Protonation amido nitrogen

When the migrating group is allyl, an additional concerted ([2,3] sigmatropic) pathway for rearrangement becomes available. In this an allylic shift must also occur. Nevertheless, the radical pathway is not always excluded. For example, rearrangement of ylids such as 36 (R = CHs.CO) leads to product 37 (R = CH3.CO) in which the allylic protons adjacent to the amido-nitrogen atom appear in emission (D. G. Morris, 1969). No polarization is observed in the much readier... 

The k pathway is three times faster in D+/D20 than in H+/H20 for la. The reverse kinetic isotope effect suggests that the rate-limiting event for the k pathway could involve protonation of an amido-nitrogen or an N-Fe bond, forming the stronger N-H bond as the weaker N-Fe bond is cleaved. The k 3 pathway is rationalized as involving pre-equilibrium peripheral protonations of the TAML macrocycle (Scheme 1). The dependence of obs on [H + ] is then given by Eq. (4), which corresponds... 

Using these values, there are several ways to compare the acidity of the protons in the boxes. The of the amide protons in acetamide is 25.5. The pA a of the amide proton in this compound should be somewhat greater, because the second amido nitrogen will reduce the resonance stabilization of the anion formed when the amide proton is removed. The precedent for the other boxed protons is the of the methylene protons of methyl cyanoacetate, 12.8. Because the carbonyl group in the given compound is an amide rather than an ester, the of its protons will be somewhat higher, but not nearly as high as the value for the amide proton. 

With ruthenium instead of iron, the 1,3-dipolar reactivity markedly increased. The same reaction pathway is followed, but the activation barriers further along the reaction coordinate are different. The reaction of 13 with one equivalent of dmad (Scheme 2f proceeds instantly at — 78 °C and, in contrast to the iron system, the bicyclo[2.2.1] adduct 14 is stable at that temperature. After protonation of the amido nitrogen with HBF4, which inhibits CO-insertion, 16 is stable at room temperature, and its X-ray structure could be determined. On warming the solution of 14 to room temperature in the presence of CO or PPh3, CO-insertion occurs, as in the iron system, to give 15. 

Complexes 25 also cycloadd the C=S double bonds in heteroallenes CS2, COS, " and aryl isothiocyanates. The reaction of 25a with CS2 (Scheme 6) gave an 80% yield of the expected bicyclo[2.2.2] complex 30, the structure of which has been established by X-ray crystallography. When the reaction was performed in the presence of water or HBF4, the amido nitrogen bridge in the initial bicyclo[2.2.1] adduct was protonated, which inhibited the insertion of isocyanide, and 31 was isolated in 80% yield. 

Perhaps the most elusive species is Mo=NNHh the presumed immediate precursor to Mo=N and ammonia upon addition of an electron. There is no experimental evidence for Mo=NNH. Calculations suggest that if Mo=NNHJ is formed, addition of an electron would lead spontaneously to MosN and NH3 [59]. Reiher considers a number of other possibiUties, among them protonation of an amido nitrogen followed by addition of an electron. 

Although the reaction mechanism has been studied via an isotopic labeling experiment, the proposed mechanism is still not clear. Thus a revised and tentative mechanism is displayed here. In this mechanism, the initial addition of an amido nitrogen to the iminium ion eliminates the charge, but the protonation of nitrogen increases the ring strain, and the three-membered ring opens. 

Why does sulfuric acid protonate the nitrogen atom of the diethylamine group of lidocaine preferentially to that of the amido group, as shown by formation of 42 ... 

Synthesis of the P2N2 macrocycle H2LI23I was performed on a nickel(II) matrix by the multistep route shown in Scheme 5-5. The ring closure was achieved by the reaction of l,3-bis(toluene-p-sulphonyloxy)propane, TsO(CH2) OTs, with [Ni(L1230)]. The mechanism involves sequential nucleophiUc attack on the toluene-p-sulphonate substituted carbon atoms of the propane by the lone pair of each of the coordinated amido nitrogen atoms. In the last step the coordination kinetic template effect should be operative. Protons liberated in the course of the reaction are removed by the excess of base. 

Ostensibly minor variations of a synthetic procedure sometimes can have significant consequences. For example, substituting KOCMe(CF3)2 for LiOC-Me(CF3)2 is believed [85] to lead to formation of the alkylidyne complex shown in Eq. 16 instead of the known [83] Mo(CH-f-Bu)(NAd)[OCMe(CF3)2]2 (Ad=ad-amantyl). A proton is likely to be transferred before formation of the final product, since it has been known for some time that both W(CH-f-Bu)(NAr)[OC-Me(CF3)2]2 and W(C-f-Bu)(NHAr)[OCMe(CF3)2]2 are stable species that cannot be interconverted in the presence of triethylamine [41]. In such circumstances the nucleophilicity of the alkoxide ion and the nucleophilicity and acidity of the alcohol formed upon deprotonation of the alkylidene will be crucial determinants of whether the imido nitrogen atom is protonated at some stage during the reaction. At this stage few details are known about side reactions in which amido alkylidyne complexes are formed. 

The imido complex [Mo2(cp)2(/r-SMe)3 (/u.-NFl)]" " 25+ undergoes an irreversible one-electron (EC) reduction [70]. Controlled potential electrolysis afforded the amido analog [Mo2(cp)2(/x-SMe)3(/x-NH2)] 26 almost quantitatively after the transfer of IF mol 25+. The amido complex was not the primary reduction product the latter was assigned as a rearranged imide radical (Sch. 18), which is able to abstract a FI-atom from the environment (supporting electrolyte, solvent, or adventitious water) on the electrolysis timescale. In the presence of protons, the reduction of 25+ became a two-electron (ECE) process. This is consistent with the protonation at the nitrogen lone pair of the primary reduction product, followed by reduction of the resulting amido cation... 

The concept of preassembly as a requirement for substitution may throw light upon the vexed question of the mechanism of the base hydrolysis reaction. It has long been known that complexes of the type, [Co en2 A X]+n can react rapidly with hydroxide in aqueous solution. The kinetic form is cleanly second-order even at high hydroxide concentrations, provided that the ionic strength is held constant. Hydroxide is unique in this respect for these complexes. Two mechanisms have been suggested. The first is a bimolecular process the second is a base-catalyzed dissociative solvolysis in which the base removes a proton from the nitrogen in preequilibrium to form a dissociatively labile amido species (5, 19, 30). 

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