Pseudo-axial

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

The ESR spectrum of the thioxanthene S, S-dioxide radical anion itself shows that the two possible conformers coexist, since the two methylene protons are not equivalent. In the case of the 9-monoalkyl derivatives, the large coupling constant observed for the 9-proton leads to the conclusion that the 9-substituent is in the boat equatorial position as in II1 F Thus the radical anions and the neutral molecule display different conformations. The protons in the 9-position of the radical anions of cis-9-methylthioxanthene S-oxides (2, n — 1, R1 = H, R2 = CH3) have an appreciable coupling constant10 which suggests that these radical anions have the substituent in the pseudo-axial position. Furthermore, in the radical anions the S—O bond is pseudo-axial. These situations are exactly the opposite of that observed for the neutral compound. 

Figure 7.23. (a) Pseudo-equatorial and pseudo-axial conformers of the dioxocarbenium ions, (b) ORTEP representations of the X-ray structures obtained for carbenium ion 49 and 50. (Taken from ref. 127.)... 

Ligand Field Theory of dx Configurations in Pseudo-Axial Symmetry... 

The coverage of the survey has though been deliberately restricted to those sandwich systems in which the pseudo-axial structure is definitely established or is strongly indicated by the available evidence, and species in which extensive distortion from this geometry is likely have thus been excluded. Consequently this has had the effect of largely limiting the coverage to the metallocenes and bisarene compounds of the 3d series since in the 4c and... 

Consequently, in Figures 5-11 are shown the appropriate Tanabe-Sugano type diagrams for dx systems (x = 2 — 8) in pseudo-axial, CmV, symmetry, the parameters DtjDs = 0.55 and C/B = 4.0 being adopted throughout. As indicated above this parameteri-sation is most appropriate for the metallocenes, for which the d-d spectra are now considered in more detail. Thereafter a brief survey is made of the limited data relating to bis-arene species, and to various mixed sandwich complexes. 

The neutral 3 dx metallocenes are thus known for x = 3 — 8, but the d9 copper complex has thus far resisted preparation, and the d2 titanocene has been found (54) to be both diamagnetic and dimeric, and is therefore excluded from consideration here. A number of cationic species, corresponding formally to Ti(Cp)2+, and V(Cp)2+, systems are however well known, but it seems very probable that these do not possess pseudo-axial symmetry (see (41) for further discussion), and very recently it has been demonstrated (55) that stable V(Cp)2+ complexes cannot be isolated without the coordination of an additional ligand to the metal. The parent systems are therefore limited to V(Cp)2, Cr(Cp)2, Mn(Cp)2, Fe(Cp)2, Co(Cp)2, and Ni(Cp)2 and the cationic species to Cr(Cp)2+, Fe(Cp)2+, Co(Cp)2+, and Ni (Cp)2+> and the d-d spectra of these systems are now considered individually. 

For d8 systems in pseudo-axial symmetry Figure 11 clearly shows that a 32 (a2 rr2 4) ground state is to be expected for all DtlB values, and this is confirmed by the observation... 

A detailed treatment of the temperature dependence and anisotropy of the magnetic moments of all the dx configurations in pseudo-axial (CooV) symmetry has though now been given by Warren (101), in which variation of the orbital reduction factor, k, and distortions from effective Cv symmetry were also considered. This has lately been followed by a similar treatment due to Cerny (102) of the d d2, d8, and d9 configurations but, although some sophistications were included the results are essentially equivalent to those of the author, and furthermore only the undistorted situation, with k = 1, was considered. Consequently the author s own treatment (101) is here briefly summarised, the theoretical approach being that most appropriate for the sandwich complexes of the 3 d series, to which the bulk of the available experimental material relates. 

For a pseudo-axial ligand field the appropriate perturbation Hamiltonian, H, is, in the absence of a magnetic field, (c.f. Section 2)... 

In principle it should be possible to determine the anisotropy of the paramagnetic susceptibility for some systems of pseudo-axial symmetry, but apart from the citation (74) of a private communication regarding the ferricenium cation, no experimental data are available for metal sandwich complexes. Such measurements should however be possible for at least some metallocenes and mixed sandwich complexes since these are found to crystallise in either a monoclinic (Fe(Cp)2 (6)) or orthorhombic (Ru(Cp)2, (Cp)V(Ch),... 

However, in pseudo-axial symmetry, all three ground states considered will be split by the combined action of the ligand field and spin-orbit coupling thus 32 -> 2+ + II,... 

---