- isomers) is formed

Conclusion: the reactions of the intermediate are consistent with a diradical structure

For a summary of later computational and experimental studies, direct detection and the ground electronic state of p-benzyne, see Sander, W. Acc. Chem. Res., 1999, 32, 669–676. 1,4-Dehydrobenzene Chemistry in the 1980’s: Mechanism of Action of Calicheamicin Antibiotic

calicheamicin γ1α core

Presumed DNA-cleaving mechanism of calicheamicin g1α (1).

Calicheamicin-antibody conjugates are now being used as anti-cancer agents: Duebel, S.; Reichert, J. M.; Peipp, M.; Gramatzki, M. Handbook of Therapeutic Antibodies, Wiley-VCH 2014, Ch. 55.

Nicolaou, K. C.; Zuccarello, G.; Ogawa, Y.; Schweiger, E. J.; Kumazawa, T. J. Am. Chem. Soc. 1988, 110, 4866-4868 Lemonade from Lemons: Alkyne Trimerization Catalyzed by (h5- C5H5)Co(CO)2

CpCo(CO)2

O or

Co

Cp

Vollhardt, K. P. C.; Bergman, R. G. J. Am. Chem. Soc. 1974, 96, 4996-4998 Attempts to Make Highly Reduced Cyclopentadienylcobalt Complexes: Early Synthesis of a Binuclear Radical Anion and Binuclear Dialkyl Complexes

[CpCoCO]2-

Co(CO)2 O O O ROTf Cp R 1. Na/THF + CpCo CoCp PPN Co Co + CpCo CoCp R Cp 2. [Ph P=N=PPh ]+ Cl- 3 3 O O O

-1 -1 IR: 1690 cm R = CH3, CH2- CF3 IR: 1790 cm

µeff = 1.69 B.M. Isolated by air-free chromatography on SiO2

Radical anion ESR spectrum (15 lines due to cobalt spin 7/2; g = 3100 G

aCo= 50 G) and molecular structure

Ilenda, C. S.; Schore, N. E.; Bergman, R. G. J. Am. Chem. Soc. 1976, 98, 255-256 Schore, N. E.; Ilenda, C. S.; Bergman, R. G. J. Am. Chem. Soc. 1976, 98, 256-258 Early use of Crossover Experiments in Organometallic Chemistry: Carbon-carbon Bond-forming Reactions of Binuclear Cobalt Complexes

Crossover experiment:

White, M. A.; Bergman, R. G. Chem. Commun. 1979, 1056-1058 Spectroscopic Observation of a Metastable Dialkylcarbonyl Species: a Question of Necessary and Sufficient Mechanistic Evidence

Thermolysis of the dinuclear dicarbonyl (no CO added):

O

Cp R 35 oC O Co Co + CpCo(CO)2 + Clusters R Cp R R O

R CH3, CH2 CH3, CH2 CF3

35 oC Monitor by 1H NMR CO CpCo R R

R CH3, CH2 CH3, Spectroscopically detectable (NMR, IR)

R CH2 CF3 isolable

Conclusion: ketone formation CAN take place via the mononuclear intermediate. But is that how ketone IS formed from the dinuclear starting material?

This question concerns the kinetic competence of the mononuclear dialkylcarbonyl species. Variable Temperature NMR Behavior of a Dinuclear Cobalt Complex

O Cp CH2CF3 Co Co

CF3CH2 Cp O

CH2CF3 2 CpCo CO

Schore, N. E.; Ilenda, C. S.; White, M. A.; Bryndza, H. E.; Matturro, M. G.; Bergman, R. G. J. Am. Chem. Soc., 1984, 106, 7451-7461 Early Study of Kinetic Competence of Detectable Organometallic Intermediates: The Fast CO-Induced Conversion of [CpCo(CH3)(CO)]2 to Acetone

O O O Cp R CO Cp CO O Co Co R 2 CpCo(CO ) + 25 o C R Co Co 2 Cp R R R Fast Cp O O O

CO

CO O O O + CO OC R 2 CpCo CpCo R R CoCp CpCo R + CoCp R CO OC CO OC

CO Reaction proceeds faster than CO O CpCo CpCo(CO2) + R R R R

The dialkyl carbonyl reacts with CO too slowly to be an intermediate here. Direct formation of the (alkyl) (acyl) complex bypasses the dialkylcarbonyl complex.

