Carbon dioxide electrophilic insertion reactions

There has been a growing interest in the utilization of CO2 as a potential Cl source for chemicals and fuels to cope with the predictable oil shortage in the near future. Insertion reactions of CO2 into M-H, M-0, M-N, and M-C bonds are well documented, where these reactions are explained in terms of the electrophilicity of CO2 il, 2). Catalytic syntheses of lactones (3-9) and pyrones (10-16) are also established by incorporation of CO2 into dienes and alkynes activated on low-valent metal complexes. Carbon dioxide shows only an electrophilicity under usual reaction conditions, but it exhibits a nucleophilicity upon coordination to low-valent metals because of the intramolecular charge transfer from metals to CO2. Metal-C02 formation may be the key species in electro- and photochemical CO2 reductions. Since the first characterization of [Ni(PCy3)2(T) (C,0)-C02)] (17), a variety of metal... 

Although the reaction requires no external catalyst, carbon dioxide is activated by the interaction of its electrophilic carbon atom and the negatively polarized carbon in the ortho (or para) position of the phenolate ring. This mechanism is supported by the fact that even under high CO2 pressure no salicylic acid is formed from phenol. The product is stabilized via an a > y proton migration. The free acid is obtained from the sodium salt in reaction with an external proton (usually from sulfuric acid). Formally, this reaction can be regarded as the insertion of CO2 into an aromatic C-H bond however, the above mechanism disproves this idea. 

Several types of anhydrides of tervalent phosphorus acids are known and have been prepared by electrophilic substitution reactions at phosphorus. Examples are the phosphi-nous acid anhydrides 52 (equation 157), 53 (equation 158), prepared from a chlorophos-phine or an aminophosphine , and 54 (equation 159) Aminophosphines react with carbon disulphide to give ionic addition compounds at low temperatures, but dithiocar-bamate anhydrides (55) at room temperature (equation 160) Aminophosphines form analogous carbamate anhydrides with carbon dioxide, but isothiocyanates give ionic addition products, not insertion products ... 

The reactions of benzyne complexes of zirconium " also occur by electrophilic attack at an M-C bond. The isolated phosphine adduct of a zironocraie-benzyne complex reacts with ketones to imdergo insertion into one of the M-C bonds and with alcohol to make an aryl alkoxo complex, as shown in Equation 12.67. An electron-rich ruthenium-benzyne complex also reacts with electrophiles, such as borzaldehyde or carbon dioxide, to form products from insertion, as shown at the top of Equation 12.68. It also reacts with weak acids, such as aniline, to form products from formal protonation at the Ru-C bond, as shown at the bottom of Equation 12.68. - This reaction with aniline could occur by initial protonation at the metal, followed by C-H bond-forming reductive elimination, or by direct protonation of the M-C bond. Initial protonation of the metal center was proposed. 

Bis(trifluoromethyl)furan 87a did not react with bromine, however lithiation allowed one to insert Br-atom regioselectively at the C2 position [143], Many other electrophilic reagents can be involved into the reaction with lithiated furan derivatives, for example, carbon dioxide, aldehydes, silyl chlorides, various Michael acceptors, etc. [74, 84, 144], Treatment of vinylation products 226, 227 with alkyl-lithium afforded the corresponding alkynes 228, 229 [64, 143],... 

The reactivity of these group 4 metal complexes has been studied to some extent. Starting from complex [(6)MCl2], the reaction with nucleophiles such as alkylhthium led to the classical reactivity at M-Cl (Scheme 32) [89]. The reactions with strong electrophiles such as isocyanates, carbon dioxide, or carbodiimide did not show the expected insertion into the M=C bond, but rather a [2+2] cycloaddition. The basicity and nucleophilicity of the C center was proved by reactions with aromatic amines, phenols, aliphatic alcohols, or methyl iodide leading to the 1,2 addition product. 

Cleavage of the iron-carbon bond in reactions of sulphur dioxide with complexes of the type [FeR( j -C5H6)(CO)2] is an electrophilic process which proceeds with a large and negative entropy of activation and with inversion of configuration at the a-carbon. The inversion of stereochemistry observed on insertion of sulphur dioxide... 

The stereochemistry of iron-carbon bond cleavage by electrophilic attack by mercury(n) chloride or bromine, or by carbon monoxide or sulphur dioxide insertion, has been probed by n.m.r. studies of products derived from /Areo-[Mc3C CHD-CHD-Fe(CO)2(A -C5H5)]. Predominant retention occurs in the reaction with mercury(n) chloride, predominant inversion in the reactions with sulphur dioxide or bromine. It is this last result which is of the most interest, for it affects discussions of the stereochemical course of oxidative additions... 

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