Electrophilic addition reactions carbon dioxide

The synthetic usefulness of reactions of lithiated methoxyallene 42 with suitable electrophiles was demonstrated by several syntheses of bioactive natural products or substructures thereof [52-58]. An interesting application was described by Fall et al. [52] after addition of alkyl iodide 55 to lithiated methoxyallene 42, deprotonation by tert-butyllithium and addition of carbon dioxide occurred at the terminal y-carbon and thus provided butenolide 57 after acidic workup. Desilylation of this intermediate with TBAF finally gave bicyclic oxepane derivative 58 in good overall yield (Scheme 8.14). 

This reaction illustrates a stereoselective preparation of (Z)-vinylic cuprates, 5 which are very useful synthetic Intermediates. They react with a variety of electrophiles such as carbon dioxide,5,6 epoxides,5,6 aldehydes,6 allylic halides,7 alkyl halides,7 and acetylenic halides 7 they undergo conjugate addition to a,6-unsaturated esters,5 6 ketones,6 aldehydes,6 and sulfones.8 Finally they add smoothly to activated triple bonds6 such as HCSC-OEt, HC3C-SEt, HC=C-CH(0Et)2. In most cases these cuprates transfer both alkenyl groups. The uses and applications of the carbocupration reaction have been reviewed recently.9 The configurational purity in the final product 1s at least 99.951 Z in the above transformations. 

In general a phenol will undergo direct carboxylation of the nucleus when the dry sodium salt is heated under pressure with carbon dioxide (the Kolbe-Schmidt reaction). Addition of the weakly electrophilic carbon dioxide is promoted by electron release from the oxyanionic site. With phenol itself the ultimate product is salicylic acid (o-hydroxybenzoic acid) predominantly ortho attack may be attributable to stabilisation of the transition state through chelation. 

Because organometallic reagents are strong nucleophiles, they react with many other electrophiles in addition to carbonyl groups. Because these reactions always lead to the formation of new carbon-carbon bonds, they are also valuable in organic synthesis. In Section 20.14, we examine the reactions of organometallic reagents with carbon dioxide and epoxides. 

Carbon disulfide is an important industrial solvent for the extraction of oils and waxes. In organic chemistry, it is widely used as a solvent for Friedel-Crafts reactions. Carbon disulfide functions as an electrophilic reagent. It is more susceptible than carbon dioxide to nucleophilic attack as the energy required to convert C=S to C-S (188 kJmol 1) is much less than required for the analogous conversion of C=0 to C-0 (305 kJmol 1). Carbon disulfide (1) thus undergoes nucleophilic additions with alcohols and phenols to yield the corresponding xanthates (2) (the xanthate reaction) (Scheme 2)... 

One final report of alkane activation has been reported by Moiseev. The mechanism of the reaction was not investigated, but this system might be classified as an electrophilic activation of methane, either of the Shilov type or of the concerted four-center type (Fig. lc) where X=triflate. Reaction of methane with cobalt(III)triflate in triflic acid solution leads to the formation of methyltriflate in nearly stoichiometric quantities (90% based on Co) (Eq. 18). Carbon dioxide was also observed, but not quantified. Addition of 02 led to catalysis (four turnovers) [79]. 

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 ... 

---