Schore, N. E.; Ilenda, C. S.; White, M. A.; Bryndza, H. E.; Matturro, M. G.; Bergman, R. G. J. Am. Chem. Soc., 1984, 106, 7451-7461 Reliable Methods for Establishing Radical Pathways: Reduction of Alkyl Halides with Hydridovanadium Anions

[CpV(CO)3H] + R X R H + [CpV(CO)3X]

Possible mechanism:

R H + V(CO)3H V(CO)3X

R X ? X SN2? H

Cp(CO)3V Cp(CO)3V R

Kinney, R. J.; Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc. 1978, 100, 7902-7915 Early Evidence for Radical Intermediates in an Organotransition Metal Hydride/Organic Halide Reduction

Cp(CO)3VH + R X R H + Cp(CO)3VX

Relative reactivity: R-I > R-Br > R-Cl

R-Hal >> R-OSO2Ar

1° ~ 2° ~ 3° ~ vinyl ~ cyclopropyl Stereochemistry:

Br H D H

[V D] CH CH3 3

(S) racemic

[V H] Br

+ [V H] Br 70% 30%

These observations are very similar to those made in the study of the reduction of organic halides by trialkyltin hydrides (R3SnH). Early Use of a “Radical Clock” in an Organotransition Metal Mechanistic Study

Conclusion: a "super tin hydride" chain mechanism is operating:

CpV(CO)3 R Br R H

CpV(CO)3Br

CpV(CO)3H R

~ 7 -1 -1 For Cp(CO)3VH + R CpV(CO)3 + R H kH = 10 M s Reaction of a Nitrosyl Cobalt Dimer with Nitric Oxide: Possible Intermediacy of CpCo(NO)2 and its Cycloaddition Reaction with Alkenes

Cycloaddition/reduction reaction could potentially lead to provide a stoichiometric alkene 1,2-diamination reaction Scope, Stereochemistry, and Reversibility of Cobalt Nitrosyl/Alkene Addition

Becker, P. N.; Bergman, R. G. Organometallics 1983, 2, 787-796 Application to Stoichiometric Alkene Diamination

o *Diastereoselectivity is improved to 80/20 using DIBAL followed by LiAlH4 at – 50 C

Becker, P. N.; White, M. A.; Bergman, R. G. J. Am. Chem. Soc. 1980, 102, 5676-5677 Becker, P. N.; Bergman, R. G. Organometallics 1983, 2, 787-796 Generation of Spectroscopically Observable CpCo(NO)2 in Solution

Conclusion: A is CpCo(NO)2, the active intermediate in the alkene addition reaction

Becker, P. N.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 2985-2995 Comparison of the Relative Rates of Organometallic Addition Reactions with Those of Organic 1,3 Dipolar Cycloadditions: An Organometallic “Click” Reaction

H Me

C4H9 Ph H

+ o 5700 56 12 7.3 1 Ph N N N, CCl4, 25 C

o CpCo(NO)2, C6H12, 20 C 1960 59 13 2.3 1 Proposed "1-3- Dipolar Addition-Like" Transition State for CpCo(NO)2 + Alkene Addition Using Metals to Cleave Carbon-Hydrogen Bonds (“C-H Bond Activation”): “Holy Grail” Issue of Accounts of Chemical Research LINUS PAULING INSTITUTE of SCIENCE and MEDICINE 440 Page Mill Road, Palo Alto, California 94306-2025

9 April 1994 Dr. Fred W. McLafferty Baker Chemistry Laboratory Cornell University Ithaca, NY 14853-1301

Dear Fred:

I am pleased that you should write to me about the series of articles "Holy Grails." I do not have any interest in any of the Holy Grails mentioned in your letter.

