Ketenes insertion reactions

The [2 + 2] and 1,3-dipolar cycloaddition reactions with several heterocycles and ketene insertion reactions have also been studied <72LA(755)177, 75MI 911-02, 77JHC81, 84MI 9II-02, 84TL273I, 85CCC1982,... 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

Carboalkoxymethylenes, like acylmethylenes, undergo rearrangement to ketenes as well as the olefin addition and C—H insertion reactions characteristic of methylenes.<37> Thus the photolysis of ethyl diazoacetate in olefinic solvents leads to substantial yields of products, which can be rationalized in terms of a Wolff rearrangement of the carboethoxymethylene followed by cycloaddition of the resulting ethoxyketene to the olefin ... 

Table 2. Insertion reaction, phosphene and ketene rearrangements of phosphorylacylcarbenes (37)...

Figure G shows some insertion reactions of carbonyl compounds. In the isocyanate and ketene cases, the addition takes place, not to the carbonyl double bond, but to the carbon—nitrogen or the carbon—carbon double bond.

The chemistry of ketenes is dominated by their high reactivity most of them are not stable under normal conditions, many exist only as transient species. Nucleophilic attack at the -carbon, [2 + 2] cycloadditions, and ketene insertion into single bonds are the most important and widely used reactions of such compounds. 

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic -hybridized carbon atom and alow eneigy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene insertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

Mulzer et al. have extended the applicability of the insertion reaction of ketene into an acetal, to that of a substituted ketene into a 1,3-dioxalane to provide an elegant synthesis of 6,6-dichloro-l,4-dioxepan-5-ones (Equation 13) <1996AGE1970>. 

Recently, Kistiakowsky and co-workers8 found that reactions of methylene, formed by photochemical decomposition of ketene or diazomethane, produce hydrocarbon molecules with considerable excess vibrational energy. The C—H insertion reaction is represented by reaction (3) ... 

The ketenimine (93) reacts only at the C=N bond to give pyrroles and pyrrolines with methylene migration (equations 102,103). Carbon (hoxide codimerizes smoothly with methylenecyclopropanes to yield lactones with alkene isomerization (equations 104, lOS). The Pd -catalyzed rearrangement of y-butyrolactone (94) to butenolide (95) indicates that this CO2 insertion reaction is reversible. Hiis is fur-ter supported by the observation that lactone (94) can serve as a TMM precursor in the codimerization with aldehydes and electron-deficient alkenes (equations 106, 107). No cycloaddition to ketenes or simple ketone carbonyls has ever been reported. 

The most common example of this type of migration is that in which the carbene is adjacent to a carbonyl group. The initial product of what is known as the Wolff rearrangement is a ketene, which cannot be isolated but is hydrolysed to the ester in the work-up. Wolff rearrangement is a typical reaction of diazoketones on heating, though these species do also undergo intramolecular C-H insertion reactions. 

CO2 analogues such as allenes, ketenes, isocyanates, carbodiimides, and carbonyl sulfide also insert. Studies of insertion reactions of Cp2ZrR2 (R = Me, CH2Ph, Ph) show that the ease of insertion follows the series Me > Py CH2Ph and Ph2C=C=0>PhN=C=0>p-MeC6H4N=C=NC6H4Me-p>C02. The data have been reasonably interpreted as evidence for precoordination before insertion occurs. A four-center transition state similar to that depicted above for the thorium system is favored. 

Insertion Reactions of an Isocyanate, Isothiocyanate, Carbodiimide, Ketene, Diketene, or Ketenimine... 

Insertion Reactions into Eiement Oxygen and Heavier Group VIB Bonds 465 11.5.6. Insertion Reactions of Ketenes, Alkenes, and Aikynes... 

Ketones and aldehydes insert into B-X bonds, and the products vary with the substituents on the substrateU Thus, with BX3 either mono-, bis-, or tn s-inserted products may be obtained in which B-0 bonds are formed. Electron-withdrawing substituents favor insertion. Reactions with ketenes lead to insertion into both the C=C and C=0 bonds . 1,3-Addition of PhBCU occurs with a-diazoethylacetates to form PhClBOC(OEt)=CRCl under mild conditions. Cyclic perfluoroketones react with... 

Insertion Reactions into Element-Halogen Bonds 11.6.4. Insertions of Aldehydes, Ketones, and Ketenes... 

Metal alkoxides undergo insertion reactions across the M—O bonds (6) when treated with unsaturated substrates such as isocyanates, isothiocyanates, aldehydes, ketones, ketenes as well as with carbon dioxide, carbon disulfide, and sulfur dioxide. 

Two types of cyclization reaction take place when a ketene silyl acetal is treated with a carbenoid generated from CHBrs and ZnEt2. ° When the substrate is aliphatic (Scheme 7), a cyclopropylcarboxy-late is formed due to a CH insertion reaction of an intermediate zinc carbenoid. With substrates having an alkene in the vicinity of the carbenoid (Scheme 7), in particular those derived from 8,e-unsaturated esters, internal cyclopropanation takes place. 

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