Scrambling Of Deuterium Research Articles

Multiplexed Conformationally Selective, Localized Gas-Phase Hydrogen Deuterium Exchange of Protein Ions Enabled by Transmission-Mode Electron Capture Dissociation.

In this article, we present an approach for conformationally multiplexed, localized hydrogen deuterium exchange (HDX) of gas-phase protein ions facilitated by ion mobility (IM) followed by electron capture dissociation (ECD). A quadrupole-IM-time of flight instrument previously modified to enable ECD in transmission mode (without ion trapping) immediately following a mobility separation was further modified to allow for deuterated ammonia (ND3) to be leaked in after m/z selection. Collisional activation was minimized to prevent deuterium scrambling from giving structurally irrelevant results. Gas-phase HDX with ECD fragmentation for exchange site localization was demonstrated with the extensively studied protein folding models ubiquitin and cytochrome c. Ubiquitin was ionized from conditions that stabilize the native state and conditions that stabilize the partially folded A-state. IM of deuterated ubiquitin 6+ ions allowed the separation of more compact conformers from more extended conformers. ECD of the separated subpopulations revealed that the more extended (later arriving) conformers had significant, localized differences in the amount of HDX observed. The 5+ charge state showed many regions with protection from HDX, and the 11+ charge state, ionized from conditions that stabilize the A-state, showed high levels of deuterium incorporation throughout most of the protein sequence. The 7+ ions of cytochrome c ionized from aqueous conditions showed greater HDX with unstructured regions of the protein relative to interior, structured regions, especially those involved in heme binding. With careful tuning and attention to deuterium scrambling, our approach holds promise for determining region-specific information on a conformer-selected basis for gas-phase protein structures, including localized characterizations of ligand, epitope, and protein-protein binding.

Broadening the Scope of the Zwitterionic 1,3-Diaza-Claisen Rearrangement through a Tethering Strategy.

Expansion of the scope of the 1,3-diaza-Claisen rearrangement beyond bridged-bicyclic tertiary allylic amines has been investigated through a tethering strategy. Isothioureas tethered to tertiary allylic amines are converted to carbodiimides through a reaction with AgOTf/Et3N. Intramolecular cyclization of the tertiary allylic amine to the carbodiimide equilibrates with a zwitterionic intermediate. Heating the carbodiimide/zwitterion affords a rearrangement product. Heating carbodiimide/zwitterion with a deuterated allyl group results in the scrambling of the deuterium label, which is consistent with an ionic mechanism involving heterolytic cleavage of the allylic C-N bond, followed by trapping of the allyl cation at either terminal carbon. The ionic mechanism is attributed to silver salt contamination since pushing deuterium-labeled carbodiimide/zwitterion through silica gel prior to heating results in clean deuterium transposition consistent with a sigmatropic mechanism, and adding back silver salts results in deuterium scrambling. Overall, the tethering strategy broadens the scope of the rearrangement to simpler allylic substrates. Density functional theory (DFT) calculations of the sigmatropic rearrangement are in agreement with reactivity trends observed with reactions run under silver-free conditions.

Photolysis-induced scrambling of PAHs as a mechanism for deuterium storage

Aims. We investigate the possible role of polycyclic aromatic hydrocarbons (PAHs) as a sink for deuterium in the interstellar medium (ISM) and study UV photolysis as a potential underlying chemical process in the variations of the deuterium fractionation in the ISM. Methods. The UV photo-induced fragmentation of various isotopologs of deuterium-enriched, protonated anthracene and phenanthrene ions (both C14H10 isomers) was recorded in a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer. Infrared multiple photon dissociation spectroscopy using the Free-Electron Laser for Infrared eXperiments was applied to provide IR spectra. Infrared spectra calculated using density functional theory were compared to the experimental data to identify the isomers present in the experiment. Transition-state energies and reaction rates were also calculated and related to the experimentally observed fragmentation product abundances. Results. The photofragmentation mass spectra for both UV and IRMPD photolysis only show the loss of atomic hydrogen from [D − C14H10]+, whereas [H − C14D10]+ shows a strong preference for the elimination of deuterium. Transition state calculations reveal facile 1,2-H and -D shift reactions, with associated energy barriers lower than the energy supplied by the photo-excitation process. Together with confirmation of the ground-state structures via the IR spectra, we determined that the photolytic processes of the two different PAHs are largely governed by scrambling where the H and the D atoms relocate between different peripheral C atoms. The ∼0.1 eV difference in zero-point energy between C–H and C–D bonds ultimately leads to faster H scrambling than D scrambling, and increased H atom loss compared to D atom loss. Conclusions. We conclude that scrambling is common in PAH cations under UV radiation. Upon photoexcitation of deuterium-enriched PAHs, the scrambling results in a higher probability for the aliphatic D atom to migrate to a strongly bound aromatic site, protecting it from elimination. We speculate that this could lead to increased deuteration as a PAH moves towards more exposed interstellar environments. Also, large, compact PAHs with an aliphatic C–HD group on solo sites might be responsible for the majority of aliphatic C–D stretching bands seen in astronomical spectra. An accurate photochemical model of PAHs that considers deuterium scrambling is needed to study this further.