[Followed by text of what Linus Pauling WAS interested in at the time] [Followed by signature] Excerpt from “Holy Grail” Issue of Accounts of Chemical Research

ARTHUR: Yes, we seek the Holy Grail (clears throat very quietly). Our quest is to find the Holy Grail. KNIGHTS: Yes it is. ARTHUR: And so we’re looking for it. KNIGHTS: Yes we are. BEDEVERE: We have been for some time. KNIGHTS: Yes. ROBIN: Months. ARTHUR: Yes...and any help we get is...is very...helpful.1

(1) Cleese, J.; Chapman, G.; Gilliam, T.; Idle. E.; Jones, T.; Palin, M. Excerpt from screenplay, “Monty Python and the Holy Grail”. 1974. Intermolecular Alkane C-H Oxidative Addition at Ir(I) Centers

Product identified by NMR; isolated by low- hv I R H Cp* (L)Ir Cp* (L)Ir(R H) temperature, inert Ir H R H Ir H atmosphere chromatography H2 Me3P H PMe3 R

“Cp*”

"L" hv R H H Ir Ir OC OC CO R Hoyano, Graham, 1982

CH3

R H: = , Me4C, , CH4 , CH2 C OH , etc.

H CH3

Selectivity: primary > secondary >> tertiary; C-H >> O-H, N-H, etc. Every organic solvent is reactive. Only successful inert solvents: liquid Xe, liquid Kr

Inspired by earlier experiments on Cp2WH2 by M.L.H. Green and co-workers

Graham experiment probably carried out several years earlier, but not published due to lack of NMR evidence and difficulty of product isolation Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1982, 104, 352-354 Kinetic Isotope Effects

Consider reaction rates:

* hu * hu Cp (L)IrH2 product Cp (L)IrH2 Deuterated product C6H12 C6D12

Little difference in overall rate of these reactions; determined by photophysics

But in mixture of deuterated and non-deuterated alkanes:

Cp*IrL intermediate reacts with CH bonds 1.38 times faster than with CD bonds. In contrast, thermal reductive elimination from Ir alkyl hydride and deuteride exhibits an inverse KIE.

Explanation: first indication of alkane complex intermediates: Infrared Flash Kinetic Studies in Liquified Noble Gas Solutions: Direct Observation of C-H Activation Intermediates on the Microsecond Timescale

hn (fast)

Cp*Rh(CO)2 Cp*(CO)Rh•Kr + CO -1 -1 (n = 1970, 2030 cm ) (nCO = 1947 cm ) CO 153-193 K

Keq CR3 Cp*(CO)Rh•Kr + R3C–H Cp*(CO)Rh– + Kr H -1 (nCO = 1947 cm )

CR CR k2 3 Cp*(CO)Rh– 3 Cp*(CO)Rh H H -1 (nCO = 2003 cm ) DH ‡ = 4.8 k/m; DS ‡ = -8 e.u.

Yeston, J. S.; McNamara, B. K.; Bergman, R. G.; Moore, C. B. Organometallics 2000, 19, 3442-3446. Schultz, R. H.; Bengali, A. A.; Tauber, M. J.; Weiller, B. H.; Wasserman, E. P.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. J. Am. Chem. Soc. 1994, 116, 7369. Weiller, B. H.; Wasserman, E. P.; Bergman, R. G.; Moore, C. B.; Pimentel, G. C. J. Am. Chem. SOC. 1989, 11 1, 8288

Very similar conclusions have since been drawn for other C–H activating system (e.g., Rh, Pt). Alkane complexes have also been observed directly in ultrafast kinetic studies with Rh and with Re complexes by low T NMR (Graham Ball). Combined Infrared Picosecond/Nanosecond Flash Kinetic Studies: Direct Observation of Intermediates in C-H Activation Reactions in Solution at Room Temperature

H

B N N Bromberg, S. E.; Yang, H.; Asplund, M. C.; Lian, T.; McNamara, B. K.; N Kotz, K. T.; Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman, R. G.; N N N Harris, C. B. Science 1997, 278, 260-263 Rh

CO H ~ B ~ N N N

N N ~ N H H Rh B R N OC 4.2 B N H N N N H N N N hn -1 N 1972 cm B N N ) N l - CO s -Complex Rh R N N o <8.3 N Rh OC H m R 8.3 / N N l OC N a H

c Rh(III) 16e H Rh

k -1 R

( 1990 cm alkyl hydride OC B H

G Dechelated N N -1 N complex 2032 cm Rh(III) 18e N N N C-H activation Rh product OC CO

Solvation by R-H Dechelation C-H Bond Re-chelation (few picoseconds) h3 h2 Activation << 200 ns 230 ns 200 ps Can C-H Activation Reactions be Carried out in Water Soluble Host- Guest Reaction Media?

