6-Hydrogen transfer

5-Hydrogen Transfer - Irradiation of valerophenone (29) in aqueous solution has been studied. The reaction follows the same path as that in hydrocarbon solution and yields acetophenone and cyclobutanols. The reaction in water arises from the triplet state. Interestingly, the formation of the cyclobutanols cis. trans ratio is 2.4 1) is more efficient in the aqueous system than in hydrocarbons. Cyclobutanols are also formed on irradiation of the butanoate derivatives (30). Hydrogen abstraction by the triplet excited state carbonyl group occurs from the alkyl groups on C2 of the butanoate chain. 

Pincock and his co-workers have studied the photochemical fragmentation reactions of the esters (31). This system has an in-built electron accepting sensitiser. When (31a-c) are irradiated in methanol the principal reaction is fission to yield the styrene (32) and p-cyanobenzoic acid. The other products formed from the reactions are the styrene addition products (33)-(35). The authors propose that the Norrish T) e II process in this instance involves a proton transfer and this occurs within the zwitterionic biradical formed as the primary intermediate on electron transfer. Further proof of the authenticity of this mechanism was obtained by irradiation of the deuteriated derivatives (31 d, e). The results of a study of the photochemical decomposition of benzyl phenylacetate, as a suspension in water over Ti02, have been reported. Bond fission is the result of irradiation of (36) in cyclohexane/ethyl acetate. A Norrish Type II hydrogen abstraction occurs with the elimination of the enone moiety. This affords a path to the CD ring system (37) of vitamin D. 

A detailed study of the photochemical reactions of the ketones (38) and (39) in the solid phase has been reported. Both of these systems readily undergo Norrish Type II hydrogen abstraction in solution and it was this fact that attracted the authors to the systems. One of the facets of the work focussed 

Upon was the use of chiral auxiliaries as counter ions of the carboxylate examples in (38a) and (39a). The authors conclude that the ionic chiral auxiliary approach is a viable general method for asymmetric synthesis. The irradiation of the biphenyl ketoamide (40) at 340 nm affords the two products (41) and (42) via the conventional Norrish Type II hydrogen abstraction process. When the reaction is carried out in the presence of an antibody microenvironment the reaction follows a different route and yields the tetrahy-dropyrazine derivative (43). The authors reason that there is interplay between conformational control and chemical catalysis that results in this high specificity. 

Type II hydrogen abstraction by the keto group from the adjacent aryl methyl group to give the biradical (60) from which both products (61) and (62) are formed. The reaction is solvent dependent. In cyclohexane solvent only the cyclobutanols (61) are formed, but in methanol a mixture of (61) and the new diketone (62) are obtained in a ratio which is dependent upon steric factors. The photochemically induced proton transfer in 3-methyl-6-hydroxy-m-phthalic acid has been reported.  

5-Hydrogen Transfer. - Both experimental and theoretical approaches have been used to study the reactivity of w-butyrophenone included in alkali-metal-exchanged zeolites. The results indicate that with smaller cations the Norrish Type I process is enhanced over the Norrish Type II reaction. Others have reported that the photochemical decomposition of w-butyrophenone in a variety of solvents follows first-order kinetics.  

Norrish Type II hydrogen abstraction to afford the usual biradicals, which can cyclize into cyclobutanols. Both the c/ -(13) and the ra -isomeric forms are possible. This particular investigation has studied the influence of antibodies (12B4, 20F10 ad 21H9) on the cyclization reaction. The authors observed that the most reactive antibody, 20F10, catalyses the formation of the cw-product 

The Norrish Type II reactivity of the acetophenone derivatives (20) has been exploited as a new photoremovable protecting system for carboxylic acids. The irradiation affords the usual 1,5-biradicals that then release the acids (21) in the yields shown in parenthesis. Irradiation times are short. Klan and co-workers have described the photochemical reactivity of 1,5-dimethylphenacyl phosphoric and sulfonic esters. 

Norrish Type II hydrogen abstraction is the predominant reaction on irradiation of the silylated ketones (27). This affords the dealkylated product (28). There is some Norrish Type I reactivity that results in the formation of the isomerized product (29) and the two ring-opened products (30) and (31). The ratio of the two reactions varies with the silyl group, with a 32 1 ratio of (28) (29) obtained from (27, R = Me) and a 13 1 ratio from (27, R = Ph or R3 = MePh2).  