Hydrogen-Deuterium Scrambling Based on Chemical Isotope Labeling Coupled with LC-MS: Application to Amine Metabolite Identification in Untargeted Metabolomics.

Identification of metabolites at the trace level in complex samples is still one of the major challenges in untargeted metabolomics. One formula in the metabolomic database is always corresponding to more than one candidate, which increases the difficulty and cost in the subsequent process of standard compound matching. In this study, we developed an effective method for amine metabolite identification by hydrogen-deuterium scrambling (HDS) based on chemical isotope labeling coupled with liquid chromatography-mass spectrometry (HDS-CIL-LC-MS). After d4-4-(N,N-dimethylamino)phenyl isothiocyanate (d4-DMAP) labeling, the labeled amine metabolites can produce HDS under collision-induced dissociation (CID). The HDS can effectively reflect the number of labile hydrogen atoms in amine metabolites and thus distinguish amine isomers with different functional groups. The developed HDS-CIL-LC-MS method was applied to the determination of amine metabolites in mice feces, in which the amine candidates obtained by the database based on chemical formula searching were reduced by 64% on average, which greatly reduces the cost of standard compound matching. Taken together, the developed HDS-CIL-LC-MS analysis was demonstrated to be a promising method for untargeted metabolomics and a novel strategy for deciphering tandem mass spectrometry (MS2) spectra.

Stereochemistry and Mechanism of the Brønsted Acid Catalyzed Intramolecular Hydrofunctionalization of an Unactivated Cyclic Alkene

Through employment of deuterium-labeled substrates, the triflic acid catalyzed intramolecular exo addition of the X-H(D) (X=N, O) bond of a sulfonamide, alcohol, or carboxylic acid across the C=C bond of a pendant cyclohexene moiety was found to occur, in each case, with exclusive formation (≥90%) of the anti-addition product without loss or scrambling of deuterium as determined by (1)H and (2)H NMR spectroscopy and mass spectrometry analysis. Kinetic analysis of the triflic-acid-catalyzed intramolecular hydroamination of N-(2-cyclohex-2'-enyl-2,2-diphenylethyl)-p-toluenesulfonamide (1a) established the second-order rate law: rate=k(2)[HOTf][1a] and the activation parameters ΔH(++)=(9.7±0.5) kcal mol(-1) and ΔS(++)=(-35±5) cal K(-1) mol(-1). An inverse α-secondary kinetic isotope effect of k(D)/k(H) =(1.15±0.03) was observed upon deuteration of the C2' position of 1a, consistent with partial C-N bond formation in the highest energy transition state of catalytic hydroamination. The results of these studies were consistent with a mechanism for the intramolecular hydroamination of 1a involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C3' position of 1a coupled with intramolecular anti addition of the pendant sulfonamide nitrogen atom to the alkenyl C2' position.

Investigation of the Thermal Decomposition of [biphen(QPiPr)Pt(alk)2]: An Entry to C–C Single Bond Activation?