12- Bergman group Ir(III) C-H Activation Chemistry Raymond group M4L6 Tetrahedral “Nanovessels” Reactivity of Host-Guest Assembly

O - CH + Me R H + Me 4 + R Ir Ir Ir - C2H4 Me P O CO Me3P 3 H Me3P 75° C R

-1 nC-O = 2000 – 2020 cm

Selectivity and cavity-blocking studies establish that the C-H activation reaction takes place inside the cluster cavity.

Leung, D.; Fiedler, D.; Raymond, K.; Bergman, R.G. Angew. Chem., 2004, 43, 963-966. Reactivity of Group 4 Metallocene Imido Complexes

R

Ph N R N Cp2Zr R N R Cp2Zr ZrCp2 Cp2Zr , N R N ZrCp2 Ph Alkene

RC CR R R N N – L Z C Y Cp'2Zr Cp'2Zr N R Cp2Zr Z L + L Ph Y N Y, Z = O, NR, CHR R' RN3 H R = t-Bu Ph R' R R' = Me, i-Pr R N L = THF, Py, Ph NH t-Bu N DMAP N Cp2Zr Cp2Zr Cp2Zr H Cp' = Cp, Cp* N N N Ph Ph

Similar synthetic and reactivity observation have been * * * made with Cp2Zr = O, Cp2Ti = N-Ph, Cp2Ti = S

Walsh, P. J.; Hollander, F. J.; Bergman, R. G. Organometallics 1993,12, 3705-3723 Zirconium Catalyzed Hydroamination of Alkynes: anti-Markovnikov N-H Addition

Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 1708-1719 Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 2753-2763 The Curtin-Hammett Principle: Stoichiometric vs. Catalytic Reaction Conditions

1) Catal. + Cp2(THF)Zr N Ar ArNH2 + Me C C i-Pr 110°C O O 2) HCl/H O 2 54% 46%

Ar N

Cp2Zr i-Pr Ar Me N HCl/H2O Cp2Zr Ar + THF N 110 oC O O Cp2Zr Me 92% 8% i-Pr

What is the cause of this difference? Is it consistent with a similar mechanism for both reactions? Reaction Coordinate for Stoichiometric Protonation of Azametallacycles

In the stoichiometric reactions, protonation of the azametallacycles is faster than cycloreversion Reaction Coordinate for Catalytic Hydroamination Reaction

Catalytic reaction: cycloreversion is both faster than protonation and reversible (“Curtin-Hammett” situation). Ratio of enamine products is determined only by the relative free energies of the transition states leading to products. Primary vs. Secondary Kinetic Isotope Effects: Does the Zr = NR Group Activate C-H Bonds Directly, or by Cycloaddition/Rearrangement?

Use of kinetic isotope effects (KIE’s) to address the above question:

NHtBu kH [R H] tBu CpCp*Zr N - THF R CpCp*Zr CpCp*Zr N tBu NDtBu THF kH [R D] CpCp*Zr (Cp* = C Me ) 5 5 R

Results:

tBu X tBu C C X

H12 vs. D12 H vs. D X = H vs. H6 vs. D6 12 12 X = H vs. X = D X = D

kH/kD 7.4 8.9 6.9 6.9 0.8

Dramatically different KIE’s for alkynyl C-H activation

Hoyt, H. M.; Michael, F. E.; Bergman, R. G. J. Am. Chem. Soc., 2004, 126, 1018-1019 Conclusion: sp2 and sp3 Hydrogens are Activated Differently

tBu NHtBu R -H(D) alkyl N CpCp*Zr R R.D.S. [Zr] H alkyl large normal Ralkyl kH / kD

CpCp*Zr=N-tBu

tBu

N NHtBu ~H R C C H(D) CpCp*Zr R CpCp*Zr fast R.D.S. C small inverse C H R kH / kD Lessons Learned

• The extra-kinetic methods for investigating mechanisms developed in physical organic chemistry provide a powerful way of thinking about how many different kinds of reactions take place, within and beyond classical organic chemistry. They have provided an important supplement to careful kinetic studies, which for many years had been the central method for investigating inorganic reaction mechanisms.

