Learn more. By means of a chiral amine catalyst, iminium ion activation of enals ensures a stereoselective radical trap. The Stetter reaction 5 is the prototypical example of such umpolung 6 reactivity: an electrophilic aldehyde is converted into a nucleophilic acyl anion equivalent e. Despite the tremendous potential of this approach, the literature contains only a few examples of enantioselective intermolecular catalytic Stetter reactions. This result highlights the intrinsic challenge of developing an enantioselective variant, which requires the chiral catalyst to override a fast racemic background reaction.
Survey of the acyl DHPs 1 and enals 2 that can participate in the acyl radical conjugate addition. Reactions performed on a 0. Yields and enantiomeric excesses of the isolated products 3 are indicated below each entry average of two runs per substrate. Ideally, the identification of two distinct chiral catalysts that specifically trigger the two mechanistically orthogonal steps of the process could enable selective access to any product enantiomer or diastereomer of 5 by judicious catalyst selection.
In contrast, these electronic properties greatly hamper the condensation of C with the adduct 3 to generate the enamine intermediate. Efforts are ongoing to expand the synthetic potential of this asymmetric acyl radical addition strategy and fully elucidate the reaction mechanism. We thank Dr. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Technical support issues arising from supporting information other than missing files should be addressed to the authors.
Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries other than missing content should be directed to the corresponding author for the article. Volume 58 , Issue 4. In addition, Samori et al. Finally, Grozema et al. The combination of a wide array of molecular properties makes an efficient ET matrix, among others relatively high IPs, high molar absorptivity in the solid state, good solubility properties, appropriate crystallization, vacuum stability, and low tendency to form clusters.
Many structures for ET purposes have been tested, mostly through trial and error; however currently ET matrices are mostly based on highly conjugated aromatic compounds such as DCTB and 9-nitroanthracene. As examples, Chait et al. However, Drewello et al. In , Luftmann et al. DCTB is useful for the ionization of fullerenes derivatives [ 3 , 25 ], polymers [ 25 , 26 ], and porphyrins [ 27 ], among others [ 28 ].
The advantages of DCTB, over other ET matrices, lie in the relatively low laser fluence needed for ionization and its ability to prevent fragmentation of labile analytes [ 5 , 23 , 25 ]. However, there are still some issues regarding the performance of DCTB such as formation of adduct ions with analytes containing amine-groups. The carbon adjacent to the dicyanomethylene functionality in DCTB can act as an electrophile for primary and secondary aliphatic amines, leading to the formation of imines [ 29 ].
However, the photophysical characteristics of 9,DPA, e. In an interesting review Wyatt [ 1 ] highlights the usefulness of MALDI for the analysis of coordination and organometallic complexes and points out the lack of research in this area despite the great benefits of the technique. For instance, porphyrins and phthalocyanines are ionized through radical cation formation and LDI is the technique of choice for their analysis; however, LDI induces demetallation and increases fragmentation in labile analytes [ 5 , 23 ].
Other analytes such as polycyclic aromatic hydrocarbons PAH , with low solubility in polar solvents, are also susceptible to electron transfer ionization. Solvent-free LDI analysis was put forward as an alternative for the ionization of these non-polar analytes; however, molecular cation abundances and survival yields in this technique are very low [ 34 ]. Considering the wide applicability of ET ionization for the analysis of polymers, fullerenes, polycyclic aromatic hydrocarbons, organometallic, and coordination complexes [ 1 ], in this contribution we report the synthesis, photophysical, and mass spectrometric characterization of eight PV derivatives with potential application as ET MALDI matrices.
Unless otherwise stated, all compounds are commercially available and were used without further purification. Methyl triphenylphosphonium bromide, methyl 4-Iodo benzoate, and sodium periodate were purchased from Panreac Barcelona, Spain. General procedure for PV synthesis using the Mizoroki-Heck cross coupling reaction. Anthracene and quinine sulphate were used as reference fluorophores for intercrossing calibration. PV derivatives, anthracene and quinine sulphate concentrations in solution were adjusted to yield a UV-vis absorbance of approximately 0.
Calibration curves were prepared in ethanol for anthracene and in a 0. EPT simulations of all PVs exhibited pole strengths above 0. For all analytes, 2. For matrices threshold energies laser power was varied from 0. Instrument calibration was performed using CHCA and a mixture of standard peptides: leu-enkephaline, bradykinin, bombesin, and renin substrate purchased from Sigma Aldrich St.