AbstractThe reaction of [(COD)PtCl2] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) yields cis‐[{biphen(QPiPr)(Q′PiPr)}PtCl2]. Treatment of trans‐[PtCl2(ethene)(py)] with biphen(QPiPr)(Q′PiPr) (Q, Q′ = O, CH2O) gives trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] for Q or Q′ ≠ O. No trans/cis isomerization can be observed for these compounds after 24 h at 105 °C in [D8]toluene. In the case of Q = Q′ = O a trans/cis isomerization takes place at room temperature. X‐ray structures of a molecule in the solid state were determined for the compounds cis/trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with Q or Q′ ≠ O. In the case of Q = O and Q′ = CH2O the closer of the two bridging C atoms in the biphenyl fragment of the ligand is only 3.42 Å away from the Pt center but does not show a coupling to it in the 13C NMR spectrum. Treatment of both cis‐ and trans‐[{biphen(QPiPr)(Q′PiPr)}PtCl2] with two equivalents of ethyl‐ or n‐propylmagnesium chloride gives cis‐[{biphen(QPiPr)(Q′PiPr)}PtAlk2]; the X‐ray structures of cis‐[{biphen(CH2OPiPr)2}PtEt2] and cis‐[{biphen(OPiPr)2}Pt(nPr)2] are discussed. These complexes give the related alkene complexes as a result of β‐H elimination/reductive elimination if heated to temperatures above 90 °C in [D8]toluene, but none of the complexes with Q or Q′ ≠ O is stable at 95 °C. The thus‐formed Pt(alkene) complex loses the alkene (ethene or propene) in a consecutive reaction. For [{biphen(CH2OPiPr)2}PtEt2], selective deuteration of the CH2O group and the Pt‐ethyl group proves that scrambling of deuterium into the iPr methyl groups occurs (by a CH activation/insertion/β‐H elimination/reductive elimination sequence) after the β‐H elimination and before the loss of ethene, which at 95 °C is almost as fast as the loss of ethene. In the final alkene‐free 14 e complex a fast dynamic C–H activation of the iPr CH3 protons by the Pt center (one H at a time) can be detected [an Isotopic Perturbation of Resonance (IPR) is found for the partially deuterated compound], which most probably prevents a further reaction of the complex (e.g. activation of the bridging C–C single bond in the biphenyl backbone or permanent coordination of PPh3). H2, however, is activated by the final complex, and the use of D2 proves that deuterium is incorporated into the iPr groups of the ligand. The reaction of [(COD)PtCl2] with o‐Br‐phen(QPiPr) gives cis‐[PtCl2{o‐Br‐phen(QPiPr)}2] (X‐ray structure reported), which forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. The reaction of trans‐[PtCl2(ethene)(py)] with o‐I‐phen(QPiPr) gives trans‐[PtCl2{o‐I‐phen(QPiPr)}2] (X‐ray structure reported), which also forms cis‐[Pt{phen(QPiPr)}2] upon treatment with sBuLi. cis‐[Pt{phen(CH2OPiPr)}2] (X‐ray structure reported) slowly isomerizes quantitatively at 95 °C in toluene to the trans complex. No product of reductive elimination can be detected up to 150 °C in mesitylene. cis‐[Pt{phen(OPiPr)}2] (X‐ray structure reported) forms two products ― the trans isomer (X‐ray structure reported) and most likely a dimeric trans‐configured complex ― upon heating to 95 °C. Again, no reductive elimination is observed regardless of whether the reaction is performed under argon or ethene atmosphere up to 150 °C in mesitylene. All materials were characterized by means of 1H, 2H, 13C, 31P, and 195Pt NMR spectroscopy, FAB mass spectrometry, FT‐IR spectroscopy, elemental analysis, and X‐ray crystallography. DFT calculations (B3PW91/LANL2DZ) are also included for the most important compounds. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Synthesis of isotopically labelled glycyl-L-prolyl-L-glutamic acid (Glypromate®) and derivatives

The related tripeptides glycyl-L-prolyl-L-glutamic acid (GPE) and glycyl-L-2-methylprolyl-L-glutamic acid (G-2-MePE) were labelled with commercially available [1,2,3,4,5-13C5, 2-15N1]-L-glutamic acid in 3 steps in excellent overall yield with high isotope incorporation. A related cyclic dipeptide was labelled with [2,2-2H2, 2-15N1]glycine giving a mixture of compounds resulting from deuterium scrambling. Copyright © 2006 John Wiley & Sons, Ltd.

