Competitive pathways

For 4,5-dialkylthiazoles, the molecular ion decomposes by two competitive pathways, either loss of HCN followed by elimination of the radical R in the position /3 to the double bond of the resulting substituted thiirene, or by p cleavage followed by elimination of HCN (119). 

Radical and ionic nitrations are often competitive pathways in strong nitrating acid rmxtures. The predominant reaction pathway is determined by the composition of the nitrating medium. Oxides of nitrogen in the nitrating medium add to... 

The potential energy surface [47] for this reaction (Fig. 5) shows many potentially competitive pathways, labeled A-F, leading to the two most exothermic product channels. Many of these pathways can be isotopically separated by reaction of 02 with HCCO in normal abundance, as diagramed in Fig. 5. Zou and Osbom used time-resolved Fourier transform emission spectroscopy to detect the CO and CO2 products of this reaction [47]. Rotationally resolved infrared (IR) spectroscopy can easily identify all the possible isotopologs. For example. Fig. 6 shows a single... 

The competitive pathway (Scheme 19, path b), which has been singled out at low temperature for 71 and 74, shows the preferential migration of the second... 

The oxidative formation of p-benzoquinones from anilides such as 7-108 was used for the synthesis of the core scaffold of the natural products elisabethin A (7-106) and pseudopterosin A aglycone (7-107) (Scheme 7.30). Exposure of anilide 7-108 to DMP [53] led to the formation of the o-imidoquinone 7-109, which underwent an intramolecular Diels-Alder reaction to give 7-110 in 28% yield after hydration. In a competitive pathway, the p-quinone 7-111 is also formed from 7-108, which on heating in toluene again underwent an intramolecular Diels-Alder reaction to give cycloadduct 7-112 in 25% overall yield. Hydrolysis of 7-112 furnished the carbocyclic skeleton 7-113 of elisabethin A (7-106). 

The one exception to this observation is the hydrolysis of bis( p-nitrophenyl) methylphosphonate which, in the presence of cycloheptaamylose, produces only 1.7 moles of phenol. Probably two competitive pathways are available for the hydrolysis of the included substrate (1) nucleophilic attack by an ionized cycloheptaamylose hydroxyl group, and (2) nucleophilic attack by a water molecule or a hydroxide ion from the bulk solution. Whereas the former process produces two moles of phenol and yields a phos-phonylated cycloheptaamylose, the latter process produces only one mole of phenol and a relatively stable p-nitrophenyl methylphosphonate anion. The appearance of less than two moles of phenol may be explained by a combination of these two pathways. Since the amount of p-nitrophenyl methylphosphonate produced in this reaction is considerably larger than expected from an uncatalyzed pathway, attack of water may be catalyzed by the cycloheptaamylose alkoxide ions, acting as general bases (Brass and Bender, 1972). 

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

It would be interesting to test with other Rh(III) complexes, whether the direct oxidation of the base (by photo-electron transfer) could also be a primary step responsible for photocleavages. Indeed, as outlined before in Sect. 5, radiation studies have shown that the radical cation of the base can produce the sugar radical, itself leading to strand scission [122]. Moreover base release, as observed with the Rh(III) complexes, can also take place from the radical cation of the base [137]. Direct base oxidation and hydrogen abstraction from the sugar could be two competitive pathways leading to strand scission and/or base release. 

In accordance with these experimental results, Wang et al. employed density functional theory calculations to comprehensively examine the possible reduction pathways for EC molecules in super-molecular structures Li+—(EC) [n = 1—5) and found that, thermodynamically, both one- and two-electron reductive processes are possible.A complete array of the possible reduction products from EC was listed in their paper considering the various competitive pathways, and they concluded that both (CH2OCO2-Li)2 and (CH2CH20C02Li)2 are the leading species in SEI, while minority species such as lithium alkox-ide, lithium carbide, and the inorganic Li2C03 coexist. 

There are multiple pathways that are currently known to inactivate lipoxygenases, ranging from competitive to allosteric to reductive inhibition. The competitive pathways have been studied by Zherebtsov, Popova, and Zyablova (2000). The allosteric process has been studied by Mogul Johansen, and Holman (2000). Reductive inhibition has been studied by Kemal Louis-Flamberg, Krupinski-Olsen, and Shorter (1987). (57 words)... 

