Allenyl systems

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

Because of this large rate factor in favor of the solvolysis of an allenyl system, it was of interest to examine the solvolytic behavior of the unsubstituted parent allenyl halide. Stang and co-workers (205) have observed good pseudo-first-order rates for the solvolysis of allenyl bromide, 244, R) = = Rj = H,... 

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

A summary of the solvolysis of allenyl and related substrates is given in Table XIX. As the data in this table indicate, the rates of solvolysis of allenyl bromide and 1-bromomethylenecyclopropane, 247a, are very comparable. Their rate of solvolysis, however, is about 10 slower than the rate of solvolysis of triphenylchloroallene under similar conditions. This rate difference between the parent and triphenyl allenyl system is not unlike the rate difference in the solvolysis of (C6Hs)3CX and CH3X. 

Type III reactions proceed by attack of a nucleophile at the central sp carbon of the allenyl system of the complexes 5. Reactions of soft carbon nucleophiles derived from active methylene compounds, such as /i-kcto esters or malonates, and oxygen nucleophiles belong to this type. The attack of the nucleophile generates the intermediates 9, which are regarded as the palladium-carbene complexes 10. The intermediates 9 pick up a proton from the active methylene compound and n-allylpalladium complexes 11 are formed, which undergo further reaction with the nucleophile, as expected, and hence the alkenes 12 are formed by the introduction of two nucleophiles. 

This transformation was further studied and the catalyst load could be decreased to 0.2 equiv.35 33 A mechanism was proposed through deuterium incorporation experiments, and the conclusion was that there was no 1,2 shift of the deuterium present in the starting material (A, Scheme 5.3) since exclusive formation of furans D deuterated on position 3 could be explained by the presence of an external source of deuterium (such as D20). Therefore, it is believed that after silver(I) coordination to the allenyl system (A, Scheme 5.3), the attack by the carbonyl oxygen may lead to an oxo cation intermediate B. Finally, proton lost would generate silver furan C that would lead to furan D after silver release (Scheme 5.3).39... 

It was concluded that the stabilization of the positive charge of 63, by delocalization over the three carbons of the mesomeric propargyl allenyl system, entailed a powerful electron releasing substituent at the allenyl end 13). However, although in theory electron releasing substituents might render 1-substituted cyclopropyl cations more stable than their 2-substituted allyl counterparts 36), the expected ring closure of a 2-substituted allyl cation such as e.g. 64 to the cyclopropyl cation 65 has not been observed experimentally, Eq. (19)37). 

PATERNO-BOCHI REACTION OF DIENYL AND ALLENYL SYSTEMS 165... 

The solvolysis product of alkyne-substituted 1-ethynyl cyclopropyl tosylates (6) appeared to be strongly dependent upon the nature of the substituent R of the triple bond. So, stabilization of the intermediate cyclopropyl cation (7), by delocalization of the positive charge over the three carbons of the mesomeric propargyl-allenyl system, entails a powerful electron-releasing substituent R effectively, when R = p-anisyl, the ring-opening of the cyclopropyl cation was totally suppressed (equation 5). ... 

Propargyl carbonate 224, which has a hydroxy group at C-5, underwent cyclization by attack of an alkoxy group at the central carbon of the allenyl system 225. The intermediate... 

R3 = COjR), futher attack of carbon monoxide at the central sp carbon of the allenyl system in 40 takes place to give the diester 42 (Scheme 11-14). 

Although no pK values of allenic sulfides RCH=C=CHSR/ are known, other experimental data indicate the basicity of the lithiated compounds RCH=C=C(Li)SR/ to be somewhat lower than that of 2-thienyllithium. Competition experiments with butyl bromide in THF or in a THF-HMPT mixture showed the metallated allenic sulfide to be much more reactive than 2-thienyl-lithium [9]. This difference might be due to the higher polarisability of the allenyl system. 

Theoretical Studies and Physical Measurements.—K sector rule for chiral allenes has been derived which relates the position of the substituent to the sign of the lowest-energy Cotton effect. A physical basis is proposed for the Lowe-Brewster rule relating the configuration of a chiral allene to the sign of its D-line rotation. Calculations have been made on the addition of allene and acetylene to vinyl cations via concerted and non-concerted pathways. The precedence for groups in the allenic diphenylphosphine (380) for eclipsing with the allenyl system have been studied by n.m.r. C N.m.r. 

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