• Much early academic work in organotransition metal chemistry focused on stoichiometric transformations, but these studies have provided an important groundwork for later developments in both homogeneous and heterogeneous catalysis.

• There are some surprising mechanistic similarities between organometallic reactions (e.g., CpCo(NO)2/alkene additions, hydride transfer reactions, carbon-hydrogen bond activations) and classical organic reactions (e.g., radical chain reactions, carbene C-H bond insertions, 1,3-dipolar cycloadditions).

• Collaboration with colleagues, at both Caltech and Berkeley, has been a crucial aspect of achieving progress in organometallic chemistry, its impact on allied fields, and its practical applications.

• Few of the studies in this talk were initiated or first rationalized on the basis of potential practical applications. In fact, if funding had been sought on that basis (e.g., for the enediyne cyclization), it is unlikely that proposal reviewers would have taken the arguments seriously. Fundamental curiosity-driven research has frequently been the source of unpredictable results leading to long range intellectual and practical benefits. Sources of Support

• National Science Foundation • National Institutes of Health • Department of Energy/Lawrence Berkeley National Laboratory • Camille and Henry Dreyfus Foundation • Research Corporation • American Chemical Society (Cope Award Fund, Petroleum Research Fund) • Chevron Co., E. I. DuPont de Nemours and Co., Union Carbide Corp., Dow Chemical Co. • California Institute of Technology; University of California, Berkeley Mentors

Prof. Jerome A. Berson Yale University Prof. Ronald Breslow Columbia University

Bergman Group Members

Lutz Ackermann Mikael Brasse James H. Davis Brian M. Adger Casey Brown Andrea M. DeLaat Dan R. Adamson Duncan W. Brown Wayne A. Dickenson Peter J. Alaimo Katherine A. Brown Wolfgang Dittmar Kateri A. Ahrendt Joseph R. Bruckner Thien M. Do Laura L. Anderson Henry E. Bryndza Daniel A. Dobbs Mitchell R. Anstey J. Michael Buchanan Jeffrey J. Doney Ellena Arceo Rebollo Peter M. Burger Vy M. Dong Bruce A. Arndtsen Elizabeth R. Burkhardt Andrew P. Duncan Mark A. Aubart Melinda J. Burn Julie Duncan Bahram Bastani Matthew D. Butts Armando Durazo Joe T. Bamberg Barbara Carl Richard Eisenberg Janel M. Baptista Michael J. Carney Eric R. Evitt Anne M. Baranger Marcus Carr Michael G. Fickes Paul N. Becker Emily A. Carter Dorothea Fiedler Thomas W. Bell William P. Carter Thomas Foo Ashfaq A. Bengali Vincent Chan Daniel J. Fox Nicholas Benson Jeffrey Chang Richard Fox Mary Beth Berger Karen Cheng Julia Friedman Kamal N. Bharucha Melanie Chiu John H. Freudenberger Trudy C. Bischoff Jens Christoffers Jane E. Frommer Lee Bishop Thomas C. Clarke J. Robin Fulton Suzanne Blum Erika Cobar Mark Gallop Gary W. Bogan Denise Colby Thomas Gianetti John N. Bonfiglio Paul B. Comita Michael W. Gribble Marco W. Bouwkamp Paul B. Condit Michael Gerisch Miriam Bowring Joseph G. Cordaro Thomas M. Gilbert W. Christopher Boyd Mark R. Crimmin David S. Glueck Maurice S. Brookhart John J. Curley Karen I. Goldberg Holger Brand Michael B. D'Amore Jeffery T. Golden Tracy A. Hanna R. Bryan S. Klassen Francois P. Mazenod 2 William Hart-Cooper Steve Klei Donald McAlister John F. Hartwig Darryl P. Klein J. Stuart McCallum Courtney J. Hastings Grant M. Kloster Lorraine P. McDonnell Jeffrey C. Hayes Theodore P. Klupinski James McGarrah Timothy J. Henry Akos Kokai William D. McGhee William H. Hersh Juergen Koller Stephan J. McLain Julia A. Hodge Caroline A. Kovac Bruce K. McNamara Andrew W. Holland Julie A. Kovacs Kristopher P. McNeill Patrick L. Holland Michael J. Krause Tara Y. Meyer Michael J. Hostetler Jamin L. Krinsky Forrest Michael William A. Howard Shane W. Krska Karen E. (Meyer) Michelman Helen M. Hoyt Jennifer Krumper Richard Ira Michelman John M. Huggins Kevin R. Kyle Aubry K. Miller Casmir S. Ilenda Marty Lail Robert E. Minto Santiago Ini Gojko Lalic Thomas A. (Andy) Mobley Eric N. Jacobsen Carl E. Larson Kevin T. Moore Andrew H. Janowicz Gino G. Lavoie Robert J. Morris Stephen R. Jenkins Sun Yeoul Lee Thomas H. Morton Donald A. Johnson Dennis H. Leung Sarah Mullins Gilbert C. Johnson Jared C. Lewis Hossein M. Naderi Jeffrey S. Johnson Chao-Jun Li John A. Napierski Miles Johnson Allegra Liberman-Martin Silvia Napierski Richard R. M. Jones Thomas P. Lockhart Michael W. Nee William D. Jones Hans F. Luecke Linda J. Newman Winslow Anne W. Kaplan Yinong Ma Jason M. Nichols Susan E. Kegley Charles B. Mallon Carmen I. Nitsche Donald R. Kelsey Peter Marsden Sara N. Paisner Robert A. Keppel Rebecca Marsh (Roth) Roy A. Periana Benjamin T. King Martha J. Marvin Martin H. F. Pedersen Robert J. Kinney Cristina Mateo Thomas H. Peterson Jaqueline L. Kiplinger Michael G. Matturro Michael Pluth 3