The amide derived from the carboxylic acid in Ugi adducts is in most cases tertiary, and therefore it cannot serve as nucleophilic partner in post-condensation transformations, unless a post-Ugi rearrangement converts it into a free amine [52, 54]. An exception is represented by Ugi adducts derived from ammonia, which give rise to two secondary amides, each of them potentially involved, as nucleophile, in nucleophilic substitution processes. Four competitive pathways are in principle possible (N- or 0-alkylations of the two amides), and the reaction is mainly driven by the stability of the formed rings. In the example shown in Fig. 12, 0-alkylation of the carboxylic-derived amide is favoured as it generates a 5-membered ring (oxazoline 62), while the alternative cyclization modes would have formed 3- or 4-membered rings [49]. When R C02H is phthalic acid, however, acylaziridines are formed instead via Walkylation [49]. In both cases, the intramolecular 8 2 reactions takes place directly under the Ugi conditions. 

To conclude this section on the effect of solvent on a-nucleophilicity, we refer to the current, rather controversial, situation pertaining to gas-phase smdies and the a-effect. As reported in our review on the a-effect and its modulation by solvent the gas-phase reaction of methyl formate with HOO and HO , which proceeds via three competitive pathways proton abstraction, nucleophilic addition to the carbonyl group and Sat2 displacement on the methyl group, showed no enhanced nucleophilic reactivity for HOO relative to This was consistent with gas-phase calculational work... 

Initial /i-scission of the 5,6-bond leads to a carbon-centered radical that is converted by efficient O2 uptake into an a-hydroxyperoxyl radical. In a subsequent step elimination of H02 /02 gives rise to Af -formyl-Af -pyruvylurea that may either hydrolyze into N(2-deoxy-/ -D-eryt/iro-pentofuranosyl)fonnamide (12) or rearrange into 5R and 55 diastereomers of 1 -(2-deoxy-/ -D-eryt/iw-pentofuranosyl)-5-methy 1-5-hydroxyhydantoin (13) according to two competitive pathways. 

The long-known rearrangement of a-bromocamphoric anhydride (209 X=Me) to (210 X = Me) has been shown to proceed via at least two competitive pathways to account for the observation, using (209 X = CDs), that the C-9 methyl group is the precursor of X [no (210 X = CDs) is obtained] but that the C-8 methyl group is the precursor of both the C-1 methyl group (71%) and the C-3 methyl group (29%) in (210 X = CH3). This paper also confirms the unequivocal methyl resonance... 

Very Shallow Traps. It has been proposed that the neutral Gua(Nl—H) radical, formed by proton transfer from the Gua radical by proton transfer from N1 of Gua to N3 of Cyt, is a shallow trap [143,144]. This proposal is based on projections from made on monomers in dilute aqueous solution, which predict that proton transfer is favored by 2.3 kJ/mol [22,145]. Ab initio calculations are in excellent agreement with this value [146,147]. So one expects that an energy of at least 0.025 eV is needed to activate the return of the proton to N1 Gua, reforming Gua . Once Gua is reformed, tunneling to nearby guanines is reestablished as a competitive pathway. Proton transfer therefore is a gate for hole transfer. Proton-coupled hole transfer describes the thermally driven transfer of holes from one Gua Cyt base pair to another. 

The PKR with 1,3-dienes implicates much more interesting features. First, there must be competitive pathways in the formation of PK product 63, Diels-Alder product 64, and homologous PKR product of bicyclo[5,3,0]-decadienone 65 (Scheme 8). 

Aqueous hetero Diels-Alder reaction was first described by Grieco, who reported the use of water as solvent for cyclocondensations of iminium salts (Larsen and Grieco, 1985). Known as being a very energy demanding reaction, the retro Diels-Alder process is usually not considered as a competitive pathway in most Diels-Alder... 

Internal hydrogen scrambling and isotope redistribution between propane and solid catalysts may proceed by a rather complex mechanism involving several pathways with different rates and activation energies. The competitive pathways cannot be distinguished if the temperature for activating propane on zeolites is... 

In summary, it may be stated that the reaction of acyl esters of aldoses and aldobioses (see Section III, p. 92) with ammonia consists of a set of competitive pathways, including intramolecular O —> N migrations of acyl groups, deacylations, and transesterifications, with formation of aldose amides and variable proportions of the free sugar as the principal products. Significant proportions of basic or insoluble polymeric substances were not observed with aldose acetates or benzoates, although occurrence of extensive browning indicates the probable formation of soluble melanoidins. 

The surface reaction consists of two competitive pathways. Their relative rates determine selectivity. The ethyl species may undergo further dehydrogenation to form ethene [Eq. (6)] or be oxidized to ethoxide and then to acetaldehyde or acetate [Eq. (7)], and possibly to carbon oxides. The formation of ethoxide is favored at lower temperatures and in the presence of water vapor (18, 19). Other surface reactions are also possible. They are discussed later. 

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