Labile Precursors Research Articles

Chemoselective Intramolecular Morita-Baylis-Hillman Reaction; Acrylamide and Ketone as Sluggish Reacting Partners on a Labile Framework.

Chemoselectivity is an important issue frequently encountered while working over labile precursors. Carbonyl compounds with a heteroatom at the β carbon are sensitive precursors because they are prone to elimination under different conditions. Morita-Baylis-Hillman (MBH) reaction, although a widespread method for C-C bond formation, has its own limitations. Acrylamide and ketone are such limitations of the MBH reaction. Using them together for an intramolecular MBH (IMBH) reaction on a labile framework prone to elimination is a significant 2-fold synthetic challenge. A highly chemoselective IMBH reaction on such precursors has been established using 1,4-diazabicyclo[2.2.2]octane (DABCO) as a promoter. The protocol leads to quick access to a diversely substituted and functionalized piperidone framework in high yields. Various substitution patterns in the form of 34 successful examples have been studied. A diastereoselective version and tolerance to various functional and protecting groups are the added advantages of the developed methodology. A tertiary carbon at the β position of ketone, however, led to complete reversal of selectivity and gave only the elimination product. Control experiments toward a better understanding of the substitution pattern, role of catalyst, and mechanistic study have been carried out. As an application of the IMBH adduct, a one-step allylic rearrangement for the dihydropyridone framework has also been demonstrated.

Kinetics of aroma formation from grape-derived precursors: Temperature effects and predictive potential

This study investigates the accumulation and degradation of aroma molecules released by acid hydrolysis of aroma precursors in winemaking grapes. A first-order kinetics model effectively interprets this accumulation, including subsequent degradation. Experimentation at three temperatures categorizes specific grape-derived aroma molecules into three stability-based groups: labile molecules from labile precursors, stable molecules from labile precursors, and stable molecules from stable precursors. While many grape-derived aromas exhibit similar patterns and levels of accumulation across temperatures, reaction rates significantly increase with temperature. The analysis of 12 samples of two grape varieties hydrolyzed at 50 °C for 5 weeks and 75 °C for 24 h confirms that fast hydrolysis accurately replicates varietal and between-sample aroma compositional differences. Moreover, the accumulated levels of 21 relevant grape-derived aromas strongly correlate with those at 50 °C, indicating that fast hydrolysis at 75 °C reliably predicts grape aroma potential.

Mechanism-based design of labile precursors for chromium(I) chemistry.

Dinitrogen complexes of the type TpR,RCr–N2–CrTpR,R are not the most labile precursors for Cr(I) chemistry, as they are sterically protected from obligatory associative ligand substitution. A mononuclear alkyne complex – TptBu,MeCr(η2-C2(SiMe3)2) – proved to be much more reactive.

Use of Labile Precursors for the Generation of Hyperpolarized Molecules from Hydrogenation with Parahydrogen and Aqueous‐Phase Extraction

Eine Hyperpolarisierung mithilfe von Parawasserstoff kann effizienter sein, wenn man von einer Vorstufe ausgeht, die dann schnell in die gewünschte Sonde umgesetzt wird. Als Beispiel wurde [1-13C]Succinat durch Hydrierung von Maleinsäureanhydrid mit Parawasserstoff und anschließende Hydrolyse hergestellt. Bei diesem Verfahren kann außerdem ein Katalysator ohne Verlust an Polarisierung entfernt werden.

Use of Labile Precursors for the Generation of Hyperpolarized Molecules from Hydrogenation with Parahydrogen and Aqueous‐Phase Extraction

Use of Labile Precursors for the Generation of Hyperpolarized Molecules from Hydrogenation with Parahydrogen and Aqueous‐Phase Extraction

Coordination Studies of a New Nonsymmetric Ditertiary Phosphane Bearing a Single Phosphaadamantane Cage