5-Hydrogen Transfer - The typical reaction of this type is exemplified by the conversion of the arylketones (23) into the cyclobutanol derivatives (24) and (25) 

The two ketones (28) and (29) are known to undergo the Norrish Type II hydrogen abstraction process, and their photochemical reactivities have now been studied in chirally modified zeolites. The zeolites were modified by stirring them with known amounts of (—)-ephedrine. Irradiation of the ketones in the zeolites brought about some enantiomeric enhancement. However, the various zeolites studied behaved differently and the NaX zeolite favoured the (+)-isomer of the product (30) while the NaY favoured the (—)-isomer. The other ketone (29) showed only low enantiomeric enhancement and gave both the cis and the trans cyclobutanols (31) and (32) in a ratio of 4 1.  

A detailed study of the photochemical reactivity of the large ring diketones (33) has been carried out. In solution irradiation brings about conventional y-hydrogen abstraction in every case except with the Cio diketone. The yields are 

Ring Size (33) m n cw-Product (34) fra i-Product (35) Open chain (36) 

This reaction has the important advantage of not involving hydrogen gas but on the downside the reaction is reversible in some cases, leading to incomplete conversions and racemisation of 39. This problem is minimised by performing the reaction in isopropanol as a solvent. There are few results with P-stereo-genic ligands, listed in Table 7.12. 

Entry 1 shows that low to moderate enantioselectivities can be obtained with simple monodentate phosphines whereas low enantioselectivities have been obtained with the P,N ligand of entry 2 or with the pincer ligand of entry 3. Much better results have been obtained with p-aminophosphines and their oxides (entries 4-6). Interestingly, it has been found that phosphine oxides provide better activities and similar enantioselectivities than trivalent phos- 

The best result has been obtained with the ligand of entry 6, 

Entry Ligand Silane Time (h) Yield (%) ee (%) References 

TangPhos, DuanPhos and BINAPINE have shown wide applicability and exceptional enantioselectivities. 

1 Hydrogan Transfer- An extensive review of asymmetric transfo hydrogenation is of interest i as is an improved version for selective ester reduction via hydrogen transfer from HSi(OEt)3 in the presence of Ti(OPr )4. Addition to either the same or opposite enantiofaces of prochiral alkenes is catalysed by RuH (S)-BINAP species from either Pr OH or H2 depending on the alkene functionality. A number of alcohol-ketone transfer hydrogenations catalysed by 

Many enzymatic reactions involve transfer protons, hydrides or hydrogen atoms (Alhambra et al 2000 Allison, 1998 Backgren et al 2000 Blum et al., 2001 Cha et al., 

1989 Bruno and Bialek, 1992 Hwang and Warshel, 1996 s Isaev and Scheiner, 2001 Musser and Theg, 2000 Sjoergen et al., 2000 Swain et al., 1958 Tripp and Ferry, 2000 Likhtenshtein and Shilov, 1976 Likhtenshtein, 1988a). 

Another method, in particular for the preparation of alcohols from ketones involves the transfer of hydrogen from a hydrogen donor. The classic example is the commercially applied Meerwein-Ponndorff-Verley reaction, which uses stoichiometric amounts of Al(O Pr)3 to produce acetone and the alkoxides of the alcohols desired [31], The catalytic version of this reaction, employing 

The product is an equilibrium mixture and an excess of 2-propanol must be used to obtain high yields. Ammonium formate or formic acid have also been used as the hydrogen donor and now the reaction to the alcohol is complete, because the thermodynamics are more favourable and because C02 leaves the reaction medium. 

Since 1980 the interest in this reaction increased because enantiospecificity was introduced and much more valuable products could be made. A wide variety of ligands was tested, such as chiral dipyridines, phenanthrolines, diphosphines, aminoalcohols, bis-oxazolines, bis-oxazolines with a third donor atom in the centre, bis-thioureas, diamines, etc [33], In 1981 the highest e.e. reported was still only 20%. For many years the best results were obtained with chiral diimines and phenanthrolines but e.e. s were below 70% [34], Pfaltz introduced bis-oxazolines for this reaction and obtained e.e. s as high as 91% [35] in 1991. 

