C2 axis of symmetry

The aminalizalion of A.A -dimethyl hydrazone 5a derived from glyoxal 1 with enantiomerically pure 1,2-diamines 7a and 7b25 26 containing a C2 axis of symmetry is a straightforward process (in contrast to the acctalization with diols). 

In order to synthesize 1,3-diphenyl-l, 3-diamines 3 containing a C2 axis of symmetry, which can be employed as auxiliaries and controller groups in asymmetric syntheses, the diastereoselective addition of organometallic reagents to racemic pyrazolines 2, prepared from cinnamaldehyde (l)23, was investigated. 

The recognition of a C2 axis of symmetry (or a plane, when the double bond was thought to have the cis configuration), allows the disconnection of the molecule into two identical moieties of indoxyl (2). The synthesis is therefore reduced to the... 

Other molecules in which the presence of a C2 axis of symmetry also simplifies the syntheses are squalene (5) rmd P-carotene (6). Notice that in such cases the simplification derives from the fact that the syntheses are convergent and reflexive i.e., syntheses which start from identical precursors [5]. In practice, the synthesis of these two substances may be carried out not only by dimerisation of two identical moieties [10] [11] -which may lead to a mixture of isomers-, but preferentially by addition of two identical fragments to a central bifunctional unit according to the scheme -1- + C. = Cjj (squalene Cji -t-Cg + Cj] = 30 [12] p-carotene ... 

The type of symmetry present in each type of metallocene initiator (C2v, C2, Cs, Ci) is listed in Table 8-5. The symmetry elements (axis and plane) for each type is indicated. An axis is a C2 axis of symmetry when rotation of 180° about that axis yields a structure indistinguishable from the original structure. The stereoselectivity of each of the two coordination... 

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

Various X-ray crystal structures of metal-ligand complexes provided evidence of the geometry of the complexes in the solid state, even though the structure of these complexes may differ in solution. The hrst crystal structure of a bis(oxazoline)-metal complex was determined in 1994 by Brown and co-workers. " This group crystallized and elucidated the structure of V,V-bis-[2-((45)-(methyl)-l,3-oxazoli-nyl)]methane-bi(ri ethene)rhodium(I), 18a, as depicted in Figure 9.3. The key features of this crystal structure include the C2-axis of symmetry, the axial positions of the methyl groups and the orientation of the ethene molecules, orthogonal to the complexation square plane. In 1995, Woodward and co-workers were able to crystallize and determine the structure of benzylbis(oxazoline) with ruthenium... 

Although reasonable asymmetric induction has been observed in the alkylation of a chiral cyclohexanone enamine, it was noted1 in 1977 that in order to obtain higher induction in this type of reaction. Clearly what is needed is an amine with a C2 axis of symmetry . (+ )-trans-... 

Figure 14.2. a) Conrotatory electrocyclic reaction showing preservation of a C2 axis of symmetry. (b) Disrotatory ring closure showing preservation of a mirror plane of symmetry. 

Figure 14.7. Orbital correlation diagram for the thermal rearrangement of benzvalene to benzene. A C2 axis of symmetry is preserved.

The ring closure of cis-butadiene to form cyclobutene can take place either in a con-rotatory or in a dis-rotatory way. In the first place the CH2 planes of the end groups rotate in the same direction keeping a C2 axis of symmetry ... 

Maleimides (17) with a substituent on the N atom have a C2 axis of symmetry. Therefore, a polymaleimide can be optically active only when the main chain has one of the two enantiomeric trans-structures (Scheme 11.3) [53,54], whereas all asymmetric centers in polybenzofuran are... 

FLC phases in the surface stabilized geometry possess a single C2 axis of symmetry, and therefore polar order and non-zero x<2) in the simple electronic dipolar model. Thus, it is not surprising that experiments aimed at measuring this property were first reported shortly after the Clark-Lagerwall invention. Early studies (14-15) described second harmonic generation in (S)-2-Methylbutyl 4-(4-decyloxybenzylideneamino)cin-namate, the first ferroelectric liquid crystal, also known as DOBAMBC (1). 

One simple practical method of assessing the possibility of the existence of non-superimposable mirror images, particularly with complex structures, is to construct models of the two molecules. The property of chirality may alternatively be described in terms of the symmetry elements of the molecule. If there is a lack of all elements of symmetry (i.e. a simple axis, a centre, a plane, or an n-fold alternating axis) the chiral molecule is asymmetric, and will possess two non-superimposable mirror image structures (e.g. 2a and 2b). If, however, the molecule possesses a simple axis of symmetry (usually a C2 axis) but no other symmetry elements, the chiral molecule is dissymmetric. Thus 4a and 4b are dissymmetric and the simple C2 axis of symmetry, of for example 4a, is shown below. If the molecule possesses a centre of symmetry (C.) or a plane of symmetry (alternating axis of symmetry (S ), the mirror images of the molecule are superimposable and the molecule is optically inactive. These latter three symmetry elements are illustrated in the case of the molecule 4c. 

The interaction of a prochiral molecule with a chiral homogeneous catalyst results in the formation of diastereomeric intermediates and transition states. High enantioselectivity is obtained if for the rate-determining step out of the many possible diastereomeric transition states, one is energetically favored. In such a situation the reaction follows mainly this path. Other possible pathways that cause dilution of optical purity are avoided. A fundamental point to note is that diastereomers, unlike enantiomers, need not have identical energies. The presence of a C2 axis of symmetry in these ligands makes some of the possible diastereomeric transition states structurally and energetically equivalent. As the... 

Ketals have also been used as an alternative to link the chiral inductor to the starting reagents. An important asymmetric induction was observed during the cycloaddition of ketals 94 having a C2 axis of symmetry, and a cyclopentenone or cyclohexenone derivative 95. Unfortunately, the observed chemical yields of 96 remain low (Scheme 22) [68]. With ketals of aliphatic enones, the selectivity decreases, and complex mixtures of isomers were observed [69]. 

A particularly interesting case of asymmetric induction occurred with d A-(2-benzoylethyl)-A-tosylglycinamides derived from pyrrolidines 158 havinj C2 axis of symmetry as the only source of chirality. A large chiral discriminati... 

Figure 28-22 The experimental x-ray crystal structure of prealbumin with bound i-thyroxine. Prealbumin is a letramer with four identical subunits, A, B, C, and D, The four identical subunits form a channel with two bound i-thyroxine molecules. The binding sites have a C2 axis of symmetry.

More recently Katsuki and coworkers have reported that (Z)-enolates of a-alkyl and a-heterosub-stituted amides such as (134), derived from pyrrolidine derivatives having a C2 axis of symmetry, undergo very diastereoselective alkylations with secondary alkyl and other alkylating agents in good to excellent chemical yields (Scheme 62) As with prolinol ether amide enolates, it appears that the direction of approach of the alkylating agent to the enolate (134) is controlled mainly by steric factors within the chiral auxiliary, i.e. chelation effects seem to be of little importance. 

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