Identical substituents

Cis-trans stereoisomerism m alkenes is not possible when one of the doubly bonded carbons bears two identical substituents Thus neither 1 butene nor 2 methyl propene can have stereoisomers... 

This involves a more uniform distribution of charge because of the identical substituents and thus lacks the stabilizing effect of the polar resonance form. The activation energy for this mode of addition is greater than that for alternation, at least when X and Y are sufficiently different. 

If a tetrahedral center in a molecule has two identical substituents, it is referred to as prochiral since, if either of the like substituents is converted to a different group, the tetrahedral center then becomes chiral. Consider glycerol the central carbon of glycerol is prochiral since replacing either of the —CH9OH groups would make the central carbon chiral. Nomenclature for prochiral centers is based on the (R,S) system (in Chapter 3). To name the otherwise identical substituents of a prochiral center, imagine... 

The three possibilities of synthesizing a C5 chain, namely, C2 +Ci + C2. C2 -t C2 -I- Cl, or Cl + C3 + Cl, lead to pyrylium salts having identical substituents in positions 2 and 4 in the second case, or 2 and 6 in the first and third cases. Despite this limitation, such syntheses are very convenient because they make the pyrylium salts easily accessible (more so than other six-membered heterocyclic aromatics) from aliphatic starting materials. 

On the other hand, 1,1,1-trisubstituted alkanes behave similarly to aldehydes, yielding pyrjdium salts (128) with identical substituents in positions 2 and 6. Thus, Dorofeenko and co-workers condensed 2 moles of acetophenone with 1 mole of benzotrichloride in the presence of perchloric acid obtaining 2,4,6-triphenjdpjTylium perchlorate with 1 mole of ethyl orthoformate, they obtained 2,6-diphenyl-pyrylium perchlorate (57) from o-hydroxyacetophenone, ortho-formic ester and perchloric acid, 4-ethoxybenzopyrylium perchlorate was formed. 

Besides acetophenone, this reaction was also applied to p-chloro- andp-methoxyacetophenone, and even to an aliphatic ketone, acetone (although the yield was stated to be only half as large as that obtained from mesityl oxide, i.e., less than 30%, Dorofeenko and co-workers reported a 45% yield of 2,4,6-trimethylpyrylium perchlorate from acetone, acetic anhydride, and perchloric acid), and is the standard method for preparing pyrylium salts with identical substituents in positions 2 and 4. The acylating agent may be an anhydride in the presence of anhydrous or hydrated ferric chloride, or of boron fluoride, or the acid chloride with ferric chloride.Schneider and co-workers ... 

In compounds bearing several different groups there wiU be a complex interaction of activation by the azine-nitrogen with activation or deactivation by the substituents (Section II, E). The complexity of the interaction is emphasized by the realization that the effects of two identical substituents in an azine (e.g., in 2,4-dichloropyrimidine) are not the same on each other (Section II,B,2,a). 

The coefficient of xV in the three expressions indicates that there exist four different derivatives of cyclopropane of the form (disubstituted cyclopropane with two identical substituents). Two of the four derivatives are mirror images of each other, that is, they form a pair of optical antipodes. If the spatial arrangement is disregarded, only two distinct cyclopropanes with formula are... 

Use hyphens to separate the different prefixes, and use commas to separate numbers. If two or more different substituents are present, cite them in alphabetical order. If two or more identical substituents are present, use one of the multiplier prefixes d, fn -, tetra-, and so forth, but don t use these prefixes for alphabetizing. Full names for some of the examples we have been using follow. 

Figure 9.18 (a) When a prochiral molecule is held in a chiral environment, the two seemingly identical substituents (red) are distinguishable, (b) Similarly, when an achiral coffee mug is held in the chiral environment of your hand, it s much easier to drink from one side than the other because the two sides of the mug are now distinguishable. 

In contrast to the steric effoits, the purely electronic influences of substituents are less clear. They are test documented by linear free-energy relationships, which, for the cases in question, are for the most part only plots of voltammetrically obtained peak oxidation potentials of corresponding monomers against their respective Hammett substituent constant As a rule, the linear correlations are very good for all systems, and prove, in aax>rdance with the Hammett-Taft equation, the dominance of electronic effects in the primary oxidation step. But the effects of identical substituents on the respective system s tendency to polymerize differ from parent monomer to parent monomer. Whereas thiophenes which receive electron-withdrawing substituents in the, as such, favourable P-position do not polymerize at all indoles with the same substituents polymerize particularly well... 

The intermolecular dimerization of nitrile oxides has been described as a procedure to prepare Fx with identical substituent both in the 3 and 4 position (Fig. 3). This procedure is a [3 -F 2] cycloaddition where one molecule of nitrile oxide acts as 1,3-dipole and the other as dipolarophile [24-26]. Yu et al. has studied this procedure in terms of theoretical calculus [27,28]. Rearrangement of isocyanates competes with the bimolecular dimerization, with the former becoming dominant at elevated temperatures. 

In the case of an sp3-hybridized carbon containing three different substituents (prochiral), the substitution of a fourth kind of substituent for one of the two identical substituents creates a new chiral center. 