Two mechanisms have been put forward, one involving P-hydride elimination and migratory insertion reactions, and the other one involving a direct, concerted transfer of the two hydrogen atoms from the alcohol donor to the reactive intermediate complex and vice versa donation to the ketone. Both pathways assume a heterolytic character for the overall transfer this is to say 

The isotope effects for transfer of hydrogen were 1.79 for transfer from OH to N and 2.86 for transfer from CH to ruthenium. The isotope effect for transfer of the doubly labelled material d% 2-propanol) was 4.88, within the experimental error. If the hydrogen atoms would be transferred in separate 

The development of supersonic molecular beam techniques created new opportunities to study tunneling effects in gas-phase isolated molecules and dimers at ultralow translational and rotational temperatures. Modern low-temperature chemistry, therefore, includes the study of chemical dynamics of molecules in various states of aggregation. 

Intra- and intermolecular hydrogen transfer processes are important in a wide variety of chemical processes, ranging from free radical reactions (which make up the foundation of radiation chemistry) and tautomeriza-tion in the ground and excited states (a fundamental photochemical process) to bulk and surface diffusion (critical for heterogeneous catalytic processes). The exchange reaction H2 + H has always been the preeminent model for testing basic concepts of chemical dynamics theory because it is amenable to carrying out exact three-dimensional fully quantum mechanical calculations. This reaction is now studied in low-temperature solids as well. 

A hydrogen bond can be loosely described as the interaction between two electronegative atoms and an intervening hydrogen atom, i.e., -XH Y-, where X and Y represent O, F, Cl, N, S, or C atoms. The potential surface that describes this interaction typically has two minima that correspond to formation of a strong XH or YH bonds. A one-dimensional potential that describes the motion of the central H atom between the two heavier atoms at a fixed distance R from each other can be represented by the empirical form 

The expansion coefficients and energy eigenvalues are found from solutions of the secular equation Hnn — SnnE = 0. The nonzero matrix elements Hnn can be expressed in terms of the coefficients of the potential 

This method of numerical quantization can be also used in the case of low barriers, for which the semiclassical approximation is no longer valid. 

RSC Catalysis Series No. 2 Chiral Sulfur Ligands Asymmetric Catalysis By Helene Pellissier Helene Pellissier 2009 

In 2001, a screening study for the enantioselective reduction of various aryl ketones was developed by Petra et al. in the presence of amino sulfide 

More recently, Zaitsev and Adolfsson have reported the preparation and application of novel chiral S/N ligands for rhodium- and ruthenium-catalysed 

On the other hand, one of the first chiral sulfur-containing ligands employed in the asymmetric transfer hydrogenation of ketones was introduced by Noyori el al Thus, the use of A-tosyl-l,2-diphenylethylenediamine (TsDPEN) in combination with ruthenium for the reduction of various aromatic ketones in the presence of i-PrOH as the hydrogen donor, allowed the corresponding alcohols to be obtained in both excellent yields and enantioselectivities, as 

In order to improve the performance of Noyori s catalytic system, Ru(II)-TsDPEN, which is very efficient but suffers from a long reaction time and a low activity in some cases, Mohar et al. have modified the diamine ligand by 

The three dialdehydes (32), (33) and (34) are photoreactive in the crystalline state. However the outcome of the reactions appears to be dependent upon the substitution pattern on the aryl ring. Irradiation of (32, X = H) and (34, X = H) gives dimers quantitatively. The structure of the dimers is illustrated by (35), 

Moorthy and MaP have reported that irradiation of the benzoyl ketones (38) results in photochemical conversion to the mixture of cyclobutanes (39) and (40). The yields are in the 31-43% range and, as can be seen from the ratios of products, there is a good degree of selectivity when the reactions are carried out in non-polar solvents. The ratios change when polar solvents are used. This change is more dramatic with the ketones (38, R = Ph), where the observed selectivity is reversed from non-polar to polar solvents. The keto derivatives (41, R = H or Ac, n = 1) undergo Norrish Type II hydrogen abstraction on irradiation at 300 nm in t-butanol as solvent. Cyclization results in the formation of the imidazolidinones (42 and 43) by cyclization within the resultant biradicals. 