Jennifer L. Polse Ian C. Stewart Donald A. Watson Grant Proulx John J. Stofko Mary P. Watson Daniela Rais Page O. Stoutland Patricia L. Watson V.J. Rajadhyaksha F. Bernd Straub Michael J. Wax Joachim M.C. Ritter Jeffrey M. Stryker Bruce H. Weiller Russell E. Roberson Louis S. Stuhl Wayne R. Weiner Lauren Rosebrugh Chaim N. Sukenik Ben-Avi Weissman Rebecca T. Ruck Kenneth S. Suslick Larry A. Wendling Jennifer Sasaki Zachary K. Sweeney Timothy T. Wenzel Jennifer L. Salsman Christopher D. Tagge Marc Weydert Christian Schade Kian L. Tan David H. White Colin J. Schaverien Michael J. Tauber Mary Ann White Jennifer M. Schomaker Michael E. Tauchert Sean Wiedemann Neil E. Schore Felicia Taw Rebecca M. Wilson Richard H. Schultz Reema K. Thalji Keith A. Woerpel Mark D. Seidler David M. Tellers Jianxin (Jason) Wu Paul F. Seidler Klaus Theopold Neal A. Yakelis Michael H. Sekera Neil C. Tomson Gilbert K. Yang Kenneth J. Shea Samuel J. Tremont Jake Yeston Kelvin K. Shen Mary K. Trost Eunice J. York Shelby A. Sherrod Andy Tsai Sirilata Yotphan Hsin-Guo Shu Ronald C. Ugolick Shuchun Joyce Yu Robert D. Simpson John E. Veltheer Cathleen M. Yung Steve J. Skoog Jerome Volkman Chen Zhao Greg A. Slough K. Peter C. Vollhardt Raymond F. Ziessel Stuart E. Smith Raymond N. Vrtis Rebecca L. Zuckerman Michael B. Sponsler Patrick J. Walsh Jeffrey G. Stack Philip M. Warner J. Ronald Stevenson Eric P. Wasserman Richter, M., New Yorker, May 7, 1984