Allene dimerization reactions

Unlike ordinary olefins, allene (1,2-propadiene) (1) undergoes thermal dimerization to afford 1,2-dimethylenecyclobutane (2), first reported by Lebedev in 1913 [1], The yield of the dimer 2 was improved up to 95% by Dolbier, who used a benzene solution of 1 for the dimerization reaction [2]. 

The reaction of tetrafluoroallene (61) with hexafluoro-2-butyne (5 equiv.) under autogenous pressure gave a mixture containing the desired [2 + 2]-cycloadduct 66 and the allene dimer 67 [64]. 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

Not only this Ni(0)-catalyzed reaction but also all reported allene dimer complexes, e.g., hexacarbonyl-p[l-3 l -3 -jj-(2,2 -biallyl)]diiron (Fe-Fe) (146), hexacarbonyl-/i-[l-3 l -3 -r)- (1,1 -diphenyl-2,2 -biallyl) ]diiron (Fe-Fe) (147), and di-/i-aeetato-/i-[l-3 l -3 -rj-(2,2,-biallyl) Jdipalladium (148) point to the formation of 2,2 -biallyl. A mononuclear Rh(I) complex containing this ligand was recently isolated (149). Accepting this biallyl formation, then the next step is the insertion to form the trimer ligand in complex IV. Thus the entire reaction paths leading to complexes I, II, and III may be depicted (Scheme 8). 

Allene is an extremely useful reagent for cycloaddition reactions giving cyelobutane derivatives. Allene dimer is also a useful and versatile starting material."... 

On the other hand, photochemical addition of alcohols to conjugated dienes, observed in the case of cholestadieiie-3,5, is probably related to the reactions observed when vitamin A acetate is irradiated in methyl alcohol. It is also possible that the photoisomerization of acyclic trienes to allenes is related to the as yet incompletely studied dimerization reactions of larger polyenes. 

A full paper describing the synthesis of 2,4-disubstituted furans in high yields through a novel oxidative cyclization-dimerization reaction between two different allenes was provided <05JOC6291>. 

The acetophenone-sensitized photochemical reactions of allene have been interpreted in terms of non-vertical energy transfer to produce the planar allene triplet. 8 Several products are formed in the reaction, among which are allene dimers and trimers. Two ketones are also obtained, one (86) of which arises by attack of the central carbon of the allene triplet on the ortho-position of the acetophenone. 

A variation of this type of reaction involves interaction of two alkyne groups via radical formation. The mechanism proposed by Garratt involves initial base-catalysed isomerization to the bis allene, which then undergoes an intramolecular allene dimerization to give the bis methylene diradical (67). 

To a solution of 15.40 g 3,3-dibromo-2,7,7-trimethyl-tricyclo[4.1.1.0 ]octane (50.0 mmol) in 100 mL dry ether was added dropwise 37.5 mL 1.6 M MeLi in ether (60 mmol) at room temperature, and the resulting solution was stirred for 2 h. The reaction mixture was quenched with water. The mixture was extracted with ether, and the organic layer was washed with saturated NaCl and dried over MgS04. After the removal of the solvent (20°C, 15 mmHg), the product mixture (8.77 g) was distilled at 38°C (5 mmHg) to provide 4.04 g the insertion product 3,7,7-trimethyltetracyclo[4.2.0.0 . 0 ]octane as a colorless liquid, in a yield of 54%. The residue was passed through silica gel (70 g), eluting with hexane to yield the head-to-head allene dimer l/ ,6/ ,85, 10/ ,l 1/ ,135-2,5,7,7,14,14-hexamethylpentacyclo [ 11.1.1.1. 0 . 0 ]hexadeca-2,4-diene, which was further purified by recrystallization from ethanol to yield 1.62 g of colorless crystals in a yield of 22%, m.p., 122.5-123.0°C. The oily second fraction was 1.19 g of a diastereomeric mixture of the head-to-tail dimer 7,7,9,11,14,14-hexamethylpentacyclo-[ll.l.l.l . 0 0 ]hexadeca-2,9-diene and head-to-head allene dimer 2,7,7,9,14,14-hexamethylpentacyclo-[l 1.1.1.l . 0 0 ]hexadeca-2,9-diene, in a yield of 15%. 