Fission of the 1,4-biradical affords acetophenone in competition with cycliz-ation. The products are obtained as racemic diastereoisomers. The other ether derivatives (41, n = 2) are also reactive and undergo formation of 1,5-biradicals on irradiation. The selectivity of the reactions was investigated in the presence of chiral hosts. The best yields were obtained for the series (41, R = H, n = 2) using the host molecule (44) in toluene at — 45°C. This gave a 70% yield of products 

The influence of chiral inductors on the photochemical cyclization of the adamantane-substituted ketones (48) in zeolites has been examined. Only the endo-products (49) are formed. The best ees are obtained for both derivatives (X = H or F) with (—)-pseudoephedrine as the chiral auxiliary. The cyclobutanols undergo retro-Aldol ring opening to afford the ketones (50). The study was extended to the more heavily substituted derivatives (51).  

Laser flash photolysis of 2-methylbenzil shows that the triplet state is produced. However, irradiation in methanol involves a different intermediate that has been shown to lead to a mixture of photoenols. Irradiation in benzene affords 

This section discusses the possibility of deep cracking of bitumen or vacuum residues at a low temperature. 

The present methods of using a hydrogen acceptor are intended to prove the ability of the substance to be a hydrogen donor. 

Shaw et al. [27] indicate that anthracene is more readily hydrogenated than other aromatic systems (e.g. naphthalene). The hydrogenation of anthracene can be described by the following equation (9.14)  

Marcel Dekker, Inc. 270 Madison Avenue. New York, New York 10016 

The results of this investigation of hydrogen transfer are presented in table 9.3. The experiment with thermal treatment of pure anthracene (E.l) demonstrated that a significant amount of dihydroanthracene was found in the used anthracene after thermal treatment. Therefore, it is important to use this fact in the discussion of the results. Experiment RE.3 is a reproducibility experiment in comparison with E.3. It is evident that the experimental method has a good reproducibility (relative divergence 4%). 

The synthetic utility of such photoenolization reactions continues to be of interest. Thus, the photochemistry of the aldehydes (26) has been examined using 

Pyrex-filtered irradiation in de-aerated acetonitrile solution. The resultant enols can be trapped by dimethyl maleate to yield the adducts (27) and (28) in the ratios and yields shown. Trapping of the enols can also be acomplished using dimethyl acetylenedicarboxylate or ethyl propiolate as the dienophile. A study of the phototautomerism of methyl salicylate (29) into (30) at 77 K has been carried out and this again involves a 1,5-hydrogen transfer. Interestingly, the authors report that triplet state emission occurs from the transient keto form (30). The influence of aryl substituents (p-Me and -MeO) upon the photochemically induced intramolecular proton transfer in salicylic acid has also been studied in detail. 

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Quasiclassical calculations are similar to classical trajectory calculations with the addition of terms to account for quantum effects. The inclusion of tunneling and quantized energy levels improves the accuracy of results for light atoms, such as hydrogen transfer, and lower-temperature reactions. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Heating with cuprous chloride in aqueous hydrochloric acid isomerizes 2-butene-l,4-diol to 3-butene-l,2-diol (98)] Various hydrogen-transfer catalysts isomerize it to 4-hydroxybutyraldehyde [25714-71-0] (99), acetals of which are found as impurities in commercial butanediol and... 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Propylene. Propylene alkylation produces a product that is rich in dimethylpentane and has a research octane typically in the range of 89—92. The HF catalyst tends to produce somewhat higher octane than does the H2SO4 catalyst because of the hydrogen-transfer reaction, which consumes additional isobutane and results in the production of trimethylpentane and propane. 

Miscellaneous Reactions. Ahyl alcohol can be isomerized to propionaldehyde [123-38-6] in the presence of sohd acid catalyst at 200—300°C. When copper or alumina is used as the catalyst, only propionaldehyde is obtained, because of intramolecular hydrogen transfer. On the other hand, acrolein and hydrogen are produced by a zinc oxide catalyst. In this case, it is considered that propionaldehyde is obtained mainly by intermolecular hydrogen transfer between ahyl alcohol and acrolein (31). 

