C-N bond scission

The method is successfully applied to selective N-C bond scission of peptides at serine or threonine residues (Eq. 3.52) [84]. 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

At higher temperatures increasing proportions of n-butane join ethane as gas-phase hydrogenation products. On lowering the temperature to room temperature for spectroscopic measurements, physically adsorbed w-butane increasingly contributes to the spectra of adsorbed species. Further temperature rises lead to increasing amounts of chemisorbed n-butyl species in all cases, and finally to gas-phase methane through C—C bond scission. 

If the aziridine cycle is fused, then rotation of one of the C-N bonds during C-C bond scission becomes possible only in one direction therefore only two of the four possibilities of ring opening can be realized depending on the rotation direction of the second free C-N bond, and the cis-ylides or trans-ylides can be formed. The /ra y-substituted aziridines are as shown in Scheme 1.35. 

This can in pan be answered because Pt/alumina, e,g. EUROPT-3 (and Pt/Re/alumina) has also been studied [7]. In n-butane hydrogenolysis on Pt/alumina the accumulation of carbonaceous deposits on the catalyst surface suppressed ethane formation (i.e. relative to that of propane formation (i.e. S3), Thus for Pt/alutnina sites responsible for central C-C bond scission in n-butane may be selectively deactivated, e.g. at 603K sample S2 S3... 

The ESR study of solids at low temperatures can yield evidence for primary photochemical processes involving radical production. Svejda and Volman (353) have obtained ESR evidence for two primary photochemical processes of CH CN one involves a C-C bond scission, the other involves a C-H bond split. They further substantiated the C-H split mechanism by observing the ethylidenimino radical (CH )HC=N, resulting from the addition of the H atom to CH CN. The photochemistry of CH CN appears rather simple compared to the radiolysis of CH CN in which methyl radicals are produced by photochemical reactions in the y-irradiated sample (354). Recently Sprague and Williams (355) have demonstrated that methyl radicals produced from CH CN in a low-temperature glass can abstract hydrogen... 

The scission of the N—C bond of coordinated isocyanates has been discussed as a useful method to prepare nitrido clusters. Under higher pressures of CO, Eq. (65) can be reversed. When [Ru6N(CO)i6] is placed... 

Because cation stabilization makes an important contribution, N-, S-, and O-substituents are often used to support C-C bond cleavage [109, 144]. C-C bond scission is readily achieved in strained carbocyclic [145] or heterocyclic [146] ring systems, with many examples stemming from cyclopropyl sulfides [147]. 

The a-scission of a N—C-bond occurs easily if a relatively stable carbon radical is formed. Thus trityl-aminyloxides 2 R = C(C6H5)3 decompose giving triphenyl-methylradicals and nitroso compound182. The reaction takes place easily if the group R is bulky, as for instance tert-butyl. 

Propin-2yl-aminyloxide 225 decomposes only when refluxed in benzene solution giving bicyclic compound 226, the structure of which was confirmed by X-ray analysis184. Apparently the first step of this reaction is again the a-scission of a N-C-bond giving l.l-dimethyl-2-propinyl radical and 2-methyl-2-nitrosopropane, compound 226 being formed in an as yet unknown sequence of reactions. 

The N—C-bond dissociation energy of di-tert-butyl-aminyloxide 2b has been estimated to be 29 kcal/Mol53 according to the high thermal stability of 2b which is decomposed by a-scission only at temperatures above 125 °C. In addition to 2-methyl-2-nitrosopropane, N.N.O-tri-tert-butylhydroxylamine was formed, the tert-butyl radical being trapped by unreacted 2b. 

Photolysis of amidinyl-N.N -dioxide 207 in aprotic solvents is quite different from photolysis in water (pp. 98-100). It is a very complex reaction188 involving an a-scission of N—C-bond, leading to radicals 235 and 236. Now the first step of the... 

In view of the experimentally observed linear relationship between n(A/AQ) and t, an Arrhenius equation has been used to determine the apparent activation Inergles for the particular decomposition reactions. We find 15 kcal/mole for N-H, 23 kcal/mole for Si-H and 31 kcal/mole for Sl-C bonds scissions. These values are very approximate, since they were evaluated assuming a linear relationship .n k = f(l/T) for the temperatures under study. As pyrolyses were carried out at only two experimental temperatures, the validity of this assumption could not be verified. 

The main oxidation products of the methyl esters of aliphatic acids containing n C atoms are methyl esters of dicarboxylic acids C4—C 3, aliphatic acids Ci— Cn-i, and keto- and hydroxy compounds [301—307]. Oxidation of acetates (140—160° C) yield acids, carbon dioxide, hydroxy, and keto compounds (see Table 17). Hydroperoxide is the primary product of oxidation. Oxidation of dimethyl esters of dicarboxylic acids gives monoesters with a lower number of C atoms in the acidic group (see Table 17). Carbon dioxide is formed in parallel with acids and monoesters [308]. All monoesters C 1 Cn 2 etc. are also formed in parallel. This suggests several mechanisms of C—C bond scission in the oxidation, an a-mechanism with only one C—C bond broken to form Cm and C — products, a /3-mechanism with two C—C bonds broken in the /3-position to form Cm, C —m— and C02, etc. The a, /3, and 7-mechanisms of C—C bond scission may be regarded as a result of peroxy radical isomerization to form labile dihydroperoxides, e.g. 

Figure 14.10 shows N Is temperature-programmed (TP) XPS spectra of glycine on Pt(l 11) (a) and Cu(l 10) (b) taken in vacuum. Clearly, Cu(l 10) is less reactive as the photoelectron signal of the intact molecule disappears at a much higher temperature compared to Pt(l 11). Other studies have shown that the desorption process on both surfaces starts with C-C bond scission, followed by desorption of the scission products from the surface [42-44]. As the decomposition on Pt(l 11) occurs at much lower temperatures, the products can be detected on the surface with XPS, whereas on Cu(llO), the higher decomposition temperature causes... 

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