Substrate effects on the mechanism of enantioselective hydrogenation using ruthenium bis(phosphine) complexes as catalyst: A mechanistic investigation of the hydrogenation of α,β-unsaturated acids and esters based on deuterium labeling studies

The mechanism of ruthenium-bis(phosphine) catalyzed enantioselective hydrogenation of olefins was examined using [Ru(( R)-BINAP)(H)(MeCN) n (sol) 3 − n ]BF 4 ( n = 0–3, sol = solvent used in reaction) as catalyst. Tiglic and angelic acids were used as standard α,β-unsaturated acid substrates; ( Z)-methyl α-acetamidocinnamate and dimethyl itaconate were used as standard α,β-unsaturated ester substrates. Isotopic labeling studies (deuterium scrambling) indicate that two distinct mechanisms are in operation for α,β-unsaturated acids versus α,β-unsaturated esters. In each case, 5-membered metallocycle intermediates are formed via olefin-hydride insertion. The mechanisms, however, deviate primarily in the activation of dihydrogen, which is strongly affected by the nature of the substrate. Hydrogenation of α,β-unsaturated acids proceed via heterolytic cleavage of dihydrogen, whereas hydrogenation of α,β-unsaturated esters proceed via homolytic cleavage of dihydrogen. A full discussion of the mechanisms is presented.

Comparative Studies with Zwitterionic Platinum(II) Bis(pyrazolyl)borate and 2,2‘-Bipyridylborate Complexes

A comparison between the mononuclear platinum complexes of three structurally different monoanionic borato ligands is presented: [Ph_2B(pyrazolyl)_2]- ([Ph_2B(pz)_2], 1), [4-Ph_3B(2,2‘-bipyridine)]- ([(4-BPh_3)bpy], 2), and [Ph_2B(CH_2PPh_2)_2]- ([Ph_2BP_2], 3). The new bipyridylborate ligand 2 is introduced in this study. The relative trans influence of these ligands has been assessed by comparison of the structural and spectroscopic (NMR) data of the platinum dimethyl complexes [[Ph_2B(pz)_2]Pt(Me)_2][NBu_4] (4), [[(4-BPh_3)bpy]Pt(Me)_2][NBu_4] (5), and [[Ph_2BP_2]Pt(Me)_2][ASN] (6). The neutral complexes [Ph_2B(pz)_2]Pt(Me)(NCCH_3) (7), [Ph_2B(pz)_2]Pt(Me)(CO) (8), [Ph_2B(pz)_2]Pt(Me)(P(C_6F_5)_3) (9), [(4-BPh_3)bpy]Pt(Me)(NCCH_3) (10), [(4-BPh_3)bpy]Pt(Me)(CO) (11), and [(bpy)Pt(Me)(CO)][BPh_4] (12) were prepared, and the carbonyl complexes 8, 11, and 12 provide information pertaining to the relative electron-releasing character of each ligand type. The CO stretching frequencies suggest that the charged borate moiety renders the borato ligands more electron-donating than their neutral analogues. Of the neutral platinum methyl solvento complexes supported by ligands 1, 2, and 3, only those of 1 display very different C−H activation propensities. Upon protonation or methide abstraction in benzene at room temperature, complex 4 rapidly activates two molecules of benzene to generate [[Ph_2B(pz)_2]Pt(Ph)_2][NBu_4] (13). Isotopic scrambling of deuterium into methane in C_6D_6 solvent suggests the intermediacy of a methane σ-adduct in this reaction. The double C−H activation reaction can be halted by addition of acetonitrile to trap the intermediate [Ph_2B(pz)_2]Pt(Ph)(NCCH_3) (14). Complex 3 also displays reactivity toward the benzylic C−H bonds of mesitylene at room temperature to form [Ph_2B(pz)_2]Pt(pzH)(CH_2C_6H_3(CH_3)_2) (15) in modest yield.

