Paul G. Williard

Formula weight prediction by internal reference diffusion-ordered NMR spectroscopy (DOSY).

Formula weight (FW) information is important to characterize the composition, aggregation number, and solvation state of reactive intermediates and organometallic complexes. We describe an internal reference correlated DOSY method for calculating the FW of unknown species in different solvents with different concentrations. Examples for both the small molecule (DIPA) and the organometallic complex (aggregate 1) yield excellent correlations. We also found the relative diffusion rate is inversely proportional to the viscosity change of the solution, which is consistent with the theoretical Stokes-Einstein equation. The accuracy of the least-squares linear prediction r(2) and the percentage difference of FW prediction are directly related to the density change; greater accuracy was observed with decreasing density. We also discuss the guidelines and other factors for successful application of this internal reference correlated DOSY method. This practical method can be conveniently modified and applied to the characterization of other unknown molecules or complexes.

Controlling Gold Nanoclusters by Diphospine Ligands

We report the synthesis and structure determination of a new Au22 nanocluster coordinated by six bidentate diphosphine ligands: 1,8-bis(diphenylphosphino) octane (L(8) for short). Single crystal X-ray crystallography and electrospray ionization mass spectrometry show that the cluster assembly is neutral and can be formulated as Au22(L(8))6. The Au22 core consists of two Au11 units clipped together by four L(8) ligands, while the additional two ligands coordinate to each Au11 unit in a bidentate fashion. Eight gold atoms at the interface of the two Au11 units are not coordinated by any ligands. Four short gold-gold distances (2.64-2.65 Å) are observed at the interface of the two Au11 clusters as a result of the clamping force of the four clipping ligands and strong electronic interactions. The eight uncoordinated surface gold atoms in the Au22(L(8))6 nanocluster are unprecedented in atom-precise gold nanoparticles and can be considered as potential in situ active sites for catalysis.

Boron trihalide-methyl sulfide complexes as convenient reagents for dealkylation of aryl ethers

Abstract High yield dealkylation of several aryl methyl ethers was observed upon exposure of these compounds to an excess of the boron trihalide-methyl sulfide complexes.

X-ray crystal structure of a lithium aldolate - a tetrameric aggregate

The aldol adduct formed from addition of pivalaldehyde to the lithium enolate of pinacolone crystallized directly from the reaction mixture and was subjected to x-ray diffraction analysis.

13C INEPT Diffusion-Ordered NMR Spectroscopy (DOSY) with Internal References

13C INEPT Diffusion-ordered NMR spectroscopy (DOSY) with an internal reference system was developed to study the aggregation state of THF-solvated LDA dimeric complex. Six components are clearly identified in the diffusion dimension, and their DOSY-generated 13C INEPT spectrum slices agree extremely well with their respective INEPT spectra. The correlation between log D and log FW of the linear least-squares fit to reference points of all components is exceptionally high: (r = 0.9985).

Internally referenced diffusion coefficient-formula weight (D-FW) analysis of 31P diffusion-ordered NMR spectroscopy (DOSY).

The development of (31)P DOSY NMR with diffusion coefficient-formula weight (D-FW) analysis is reported. Commercially available trialkyl phosphine internal references were used in a model system to establish the molecular weight of a phosphorous containing organolithium compound. The feasibility of (31)P DOSY D-FW studies is established. This extension of DOSY D-FW analysis expands its applicability to solution structure studies of a wide variety of compounds.

Characterization of a chiral enolate aggregate and observation of 6Li-1H scalar coupling.

A chiral enolate aggregate 1 containing a lithium enolate and a chiral lithium amide was systematically investigated by various NMR techniques. (1)H and (13)C DOSY at 25 and -78 degrees C provide its solution structure, aggregation number, and formula weight. Multiple 2D (6)Li NMR techniques, such as (6)Li-(6)Li EXSY, (6)Li-(1)H HOESY, were utilized to investigate its stereochemical structure. The configuration of the enolate in complex 1 was confirmed by (6)Li-(1)H HOESY and trapping with TMS-Cl. A unique (6)Li-(1)H coupling through the Li-N-C-H network was observed. This scalar coupling was corroborated by (6)Li-(1)H HMQC, deuterium labeling experiments, and selective (1)H decoupling (6)Li NMR. The stereostructure of 1 provides a model for the origin of enantioselectivity of chiral lithium amide-induced enolate addition reactions.

Synthesis and Structure Determination of a New Au20 Nanocluster Protected by Tripodal Tetraphosphine Ligands

We report the synthesis and structure determination of a new Au20 nanocluster coordinated by four tripodal tetraphosphine (PP3) ligands {PP3 = tris[2-(diphenylphosphino)ethyl]phosphine}. Single-crystal X-ray crystallography and electrospray ionization mass spectrometry show that the cluster assembly can be formulated as [Au20(PP3)4]Cl4. The Au20 cluster consists of an icosahedral Au13 core and a seven-Au-atom partial outer shell arranged in a local C3 symmetry. One PP3 ligand coordinates to four Au atoms in the outer shell, while the other three PP3 ligands coordinate to one Au atom from the outer shell and three Au atoms from the surface of the Au13 core, giving rise to an overall chiral 16-electron Au cluster core with C3 symmetry.

6Li diffusion-ordered NMR spectroscopy (DOSY) and applications to organometallic complexes.