AbstractThe new nonsymmetric ditertiary phosphane, Ph2P(CH2)2PAd (1), was prepared in one‐step from Ph2PCH=CH2 andH‐PAd (H‐PAd = 1,3,5,7‐tetramethyl‐2,4,8‐trioxa‐6‐phosphaadamantane) by a hydrophosphination reaction using 2,2′‐azo‐bisisobutyronitrile (AIBN) as free radical initiator. The sterically encumbered phosphaadamantane cage in 1 was found to influence the coordination capabilities of this ligand. The reaction of 1 with [PdCl2(cod)] or [Pt(CH3)2(cod)] (cod = cycloocta‐1,5‐diene) gave the corresponding κ2‐P,P′‐chelate complexes cis‐[PdCl2(1)] (2) and cis‐[Pt(CH3)2(1)] (3), respectively. The dinuclear gold(I) complex [Ph2P(AuCl)(CH2)2PAd(AuCl)] (4) was prepared from 1 and 2 equiv. of [AuCl(tht)] (tht = tetrahydrothiophene). In contrast, the cleavage of the chloro‐bridged dimers {RuCl2(η6‐p‐cymene)}2 or {MCl2(η5‐Cp*)}2 (M = Rh, Ir) with 1 gave the κ1‐P‐monodentate complexes [RuCl2(η6‐p‐cymene)(1)] (5), [RhCl2(η5‐Cp*)(1)] (6) and [IrCl2(η5‐Cp*)(1)] (7), respectively, in which the ‐PAd group is non‐coordinating. Chloride abstraction from 6 (or 7) can be accomplished upon addition of Na[SbF6] to generate the cationic κ2‐P,P′‐chelate complexes 8b (and 9b). Alternatively 8a (or 9a) could be observed, as their chloride salts, by 31P{1H} NMR spectroscopy upon addition of several drops of CH3OH to CDCl3 solutions of 6 (or 7). The reaction of 5–7 with [AuCl(tht)] gave the dinuclear complexes [κ2‐P,P′‐μ‐RuCl2(η6‐p‐cymene){Ph2P(CH2)2PAd(AuCl)}] (10), [κ2‐P,P′‐μ‐RhCl2(η5‐Cp*){Ph2P(CH2)2PAd(AuCl)}] (11) and [κ2‐P,P′‐μ‐IrCl2(η5‐Cp*){Ph2P(CH2)2PAd(AuCl)}] (12). Reaction of two equiv. of 5 with the labile precursors [PdCl2(CH3CN)2] or [PtCl2(PhCN)2] gave instead the novel trinuclear Ru2Pd and Ru2Pt complexes trans‐[{κ2‐P,P′‐μ‐RuCl2(η6‐p‐cymene){Ph2P(CH2)2PAd}}2PdCl2] (13) and trans‐[{κ2‐P,P′‐μ‐RuCl2(η6‐p‐cymene){Ph2P(CH2)2PAd}}2PtCl2] (14), respectively. All new compounds have been fully characterised by spectroscopic and analytical methods. Furthermore the structures of 3·CHCl3, 4, 5, 7·CHCl3, 10·CH2Cl2·0.5C2H10O and 13·2CH2Cl2 have been elucidated by single‐crystal X‐ray crystallography. The X‐ray structures of 10·CH2Cl2·0.5C4H10O and 13·2CH2Cl2 demonstrate how nonsymmetric ditertiary phosphane complexes bearing one pendant phosphaadamantane moiety can be used as metalloligands in the controlled syntheses of novel bi‐ and trimetallic complexes.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

New pentafluorophenyl complexes with phosphine-amide ligands

A series of nickel(II) and palladium(II) complexes containing one or two pentafluorophenyl ligands and the phosphino-amides o-Ph 2PC 6H 4CONHR [R = i Pr ( a), Ph ( b)] displaying different coordination modes have been synthesised. The chelating ability of these ligands and the influence of both coligands and the metal centre in their potential hemilabile behaviour have been explored. The crystal structure of ( b) has been determined and reveals N–H⋯O intermolecular hydrogen bonding. Bis-pentafluorophenyl derivatives [M(C 6F 5) 2( o-Ph 2 PC 6H 4C O-NHR)] [M = Ni; R = i Pr ( 1a); R = Ph ( 1b); M = Pd; R = i Pr ( 2a); R = Ph ( 2b)] in which ( a) and ( b) act as rigid P, O-chelating ligands were readily prepared from the labile precursors cis-[M(C 6F 5) 2(PhCN) 2]. X-ray structures of ( 1a), ( 1b) and ( 2a) have been established, allowing an interesting comparative structural discussion. Dinuclear [{Pd(C 6F 5)(tht)(μ-Cl)} 2] reacted with ( a) and ( b) yielding the monopentafluorophenyl complexes [Pd(C 6F 5)Cl{ PPh 2(C 6H 4–C ONH–R)}] (R = i Pr ( 3a), Ph ( 3b)) that showed a P, O-chelating behaviour of the ligands, confirmed by the crystal structure determination of ( 3a). New cationic palladium(II) complexes in which ( a) and ( b) behave as P-monodentate ligands have been synthesised by reacting them with [{Pd(C 6F 5)(tht)(μ-Cl)} 2], stoichiometric Ag(O 3SCF 3) and external chelating reagents such as cod [Pd(C 6F 5)(cod){ PPh 2(C 6H 4-CONH-R)}](O 3SCF 3)(R = i Pr ( 4a), Ph ( 4b)) and 2,2′ -bipy [Pd(C 6F 5)(bipy){ PPh 2(C 6H 4-CONH-R)}](O 3SCF 3) (R = i Pr ( 5a), Ph ( 5b)). When chloride abstraction in [{Pd(C 6F 5)(tht)(μ-Cl)} 2] is promoted by means of a dithioanionic salt as dimethyl dithiophospate in the presence of ( a) or ( b), the corresponding neutral complexes [Pd(C 6F 5){S(S)P(OMe) 2}{ PPh 2(C 6H 4-CONH-R)}] (R = i Pr ( 6a), Ph ( 6b)) were obtained.