Conformationally dependent chirality is a common feature of molecules that contain carbon atoms with identical substituents. Another example is presented in Fig. 7.10. In dihydroxymethane, the conformation in which the H-O-C-O angles are trans, has a plane of symmetry, and the carbon atom is of the type C(a2b2). In contrast, when one of the O-H bonds is rotated out of the plane, the structural parameters change in an asymmetric way. The system is now of type C(aa bb ) and the mirror images cannot be superimposed. 

Pyramidal nitrogen is favorable for slow inversion. In this case, two methyl groups in the SilV Bu fragment are nonequivalent, whereas two SiMej groups are, on the contrary, equivalent. However, any two identical substituents at the nitrogen atom become nonequivalent in the presence of the asymmetric center G attached to the C,C double bond. 

In the examples presented so far, only two enantiomeric products have been possible in each case, since the substrates have all contained identical substituents on the Cl and C3 positions. However, a more complex situation occurs when the allyl system is unsymmetrically-substituted, as in 43a or 43b (Scheme 12).1161 Here, nucleophilic addition to the corresponding ri3-allyl intermediate 44 may afford an achiral, linear product 45, in addition to the pair of enantiomeric branched products 46. 

The cumulative effects of multiple substituents have been studied at length in search of particularly stable radicals. It is generally found that the repetitive addition of identical substituents leads to a stepwise decrease in RSE values. This is well illustrated by the comparison of the methyl, ethyl, isopropyl, and ferf-butyl radicals with RSE values of 0.0, - 13.8, - 23.3, and - 28.3 kj/mol. Thus, while the stability of the alkyl radicals clearly increases with the number of alkyl substituents attached to the radical center, the substituent ef-... 

In naming identical substituents on the longest carbon chain, be sure to use repeating location numbers, separated by commas (2,2-dimethyl). 

The FMO model is sometimes unable to correctly predict the regioisomer to be obtained from cycloadditions of dienes having either two different substituents or two identical substituents at two different positions. For example, substituents at C(l) have proven to... 

It is evident from the data in Table 6 that, with only one exception (entry 13), the combination of two captor or two donor substituents does not produce an additive effect, whereas, without exception, the captodative combinations display synergetic behaviour. Thus, the delocalization of the unpaired spin density in captodative radicals is markedly increased in comparison to pure additive superposition of capto and dative effects. This result is all the more significant since two identical substituents do not... 

To correctly address the problem of identification of target-specific privileged motifs, one should take into account the phenomenon of bioisosterism [26]. Thus, several different bioisosteric structures can constitute only one distinct privileged structural motif. In order to include all possible bioisosteric analogs into one cluster, we use a special algorithm of ChemoSoft based on a collection of rules for bioisosteric conversions described in literature. AH bioisosteric analogs are considered similar with similarity coefficient 1 if they have identical substituents around the central bioisosterically transformed fragment. 

The condensation of acetone and 1,2-diarylbiguanides bearing identical substituents yields the same alkali-stable product (CXXXVI) under both acidic and basic conditions (95). 

In the NMR spectra of 7-azabicyclo[2.2.1]heptadienes, with identical substituents at C-2 and C-3 as in structure 9, the bridgehead protons couple with the vinylic protons at C-5 and C-6 in a degenerate A2X2 system such that their two signals appear as triplets. This is likewise characteristic of carbocyclic norbornadiene derivatives. ... 

The condition of handedness or lack of superimposabil-ity of molecules differing only with respect to the stereochemical arrangement of identical substituents around a tetrahedral center. In 1893, Lord Kelvin succinctly defined chirality I call any geometrical figure, or any group of points, chiral, and say that it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide with itself. ... 

A stereochemical property of compounds arising from the ability of an enzyme s active site to distinguish between two chemically identical substituents covalently bound to a tetrahedral center (usually carbon and, in some cases, phosphorus). Prochirality is also termed prostereoisomerism. The classical example is citrate with its two carboxymethyl group substituents. Likewise, the Cl carbon atom of ethanol has two prochiral hydrogens. See Chirality Chirality Probes... 

At the time of the earlier review (66HC1155), it was already known that combinations of arenes with sulfur, or with sulfur mono- or dichlorides in the presence of Lewis acids (IV,B,1), or of aryl thiols, diaryl sulfides, or disulfides (IV,B,2 and 3) again heated with Lewis acid catalysts, generate thianthrenes, sometimes in acceptable preparative yields. A complimentary method is the treatment of aryl thiols with c. H2SO4. Routes from arenes and aryl thiols almost certainly involve the initial formation of diaryl sulfides. All these methods inevitably give symmetrical thianthrenes carrying identical substituents on each benzene ring (Scheme 9), unless the second sulfur is introduced in a controlled fashion into a preformed, unsymmetrical diphenyl sulfide. 

For molecules with two stereogenic centers, the traditional descriptors are erythro and threo to which, henceforth, a c (short for carbohydrate) is added for a reason that will become apparent (Table 10). They are unambiguously defined for simple sugars. The backbone is given by the carbon chain the reference conformation is the conformation with all carbon atoms eclipsed. This statement is identical with the requirement of a Fischer projection. If two identical substituents X are located on the same side/opposite sides of the plane traced by the backbone, then the c-erythrojc-threo configuration is assigned. 

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