Cumulenes, cycloadditions

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

Transition-metal allenylidenes are prone to undergo cycloaddition and related cyclization reactions involving both M=Co-, Co,=Cp, and Cp=Cy bonds of the cumulenic chain. In some cases, cyclization/cycloreversion pathways have been observed leading to the final isolation of acyclic products. 

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. 

The homologous cumulene 44 prepared by an analogous vinylidene insertion reaction is spontaneously transformed to the dimer 45.46 By contrast the stable tert-butyl derivative 46 undergoes thermal cycloaddition at one of the inner double bonds giving 47. The structure of this dimer was confirmed by X-ray diffraction.47... 

Cyclodimerization of unsymmetrically substituted butatricncs such as 12 give both head-to-head and hcad-to-tail cycloaddition products. The structure of the head-to-head dimer was confirmed by its independent synthesis from the mixed cycloaddition of cumulenes 8 and 10,21 22 These dimerizations proceed by discrete nickel cyclopentanes which was established by the isolation of the 2-bispyridinenickel complex of the l.l,4,4-tetramelhylbuta-l,2,3-triene dimer.23 4-Radialenes with extended conjugation, potential organic conductors and semiconductors, have been prepared by similar methods as illustrated by the examples below.24,25... 

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. 

A problem with DFT that is not restricted to intermolecular complexes is what might be called overdelocalization . In part because of problems in correcting for the classical self-interaction energy, many functionals overstabilize systems having more highly delocalized densities over more localized alternatives. Such an imbalance can lead to erroneous predictions of higher symmetry structures being preferred over lower symmetry ones, as has been observed, for instance, for phosphoranyl radical structures (Lim et al. 1996), transition-state structures for cationic [4-1-3] cycloadditions (Cramer and Barrows 1998), and in the comparison of cumulenes to poly-ynes (Woodcock, Schaefer, and Schreiner 2002). It can... 

The C=N bond in cumulenes can also act as the two-atom fragment, and reaction of aroyl isocyanates with carbodiimides gives imino-l,3,5-oxadiazinones (246) (79BSF(2)499). The reaction is not a simple [4+2] cycloaddition, as the kinetic product is the [2+2] adduct which then rearranges. 

Keywords cumulene derivative, [2+2]cycloaddition, [4]radialene tetracarboxylic acid... 

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 dimerization of the diphosphallene has an interesting parallel with the carbodiphosphoranes. The chloro-substituted A5 phospha-cumulene reacts under cycloaddition to give the diphosphacyclobuta-diene dichloride [Eq. (56)] (120a, 120b). 

N-Alkyl- and N-aryl-ketenimines are poor electrophilic partners and do not react with benzylidene-aniline or dicyclohexylcarbodiimide." The introduction of an electron-withdrawing substituent (tosyl or cyano ) on the nitrogen atom enhances the electrophilic character of the cumulene and cycloadditions to imines occur under very mild conditions (Scheme 42). In most cases, the reactions are trans stereoselective. 

While cumulene V might evolve directly into dimethylfiimarate derivative III by proton transfer, alternatively, it might evolve into cyclobutene VI, whereby the desired C-6 methyl acetate unit is expediently formed. Such a structure has very little chance to survive in a hot reaction medium and three decomposition reactions are likely to take place (1) return to starting materials (2) a metathesis-type retro(2-l-2]cycloaddition to diene VII, something that finds... 

The products obtained by cycloaddition of xanthene or thioxanthenethiones to allenes or cumulenes isomerize to thietes. ... 

Cycloadditions of electrophilic olefins or cumulenes to vinylcyclopropanes should also be mentioned in this context, since in most cases stepwise reactions starting with an electrophilic attack are very likely. One example operating with chlorosulphonyl isocyanate and providing a seven-membered lactam is depicted in equation 152 however, so far most reactions in this area have a limited synthetic potential. 

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]. 

Treatment of a solution of 1 in toluene with the butyne derivative at -30°C affords the polycyclic silaheterocycle VII in high yield. For the above reaction we propose the following pathway (see Scheme 4) First a nucleophilic attack of 1 to one of the alkyne carbon atoms takes place under formation of the dipolar intermediate 9, which is stabilized by migration of a fluorine atom from a CFs group to the silicon atom. The resulting cumulene 10 is not stable under the reaction conditions and reacts instantly with one of the Cp ligands in a [2+2] cycloaddition to form the final product VII... 

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 /3-lactams, the key functional group in the penicillin and cephalosporin antibiotics, as is the [2 + 2] cycloaddition of aketene and an imine. 

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. 

To summarize, most [2 + 2] cycloadditions are light-promoted. The only concerted thermal [2 + 2] cycloadditions involve a ketene or other cumulene, or a compound in which a heavy atom such as P or a metal is doubly bound to another element. 

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