Substitutions of carbonyls

Substitution mechanisms of the carbonyls of organometallic complexes have been most extensively investigated. These complexes are easy to pre-paie, are usually very stable and can be well characterized by IR spectra. 

Let us consider the [NifCO) complex. Nickel can accept 4 electron pairs from donor ligands. Thus, its valence shell acquires a total of 18 electrons, i.e., the noble gas configuration the coordination number Z = 4. Under such conditions, nickel has no adequate orbitals for accepting an additional electron pair, required for an associative mechanism. Indeed, mechanistic studies have shown that the substitution mechanism of carbonyls is of the dissociative type, as shown by the following reaction scheme  

The electron in the n orbital is relatively easily released to give nitrosonium ion (NO ). Since the electron is supplied from an antibonding orbital, the NO ion has a bond distance of 106 pm, which is shorter than the bond distance in the NO molecule (114 pm). 

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The facile nitration of a wide variety of ketones with TNM in Table 2 is illustrative of the synthetic utility of enol silyl ethers in facilitating a-substitution of carbonyl derivatives. It is necessary to emphasize here that the development of a strong charge-transfer (orange to red) coloration immediately upon the mixing of various ESEs with TNM invariably precedes the actual production of a-nitroketones in the thermal nitration (in the dark). The increasing conversion based on the time/yields listed in Table 2 qualitatively follows a trend in which electron-rich ESE from 6-methoxy-a-tetralone reacts faster than the relatively electron-poor ESE from cyclohexanone. 

Another useful reducing agent is sodium tetrahydroborate, NaBH4, although in this case reduction can be complicated by progressive substitution of carbonyl groups with hydrogen atoms l47 ... 

Oxidative addition to complex 1 is the slowest and rate-determining step in the reaction scheme and also it is a singular step, involving the conversion of the catalyst resting state to a more reactive 2. An obvious way to obtain a faster catalyst is the substitution of carbonyl ligands in 1 by electron-donating phosphines, as organometallic chemistry tells us this variation never fails. Indeed, several variants that are indeed fester are known [11], but none of them has found application. 

Rhenium(0) compounds are rare and frequently lie in the realm of the organometallic chemistry. A simple example is decacarbonyldirhenium(0) in which two staggered, square-pyramidal Re(CO)5 fragments are held together by a single rhenium-rhenium bond. Substitution of carbonyl ligands is possible by tertiary phosphines and arsines, silanes and isocyanides, and binuclear Re-Re, Mn-Re, and Co-Re complexes have been studied. " Successive replacement of CO ligands can readily be observed by vibrational spectroscopy. This has been demonstrated... 

Condensations with carbonyl compounds phenol-formaldehyde resins. Acid or base catalyzes electrophilic substitution of carbonyl compounds in ortho and para positions of phenols to form phenol alcohols (Lederer-Manasse reaction). 

SCHEME 56. Two complementary protocols for Pd- or Ni-catalyzed -substitution of carbonyl compounds... 

Contrary to early reports,16 pentacarbonylhydridorhenium is air stable.6 The neat liquid, however, is moderately sensitive to light, turning yellow with formation of Re3H(CO)14.9 Pentacarbonylhydridorhenium is stable up to 100 °C, whereupon it decomposes to form Re2(CO)10 and H2.1 Substitution of carbonyl groups in ReH(CO)5 by tertiary phosphines and other Lewis bases is well known and apparently occurs by a radical chain pathway.6 More detailed surveys of the reactivity of ReH(CO)5 have been summarized elsewhere.10,11... 

Naturally, the ideal source of starting materials for homoleptic metal isocyanide compounds is via metal carbonyl complexes, but previously only with the two carbonyls Ni(CO)4 (24) and Co2(CO)g (25) has direct substitution of all carbonyl groups been effected. Recently, however, remarkable discoveries by Coville and co-workers (26-31) on the transition-metal-catalyzed substitution of carbonyl groups in monomeric and cluster compounds have shown that Fe(CNR)s, Mo(CNR)6, and Ir4(CO)5(CNR)7 (32) can be prepared in high yield by stepwise substitution from the parent carbonyl. 

There are few other examples of complete substitution of carbonyl groups from a homoleptic metal-carbonyl complex by isocyanide ligands [cf. Ni(CO)4 (24), Fe(CO)s (26), and Mo(CO)6 (124)]. The corresponding butyl isocyanide derivative Co CNBuOg was formed by reduction of [Co(CNBu )5]PF6 with potassium amalgam (19). 

Rhodium(I) or polymer supported rhodium(I) compounds catalyzed the formation ofFefCO - CNR L (x = 1 - 3 R = Bu, xylyl L = MA, citra-conic anhydride, acrylamide) (29, 30), and the dimer [CpFe(CO)2]2 catalyzed the stepwise substitution of carbonyl groups in CpFeI(CO)2 to give CpFeKCO - CNR) (x = 1,2 R = Bu, xylyl) in 60-80% yields. A nonchain free-radical mechanism was proposed for the latter reaction (28). The compounds CpFeX(CO)2 x(CNR)x (x = 1,2 X = halide, SiMe3) are known for a range of alkyl and aryl isocyanides (169-171). 

Fig. 4 Left Fluorescence, MALLS (90°) and RI signals from a CCOA-labeled cellulose sample. Right Differential molecular weight distribution and degree of substitution of carbonyl groups (DSco) as calculated from the detector outputs...

Photochemical activation of transition metal carbonyls has been used as a preparative tool for substitution of carbonyl ligands by donor molecules or unsaturated hydrocarbons for many years (7-6). The advantage of photochemical activation in comparison with thermal activation is the possibility of conducting reactions at fairly low temperatures. Hence even thermolabile products can be prepared and isolated by appropriate treatment of the reaction mixtures. However, due to the various activation modes of transition metal carbonyls by UV light, often more than one product is obtained, and chromatographic separation is necessary. Limitations are set primarily by the amount of substance which can be irradiated in solution at one time. 

CHAPTER 22 Condensations and Alpha Substitutions of Carbonyl Compounds... 

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