A second process has two steps. The first step is oxidation of propylene [115-07-1] to acrolein and the second step is reduction of acrolein to ahyl alcohol by a hydrogen transfer reaction, using isopropyl alcohol (25). 

Stabilized by hydrogen transfer. The stabilized free radicals undergo secondary cracking reactions as they come in contact with the hot coke. 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Coke deposition is essentially independent of space velocity. These observations, which were developed from the study of amorphous catalysts during the early days of catalytic cracking (11), stiU characteri2e the coking of modem day 2eohte FCC catalysts over a wide range of hydrogen-transfer (H-transfer) capabihties. 

Structure and Mechanism of Formation. Thermal dimerization of unsaturated fatty acids has been explaiaed both by a Diels-Alder mechanism and by a free-radical route involving hydrogen transfer. The Diels-Alder reaction appears to apply to starting materials high ia linoleic acid content satisfactorily, but oleic acid oligomerization seems better rationalized by a free-radical reaction (8—10). 

Clay-catalyzed dimerization of unsaturated fatty acids appears to be a carbonium ion reaction, based on the observed double bond isomerization, acid catalysis, chain branching, and hydrogen transfer (8,9,11). 

Salicylonitrile is believed to arise by direct cleavage with subsequent hydrogen transfer, while the benzoxazoles were produced by an isocyanide intermediate (73JA919, 74HCA376). Photolysis in D2O tends to confirm this possibility and rule out an azirine intermediate (39), due to deuterium corporation into the molecule (Scheme 10) (74HCA376). 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

This reasoning was set forth by Johnston and Rapp [1961] and developed by Ovchinnikova [1979], Miller [1975b], Truhlar and Kupperman [1971], Babamov and Marcus [1981], and Babamov et al. [1983] for reactions of hydrogen transfer in the gas phase. A similar model was put forth in order to explain the transfer of light impurities in metals [Flynn and Stoneham 1970 Kagan and Klinger 1974]. Simple analytical expressions were found for an illustrative model [Benderskii et al. 1980] in which the A-B and B-C bonds were assumed to be represented by parabolic terms. 

Fig. 18. Rate constant calculated with the use of (2.80a) plotted against (m/mH). The hydrogen transfer rate is assumed to be 10 s the effective symmetric vibration mass 125mH. The ratio of force constants corresponding to the intra (Kq) and intermolecular (K,) vibrations is (Ki/Ko) = 2.5 x 10 , 5 x 10 and l.Ox 10 for curves 1-3, respectively.

In addition, the frequency cooo, as well as the tunneUng distance can also be extracted from the same empirical data. Thus all the information needed to construct a PES is available. Of course, this PES is a rather crude approximation, since all the skeleton vibrations are replaced by a single mode with effective frequency cooo and coupling parameter C. From the experimental data it is known that the strong hydrogen bond (roo < 2.6 A) is usually typical of intramolecular hydrogen transfer. 

Hydrogen transfer in excited electronic states is being intensively studied with time-resolved spectroscopy. A typical scheme of electronic terms is shown in fig. 46. A vertical optical transition, induced by a picosecond laser pulse, populates the initial well of the excited Si state. The reverse optical transition, observed as the fluorescence band Fj, is accompanied by proton transfer to the second well with lower energy. This transfer is registered as the appearance of another fluorescence band, F2, with a large anti-Stokes shift. The rate constant is inferred from the time dependence of the relative intensities of these bands in dual fluorescence. The experimental data obtained by this method have been reviewed by Barbara et al. [1989]. We only quote the example of hydrogen transfer in the excited state of... 

The temperature dependence of the rate constant of rapid hydrogen transfer, measured by Mordzinski and Kuhnle [1986], is given in fig. 1. A similar dependence has been found by Grellmann et al. [1989] for one-proton transfer in half of the above molecule (6.16), which does not include the two rightmost rings. 

During the high-temperature operations, intermolecular hydrogen transfer reactions occur, transforming some indene to indane. The high indane concentration in the resin feedstock causes low yield and poor quality in the polymerization process. The indene loss can be reduced by decreasing the temperature and the residence time during distillation. 

Chloranil (2,3,5,6-tetrachlorobenzoquinone) and DDQ were included by Braude in an investigation of hydrogen transfer reac-... 

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