Positive adduct formation

The position of the proton transfer equilibrium for the Schiff bases being derivatives of rac-2-aminobutane [24] or rac-a-methylbenzylamine [25] and their adducts with dirhodium complex has been estimated in CDCI3 solution on the basis of measurements of deuterium isotope effects on 15N chemical shift.12 It was shown that adduct formation significantly influenced the position of the equilibrium which was manifested by AN(D) values. 

Sulfite is an extremely good nucleophile for activated aromatic systems and reaction with l-substituted-2,4,6-trinitrobenzenes (1) may result in cr-adduct formation or in displacement of the 1-substiment as shown in Scheme 1. When X = OEt or SEt, adducts (2) and (3) formed by reaction at unsubstituted positions are long-lived. 

A kinetic smdy of the formation of zwitterionic adducts (28) from 1,3,5-trinitrobenzene and diazabicyclo derivatives indicates that reactions are surprisingly slow, with rate constants many orders of magnitude lower than those for related reactions with primary or secondary amines. The use of rapid-scan spectrophotometry was necessary to study the kinetics of reaction of 4-substimted-2,6-dinitro-A -n-butylanilines (29) with n-butylamine in DMSO the two processes observed were identified as rapid deprotonation to give the conjugate base and competitive a-adduct formation at the 3-position. The reactions of MAf-di-n-propyl-2,6-dinitro-4-trifluoromethylaniline (30), the herbicide trifluralin, and its A -ethyl-A -n-butyl analogue with deuteroxide ions and with sulfite ions in [ H6]DMS0-D20 have been investigated by H NMR spectroscopy. With deuteroxide a-adduct formation at the 3-position is followed by... 

However, in view of the results mentioned earlier, direct attack of the amide ion on position 2 seems highly unlikely. An Sn(ANRORC) mechanism, starting with an attack of the amide ion at position 6 containing the amino group, seems to be involved. Adduct formation at a position already occupied by an amino group is not unprecedented. The conversion of 4-amino-2-bromoquinoline into 4-amino-2-methylquinazoline (72RTC841) and of 4-amino-2-bromo-l,5-naphthyridine into 4-amino-2-methyl-l,3,5-... 

Investigation of the deethylation with N-labeled ammonia showed that in 18 the label is incorporated for about 100% in the pyrimidine ring. Evidently the Sn(ANRORC) mechanism is operative in the replacement of the quaternary ring nitrogen by the amino nitrogen. The initial step in this deethylation reaction will probably be adduct formation on C-6, i.e., 17. Addition at C-2 is less likely because of the presence of the phenyl group at that position (Scheme III.12). 

Norbornadiene is also known to add electrophilic or free-radical reagents across the 2- and 6-positions with formation of nortricyclene derivatives (85). Neither type of adduct, 84 nor 85, has yet been obtained from 7-azabicyclo[2.2.1]heptadiene derivatives, but aromatiza-tion of the latter is induced by some electrophilic reagents (see Section II,E). 

One very important use of E and G numbers is the calculation of heats of interaction for systems which have not been examined experimentally. We are now in a position to predict almost 1000 enthalpies of adduct formation by combining the parameters in Tables 3 and 4 according to Eq. (13). The trimethylaluminium enthalpies are for the monomer... 

The addition of benzyne to Cgg leads to the formation of [2+2] qfcloadducts [330-332]. The in situ generation of benzyne can be achieved by diazotization of anthraniKc acid at room temperature with isoamyl nitrite, by diazotization of the potassium salt of anfhranilic acid in the presence of 18-crown-6, or by the oxidation of 1-aminobenzotriazole with lead tetraacetate (Scheme 4.54). Adduct formation occurs in the 1,2-position, leading to the corresponding ring-closed structure. 

Theoretical calculations support the expectation that the preferred site of initial OH attack is ortho to the methyl group (Andino et al., 1996), but addition to the other positions also occurs. If the OH-aromatic adduct, which contains 18 kcal mol-1 excess energy, is not stabilized, it decomposes back to reactants, reaction ( — 62). The existence of the adduct in the case of the OH-benzene reaction has been observed spectroscopically (Fritz et al., 1985 Knispel et al., 1990 Markert and Pagsberg, 1993 Bjergbakke et al., 1996). As expected for such a mechanism, the rate constants at temperatures below 300 K exhibit a pressure dependence at lower pressures. At higher temperatures, the rate of decomposition of the excited adduct back to reactants is higher, so the net contribution of adduct formation to the overall reaction is small compared to H-abstraction. 

Substituting the 2- and 5-positions of hexane with methyl groups prevents dione production, adduct formation, and subsequent neurotoxicity [13]/... 

The equilibrium and kinetic patterns change when the steric environment at the CH reaction center consists of two flanking nitro groups for both the heterocyclic and homocyclic adducts. In such a case the difference in the substrate tendency toward adduct formation is dominated by the different electronic effect of an aza group and of a nitro group at the para position (see Section II,B,5,a). 

Comparison of Rate Constants and of Equilibrium Constants for Adduct Formation on Attack of MeO" Ion in MeOH at CH and COMe Positions as Expressed by k /kOMc and KH/KOMe Ratios. Respectively"... 

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