The reaction would appear to be an intramolecular allene dimerization since both products could reasonably be derived from bicyclo[6.2.0]deca-1,7-diene (Scheme 11.88). 

Allene dimerization is a frequently used route to bismethylenecyclobutanes and can be highly selective, as in the dimerization of (248 X = Cl or Br). Only (249) is formed, although 14 isomers are possible, via a bis-allyl biradical. The same biradical is involved in the thermal rearrangement of (249) to (250), and a graphical approach has been developed for predicting the fates of such radicals. On the energy surface which contains all of the species, the first products should be those which are one allowed reaction step away from a given biradical. ... 

Methylenecyclobutanes and Cyclobutenes.— The mechanism of allene dimerization has been further investigated. Dimerization of 1,1-dimethylallene at 150 C yielded the methylenecyclobutanes (86), (87), and (88). The product distribution at low total conversions was as indicated below. Under the same conditions, thermolysis of the tetrahydropyridazine (89) gave the same product distribution, affording strong evidence of a common intermediate, presumably the 2,2 -bis(l,l-dimethylallyl) radical, in the two reactions. 

Alkyl methacrylates fumarate copolymerization, 279 MA copolymer-alcohol reactions, 284 MA copolymer-amine reactions, 284 MA copolymerization, 520-531, 535, 542 maleate copolymerization, 279 Alkyloxyamines, reaction with MA, 83 A -Alkyl-2-pyridine ylids, MA adducts, 217 7V-Alkylpyrroles, MA alkylation, 237 Alkyl thioglycolates, reaction with maleates, 49 Alkylthiosuccinic acids, 52, 55 ii/-Alkylthiosuccinic anhydrides, melting points, 50 Alkylureas, reaction with MA, 87 Allene, dimer-MA Diels-Alder diadduct, 109 N-Allylacetamide, MA copolymerization, 312 Allyl acetate... 

Silaallenes are of recent vintage. While 2-silaallenes and silazaallenes have only been encountered as intermediates, stable 1-silaallenes with 2,4,6-triisopropylphenyl substituents on silicon have been synthesized recently While 1-silaallenes with highly sterically hindered substituents are stable at room temperature, Me2Si=C=C(SiMe3)Ph exhibits a lifetime of about 25 ps at room temperature in nitrogen-saturated hexane solution. Also, the first stable allenes with double-bonded phosphorous with one 2,4,6-triisopropylbenzene substiment on silicon have been synthesized. In view of the instability of silaallenes, their dimerization reactions are mainly observed. 

Push-puir allenes, such as the shown cyclopentadienylidene allenes 74, dimerize in solution on attempted synthesis to give two major dimers 75 and 76 and one minor dimer 77. In contrast, fulvenallenes do not undergo a dimerization reaction. 

Thus in the dimerization reactions of allenes, or in their cyclo-addition reactions to olefins, non-concerted pathways appear to be favoured. The allene dimers are always of the head-to-head, tail-to-taU, type which suggests that 1,4-diradical intermediates are formed in these reactions. The stereospecific addition that may attend these and related reactions may merely be an indication of the lack of conformational flexibility of the diradical intermediates because of steric crowding. 

Oxidative cleavage of the complex 549 with CuCri affords 2,3-bis(chloro-methyl)-1,3-butadiene (550) and regenerates PdCri. Thus the preparation of this interesting dimerization product 550 can be carried out with a catalytic amount of PdCl2 and two equivalents of CuCb in MeCN[495], Similarly, treatment of allene with PdBr2 affords the dimeric complex 551. Treatment of this complex with 2 equiv, of bromine yields the dibromide 552. The tetra-bromide 553 is obtained by the reaction of an excess of bromine[496]. Similarly,... 

Like butadiene, allene undergoes dimerization and addition of nucleophiles to give 1-substituted 3-methyl-2-methylene-3-butenyl compounds. Dimerization-hydration of allene is catalyzed by Pd(0) in the presence of CO2 to give 3-methyl-2-methylene-3-buten-l-ol (1). An addition reaction with. MleOH proceeds without CO2 to give 2-methyl-4-methoxy-3-inethylene-1-butene (2)[1]. Similarly, piperidine reacts with allene to give the dimeric amine 3, and the reaction of malonate affords 4 in good yields. Pd(0) coordinated by maleic anhydride (MA) IS used as a catalyst[2]. 

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