Ketenes and cumulenes

A general consideration of reactive intermediates in 1956 by Leffler suggested that assignments of reaction mechanisms very often assume intermediates that have not been isolable for direct study and the physical reahty of such intermediates depends on their relation to similar substances that do happen to be stable enough to study directly. Study of reactive intermediates focuses not only on ions, free radicals, and carbenes but also on other reactive species such as benzynes and ketenes, which were recognized around 1900. With advances in experimental methods of generation and detection of such species, as well as improvements in 

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Chemistry of Ketenes, Allenes and Related Compounds", S. Patai (ed.), John Wiley and Sons, Chichester, New York, Brisbane, Toronto, (1980). lOe. H. Hopf, "The Preparation of Allenes and Cumulenes", Chapter 20 in "The Chemistry of Ketenes, Allenes and Related Compounds", S. Patai (ed.), John Wiley and Sons, Chichester, New York, Brisbane, Toronto, (1980). 

Using standard references and protocol, we find the three reactions are respectively endothermic by ca 2, 8 and 6 kJmol-1, or ca 2, 4 and 3 kJmol-1 once one remembers to divide by 2 the last two numbers because the allene is dialkylated. So doing, from equations 10 and 11 we find an average ca 3 kJmol-1 (per alkyl group) lessened stability for alkylated allenes than the correspondingly alkylated alkenes. This is a small difference that fits most naturally in the study of substituted cumulenes such as ketenes and ketenimines, i.e. not in this chapter. But it is also a guideline for the understanding of polyenes with more cumulated double bonds. 

There are many reactions in which pyridines are used as bases. However in a large number of reactions only pyridine itself is reactive. a-Substituted pyridines behave differently, e.g. in the catalysis of acylation reactions with acyl chlorides or anhydrides [45]. The sterical hinderance of the a-substituents decelerates reactions in which a pyridine reacts as a nucleophile. A reaction which can be base-catalyzed by a-substituted pyridines is the addition of alcohols to hetero-cumulenes such as ketenes and isocyanates. Therefore this reaction was investigated as a model reaction for base catalysis by concave pyridines. 

The [2+2] cycloadditions can be concerted under thermal conditions provided that the interaction between the Ji-systems takes place in a supra-antara mode (Fig. 1). This [27is + 27+] mechanism [20] is sterically very demanding and, therefore, it should be facilitated by cumulenes possessing s/ -hybridized electrophilic carbon atoms. This makes ketenes and isocyanates suitable candidates for concerted symmetry-allowed thermal [2+2] cycloadditions. However, the presence of heteroatoms in both possible [2+2] reactions leads in turn to different stepwise mechanisms in which the electrophilic nature of the v/ -hybridized carbon atoms of ketenes and isocyanates plays a crucial role (Scheme 2). According to these mechanisms, zwitterionic intermediates (6) and (7) are plausible via formation of C-N or C-C bonds, respectively. 

The thermal [2-1-2] cycloaddition of cumulenes with alkenes, imines or carbonyl compounds is one of the most useful methods of four-membered ring formation. The cycloaddition of ketenes with alkenes to give cyclobutanones represents a reaction of general importance. According to Woodward and Hoffmann, these reactions proceed via a [ttIs+ttIi,] pathway [24]. Dihaloketenes are more reactive than simple ketenes and readily react with electron-rich olefins [25]. 

If you see a four-membered ring, think [2 + 2] cycloaddition, especially if the ring is a cyclobutanone (ketene) or light is required (photochemically allowed). Ketenes and other cumulenes undergo [2 + 2] cycloadditions with special facility. An oxetane (four-membered ring with one O) is often obtained from the [2 + 2] photocycloaddition of a carbonyl compound and an alkene. 

The thermal [2 + 2J cycloaddition of cumulenes with alkenes, imines, and carbonyl compounds is one of the most useful routes to four-membered ring compounds. Ketenes and keteniminium salts add to alkenes to give cyclobutanones (Houben-Weyl Vol. IV/4 pp 174 205) 41 al-lenes add to alkenes to give methylenecyclobutanes (Houben-Weyl Vol. IV/4 pp 151-173), ketenes add to carbonyl compounds to give //-lactones (Houben-Weyl Vol. VI/2 pp 520-527), ketenes add to imines to give /1-lactams, and isocyanates add to alkenes to give //-lactams (Houben-Weyl Vol. V/lb p 1098). 

Other cumulenes such as isocyanates RN=C=0 can also undergo thermal [2 + 2] cycloadditions. The [2 + 2] cycloaddition of an isocyanate and an alkene is a useful route to jS-lactams, the key functional group in the penicillin and cephalosporin antibiotics, as is the [2 + 2] cycloaddition of a ketene and an imine. 

A detailed discussion of the structural chemistry of allenes and cumulenes (1 b) has revealed that upon arbitrary substitutions the cumulenic double bonds in allenes, butatrienes, and ketenes retain their linear arrangements and their bond lengths within experimental errors. Furthermore, the antiplanar arrangements of the ligands in allenes (and surely also in pentatetraenes) are not affected by substitutions. 

Most remarkably, the central atom C resonances (C2O of thioketenes are far more sensitive towards substituents than the resonances of the other cumulenic sp central atoms. The substituent induced variations cover about 60 ppm, in contrast to 40 ppm for correspondingly substituted ketenes and about 10 ppm for similar allenes. 

The present section is concerned with the electron density distributions in monosubstituted cumulenes on a CNDO/S level (which is equivalent to a discussion on an ab initio STO-3G level (Section II.C.I)). Emphasis is on the ir electronic systems in allenes, ketenes, and butatrienes. Of particular importance with respect to ir electron densities is the fact that the cumulenic functionalities under consideration act as ir donors as may be deduced from the C chemical resonance positions of the para carbon atoms in phenyl substituted cumulenes (8,89fc,89[Pg.412]

Ketenes and other cumulenes as reactive intermediates 13CRV7287. Lanthanides (II) other than Sml2 as reductive agents, in particular, for heterocycles 12AG(E)9238. 

In [2+2] cycloaddition reactions of carbon cumulenes, often only one four-membered ring compound is obtained. This reaction is of considerable importance in the synthesis of 8-lactams from ketenes and C=N double bond containing substrates. The j8-lactam structure is present in a variety of antibiotics. Also, j8-thiolactams are obtained from thioketenes and imines. 

The other major approach to systems of this type is cycloaddition of cumulene ylides with vinyl isocyanate and vinyl isothiocyanate. Thus, Ph3P=C=C=0 reacts with these two reagents to give 160 and 161 respectively while the corresponding reactions of Ph3P=C=C=NPh afford 162 and 163 <88T543>. By using styryl isothiocyanate with the ketene ylide, 164 was obtained. 

Cumulenes such as butatrienes and hexapentaenes can undergo cycloaddition at several possible double-bond sites. The electrophilic l,l-diphenyl-4,4-bis(trifluoromethy )butatriene (34), however, reacts with ketene acetals and geminal enediamines at the central double bond exclusively.25 In the case of the ketene acetal cycloadduct 35 (R1 = H R2 = R3 = OMe). acid-catalyzed hydrolysis gives the cyclobutanone. 

Virtually all reactions involving cyclobutane formation via cycloaddition of a cumulene to another C —C double-bond system involves excitation of this latter moiety, e.g. an enone or a quinone, and not of the allene or ketene itself.1 Earlier examples of such reactions have been discussed in Houben-Weyl, Vol. 4/5 b, pp 926 931. 

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