Priority sequences

Figure 2-71. The ordered list of 24 priority sequences of the ligands A-D around a tetrahedral stereocenter, The permutations can be separated into two classes, according to the Cl P rules the R stereoisomer is on the right-hand side, and the S stereoisomer on the left.

Availability constraints the macroscopic limits on material resources and the availability or up-time of equipment. Availability of raw materials is an obvious constraint at scheduling. Obviously, no catalytic hydrogenation can be done if the catalyst is unavailable. Simultaneous operation of certain tasks is restricted by the limited availability of common utilities such as steam, electricity, or labour. The priority sequence in a product chain needs to be respected by ensuring that intermediate products are manufactured in time to be available when required by a batch of the consecutive product. 

Hi) A priority sequence is determined for substituents at the asymmetric centre. 

In general, the d,l convention signals a configuration change, if any, as follows d- d or L— L means no change (retention), and d— i or l- d means change (inversion). The RS convention can also be used, but it requires working out the priority sequences. 

Chirality symbols based on the priority sequence. The procedure is applied as for tetrahedra, but modified because there is a unique principal axis. The structure is oriented so that the viewer looks down the principal axis, with the ligand having the higher priority closer to the viewer. Using this orientation, the priority sequence of ligating atoms in the [horizontal] plane is examined. If the sequence proceeds in a clockwise fashion, the chirality symbol C is assigned. Conversely, if the sequence is anticlockwise, the symbol A is assigned. 

A general method of stereochemical notation based on the ligand priority sequence rule of Cahn, Ingold, and Prelog (CIP),8,9 developed at Chemical Abstracts Service,0 1 has been in use in Chemical Abstracts Chemical Substance Indexes for mononuclear coordination compounds since 1972. 

The understanding of R and 5 is simple the problems are in assigning the priority sequences for actual substituents. The rules follow ... 

Priority is given to the substituent atoms that have the highest atomic number. This means that four different atoms arranged tetrahedrally about the chiral center have a priority sequence that decreases with decreasing atomic number. For example, the sequence among the halogens is I > Br >... 

The first-atom priority sequence is Cl > C and C > H. We now need to order —CH2CH3 and —C=CH and, in doing this, we compare the three atoms attached to the first carbon of the ethyl group (C, H, H) with the three attached to the first carbon of the ethynyl group [C, (C), (C)]. On this basis, ethynyl comes ahead of ethyl, and the overall sequence is Cl > C=CH > CH2CH3 > H, so 8 will have the S configuration. 

Exercise 19-8 Determine by the sequence rules the priority sequence in the pairs of groups listed. 

An order of precedence is established for the two atoms or groups attached to each end of the double bond according to the sequence rules of Section 19-6. When these rules are applied to l-fluoro-l-chloro-2-bromo-2-iodoethene, the priority sequence is ... 

According to this proposal, and in addition to the rules of Cahn et al., the priority sequence of the four ligands in the Fe complex 19b in Scheme 26 is C5H5 > P(C6H5)3 > COOC 0H,9 > CO, and the configuration at the Fe atom can be specified by use of the sequence rule as (S) (106, 107). For the comparison of literature data, it is necessary to note that in some... 

The familiar carbon chiral center has four different substituents as shown in 3-hydroxybutanoic acid (compound e in Figure 3.43). This chiral center is designated R in accordance with the well-known priority-sequence rules in the enantiomeric compound, the chiral center is designated 5. Both enantiomers give the same NMR spectrum in an achiral solvent, as does the racemate. Because of the chiral center, there is no symmetry element, and the methylene protons are diastereotopes. 

Tables 3-11 are organized according to the a-heteroatom. Where there are two different hetero-substituted groups present in the carbanions, the listing in the tables is in accord with the priority sequence N, P, O, S, Se and halogen (as heteroatoms), COjR, CN, and metalloids. This takes no regard of the stabilizing effectiveness of the hetero-substituted groups present. Table 12 lists lithioformamides and -thioformamides.

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