The development of (6)Li diffusion-ordered NMR spectroscopy (DOSY) is reported. This technique is applied to (6)Li organometallic complexes. (6)Li DOSY provides a facile means of identification of peaks in the (6)Li spectrum, as well as evidence of mixed aggregates based on relative diffusion coefficients. (6)Li data is correlated to (1)H diffusion experiments through (6)Li{(1)H} HOESY and/or (1)H{(6)Li} HMBC experiments to obtain formula weight information of Li aggregates.

Structures of lithium salts of 2,3,3-trimethylindolenine and its 5-methoxy derivative in solution and the solid state.

An X-ray crystal structure of lithium 2,3,3-trimethylindolenide therate shows it to be a disolvated dimer having a 7)-azaallyl-type structure. The structures of the salt in several solvents have been established by studies of I3C chemical shifts, 6Li,15N spinspin splitting, 7Li quadrupole splitting constants, and apparent degrees of aggregation determined by vapor pressure barometry. It is the ?3-azaallyl dimer disolvate in diethyl ether, a tetrasolvated dimer in dioxolane, a mixture of monomer and tetrasolvated dimer in tetrahydrofuran, and a monomer in pyridine. The species are characterized by ’Li quadrupole splitting constants of 230, 156, 180-190 (0.27-0.75 M), and 217 kHz, respectively. In diethyl ether with 4 equiv of hexamethylphosphoric triamide, the salt is a mixture of monomeric and triple ion species. Lithium 5-methoxy-2,3,3-trimethylindolenide forms similar species except that, in tetrahydrofuran, the tendency for dimer formation is enhanced, which leads to a higher proportion of Cto N-methylation in its reaction with methyl chloride in that solvent. In recent years there has been considerable interest in the potential of imine anions, particularly (1-azaallyl)lithium reagents, in organic ~ynthes is .~ Such reagents offer advantages over the corresponding enolates in that they generally react with electrophiles much faster than they undergo proton transfer with the resulting imines so that monoalkylation, for instance, is not complicated by concomitant dialkylation. Furthermore, certain (1 -azaallyl)lithium reagents,” lithioimino and hydrazones9 form the basis of one of the most useful classes of chiral auxiliaries employed in organic synthesis. Imine anions are ambident and undergo reactions a t both their Cand N-termini.lo Frequently, reactions are performed on the lithium salts in ether or tertiary amine solvents, and it is probable that in these solvents the salts, like those of aromatic secondary amines,” are monomeric or dimeric tight ion pairs, the structures of which determine reactivity and regioand stereochemistry. In order to extend our studies1* of the relation of the solution structure of lithium salts involving organic ambident anions to reactivity and regiochemistry to include imine anions, we have chosen the salts la and b since they are readily generated from the corresponding imines by treatment with alkyllithium reagents and have fixed (anti) stereochemi~try.’~ In this paper, we present the results of structural studies of the salts in several solvents as well as an X-ray crystal structure of the etherate of la. In addition, we report some preliminary results that establish that these salts can undergo both (1) The Pennsylvania State University. (2) Brown University. (3) Whitesell, J. K.; Whitesell, M. A. Synthesis 1983, 517 and references (4) Meyers, A. I.; Williams, D. R.; Druelinger, M. J . Am. Chem. SOC. (5) Whitesell, J. K.; Whitesell, M. A. J . Org. Chem. 1977, 42, 377. (6) Hashimoto, S.; Koga, K. Chem. Pharm. Bull. 1979, 27, 2760. (7) Meyers, A. I.; Knaus, G.; Kamata, K.; Ford, M. E. J . Am. Chem. SOC. (8) Schollkopf, U. Pure Appl. Chem. 1983, 55, 1799. (9) Enders, D.; Eichenauer, H. Angew. Chem., I n t . Ed. Engl. 1976, 15, 549. (IO) Heiszwolf, G. J.; Kloosterziel, H. Red . Trau. Chim. Pays-Bas 1970, 89, 1217; Ahlbrecht, H.; Liesching, D. Synfhesis 1976, 746. (1 1) Jackman, L. M.; Scarmoutzos, L. M. J . Am. Chem. SOC. 1987, 109, 5348. (12) (a) Jackman, L. M.; Lange, B. C. J . Am. Chem. Soc. 1981,103,4494. (b) Jackman, L. M.; Dunne, T. S. Ibid. 1985, 107, 2805. (13) In general, C-alkylation can give rise to syn and anti isomeric iminesl4-I6 and N-alkylation to (E)and (Z)-enamines.15J7 (14) Fraser, R.; Banville, J.; Dhawan, K. J . Am. Chem. SOC. 1978, 100, 7999. Frazer, R.; Banville, J. J . Chem. SOC., Chem. Commun. 1979, 47. (15) Knorr, R.; Low, P. J . Am. Chem. SOC. 1980, 102, 3241. (16) (a) Wanat, R. A,; Collum, D. B.; Van Duyne, G.; Clardy, J.; DePue, R. T. J . Am. Chem. SOC. 1986, 108, 3415. (b) Kallman, N.; Collum, D. B. Ibid. 1987, 109, 7466. (17) Lee, J. Y.; Lynch, T. J.; Mao, D. T.; Bergbreiter, D. E.; Newcomb, M. J . Am. Chem. SOC. 1981, 103, 6215, and references cited therein. cited therein.