Natural organic matter fractionation: XAD resins versus UF membranes. An investigation into THM formation

An increase in reservoir water colour over the last 10 years has been impacting on the ability of water treatment works (WTWs) in the UK to meet THM regulations. The problem is exacerbated in autumn particularly after heavy rainfall. It appears that residual organics are labile precursors to disinfection by-products (DBPs). One WTW was investigated to determine a link between raw water characteristics and reactivity with chlorine. Bulk water parameters were measured but the information obtained was limited. To further characterise the water, samples of reservoir water were fractionated using XAD resins and UF membranes. The resulting fractions were subjected to analysis of dissolved organic carbon (DOC), UV absorbance at 254 nm (UV254), specific UV absorbance (SUVA) defined as the ratio of UV254 to DOC and trihalomethane formation potential (THM-FP). The samples were also characterised using high performance size exclusion chromatography (HPSEC). Both methods of fractionation described here have limitations in explaining the bulk water behaviour. The relationship between SUVA and THM-FP was discussed. HPSEC showed that the size distribution of the resin separated and membrane separated fractions was similar indicating that the molecular weight cut-off (MWCO) of the membranes is not sharp.

Synthesis, Solution‐State and Solid‐State Structural Characterization of Monocationic Nitrido Heterocomplexes [M(N)(DTC)(PNP)]+ (M = 99Tc, Re; DTC = Dithiocarbamate; PNP = Heterodiphosphane)

AbstractMono‐cationic nitrido heterocomplexes of general formula [M(N)(DTC)(PNP)]+ (where M is 99Tc or Re, DTC is the mono‐anionic form of a dithiocarbamate ligand, and PNP is a diphosphane ligand with a tertiary amine‐containing five‐membered spacer) were prepared by ligand‐exchange reactions with the labile precursors [M(N)Cl2(PPh3)2] in dichloromethane/alcohol mixtures. The molecular structure of the representative rhenium complex [Re(N)(dedc)(pnp2)][PF6] (1) displays a distorted, square‐pyramidal geometry with the dithiocarbamate sulfur and the diphosphane phosphorus atoms spanning the four coordination positions on the equatorial plane. If the additional interactions between the nitrido nitrogen and the weakly bonded trans N‐diphosphane heteroatom, the molecular geometry can be viewed as pseudo‐octahedral. The structure in solution, as established by multinuclear NMR spectroscopy and ESI spectrometry, is monomeric, and identical to that shown in the solid state. Replacement of the phenyl groups on the phosphorous atoms in complexes 1, 2, 5, and 6 with alkyl groups modified neither the course of the reaction nor the composition of the resulting complexes. These results, together with the observation that no symmetrical complexes containing two identical bidentate ligands were produced in these reactions, strongly supports the conclusion that a mixed coordination sphere, composed by a combination of π‐donor and π‐acceptor atoms around the [M≡N]2+ group, constitutes a highly stable system. Compounds containing dangling alkyl‐substituted groups in the outer sphere (3, 4, 7, and 8) were fully characterized by multinuclear NMR spectroscopy and ESI mass spectrometry. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

An entry to half-sandwich complexes of first row transition metals (Ni, Co, Mn) containing the Kläui tripodal ligand, LOMe: synthesis and crystal structures of oligomeric, [LOMeM(μ-X)(L)]n (n = 2, 4), and monomeric complexes, LOMeM(L2)(X)

