The purity of laboratory chemicals with regard to measurement uncertainty

The purity P of laboratory chemicals is often declared in the form P > or = xy% (e.g., P > or = 97%). With a randomly chosen set of 40 compounds we found that their purity is generally closer to 100% than to the lower limit. The distribution of the purity data as found in the laboratory depends on the analytical technique used. Whereas purities determined by chromatography do not exceed 100% (because the sum of all observed peak areas is set to 100%), the purities obtained by titration can exceed 100% (because the functionality of the compound is measured). Therefore, the data for these two groups need to be dealt with in different ways. For purities based on titration we propose to use a rectangular distribution with a range from Pmin to 101%, an expected purity value which is the mean and a standard uncertainty of the purity u(P) of 29% of the range. Purities determined by chromatography can be described with a triangular distribution (ramp function). One leg of the triangle represents the range from Pmin to 100% and the right-angle is located at 100%. The expected value is the median and the uncertainty u(P) is 24% of the range. These proposals match the experimental data well.     

Directed evolution experiments reveal mutations at cycloartenol synthase residue His477 that dramatically alter catalysis

[reaction: see text] Cycloartenol synthase cyclizes and rearranges oxidosqualene to the protosteryl cation and then specifically deprotonates from C-19. To identify mutants that deprotonate differently, randomly generated mutant cycloartenol synthases were selected in a yeast lanosterol synthase mutant. A novel His477Asn mutant was uncovered that produces 88% lanosterol and 12% parkeol. The His477Gln mutant produces 73% parkeol, 22% lanosterol, and 5% Delta(7)-lanosterol. These are the most accurate lanosterol synthase and parkeol synthase that have been generated by mutagenesis.     

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.     

Asymmetric base-mediated epoxide isomerisation

The base-mediated rearrangement of epoxides into allylic alcohols is a well-known synthetic transformation. The first enantioselective version of the reaction using a chiral base was reported in 1980. Since then, the reaction has received a lot of attention mostly due to the great usefulness of chiral allylic alcohols in organic synthesis. Major breakthroughs in the area were the first report on using a sub-stoichiometric amount of chiral base, and the development of chiral bases for a true catalytic reaction protocol. The present review covers the time from when the first asymmetric epoxide isomerisation reaction was reported (1980) up to now, focusing on the period 1997-2001.     

Synthesis, reactions and structural features of monofluorinated cyclopropanecarboxylates

Monofluorinated cyclopropanecarboxylates are available in racemic or optically active form by transition metal-catalyzed reactions of vinylfluorides with diazoacetates. From α-fluorostyrene and tert-butyl diazoacetate in the presence of 2 mol% of an enantiopure bis(oxazoline) copper complex, a 81:19 mixture of tert-butyl trans- and cis-2-fluoro-2-phenylcyclopropanecarboxylates was obtained with high enantiomeric excess (ee) of 93 or 89%, respectively. The corresponding racemic ethylesters were used as starting materials for the synthesis of carboxamides, of the cis- and trans-isomers of analogues of tranylcypromine, an anti-depressive drug and several of its homologous fluorinated cyclopropylmethyl and cyclopropylethyl amines. Corresponding enantiopure cyclopropylmethanols and several of their derivatives were synthesized also. Solid state structures of a selection of these compounds were examined by X-ray crystallography. Particularly, the cis-configurated fluorinated phenylcyclopropane derivatives showed extremely close intermolecular CH?FC contacts. The shortest of such distances (2.17 Å) was found in the N-(4-bromophenyl)carbamate of (1S,2R)-(2-fluoro-2-phenylcyclopropyl)methanol.     

Photocycloaddition of Cyclohex‐2‐enones to 2‐Methylbut‐1‐en‐3‐yne

Irradiation (350 nm) of 2‐alkynylcyclohex‐2‐enones 1 in benzene in the presence of an excess of 2‐methylbut‐1‐en‐3‐yne ( 2 ) affords in each case a mixture of a cis‐fused 3,4,4a,5,6,8a‐hexahydronaphthalen‐1(2H)‐one 3 and a bicyclo[4.2.0]octan‐2‐one 4 (Scheme 2), the former being formed as main product via 1,6‐cyclization of the common biradical intermediate. The (parent) cyclohex‐2‐enone and other alkylcyclohex‐2‐enones 7 also give naphthalenones 8 , albeit in lower yields, the major products being bicyclo[4.2.0]octan‐2‐ones (Scheme 4). No product derived from such a 1,6‐cyclization is observed in the irradiation of 3‐alkynylcyclohex‐2‐enone 9 in the presence of 2 (Scheme 4). Irradiation of the 2‐cyano‐substituted cyclohexenone 12 under these conditions again affords only traces of naphthalenone 13 , the main product now being the substituted bicyclo[4.2.0]oct‐7‐ene 16 (Scheme 5), resulting from [2+2] cycloaddition of the acetylenic C−C bond of 2 to excited 12 .     

