Phip

Hydroboration of conjugated dienes proceeds without a catalyst to give 1,2-adducts. However, the less reactive catecholborane reacts with isoprene with catalysis by Pd(PhiP)4, yielding the 1,4-adduct 73[66]. 

J-unsaturated ester is formed from a terminal alkyne by the reaction of alkyl formate and oxalate. The linear a, /J-unsaturated ester 5 is obtained from the terminal alkyne using dppb as a ligand by the reaction of alkyl formate under CO pressure. On the other hand, a branehed ester, t-butyl atropate (6), is obtained exclusively by the carbonylation of phenylacetylene in t-BuOH even by using dppb[10]. Reaction of alkynes and oxalate under CO pressure also gives linear a, /J-unsaturated esters 7 and dialkynes. The use of dppb is essen-tial[l 1]. Carbonylation of 1-octyne in the presence of oxalic acid or formic acid using PhiP-dppb (2 I) and Pd on carbon affords the branched q, /J-unsatu-rated acid 8 as the main product. Formic acid is regarded as a source of H and OH in the carboxylic acids[l2]. 

Aoki, H. et al., Inhibitory effect of anthocyanin colors on mutagenicity induced by 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), Foods Food Ingred. J. Jpn., 209, 240, 2004. 

Duckett381 reported on the use of parahydrogen-induced polarization (PHIP) to delineate the pathways involved in the catalytic hydrogenation of alkenes and alkynes by [Ru3(CO)12 x(PPh3)x] (x = 1 or 2) and showed that the mechanism is highly dependent on the solvent. Bassett and... 

Scheme 8.5 Main species involved in the hydrogenation of diphenylacetylene catalyzed by ruthenium clusters, as determined by PHIP methods (CO ligands omitted for clarity).

Other important topics, such as the use of para-hydrogen-induced polarization (PHIP) NMR, are discussed in more detail elsewhere in this book. Basically, this approach enhances the NMR signal one thousandfold, thus allowing the detection of intermediates that go unnoticed when using classicaF NMR techniques. PHIP is particularly suited for homogeneous hydrogenation research because a prerequisite of the method is that both former para-hydrogen nuclei must be present (and J-coupled) in the molecule of interest. 

Structure elucidation of various species present in the reaction mixtures typical tools used for this purpose include chemical shifts, coupling constants, PHIP-NMR, and 2D-NMR. 

An example of these pressure studies is provided by the studies of Elsevier et al. [31], who investigated the dependence of the hydrogenation rate of 4-octyne by a Pd-catalyst on the dihydrogen pressure, which was varied between 0 and 40 bar. The hydrogenation rate was shown to depend linearly on the dihydrogen pressure. In order to elucidate the reaction mechanism, the dependence of the reaction rate on substrate and catalyst concentration, and on the temperature, was also measured. NMR experiments with deuterium gas as well as PHIP-ex-periments were also carried out. 

This concept has originally been named PASADENA (Parahydrogen And Synthesis Allow Dramatically Enhanced Nuclear Alignment) [6], but the spectroscopic method based on this phenomenon has subsequently also been called PHIP (Para-Hydrogen Induced Polarization) [7]. In this chapter the abbreviation PH IP will be used throughout. 

The signal enhancement due to this approach can, in principle, be as high as 105-fold - that is, equal to the reciprocal Boltzmann factor however, the experimentally achievable enhancement factors typically range between 10 and 103. Thanks to this increase in sensitivity, the PHIP phenomenon, therefore, provides for a powerful tool to investigate the fate of the dihydrogen, the catalysts, and of the substrates during hydrogenation reactions. 

Initially in this chapter, the various features of the PHIP phenomenon, of the apparatus to enrich parahydrogen and orthodeuterium, and of the computer-based analysis or simulations of the PHIP spectra to be observed under specific assumptions will be outlined. In the following sections, comparisons of the experimentally obtained and of the simulated spectra reveal interesting details and mechanistic information about the hydrogenation reactions and their products. 

Figure 12.3 outlines the essential features of the PASADENA/PHIP concept for a two-spin system. If the symmetry of the p-H2 protons is broken, the reaction product exhibits a PHIP spectrum (Fig. 12.3, lower). If the reaction is carried out within the high magnetic field of the NMR spectrometer, the PHIP spectrum of the product consists of an alternating sequence of enhanced absorption and emission lines of equal intensity. This is also true for an AB spin system due to a compensating balance between the individual transition probabilities and the population rates of the corresponding energy levels under PHIP conditions. The NMR spectrum after the product has achieved thermal equilibrium exhibits intensities much lower than that of the intermediate PHIP spectrum. 

Fig. 12.3 Regular NMR (upper) and high-field PHIP-NMR spectrum (lower) of a two-spin AX system.

Fig. 12.4 Normal NMR, high-field (PASADENA) and low-field (ALTADENA) PHIP.

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