Methyl iodide relative nucleophilicity toward

The [Rh(CO)2I2] ion is clearly an important species in systems derived from several different catalyst precursors fortuitously, it is a relatively nucleophilic rhodium species. Thus it reacts with methyl iodide at room temperature, whereas the related uncharged species, [Rh(CO)2Cl]2, is unreactive toward methyl iodide at low temperatures. This difference between neutral and charged species is also evidenced markedly in the relative reactivities of [RhL2(CO)X] and [RhL(CO)X2] toward methyl iodide, where a difference of five orders of magnitude has been observed (19). 

Another key feature of the metal complex cycle in which the iodide acts most effectively is the nature of the active catalyst itself. The oxidative addition step is considered to be nucleophilic in nature, based on activation parameters and relative rate data (23, 24a) (Section II,C), and the presence of a negative charge on the metal center appears to significantly enhance the nucleophilicity (and hence reactivity toward methyl iodide) of the metal relative to neutral rhodium(I) species (20). Extrapolations of available data (24-26) indicate that, at 25°C, the diiododicarbonylrhodium(I) species has a Pearson nucleophilicity parameter (25) toward methyl iodide of 5.5. In relation to other common nucleophiles, this value corresponds to nucleophilic reactivity toward methyl iodide comparable to that of pyridine (n = 5.2), an order of magnitude greater than chloride (n = 4.4), and two orders of magnitude slower than iodide (n = 7.4). 

We can restate the HSAB principle. Hard nucleophiles favor binding with hard electrophiles soft nucleophiles favor binding with soft electrophiles. Most nucleophilicity charts show relative rates of nucleophilic attack with methyl iodide as the electrophile. A carbon-iodine bond is very soft because the electronegativity difference is nearly zero (Section 1.2). Therefore softer (less electronegative, more polarizable) atoms have lone pairs that are better electron sources toward soft electrophiles. 

Kevill and Lin s (27) earlier study of the ethanolysis of the triethyloxonium ion, at 0.0 °C, has been extended to a consideration of the competition between the solvent and added nucleophile, either anionic or neutral, for reaction with the substrate (equation 11). This study is related to earlier studies, at 25.0 °C, of competition between water and added nucleophile for reaction with methyl bromide (36), methyl iodide (37), or the cyclic pen-tamethyleneiodonium ion (38) and of competition between methanol and added nucleophile for reaction with methyl iodide (39) or trans-Pt(py)2Cl2 (39). The nucleophilicities are usually expressed, relative to the solvent, in terms of the Swain-Scott equation (36) (equation 12). In equation 12, k and k0 are second-order rate coefficients for reaction of a substrate with the added nucleophile and with the solvent, n is a measure of the nucleophilicity of the added nucleophile, and s is a measure of the sensitivity of the substrate toward changes in nucleophilicity. The value of s is taken as unity for the standard substrate. 

These observations reported in the mid-30 s on the relative reactivities of the C-4 and C-6 tosylates toward a nucleophile such as sodium iodide, lay dormant until 1963, when it was found (60) that treatment of methyl 2,3-di-0-benzoyl-4,6-di-0-methylsulfonyl-a-D-glucopyranoside (14) with potassium thiocyanate at 130°C. for 48 hours afforded a 40% yield of the corresponding 4,6-dithiocyanato derivative 15. 

Fig. 26. Correlation of relative proton affinities of pyridines (APA) with their solution nucleophilicities (N) toward methyl fluorosulfonate in 2-nitropropane at 25°C (squares) and ethyl iodide in nitrobenzene at 60°C (circles) (8IJOC635).

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