Dimerization, allenes

The conformationally locked [46] bisallene 127 was first prepared by subjecting the allene dimer 126 to the DMS allene synthesis (Scheme 5.17) [47]. 

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]. 

Aryl-substituted allenes dimerize giving symmetrical and nonsymmetrical bismethylenecy-clobutanes. 

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."... 

Trimethylenemethane biradicals have been proposed as intermediates in the photodecomposition of fluorine-substituted 4-methylenepyrazolines both in the gas phase and in solution.Evidence for the intermediacy of two trimethylenemethane biradicals on direct irradiation of the bi-(l-pyrazolin-4-ylidene) (29) has also been reported. Benzophenone-sensitized irradiation of the same pyrazoline, however, takes a different course and affords the allene dimer (30), presumably via the tetramethylene-ethane biradical (31). 

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%. 

Dimethylenecyclobutane Degenerate Rearrangement, Tetramethyleneethane, and the Allene Dimerization... 

The question of whether the bisallyl biradical is involved in the allene dimerization has been answered in the affirmative. Dimerization of 1,1-dimethylallene at 150°C and thermolysis of 4,5-dimethylene-3,3,6,6-tetramethyl-3,4,5,6-tetrahydropyridazine at the same temperature gave the three isomeric allenes in the same ratio. Further, direct photolysis of the azo compound just above room temperature gave the same product distribution when the thermal data was extrapolated to that temperature. 

Finally, the relative energies of all species on this energy surface need to be placed. Unfortunately, the activation energy for the allene dimerization is unknown. 

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. 

The analyses of the two head-to-head allene dimers (11) and (12), respectively formed by dimerization of l,l-diphenyl-3-chloroallene and of l-mesityl-3-chloroallene, represent the first studies of compounds containing... 

Phenyl-3,3-biphenylene-allene dimer, 40B, 142 C1HH30F2, anti-1,2-Difluoro-3,4,5,6,7,8-hexaphenyltricyclo-[4.2.0.02-5]octa-3,7-diene, 27, 786 C60H80O4 0.5 C4H10O, 3,3a ,5,5 -Tetra-t-butyl-2 -(3,5-di-t-butyl-4-hydroxyphenyl)-7 -[1 (3,5-di-t-butyl-4-hydroxyphenyl)ethylidene]-3a ,4 ,7 7a -tetrahydrospiro[cyclohexa-2,5-diene-1,1 -[IHjindene]- 4,4 -dione diethyl ether solvate, 46B, 110... 

AUene Cycloadditions. —The consensus of opinion is that allene dimerization to 1,2-dimethylenecyclobutanes is a two-step process which proceeds via an intermediate bisallyl biradical (315). Preference for formation of those stereoisomers having larger groups (R) on the inside positions of the double bonds of the product (316) can be explained on this model by assuming that they occupy the less hindered inward positions in (315). Dimerization of 1,3-diadamantylallene was studied since models suggested that a product such as (316 R = adamantyl) would be impossible to form. In fact dimerization... 

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... 

Apparently the allene dimer intermediate diradical is trapped by the acetylene dicar-boxylates. 

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