Br0nsted-type plot

FIGURE 1. Br0nsted-type plot for reactions of p-nitrophenyl acetate (PNPA) with anionic oxygen nucleophiles. The a-nucleophiles are shown as solid circles. Data taken from Jencks and Gilchrist, J. Am. Chem. Soc., 90, 2622 and reprinted with permission. Copyright (1968) American Chemical Society... 

IV. LEVELING EFFECT OF a-NUCLEOPHILICITY BR0NSTED-TYPE PLOTS... 

It was later shown that one could reformat the log k vs mol% DMSO plot as a log k vs pX a plot , i.e. a Br0nsted-type plot. This transformation was effected through the effect of DMSO composition on pTsTa values, i.e. a novel Br0nsted-type plot , as illustrated in Figure 6 for the reaction of the Ox /T-ClCeHtO" pair with 4-nitrophenyl diphenylphos-phinate in the DMSO-H2O system. 

A different type of solvent effect on reactivity was observed by Terrier and coworkers for the reaction of oximates with the bis(4-nitrophenyl) phenylphosphonate (BPNPPP) . As shown in Figure 7, the Br0nsted-type plot is bell-shaped, i.e. oximates with pTsTa > 9 exhibiting lower reactivity than more weakly basic oximates . ... 

FIGURE 6. Novel Br0nsted-type plots log k vs pA"a for the reaction of butane-2,3-dione monoxi-mate (Qx , ) and 4-chlorophenoxide (4-ClC5H40 , O) with 4-nitrophenyl diphenylphosphinate (PNPDPP) at 25.0 °C. Reprinted with permission from Tarkka and Buncel, J. Am. Chem. Soc., 117, 1503. Copyright (1995) American Chemical Society... 

FIGURE 7. Br0nsted-type plot for the reaction of the oximates with the bis(4-nitrophenyl) phenyl-phosphonate at 25 °C in aqueous solution. From Terrier et al, Chem. Commun., 600 (2003). Reproduced by permission of The Royal Society of Chemistry... 

Aminolysis of a series of aryl 2,4-dinitrophenyl carbonates by a series of quinuclidines gave linear Br0nsted-type plots, the magnitudes of their slopes confirming their mechanisms as concerted.40 A comparison41 of the aminolysis, by primary amines, of 4-nitrophenyl phenyl carbonate (31 X = O) with its thiono analogue (31 X = S) is discussed in the section Thioacids, Thioesters, Thiolactones, and Thiocarbonates below. 

The rates of aminolysis of Y-phenyl diphenylphosphinates (39) (Scheme 14) by a series of alicyclic secondary amines were determined in 20 mol% DMS0-H20 at 25 °C. The phosphinates were less reactive than the corresponding Y-phenyl benzoates (the rates of which had been reported earlier). The reactions of 2,4-dinitrophenyl diphenylphosphinate [39 Y = 2,4-(N02)2l with alicyclic secondary amines resulted in a linear Br0nsted-type plot, whereas the corresponding reactions of 2,4-dinitrophenyl... 

A kinetic study of the aminolysis of / -cresyl / -nitrophenyl carbonate by B11NH2 in C6H6 at 27 °C in the presence of pyridine, Et3N, or imidazole has shown that each of the three bases are very effective catalysts.38 A concerted mechanism of aminolysis of di(4-nitrophenyl) carbonate (37 X = N02) by anilines at 25 °C in 44% EtOH-water was indicated by a linear Br0nsted-type plot showing ft = 0.65. However, the aminolysis of two related compounds, 4-methylphenyl (37 X = Me) and 4-chlorophenyl 4-nitrophenyl carbonate (37 X = Cl), with = 0.85 and 0.78, respectively, proceeded by a stepwise mechanism.39... 

Curvature in a Br0nsted-type plot is sometimes attributed to a change in transition state structure. This is not a change in mechanism rather it is interpreted as a shift in extent of bond cleavage and bond formation within the same mechanistic pattern. Thus, Ba-Saif et al. found curvature in the Br nsted-type plot for the identity reactions in acetyl transfer between substituted phenolates this reaction was shown earlier. They concluded that a change in transition state structure occurs in the series. Jencks et al." caution against this type of conclusion solely on the evidence of curvature, because of the other possible causes. 

A qualitative difference in the type of solvation (not simply in the strength of solvation) in a series of nucleophiles may contribute to curvature. Jencks has examined this possibility. " -" An example is the reaction of phenoxide, alkoxide, and hydroxide ions with p-nitrophenyl thiolacetate, the Br0nsted-type plot showing Pnuc = 0-68 for phenoxide ions (the weaker nucleophiles) and (3nuc = 0.17 for alkoxide ions. It is suggested that the need for desolvation of the alkoxide ions prior to nucleophilic attack results in their decreased nucleophilicity relative to the phenoxide ions, which do not require this desolvation step. 

Discuss possible reasons for the curvature in the Br0nsted-type plot of Fig. 7.5 for the nucleophilic reactions of oxygen nucleophiles. 

The reaction mechanism may change in the reaction series, and this should reveal itself in the Br0nsted-type plot as a change in slope. As noted above, most acyl transfers seem to take place via the two-step process shown in Scheme II. A direct displacement is also possible, as in Scheme III. 

Figure 7-5. Br0nsted-type plot for nucleophilic reactions of p-nitrophenyl acetate. Key , simple imidazoles in 28.5% ethanol at 30°C. P = 0.80 (data from Ref. 197] O, oxygen anions, in water at 25 , P = 0.95 for linear portion [data from Ref. 119, 198] , a effect nucleophiles. Several of the nucleophiles are identified.

