Endo-exo isomerization

CpMo(CO)2( ] -allyl) complexes show a planar allyl rotation analogous to that found for the diene in [CpMo(CO)2( ]" -diene)]+ complexes. Endo-exo isomerism is observed and interconversion occurs rapidly at room temperatme... 

Change of configuration around the C(l)-C(2) or the C(l)-C(3) bond does not necessarily afford a ring-opened species as an intermediate. Conversion of 523 into 525 occurred via iminium ion (524), which is attacked by the nucleophile from the more hindered position forming the thermodynamically more stable product (equation 134) For endo-exo isomerization, see equation 91). Isomerization of 523 (R = Ph, R = H, X = CN) was discussed in terms of a diradical intermediate stabilized by the capto-dative substituent combination... 

CHsCN-catalyzed process. This exchange presumably proceeds via an intermediate bis-CH3CN complex in which the alkyl group has a normal structure. This is the only cation to date for which such endo/exo isomerism has been observed (79). 

The NMR spectra of the homopolymers obtained from the endo and exo adducts at 260 C were similar to each other and different from those obtained from the homopolymers of either adduct polyma -ized at lower temperatures. The spectra contained peaks arising from both endo and exo isomers, indicating that the adducts had undergone some endo-exo isomerization. No unsaturation was shown in the spectrum of either homopolymer obtained at 260 C (Figure 2). 

The retrograde dissociation of the Diels-Alder adduct to generate the diene and dienophile, followed by the polymerization of the comonomer charge transfer complex, has previously been proposed (12-15) in the polymerization of the CPD-MAH adduct, in the presence of peroxides having short half-lives at the elevated polymerization temperatures. Under these conditions, the ground state complex, presumably involved in endo-exo isomerization, is converted to the excited state complex which undergoes the indicated polymerization. 

Subsequent to the presentation of the results reported herein (21), 13C and 270 MHz H-NMR studies of the thermally induced polymerization of the N-phenyl-5-norbornene-2,3-dicarboximides (22) and JC-NMR studies of norbornene end-capped polyimide prepolymers (23) were reported. Endo-exo isomerization of the adducts takes place at 200 C and above. Retrograde dissociation... 

In bicyclohexenyl radicals additionally substituted at C-6, an endo - exoisomerization was found to proceed with a rate comparable to the walk rearrangement (120). If one starts with endo-78-3-d (R = CH2OH), the reaction product consist of a mixture of exo-78-2- and -3-d. Both reactions, walk rearrangement and endo - exo isomerization, have been assumed to involve an intermediate cyclopentadienylmethyl radical of type 79. In view of MO considera-... 

Figure 14. Stereochemistry of the photochemically induced walk rearrangement in homo-tropylium cations. Gibbs activation energy of the thermal endo - exo isomerization is in kcal/mol (124B, 127).

In the case of [( -allyl)Fe(CO)4]+ cations (138), there is no possibility for endo exo isomerism. Crystal structures of this type of complex have been reported, and are similar to those of (135). Noteworthy in the character of these complexes is their geometric stability the syn and anti isomers do not interconvert unless heated to 60-70 °C for extended periods of time. ... 

Fig. 6. (A) A 270 MHz 180°-t-90° inversion recovery determination of relaxation times in an endo-exo isomeric mixture of (T -allyl)Fe(CO)3l. The value of rfor each trace is the delay (seconds) between the 180° and 90° pulse. The doublets with the greater spacing at 2.33 and 3.788 are assigned to the anti protons of the exo and endo isomers, respectively. Those at 3.69 and 4.158 are assigned to the syn resonances. The central proton resonances are at lower field. These traces demonstrate that the relaxation times for the syn and anti protons in both isomers are equal.

The metal center in (i9 -C5H5)M(L)(CO)(allyl) can be considered pseudotetrahedral and characterized by the (R) and (5) nomenclature. Neutral chiral complexes of this type have been prepared previously, e.g., phosphine carbonyl derivatives (21, 75) and nitrosyl halide derivatives prepared by carbonyl displacement from nitrosyl cations by iodide (57, 60). Each of the diastereomers produced by the addition of a prochiral allyl moiety to the chiral metal center exhibit endo-exo isomerism. The endo-exo equilibration depends on charge and the neutral nitrosyl iodides rearrange rapidly compared to the cationic nitrosyls. 

There is remarkable stereospecificity in nucleophilic additions at the -allyl groups of the isomeric compounds (18) and (19) (Scheme 20). The barrier to endo-exo isomerization is high so that they may be separately prepared and nucleophilic additions at each studied. Nucleophilic attack at the different ends of the allyl group in (18) would give different diastereomeric forms of the alkene... 

Dioxolane-type ndo-benzylidene acetals were the main products in the kinetically controlled reactions of several hexopyranosides carrying 2,3- or 3,4-cis-hydroxyl groups with dimethoxytoluene and p-tolenesulfonic acid without added solvent. It was noted that endo-exo isomerization was faster for derivatives with unprotected hydroxyl groups vicinal to the acetal thus, methyl a-L-ihanuiopyranoside gave a 1 1 mixture of 14 and 15, while from the 4-0-benzylated starting conqx>und the endo-product 16 was formed stereospecifically. ... 

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