Isomerization and Deuterium Scrambling Evidence for a Change in the Rate-Limiting Step during Imine Hydrogenation by Shvo's Hydroxycyclopentadienyl Ruthenium Hydride

Hydroxycyclopentadienyl ruthenium hydride 5 efficiently reduces imines below room temperature. Better donor substituents on nitrogen give rise to faster rates and a shift of the rate-determining step from hydrogen transfer to amine coordination. Reduction of electron-deficient N-benzilidenepentafluoroaniline (8) at 11 degrees C resulted in free amine and kinetic isotope effects of k(OH)/k(OD) = 1.61 +/- 0.08, k(RuH)/k(RuD) = 2.05 +/- 0.08, and k(RuHOH)/k(RuDOD) = 3.32 +/- 0.14, indicative of rate-limiting concerted hydrogen transfer, a mechanism analogous to that proposed for aldehyde and ketone reduction. Reduction of electron-rich N-alkyl-substituted imine, N-isopropyl-(4-methyl)benzilidene amine (9), was accompanied by facile imine isomerization and scrambling of deuterium labels from reduction with 5-RuDOH into the N-alkyl substituent of both the amine complex and into the recovered imine. Inverse equilibrium isotope effects were observed in the reduction of N-benzilidene-tert-butylamine (11) at -48 degrees C (k(OH)/k(OD) = 0.89 +/- 0.06, k(RuH)/k(RuD) = 0.64 +/- 0.05, and k(RuHOH)/k(RuDOD) = 0.56 +/- 0.05). These results are consistent with a mechanism involving reversible hydrogen transfer followed by rate-limiting amine coordination.

Factors affecting gas-phase deuterium scrambling in peptide ions and their implications for protein structure determination.

In this report, we evaluate the validity of using hydrogen/deuterium exchange in combination with collision-induced dissociation mass spectrometry (CID MS) for the detailed structural and conformational investigation of peptides and proteins. This methodology, in which partly deuterated peptide ions are subjected to collision-induced dissociation in the vacuum of a mass spectrometer, has emerged as a useful tool in structural biology. It may potentially provide quantitatively the extent of deuterium incorporation per individual amino acid in peptides and proteins, thus providing detailed structural information related to protein structure and folding. We report that this general methodology has limitations caused by the fact that the incorporated deuterium atoms migrate prior or during the CID MS analysis. Our data are focused on a variety of transmembrane peptides, incorporated in a lipid bilayer, in which the near-terminal amino acids that anchor at the lipid-water interface are systematically varied. Our findings suggest that, under the experimental conditions we use, the extent of intramolecular hydrogen scrambling is strongly influenced by experimental factors, such as the exact amino acid sequence of the peptide, the nature of the charge carrier, and therefore most likely by the gas-phase structure of the peptide ion. Moreover, the observed scrambling seems to be independent of the nature of the peptide fragment ions (i.e., protonated B and Y' ' ions, and sodiated A and Y' ions). Our results strongly suggest that scrambling may be reduced by using alkali metal cationization instead of protonation in the ionization process.

A Deuterium-Labeling Study on the Dehydrogenation of 2-Propanol with Ru–Pt/Carbon Catalyst in the Liquid-Film State Effective for the 2-Propanol/Acetone/Hydrogen Chemical Heat-Pump System

Abstract A unique feature of the dehydrogenation of 2-propanol with Ru–Pt/carbon catalyst in the liquid-film state has been elucidated by means of various kinds of deuterium labeling. A quite large kinetic isotope effect for 2-propanol-d8 (kH/kD = 14.6) suggests that fission of the methine C–H bond in the isopropoxy intermediate is rate-determining in the overall process. The dominant formation of H2 for 2-propanol-O-d1 is explained in terms of rapid scrambling of deuterium together with a large kinetic isotope effect for fission of the methine C–D bond. This picture is supported by both the absence of an appreciable kinetic isotope effect for 2-propanol-O-d1 and the deuterium distribution among the liquid-phase components. The extent of hydrogen transfer from 2-propanol to acetone-d6 during the dehydrogenation reaction suggests suppression of the inhibitive adsorption of acetone, leading to a high catalytic activity in the region of high acetone concentration, which is essential for a 2-propanol/acetone/hydrogen chemical heat-pump system with high thermal efficiency.