If an atom containing one or more unshared pairs is adjacent to the attacking atom on the nucleophile, the nucleophilicity is enhanced. Examples of such nucleophiles are HO2, Me2C=NO, NH2NH2, and so on. This is called the alpha effect (a-effect and a broader definition is a positive deviation exhibited by an a-nucleophile from a Brpnsted type nucleophilicity plot, where the reference (or normal) nucleophile is one that possesses the same basicity as the a-nucleophile, but does not deviate from the Br0nsted-type plot. Several reviews of the a-effect have been published previously, ... 

The answer to this question only became apparent when it was observed that the reactions of substituted quinuclidines with 2,4-dinitrophenyl phosphate, p-nitrophenyl phosphate and phosphorylated pyridine show a decrease in rate with increasing basicity of the attacking quinuclidine (77). These second-order reactions clearly cannot have a negative amount of bond formation with the nucleophile in the transition state, so that this result forced us to think harder about what could cause zero or negative slopes in Br0nsted-type plots against the pK of the nucleophile. 

Figure 1. Br0nsted-type plot of log k against the pK of oxygen nucleophiles for reactions with phosphorylated y-picoline monoanion. The solid line has a slope of = 0.13 and the dashed line has a slope of 0.25 UP).

Figure 1. Br0nsted-type plots for the reactions of substituted quinuclidines with the dianions of 2,4-dinitrophenyl phosphate complexed with calcium (upper line), 2,4-dinitrophenyl phosphate (middle line), and p-nitrophenyl phosphate (lower line) at 39 °C, ionic strength 1.0 (KCl) ( ) and ionic strength 1.5 (KCl) (o) (Reproduced from reference 12. Copyright 1986 American Chemical Society.)...

The Br0nsted-type plot of the reaction between a series of 18 Y-phenyl benzoates (6) (Scheme 3) with CN in H2O/DMSO (4 1) at 298 K was linear with jSjg = -0.49. A Yukawa-Tsuno plot was also linear with Py = 137 and r=0.34 and these data pointed to a concerted mechanism in the formation of benzoyl cyanide (8). ... 

The reaction of a series of anilines with methyl 2,4,6-trinitrophenyl carbonate in water proceeded via a concerted mechanism as revealed by a linear Br0nsted-type plot with slope of 0.7. A Yukawa-Tsuno plot was linear for the alkaline hydrolysis of a series of Y-phenyl phenyl carbonates with p = 1.21 and r = 0.33, consistent with a concerted mechanism. 

The Br0nsted-type plot for the aminolysis of 2,4-dinitrophenyl phenyl carbonate by a series of pyridines in H2O/DMSO (4 1) at 298 K exhibited a downward curvature ()32 = 0.84, /Sj = 0.16) centred at pK 8.5, typical for reactions proceeding through a stepwise mechanism with a change in the rate-determining step from breakdown to formation of the tetrahedral zwitterionic intermediate. ... 

The Br0nsted-type plots for the pyridinolysis, aminolysis (R2NH), and phenolysis in 44% EtOH/water at 298 K of the insecticide, diethyl 4-nitrophenyl phosphate (paraoxon), were linear with slopes, respectively, of jS = 0.43, 0.39, and 0.21. No... 

Kinetic studies of Michael addition of alicyclic secondary amines to ethyl propiolate in H2O and MeCN have demonstrated a substantial solvent effect on reactivity and transition-state structure. The amines were found to be less reactive in MeCN, although they are by 7-9 units more basic in the aprotic solvent. The reaction rates for morpholine and deuterated morpholine proved to be identical, which rules out both a stepwise mechanism in which proton transfer would occur in the RLS and a concerted mechanism in which nucleophilic attack and proton transfer would occur through a four-membered cyclic transition state. Consequently, a stepwise mechanism with proton transfer occurring after the RLS is probable. Br0nsted-type plots were found to be linear with = 0.29 and 0.51 in H2O and MeCN, respectively, indicating that bond formation is not advanced significantly in the RLS. The small value is also consistent with the absence of isotope effect. ... 

FIGURE 3.3 Br0nsted-type plot of log versus p for the reaction of nucleophiles with methyl p-nitrophenyl sulfate. The a-nucleophiles are shown as solid circles. Source Buncel et al. (1980) by permission of the American Chemical Society. 

The kinetics of the aminolysis of 4-nitrophenyl acetate by piperidine were studied in nine ILs and compared to data in nine organic solvents. The rates in the ILs were about equivalent to those in MeCN and THF, and for [Bmim]BF4, the rate was only sevenfold less than that in DMSO (dimethyl sulfoxide). A Br0nsted-type plot for the aminolysis of [Bmim]BF4 by four secondary amines (piperidine, morpholine, formylpiperazine, and l-(2-hydroxyethyl)piperazine), which could only be generated after determination of their pKa values (which was accomplished by cyclic voltammetry at a P-Pt electrode), was linear (P = 0.77) and revealed that the mechanism was stepwise, different from the concerted mechanism found in conventional solvents. ... 

Attack occurred at the C( 1) aromatic carbon as well as the P=0 group for reactions of diethyl 2,4-dinitrophenyl phosphate with phenoxides, secondary alicyclic amines, and pyridines. The absence of breaks in the Br0nsted-type plots for attack at P=0 pointed to a concerted mechanism, but values in the range 0.32-0.71 for attack at the aromatic C(l) were consistent with stepwise mechanisms and formation of Meisenheimer complexes." ... 

The Br0nsted-type plot for the alkaline hydrolysis in water at 298 K of a series of Y-phenyl diphenylphosphinates Ph2P(=0)0C6H4Y was linear with fii = -036. 

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