Allyl anions, calculations reactions with

Theoretical calculations of the reactions of CH3SSR (R = H or CH3) with fluoride, hydroxide or allyl anion in the gas phase have been performed to determine the mechanism for both elimination and substitution reactions.6 The elimination reactions have... 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

The generally slow conrotatory ring-opening reaction of cyclopropyl anions as compared to the fast isomerization of the corresponding allyl anions has been supported by means of MO calculations with the parent anions (Table 31) . 

It was shown that the ring-opening reaction e.g. of 10 to give 13 (and/or 11 which is not clear from the available data) is about 1500 times slower than the isomerization of the allyl anion 13 (and/or 11) to give the more stable 12. A similar situation occurs when one starts from the cyclopropyl anion 1418). These experimental data thus nicely confirm the results of the theoretical calculations discussed in connection with Table 1. 

The rotational barriers of allylpotassium 23c (16.7 kcal/mol) and allylcesium 23d (18.0 kcal/mol) are much higher than those of 23 a and 23b. This observation and the question ofwhether aggregates are involved in the rotational process, clearly emphasize once more the important influence of the gegenion in carbanion reactions. The experimental results with 23a-d thus lead only to a lower limit (18.0 kcal/mol) for the rotational barrier of the allyl anion in solution. Incidently, this value comes close to the lowest calculated (MP2/4-31 + G//4-31 + G) value of the allyl anion rotational barrier (22.2 kcal/mol)33). 

A crystal structure determination of the [(C7H7)Fe(CO)3] anion as its PhiAs salt confirms an allyl co-ordination of Fe(CO)a while the diene remains unco-ordinated these results are in agreement with MO calculations. -Reaction of [(C7H7)Fe(CO)3] with ([cyclohexadienyl)Fe(CO)3]+ yields (66) and an analogous reaction occurs with [(cyclohexadienyl)Fe(CO)a(CNEt)]+. Fea(CO)fl reacts with (67) to yield (68a,b) formed by loss of oxygen from the N—O bond similar reactions occur with (69). Various arenetropilidenes... 

The 5n2 reactions of y-substituted allyl chlorides where the nucleophile is a free anion and an ion pair were studied theoretically at the HF/6-31+G level of theory.10 The calculations showed that the free ion, S N2 reaction proceeds through a transition state (7) with significant positive charge on Ca and little conjugation with the n-system. Placing substituents on Cy of the free ion reactions gave computed Hammett p values of -3.3 and -4.6 for the trans- and ds-conformation reactions, respectively. The transition states (8) for the ion pair reactions, on the other hand, have considerable positive charge on both Ca and Cy and the computed Hammett p value is +18.8. 

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