Synthesis of deuterated C-6 and C-9 flavour volatiles

Regiospecific deuterogenation of alkynols and chain extension of deuterated C-6 alkyl halides by alkylation with malondialdehyde derivatives was used to synthesise a series of deuterated C-6 and C-9 flavour volatiles. Reduction of 3-hexyn-1-ol with deuterium gas using Wilkinson's catalyst required protection of the alcohol as the tert-butyldimethylsilyl ether to speed reaction and minimise scrambling of deuterium. Deuterogenation of 5-hexyn-1-ol proceeded cleanly without protection to give 5,5,6,6-2H4-hexanol which was converted to 5,5,6,6-2H4-hexyl bromide and the corresponding Grignard reagent. Reaction of this Grignard reagent with 3-trimethylsiloxy-2-propenal gave 8,8,9,9-2H4-2E-nonenal. Deuterogenation of 3-hexyn-1-ol with Lindlar catalyst gave 3,4-2H2-3Z-hexenol which was converted to the corresponding iodide and used to synthesise 6,7-2H2-2E,6Z]-nonadienal and 6,7-2H2-2E,6Z]-nonadienol. Wilkinson reduction of the tert-butyldimethysilyl derivative of 3-nonyn-1-ol was used to synthesize 3,3,4,4-2H4-nonanol and 3,3,4,4-2H4-nonanal. Copyright © 1999 John Wiley & Sons, Ltd.

Diastereomeric and Isotopic Scrambling in (Hydrido)alkyliridium Complexes. Evidence for the Presence of a Common "Alkane Complex" Intermediate

An alternative explanation for the scrambling of isotopic labels in the alkylmetal hydride complexes is the direct dyotropic rearrangement over the transition state. In an attempt to clarify the mechanism involved in these deuterium-scrambling reactions we have carried out kinetic investigations of the interconversion of diastereomeric (hydrido)(dimethylcyclopropyl)iridium complexes 1a and 1b and the deuterium scrambling of their {alpha}-deuterio analogs. A dramatic difference in behavior of the Rh and Ir systems yields significant conclusions regarding the reaction coordinate for alkane oxidative addition reactions to metal centers. Comparison of the results from the iridium system discussed here and the analogous rhodium system studied earlier yields the following interesting conclusion: with iridium, the barrier for interconversion of 4a and 4b is comparable to the barrier for C-H insertion to re-form 1a,b whereas the corresponding barrier for interconversion of the rhodium analogs of 4a and 4b is high compared to the barrier for C-H insertion. This difference may be caused by the different temperatures at which the two reactions are run; another possibility, however, is that deuterium scrambling in the rhodium system occurs by a dyotropic rearrangement. 27 refs., 3 figs.

Dimethyl ether and dimethyl‐d6 ether chemical ionization mass spectrometry of polynuclear aromatic hydrocarbons

AbstractDimethyl ether (DME) chemical ionization mass spectrometry of a number of polynuclear aromatic hydrocarbons (PAHs) gave rise to a variety of fragment–molecule adduct ions resulting from ion–molecule reactions with the reagent gas. In 18 out of 22 cases, it was possible to differentiate among non‐nitrogenous PAHs on the basis of the identities and relative abundances of these adduct ions alone. For 14 nitrogen‐containing PAHs, adduct ion profiles were less distinct and differentiation among isomers was limited but possible in certain cases. With DME‐d6 as the reagent gas, the corresponding perduterio adduct ions were seen in substantially reduced abundances relative to their unlabeled counterparts, and there was no scrambling of deuterium. For the majority of the non‐nitrogenous PAHs and all of the nitrogen‐containing PAHs the most abundant ion was the methoxymethylene cation [M + 45]+. Collision‐induced dissociation studies of these ions demonstrated that fragmentation can take place according to three major pathways.

Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene

The photochemistry and photophysics of 1-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2–0.3 range in cyclohexane and acetonitrile solution. 1-Phenylcyclobutene undergoes [2+2]-cycloreversion [Formula: see text] to yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solution [Formula: see text] in addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1,3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl σ-bond framework during the reaction, in almost exact correspondence with that developed in the π system during photoprotonation. No deuterium scrambling is observed in 1-phenylcyclobutene-2,4,4-d3 after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet–triplet absorption spectra and triplet lifetimes of 1-phenyl-, 1-(para-methylphenyl)-, and 1-(para-trifluoromethylphenyl)cyclobutene in hydrocarbon solution are also reported. Keywords: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition

Labeling studies of some carbonylation reactions of styrene with cationic palladium complexes

