Nitrogen electrophilic sources

The a-amination of ketones can be achieved using Lewis acids in combination with electrophilic sources of nitrogen. As nitrosobenzene also functions as... 

Before we talk about this group of aromatic Sfjl reactions in more detail, let s consider how to make the diazonium salt. The reagent we need is the reactive nitrogen electrophile NO. You met NO in Chapter 20, but to remind you, it forms when the nitrite anion (usually sodium nitrite) is treated with acid at around 0 °C. Protonation of nitrite gives nitrous acid, HONO protonation again gives a cation, which can lose water to form NO. Butyl nitrite (or other alkyl nitrites) can also be used as a source of NO. ... 

There are conflicting generalizations in the heterocyclic literature as to the relative reactivity of a- and y-positions in azines toward nucleophiles. Variations in the relative reactivity are attributed in this and subsequent sections to specific factors operating in addition to activation by azine-nitrogen. Another possible source of variation may be a decrease in selectivity with increasing reactivity of one or both reagents, an effect established in electrophilic aromatic... 

Reactions involving the catalytic reduction of nitrogen oxides are of major environmental importance for the removal of toxic emissions from both stationary and automotive sources. As shown in this section electrochemical promotion can affect dramatically the performance of Rh, Pd and Pt catalysts (commonly used as exhaust catalysts) interfaced with YSZ, an O2 ion conductor. The main feature is strong electrophilic behaviour, i.e. enhanced rate and N2 selectivity behaviour with decreasing Uwr and , due to enhanced NO dissociation. 

Like nitrogen fluorides, compounds having an O—F bond are also very strong oxidants and, when utilized as such, are effective sources of electrophilic fluorine. Because the O—F bond is so weak, the chemistry of such compounds approaches that of F2 itself, and such chemistry can be free radical or electrophilic in nature, depending on the conditions and the reactants. 

Individual substitutions may not necessarily be true electrophilic aromatic substitution reactions. Usually it is assumed that they are, however, and with this assumption the furan nucleus can be compared with others. For tri-fluoroacetylation by trifluoroacetic anhydride at 75 C relative rates have been established, by means of competition experiments 149 thiophene, 1 selenophene, 6.5 furan, 1.4 x 102 2-methylfuran, 1.2 x 105 pyrrole, 5.3 x 107. While nitrogen is usually a better source of electrons for an incoming electrophile (as in pyrrole versus furan) there are exceptions. For example, the enamine 63 reacts with Eschenmoser s salt at the 5-position and not at the enamine grouping.150 Also amusing is an attempted Fischer indole synthesis in which a furan ring is near the reaction site and diverted the reaction into a pyrazole synthesis.151... 

The electron capture detector is another type of ionization detector. Specifically, it utilizes the beta emissions of a radioactive source, often nickel-63, to cause the ionization of the carrier gas molecules, thus generating electrons that constitute an electrical current. As an electrophilic component, such as a pesticide, from the separated mixture enters this detector, the electrons from the carrier gas ionization are captured, creating an alteration in the current flow in an external circuit. This alteration is the source of the electrical signal that is amplified and sent on to the recorder. A diagram of this detector is shown in Figure 12.13. The carrier gas for this detector is either pure nitrogen or a mixture of argon and methane. 

Iodine was found to be an efficient catalyst for the aziridination of alkenes (Scheme 6) utilizing chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide) as the nitrogen source. For example, when 2 equiv. of styrene (45a) were added to chloramine-T in the presence of a catalytic amount of iodine (10mol%) in a 1 1 solvent mixture of acetonitrile and neutral buffer, the corresponding aziridine (46) was obtained in 91% yield. The reaction proved to work with other acyclic and cyclic alkenes, such as oct-l-ene and cyclohexene. The aziridination of para-substituted styrene derivatives (45b-e) demonstrated that, as expected for an electrophilic addition, electron-rich alkenes reacted faster than electron-poor alkenes. However, with 1 equiv. of I2, mainly iodohydrin (47) was formed. A catalytic cycle has been proposed to account for the fact that only a catalytic amount of iodine is required (Scheme 1) ... 

There are no reports on electrophilic attack at ring carbon or nitrogen. Regarding ring sulfur atom, it has been reported that oxathiazolones (49), act as sulfur sources in their reaction with Mn(CO)s yielding a sulfido cluster of the type [Mn3(CO)9(/i3-S)2] by elimination of PhCN and CO2 <930M1009>. 

While the detailed mechanism of these rhodium-catalyzed cyclizations is not known, a working hypothesis that accommodates all of the observations to date is as follows. The diazo ketone can be considered to be a stabilized ylide, 14. Association of the Lewis acidic LUMO of the rhodium(II) carboxylate with the locally electron-rich ylide yields 15. Loss of nitrogen would then give the highly electrophilic intermediate 16. In nondonating solvents, the richest source of electron density available to this reactive species is the remote C—H bond. Complexation with the electron density in this bond gives 17, which collapses to the cyclopentanone product. 

Nucleophilic attack at nitrogen is rare in these systems. However, the inorganic trithiazyl trichloride (87) acts as an apparent source of electrophilic nitrogen on reaction with certain organic substrates. Reaction with electron-rich alkenes such as stilbene gives 3,4-diphenyl-1,2,5-thiadiazole (77JCS(Pl)916>. 

A 70% solution of anhydrous hydrogen fluoride in pyridine (a rather weak nitrogen base). Weak acids require a strong acid catalyst to initiate electrophilic attack. cln aqueous solution serves as source of HOX, X = Br or Cl. 

The direct a-amination of aldehydes by azodicarboxylates as the electrophilic nitrogen source can be catalyzed by, for example i-proline 3a, to give the a-hydrazino aldehydes 4 having (R -configuration in moderate to good yields and with excellent enantioselectivities (89-97% ee) (Scheme 2.27) [4]. The optically active a-hydrazino aldehydes 4 are prone to racemization, and it was found beneficial to reduce them directly with NaBFU to stereochemical stable compounds which, by treatment with NaOH, can cyclize to form the N-amino oxazolidinones 5 in a one-pot process. The N-amino group in 5 could be cleaved with Zn/acetone to give the oxazolidinone 6 (Scheme 2.27). 

Azodicarboxylates are efficient sources of positive nitrogen used in the preparation a-hydrazino and a-amino acids starting from chiral enolates. Di-rm-butyl 4a [3b] (DTBAD) and dibenzyl 4b [3c] (DBAD) azodicarboxylates are the most commonly used reagents for diastereoselective electrophilic animation (Scheme 11). Both compounds are available commercially. 

Why is 5 so much more stable than normal silylenes In part the stability probably results from electron-donation by the nitrogen atoms into the vacant p orbital on silicon, making the silicon distinctly less electrophilic. But another source of stability may be aromatic delocalization in the five-membered ring Silylene 5 has six n electrons two from each nitrogen atom, one from each carbon and zero from silicon (since the two nobonding electrons are in an in-plane, c-type orbital). 

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