Über das Lithiumchloromolybdat Li[Mo6Cl13]

On the Lithium Chloromolybdate Li[Mo₆Cl₁₃] Li[Mo₆Cl₁₃] was obtained as single phase product from a solid state reaction of MoCl₅, Mo powder, and LiCl at 800 °C. The structure as refined by single crystal X‐ray diffraction, contains one‐dimensional [Mo₆Cl Cl Cl ]^– chains, formed by Cl^a–a bridges. Lithium ions are located in tunnels along the chain‐direction, each of them being surrounded by a distorted tetrahedral arrangement of outer chlorine ligands (Cl^a) belonging to four different clusters.     

Paratropicity and antiaromaticity: role of the homo-lumo energy gap

Doubly charged systems derived from fused benzenoid polycycles reveal an unquenched delocalization of 4 n π-electrons and hence are predicted to possess antiaromatic character. The magnitude of the paratropic ¹H NMR chemical shifts, due solely to the paramagnetic secondary field sustained in these species, was found to depend linearly upon the magnitude of LUMO-HOMO energy gaps of the corresponding systems. The existence of such a correlation enables a comprehensive treatment of the various factors which determine the antiaromatic character and the subtle interrelations between those factors. This, in turn, leads to a deeper understanding of antiaromaticity.     

Step-scan FTIR time-resolved spectroscopy study of excited-state dipole orientation in soluble metallopolymers

Step-scan FTIR time-resolved spectroscopy (S2FTIR TRS) in acetonitrile-d3 has been used to probe the acceptor ligand in metal-to-ligand charge transfer (MLCT) excited states of amide-substituted polypyridyl complexes of RuII and in analogues appended to polystyrene. On the basis of ground-to-excited state shifts in v(C = O) of -31 cm-1 for the amide group in [RuII(bpy)2(bpyCONHEt')]2+ (bpyCONHEt' = 4'-methyl-2,2'-bipyridine-4-carboxamide-Et'; Et' = -CH2CH2BzCH2CH3) (1) and in the derivatized polystyrene abbreviated [PS-[CH2-CH2NHCObpy-RuII(bpy)2]20]40+ (3), the excited-state dipole is directed toward the amide-containing pyridyl group in the polymer side chain. Smaller shifts in v(C = O) of -17 cm-1 in [RuII(4,4'-(CONEt2)2bpy)2-(bpyCONHEt')]2+ (2) and in the derivatized polystyrene abbreviated [PS-[CH2CH2NHCObpy-RuII(4,4'-(CONEt2)2bpy)2]20]40+ (4) indicate that the excited-state dipole is directed toward one of the diamide bpy ligands. The nearly identical results for 1 and 3 and for 2 and 4 show that the molecular and electronic structures of the monomer excited states are largely retained in the polymer samples. These conclusions about dipole orientation in the polymers are potentially of importance in understanding intrastrand energy transfer dynamics. The excited-state dipole in 3 is oriented in the direction of the covalent link to the polymer backbone, and toward nearest neighbors. In 4, it is oriented away from the backbone.     

Mechanistic control of product selectivity. Reactions between cis-/trans cis-/trans-[OsVI(tpy)(Cl)2(N)]+ and triphenylphosphine sulfide

Reactions between the Os(VI)-nitrido complexes cis- and trans-[Os(VI)(tpy)(Cl)2(N)]+ (tpy is 2,2':6',2"-terpyridine) and triphenylphosphine sulfide, SPPh3, give the corresponding Os(IV)-phosphoraniminato, [Os(IV)(tpy)(Cl)2(NPPh3)]+, and Os(II)-thionitrosyl, [Os(II)(tpy)(Cl)2(NS)]+, complexes as products. The Os-N bond length and Os-N-P angle in cis-[Os(IV)(tpy)(Cl)2(NPPh3)](PF6) are 2.077(6) A and 138.4(4) degrees. The rate law for formation of cis- and trans-[Os(IV)(tpy)(Cl)2(NPPh3)]+ is first order in both [Os(VI)(tpy)(Cl)2(N)]+ and SPPh3 with ktrans(25 degrees C, CH3CN) = 24.6 +/- 0.6 M(-1) s(-1) and kcis(25 degrees C, CH3CN) = 0.84 +/- 0.09 M(-1) s(-1). As found earlier for [Os(II)(tpm)(Cl)2(NS)]+, both cis- and trans-[Os(II)(tpy)(Cl)2(NS)]+ react with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3. For both complexes, the reaction is first order in each reagent with ktrans(25 degrees C, CH3CN) = (6.79 +/- 0.08) x 10(2) M(-1) s(-1) and kcis(25 degrees C, CH3CN) = (2.30 +/- 0.07) x 10(2) M(-1) s(-1). The fact that both reactions occur rules out mechanisms involving S atom transfer. These results can be explained by invoking a common intermediate, [Os(IV)(tpy)(Cl)2(NSPPh3)]+, which undergoes further reaction with PPh3 to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and SPPh3 or with [Os(VI)(tpy)(Cl)2(N)]+ to give [Os(IV)(tpy)(Cl)2(NPPh3)]+ and [Os(II)(tpy)(Cl)2(NS)]+.