The racemic form of compound 42a crystallizes in the triclinic space group PI with two independent molecules in the asymmetric unit. The structure analysis firstly confirms the constitution of compound 42a and secondly, indicates that in each of the independent molecules the two stereogenic carbon atoms have the same stereochemistry, so that equal numbers of RR and SS molecules are accommodated by the centrosymmetric space group.
For a molecule of type (Cabc) 2 in which the two stereogenic carbon
atoms are of the same hand, then if a, b and c represent mono-atomic substituents, there will be a two-fold rotation axis normal to the central
C-C bond, regardless of the conformation adopted by the substituents: only if a, b and c represent poly-atomic substituents with internal conformational degrees of freedom can the molecular symmetry be less than C2 (Figure 2.4). In both independent molecules of rac-42a the
torsional degrees of freedom, particularly those involving the aryl rings (appendix V) preclude exact C2 symmetry: however. Figure 2.5
demonstrate clearly the approximate C2 axes, normal to the C12-C13 and C22-C23 bonds and lying approximately in the plane of the page. Not only is the molecular symmetry only approximately C2 but, in addition,
the four independent -CH(Fc)COPh fragments within the asymmetric unit are all conformationally distinct.
o II Pc— CH — C ~ P h
I
Fc— CH — C - Ml II O Let H = a, Fc = b and COPh = c 'b c* C2Figure 2.4: A schematic representation that a molecule of type (Cabc)2 has a twofold rotation axis regardless of the positions o f the substituents on both the carbons.
C22 C23
C251 021
Figure 2.5: Single crystal X-ray structure o f racemic 2,3-diferrocenyl-l,4- diphenylbutane-l ,4-dione 42a; showing both the coupled carbons C l2 and C l3 (C22 and C23) in RR and SS conformation
Figure 2.6: Single crystal X-ray structure of racemic 2,3-diferrocenyl- 1,4-bis(biphenyl)butane-l ,4-dione 42b.
Figure 2.7: Single crystal X-ray structure of meso 2,3-diferrocenyl-l ,4- bis(biphenyl)butane-l A-dione 42b»
The structures of the two forms of compound 42b show that these are indeed the racemic and meso diastereoisomers. Rac-^2h is triclinic, PI, with equal numbers of RR and SS molecules in the unit cell and, just as found for rac-42a, the molecules have approximate but not precise two fold rotational symmetry (Figure 2.6 and crystal data in appendix VI).
The meso form of 42b (Figure 2.7) adopts a conformation which is approximately, though not precisely, centrosymmetric, consistent with its
RS configuration as shown in the schematic representation in Figure 2.8, crystal data are presented in appendix VII. In each of rac-42a, rac-42b and m esoA lh, the molecules have the central H-C-C-H fragment in a
trans conformation; this conformation uniquely minimises the steric repulsions between the four large substituents bonded to the central C2 fragment. All other staggered conformations require the four large substituents to be adjacent, while if the H-C-C-H is trans, only pairwise interactions occur between these large substituents.
H
CObiPh
PhbiOC
H
Figure 2.8: Schematic representation along CH-CH bond in meso-42b that shows the molecule is almost centrocemitric.
2.1.2 Bifunctional 1,2-diketones.
Ferrocene-containing bifunctional diketones were synthesised with a view to further condense with di- or tetra-amines to yield bis- and poly- quinoxalines. Two approaches were investigated for the synthesis of such diketones, in the first approach a bisiodosalt, 4,4’-dicarbonyl- bis[ferrocenylmethyl(triphenylphosphonium iodide)] 46, like that of 38
under the same reaction conditions, was attempted to synthesise by the reaction of 4,4'-biphenyldicarbonyl chloride 45, that was synthesised itself by the action of oxalylchloride on 4,4’-biphenyldicarboxylic acid 44
and ferrocenyl phosphorane 37. The reaction was not successful and a mixture of products were found therein, but none of them was found to be
46, one component of that was characterised as the starting material 36,
but no attempts were made to properly characterise all the products obtained.
45X = C r (46)
In the other approach bisacyl ferrocene Fcd(COCH2Ph) 2 47,
synthesised in one step by Friedel Crafts acylation, was oxidised to ferrocene-l,T-diylbis(phenylethanedione) 48 by using very active manganese dioxide at 100 ^C. The choice of solvent and temperature are very critical in this oxidation; lower temperatures show either no reaction at all or very little oxidation to the required product, or oxidation of the methylene group attached to the one ring only, while at temperatures above 105 °C decomposition is observed. 1,1,2-trichloroethane or
chlorobenzene were found to be the right solvents for this oxidation with a narrow temperature band of 95-105 “C.
^-COCH 2Ph ^;;^^;r-COCOPh
Fe Fe
COCH gPh <^^>-COCOPh
(47) (48)
The course of reaction was followed by TLC; in the first few hours of the reaction two red spots appeared other than the starting material, one of them of higher Rf than the other. The lower Rf value spot, presumably the mono-oxidised intermediate, disappeared slowly by the end of the reaction time and finally only one red spot with no unreacted starting material was observed on TLC with the bisdiketone 48 as the sole product in moderate yield. The compounds 47 and 48 were characterised by NMR and elemental analysis; further condensation of bisdiketone 48 with aromatic di- and tetra-amines to yield quinoxaline compounds is also a proof of its chemical nature (chapter 3).
2 .2
EKPERIMENTflL DETAILS
NMR spectra were recorded at ambient temperatures, all i^C spectra were recorded as decoupled, in CDCI3 solution unless stated otherwise,
on a Bruker AM-300 spectrometer operating at 300.135 MHz for ^H and 75.469 MHz for Elemental analysis was carried out in the microanalytical laboratory of the School of Chemistry. Diethyl ether and light petroleum (b.p. 40-60 °C) were dried over sodium wire, and dichloromethane was distilled over calcium hydride prior to use. All the other chemicals were used as received unless stated otherwise.