Transition metal nucleophiles

In contrast to their hydrocarbon analogues, many unsaturated fluorocarbons react with anionic transition metal nucleophiles to yield products arising from net displacement of fluoride ion. This synthetic approach has been successful in producing a host of fluorovinyl and fluoroaryl complexes (95-110). 

Reactions of transition metal carbonyls with an In(I) center also take place by oxidative addition here InX inserts into the M-X bond. Oxidative addition to the low-valent halides of Ga, In, or T1 thus provides a useful route to compounds with group IIIB-metal bonds This approach is particularly useful when the salt elimination route cannot be employed because a suitable transition metal nucleophile is lacking. 

Transition metal complexes are often very good nucleophiles and qualify as being supersoft under Pear son s HSAB classification for example, reaction with soft methyl iodide can be as much as 3 X 105 times faster than the reaction with hard methyl tosylate. Because soft nucleophiles are those with large a values in the Edwards equation, that rates for the transition metal nucleophiles are effectively correlated with oxidation potentials is not surprising. In the last chapter in this section, Chapter 16, Pearson uses recently obtained values of pKa for transition metal complexes to test the full Edwards... 

Rates of reaction of transition metal nucleophiles correlate both with oxidation potentials for MLn and with the pKa values of the corresponding acids, HMLn. Therefore, the two parameters, E° and H, in the Edwards equation are not independent parameters. The same result is found for other nucleophiles, if the donor atom is C,N, O, or F. However, for bases with heavier donor atoms, E° and H are not as correlated with each other. For transition metal complexes, soft ligands, L, increase acidity and decrease nucleophilic reactivity. 

Characteristically, transition metal nucleophiles react much faster with methyl iodide than with methyl tosylate. Rate constant ratios ranging from 30 to 3 X 105 have been found (3). Such behavior qualifies transition metal complexes to be called supersoft nucleophiles (5). Even larger ratios are found for reagents such as Co(CN)53, up to 109. Such large ratios are found only for free-radical pathways (6) and may be used as a mechanistic probe. 

A pioneering study of transition metal nucleophiles was made by Dessy et al. (7). These workers measured not only rates of reaction of various MLn with CH3I but also the oxidation potentials at a platinum electrode. A good linear relation was found when log k2 was plotted against EV2 for various nucleophiles. 

Returning to transition metal nucleophiles, the experimental evidence already cited indicates that very likely pKa and E° are strongly correlated. 

Metathetical reactions are the common synthetic routes to phosphido derivatives of early transition metals nucleophilic substitution of halide on the metal by anionic phosphido species has been widely used in group 4 chemistry. ... 

In general, the most convincing evidence that the reaction of an alkyl halide or tosylate with a metal complex adopts a nucleophilic displacement (8 2) mechanism is the demonstration that inversion of the configuration of the saturated carbon centre has occurred as a consequence of this process. However, as will be seen, inversion of the configuration of the carbon atom is not an essential consequence in the reactions of coordinatively-unsatur-ated. transition-metal nucleophiles. 

An aspect of the metal s coordination environment and nucleophilicity is the influence that iodide ion can have on the reactions of certain transition metal nucleophiles with Mel. This is an aspect of particular relevance to the homogeneously catalysed carboxylation of methanol to acetic acid which employs a rhodium iodide-promoted catalyst (Forster, 1979 and references therein). 

The reactions of a,(o-dihalogenoalkanes with transition-metal nucleophiles proceed by a variety of different routes. In the simplest possible pathway only one end of the alkyl-group is substituted (39) M = Mo or W, X = Br... 

The reactions of [(ti -C5H5)M (CH2)X (CO)j] (M = Mo or W, X = Br or I) with transition metal nucleophiles is not a general method for making heterobimetallic carbene complexes, since the products can be homobimetal-lic species involving displacement of the originally alkylated metal centre as shown in Fig. 7. 

Probably the most important industrial application of a transition-metal nucleophile is the Monsanto process for carbonylating methanol, using soluble rhodium-carbonyl complexes in the presence of iodide. The catalyst is in fact [Rhl2(CO)2] (Forster 1979, and references therein), and the catalytic cycle is shown in Scheme 21. The substrate for the rhodium catalyst is methyl iodide, which oxidatively adds to yield [Rh(Me)(I)3(CO)2] . 

In this section, we have concentrated on the use of transition-metal nucleophilic-carbene catalysts to bring about ring-closing alkene metathesis reactions. These same types of compounds can also be used to catalyze a remarkable reaction called ringopening alkene metathesis polymerization (ROMP). A special value of ROMP is that it can be used to prepare highly unsaturated polymers. For a discussion of ROMP techniques, see Section 29.6E. 

In an attempt to see if the mechanism of oxidative addition can be correlated with relative reactivities, Pearson and Figdore have measured rate constants and activation parameters for the reactions of methyl iodide and methyl tosylate (MeOTs) with a number of transition metal nucleophiles, mci and mcots both vary enormously (see Table 8.10). It is curious that the A// and A5 for the... 

Table 8.10. Nucleophilic Reactivities at 25 C for Various Transition Metal Nucleophiles with Methyl Iodide and Methyl...

Carhon nucleophiles of pXa 10-20 have heen most studied and often work hest with these substrates (eq 1), hut enamines, cyclopentadiene anions, enolates (eq 2), organotin, -thaUium, -zinc, -aluminum, -lithium, and -zirconiums, as weU as Grignards (eq 3) and horates, also add to r-allylpaUadium complexes. Heteroatom nucleophiles also add successfully these include amines (eq 4), amides, azides, magnesium amides, sulfonamides, alcohols, acids (eq 5), nitrites, sulhnic acids, thiols, phosphines, and phosphites. Limited use of transition metal nucleophiles is also known. ... 

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