The autoexecution method uses a random walk algorithm, which takes 10 laser shots in the same coordinates, then the target moves to another random position until it completes the requested total shots, in this case If the summed spectra fit the evaluation parameters, they are added to the total spectrum, if they do not then another acquisition takes place; each reported analysis corresponds to the sum of spectra. Experimental and theoretical isotope patterns, calculated with ChemCalc [ 41 ], were compared to verify compound identification. Many reports regarding photophysical properties of oligo PVs, particularly optical absorption and photoluminescence, are available in literature [ 12 , 19 , 42 , 43 , 44 ].
There is general agreement that substituent types and their placement in the PV core strongly influence PV derivatives optical performance [ 12 , 13 , 42 , 43 ]. Typically, the presence of alkyl or alkoxy groups on the central ring improves compound solubility [ 12 , 45 ]; while substituents in the peripheral rings induce shifts in UV-vis absorption and fluorescence emission wavelengths.
Photophysical properties for PVs derivatives. Dashed and dashed-dotted lines correspond to the laser wavelength nm and the PV core maximum absorption wavelength nm , respectively. According to Figure 1 all absorption maxima for PV derivatives lie to the right of the nm value. Finally, PV-NO 2 has the most dramatic shift to red wavelengths: 92 nm from the nm value, due to effect of the electron withdrawing functional group.
These values indicate efficiently absorption of nm photons by the PV derivatives in liquid phase and perhaps a good mass spectrometric behavior as MALDI matrices. We have to keep in mind that according to the coupled photophysical and chemical dynamics CPCD model of UV MALDI, primary ionization events and also much of the secondary reactions occur in a condensed-like phase and that ion yields in MALDI strongly depend on the solid state absorption properties of the matrix and the laser characteristics [ 2 , 49 , 50 , 51 ].
Hence, matrix photophysical properties in condensed phase are the ones that play a crucial role in primary ions generation through processes involving excitation, migration and concentration of energy [ 50 ]. From a mass spectrometric point of view both yields are important depending on the desired reaction channel during the ionization step. Interestingly observations by Sierra et al. Inserts show experimental isotopic distributions. We also performed LDI experiments under acidic conditions for the eight PV derivatives, using various amounts of TFA, to assess the PVs ability of forming protonated molecules or adducts.
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PVs, on the other hand could be potentially useful for characterization of either low or high mass analytes. Above the detection threshold ion abundances increase rapidly with increased laser power and then level out, as observed in the ion appearance curves for the PV series in Figure 3. For the tested PV derivatives m ranges from 5. Clearly, under identical experimental conditions PV derivatives have not only the ability to efficiently absorb energy but also to produce more ions, when compared to the standard DCTB matrix. Analytes IPs correspond to reported values left [ 13 , 14 , 15 ].
Limbach et al. Interestingly they also observed that analyte ion abundances are directly related to the IP differences between matrix and analyte, with higher differences indicating a more favorable electron transfer process [ 52 , 53 ]. Commonly IP values for commercially available ET matrices range from 7.
Our calculations indicate that IPs for the PV derivatives range from 6. IP values for the tested analytes see below are also included in Figure 4 and correspond to reported values [ 55 , 56 , 57 ].
At the lowest tested NP concentration 0. When the analyte:matrix ratios vary, from to and , the analyte amount on target decrease from 25 to 5. High analyte:matrix ratios, in the order of to , are commonly used on ET ionization experiments [ 3 , 6 , 30 , 53 ]. In addition, low analyte:matrix ratios , 2. This molecule exhibits low ionization efficiency and only the use of relatively high analyte:matrix ratios and , corresponding to 25 and 5.
We believe that this fact is a combination of the excellent photophysical properties of PV-CN, such as the highest molar absorptivity of all tested PV derivatives 72, Lmol —1 cm —1 , low energy threshold 0. Figure S15 also indicates that the ideal working energies for the PV matrices lie around 0. Rolla et al. Willis, L. Klassen, D. Tuma, M. Sorrell, and G. Dou, S. Li, Z. Wang et al. Smathers, J. Galligan, B. Stewart, and D. Poli, F. Petersen and J. Galligan, R. Smathers, K. Fritz, L. Epperson, L. Hunter, and D. Perluigi, R. Coccia, and D. Yoritaka, N. Hattori, K. Uchida, M. Tanaka, E.
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- Far from the Madding Crowd (Penguin Classics).
- Oligo p-Phenylenevinylene Derivatives as Electron Transfer Matrices for UV-MALDI.
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