The dicarbonylation reaction of E-β-deuteriostyrene to syndiotactic poly(1-oxo-2-phenyltrimethylene) as well as to dimethyl-2-phenylbutanedioate and dimethyl-2,5-diphenyl-4-oxoheptanedioate using Pd(CF 3COO) 2/2,2′-bipyridine as the catalyst precursor in the presence of 1,4-benzoquinone in methanol takes place stereospecifically in a syn-fashion with complete retention of the label. The same result was found for the dicarbonylation to dimethyl 2-phenylbutanedioate catalyzed by [Pd(CF 3COO) 2(Diop)]. In the absence of the oxidant the latter catalytic system produces methyl 2- and 3-phenylpropionates for which some scrambling of deuterium is observed when using either α-deuteriostyrene or CH 3OD as the labeled substrate. [Pd(CH 3CN) 4][BF 4] 2 modified with different ligands catalyses the formation of E-1,5-diphenylpent-1-en-3-one or of E-1,4-diphenylpent-1-en-3-one in tetrahydrofuran as the solvent. The label distribution using E-β-deuteriostyrene as the substrate (or styrene in the presence of dideuterium) suggests that in the synthesis of ketones catalyzed by [Pd( p-CH 3C 6H 4SO 3) 2(Dppp)]·2H 2O the regioselectivity of the first inserted olefin unit does not determine the ketone regioisomer; rather which regioisomeric product preferentially forms depends on the rate of carbon monoxide insertion in either the branched or linear metal-hydrocarbyl intermediate. β-Hydrogen elimination is very rapid both after the first and the second olefin insertion.

The far-UV photochemistry of five- and six-membered-ring monosilacycloalkenes in alcohols: stereospecific .beta.-cleavage to silyl ethers and stereoselective formation of silacycloalkyl ethers

Photoalcoholysis of 1,1-dimethylsilacyclopent -2-ene (9) at 214 nm in methanol, 2,2,2-trifluoesthanol, and tert-butyl alcohol affords alkoxy(3-butenyl)dimethylsilanes 11 and 3-alkoxy-1,1-dimethylsilacyclopentanes 12. Also fromed are alkoxy(cyclopropylmethyl)silanes 13 from apparent 1,3-C migration. In pure, delurated alcohols (CH 3 OD and CF 3 CD 2 OD) adducts 11 and 12 become trans deuterated, whereas in dilute solutions of methanol-O-d in pentane, scrambling of deuterium between the trans and cis C-2 positions of 12 is observed without corresponding loss of stereospecificity for formationh of silyl ether 11

Aspects of the hydrozirconation-isomerisation reaction

Aspects of the hydrozirconation-isomerization of nonfunctionalised olefins are discussed. Cyclooctene and 1-methyl-1-cyclohexene were previously reported not to react with Cp 2Zr(H)Cl. In the present work after treatment of the former with Cp 2Zr(D)Cl, and subsequent hydrolysis, no cyclooctane could be detected. Although the olefin seemed not to have reacted, it was deduced that the corresponding cyclooctylzirconium was in fact formed, since the amount of deuterium incorporated in cyclooctene was roughly the same as the amount of zirconium deuteride used. Total scrambling of deuterium was observed. Cycloheptene and 2,2,11,11-tetramethyl-6-dodecene gave similar results. 1-Methyl-1-cyclohexene, on the other hand, was very unreactive. No evidence for deuterium incorporation in the recovered olefin (80–90%) was obtained. When Z-5-decene was used as substrate, zirconium migration towards the terminal carbon, and cis-trans isomerisation, were slower than expected. No internal alkylzirconium derivatives could be trapped. Competing hydrogenation of 1- and Z-5-decene is favoured when a deficiency of the zirconium hydride is used.

Synthesis of methyl [2‐13C‐2,2‐d2]‐ and [3‐13C‐3,3‐d2]tetracosanoate

AbstractThe synthesis of methyl [2‐13C‐2, 2‐d2]‐ and methyl [3‐13C‐3, 3‐d2]tetracosanoate is reported. The carbon‐13 was introduced by cyanation of 1‐bromodocosane and 1‐bromoheneicosane using 18‐crown‐6 ether and potassium [13C]cyanide in hexamethylphosphoramide. Cyanation reactions with carbon‐13 proceeded at better than 90% yield by this method. Triphasic catalysis was used to introduce the CN group in successive homologations of the labeled compounds. This method was found to be label‐sparing with typical yields being 95%. No scrambling of deuterium was observed.