A series of half-sandwich complexes of Ni, Co and Mn containing the Klaui tripodal ligand LOMe [κ3-(η5-C5H5)Co{P(O)(OCH3)2}3] are prepared by reaction of the labile precursors, LOMeM(L)(κn-NO3) [M = Ni, Co; L = acetone (1, n = 2), PPh3 (2, n = 2), κ2-bipy (8, n = 1), κ2-tmeda (9, n = 1)] and [LOMeMn(py)(μ-NO3)]2 (15), with anionic N- and O-nucleophiles. Oligomeric complexes are formed from complexes 1, 2, and 15via elimination of the κ1-ligands (L). Reactions with NaN3 and sodium aryloxide (NaOAr) afford the tetrameric μ3-azido complexes, [LOMeM(μ3-N3)]4 (3), and the dimeric μ-aryloxo complexes, [LOMeM(μ-OAr)(X)]2 (5, 6: Ar = 2,6-Me2-C6H3, 2,4,6-Me3-C6H2, 2- and 4-NO2-C6H4, C6F5; X = none, ROH), respectively, but the sandwich complexes (LOMe)2M result from reactions with other O-nucleophiles such as NaOH, NaOR, and NaOAr [Ar = C6H5, 4-MeC6H4, 2,6-X2-C6H3 (X = F, Cl)]. Mononuclear complexes LOMeM(κ2-L)(κ1-NO3) 8 and 9 are obtained by treatment of 1 or 2 with bidentate ligands and further converted to LOMeM(κ2-L)-X-type azido (10, 11), halo (12), aryloxo (13) and MeCN-coordinated cationic complexes (14).

A reexamination of amino acids in lunar soils: Implications for the survival of exogenous organic material during impact delivery

Using a sensitive high performance liquid chromatography technique, we have analyzed both the hot water extract and the acid hydrolyzed hot water extract of lunar soil collected during the Apollo 17 mission. Both free amino acids and those derived from acid labile precursors are present at a level of roughly 15 ppb. Based on the D/L amino acid ratios, the free alanine and aspartic acid observed in the hot water extract can be entirely attributed to terrestrial biogenic contamination. However, in the acid labile fraction, precursors which yield amino acids are apparently present in the lunar soil. The amino acid distribution suggests that the precursor is probably solar wind implanted HCN. We have evaluated our results with regard to the meteoritic input of intact organic compounds to the moon based on an upper limit of ≤ 0.3 ppb for α-aminoisobutyric acid, a non-protein amino acid which does not generally occur in terrestrial organisms and which is not a major amino acid produced from HCN, but which is a predominant amino acid in many carbonaceous chondrites. We find that the survival of exogenous organic compounds during lunar impact is ≤ 0.8%. This result represents an example of minimum organic impact survivability. This is an important first step toward a better understanding of similar processes on Earth and on Mars, and their possible contribution to the budget of prebiotic organic compounds on the primitive Earth.

Atropo-enantioselective biaryl synthesis by stereocontrolled cleavage of configuratively labile lactone-bridged precursors using chiral H-nucleophiles

The atropo-enantioselective synthesis of axially stereogenic, sterically shielded biaryl systems is described, by stereocontrolled ring opening of the corresponding lactone-bridged, still configurationally labile precursors. The atropenantiomer excesses range up to 97% ee.

Isolation of ribosomal RNA precursors from lung

A method is presented for the isolation of ribosomal RNA precursors from lung. The very labile precursors of ribosomal RNA in mouse lungs can be isolated and preserved when lung tissue (i) is frozen in liquid nitrogen and pulverized, (ii) is homogenized in hypotonic buffer, (iii) nuclei are rapidly isolated after being sieved through multiple screens, (iv) nuclei are treated with Tween-40 and Nadeoxycholate, (v) nuclei are digested with DNase, and (vi) partially purified nucleoli are pelleted through a sucrose gradient. Nucleolar RNA was extracted with phenol-SDS-chloroform. The RNA was separated on polyacrylamide gels. Absorbance and radioactive profiles of the RNAs on the gels can be obtained. Therefore, the specific activities of the ribosomal RNA precursors can be calculated.

Metabolism of ribosomal precursor ribonucleic acid in kidney.

The labile precursors of ribosomal RNA in mouse kidney are preserved when nuclei rapidly isolated after sieving through multiple screens are swollen and cleansed in the presence of an RNase inhibitor before digestion with DNase and phenol extraction. The kinetics of nucleolar labeling analyzed on polyacrylamide gels show that 36S RNA is the major intermediate product in the catabolism of the original 45S RNA precursor to 32S RNA, from which 28S RNA is derived. Each kidney nucleus contains about 200-600 molecules of 45S RNA; the turnover time of the 45S pool is about 3 +/- 2 min. Compared with HeLa cells, kidney nuclei have a different major intermediate product and a much smaller and more rapidly turning-over pool of ribosomal precursor RNA.

The stereochemistry of bicyclo[3.2.1]octane—XI : The addition of monohalocarbene to norbornene

Monochloro- and monobromocarbene undergo exo addition to norbornene to give exo-anti-3-halotricyclo[3.2.1.0 2,4octene-2. The exo-anti adducts were found to be remarkably stable, in contrast to the exo-syn adducts, which, though not observed, were presumed to be the labile precursors of the rearranged products. The reasons for these stereochemical differences are discussed.