Two Different Substituents

We can calculate the steric energy of the conformations of cyclohexanes substituted with two different substituents using the same techniques described above. For example, consider the two conformations of c -4-methylcyclohexanol. 

Draw the chair conformations of c -l-chloro-4-methylcyclohexane and determine the most stable conformation. Use the data in Table 4.6 to determine the energy difference for the two conformations. 

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If a disubstituted cyclohexane has two different substituents then the most stable conformation is the chair that has the larger substituent m an equatorial orientation This IS most apparent when one of the substituents is a bulky group such as tert butyl Thus the most stable conformation of cis 1 tert butyl 2 methylcyclohexane has an equatorial tert butyl group and an axial methyl group... 

When two different substituents are attached to each carbon atom of the double bond, cis-trans isomers can exist. In the case of c T-2-butene (Fig. 1.11a), both methyl groups are on the same side of the double bond. The other isomer has the methyl groups on opposite sides and is designated as rran5--2-butene (Fig. l.llb). Their physical properties are quite different. Geometric isomerism can also exist in ring systems examples were cited in the previous discussion on conformational isomers. 

The number of naphthalene derivatives is very large, since the number of positional isomers is large 2 for monosubstitution, 10 for disubstitution—same substituent, 14 for disubstitition—different substituents, 14 for trisubstitution—same substituent, 42 for trisubstitution—two different substituents, 84 for trisubstitution—three different substituents, and so on with multiplying complexity. 

A carbon atom in a ring can be a chirality center if it bears two different substituents and the path traced around the ring from that carbon in one direction is different from that traced in the other. The car bon atom that bears the methyl group in 1,2-epoxypropane, for exfflnple, is a chuality center. The sequence of groups is O—CH2 as one proceeds clockwise around the ring from that atom, but is CH2—O in the counterclockwise direction. Similarly, C-4 is a chirality center in limonene. 

The halogen atom in benz-chloro substituted quinazolines is very stable (as in chlorobenzene), whereas the halogen atoms in positions 2 and 4 show the enhanced reactivity observed with halogen atoms on carbon atoms placed a and y to heterocyclic ring nitrogens. The chlorine atom in position 4 is more reactive than in position 2, and this property has been used to introduce two different substituents in the pyrimidine ring. ... 

Tire presence of two different substituents on the ring carbon atom of dithiirane 1-oxides provides two asymmetric centers, i.e., four stereoisomers. 

Two different substituents at the P of different length Amorphous elastomers... 

Similarly, energy-transfer processes, together with electron transfer and hydrogen abstraction reactions could be induced in poly(organophosphazenes) in an intramolecular way by preparing POPs geminally substituted at the same phosphorus with two different substituent groups. 

When the two groups in disubstituted benzenes are different, the same three isomers are possible that are possible when the substituents are the same. Compounds with two different substituents are usually named as positional derivatives of a monosubstituted (parent) compound. Above, the common (and commercial) name for methylbenzene is toluene, and the chlorinated derivatives are named as shown above. However, the same two chlorinated derivatives can also be properly named 2-chloromethylbenzene and 4-chloromethylbenzene. In this case, for naming, the parent compound is methylbenzene and it is understood that the methyl group is in the 1-position. The terms ortho- (1,2-), meta- (1,3-), and para- (1,4-) are also sometimes used for example, 2-chlorotoluene can be called ortho-c Aoioio -uene. This can be very confusing, but in the chemical industry, outside of the research labs, the common names for the parent compounds are almost always used. 

Disilene 2 having a bulkier substituent, 2,4,6-triisopropylphenyl, was obtained similarly.7 When trisilanes bearing two different substituents on the central silicon atoms are used, a mixture of E- and Z-isomers of disilenes is obtained (Eq. 3).8-10 Attempts to synthesize disilenes with other substituents using trisilane Tip(R)Si(SiMe3)2 (R = H, F, Cl, or 1-pyrroyl) were unsuccessful.10... 

Dimerization of the stannanethiones and stannaneselones with two different substituents resulted in a mixture of the cis and trans isomers. 

All of the soluble polymers (1 and 3-6) give high resolution NMR spectra (1H, 13C, and 31P) that are completely consistent with their proposed structures. As observed for other types of poly(phosphazenes), the 31P chemical shifts of these alkyl/aryl substituted polymers are consistently ca. 15-30 ppm upfield from those of the analogous cyclic trimers and tetramers. Some important structural information is provided by 13C NMR spectroscopy, particularly for the phenyl/alkyl derivatives 3 and 4. These polymers are rare examples of phos-phazenes that contain two different substituents at each phosphorus atom in the chain. Thus, they have the possibility of being stereoregular. The fact that the structures are completely atactic, however, is confirmed by the observation of three doublets in the P-Me region of the 13C NMR spectrum (ca. 22 ppm) in a 1 2 1 intensity ratio. 

This chapter is organized according to the position of the phosphine groups P, as depicted in Fig. 25.2. It is important to realize that many of the ligands described here have both planar (Cp ring with two different substituents) as well... 

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... 

Spino and colleagues134 studied the Diels-Alder reactions of vinylallenes aiming to synthesize six-membered rings with a tetrasubstituted exocyclic double bond, which were to be employed as precursors of quassinoids. Some representative results of their investigations have been summarized in Table 5 (equation 56). Due to the presence of two different substituents at the allene terminus of 200, facial differentiation occurred, which resulted in non-equivalent amounts of geometrical isomers 201 and 202. The major isomers obtained in each case were formed by endo attack of maleic anhydride 144 at the less hindered face of the diene. 

While most polymers contain only one chiral or asymmetrical center in each repeat unit, it is possible to have diisotacticity where two different substituents are present at chiral centers. These isomers are labeled erythro- and threodiisotactic and erythro- and threosyndiotactic isomers (Figure 2.6). This topic is further described in Appendix J. 

While monosubstituted alkenes usually react with high regioselectivity, it is not true for disubstituted alkenes. Formation of mixtures of type 163 and 164 (equation 105) has been observed in most cases when unsymmetrical alkenes bearing two different substituents possess similar stereoelectronic properties. In general, regioselectivity is controlled by a combination of HOMO-LUMO interactions, steric effects and hydrogen bonding between suitable substituents in both alkene and nitrone molecules . ... 

Chiral allenes with two different substituents at C-5 gave good diastereoselectivities when oxidised by RuCl3/aq. Na(IO )/CH3CN-EtOAc/0°C (Fig. 3.7) [16]. Several... 

Additional examples of similar reactions involving two different substituents (R) on the pyridazine ring have also been reported <2003BKC1181>. 

The C=C consists of a cr bond and a tt bond. The tt bond is in a plane at right angles to the plane of the single cr bonds to each C (Fig. 6-1). The tt bond is weaker and more reactive than the a bond. The reactivity of the tr bond imparts the property of unsaturation to alkenes alkenes therefore readily undergo addition reactions. The tt bond prevents free rotation about the C=C and therefore an alkene having two different substituents on each doubly bonded C has geometric isomers. For example, there are two 2-butenes ... 

Synthesis of 2,4-disubstituted 1,3,5-triazines bearing two different substituents 522... 

Triazines containing two different substituents are formed by a variety of methods. The guidelines described above are applicable to these compounds also (see Section 2.20.4.9.4). The condensation of imidates with amidines provides a valuable route to 6-substituted 2,4-dialkyl-l,3,5-triazines (see Section 2.20.4.6.2). 

In this section, the classification of carbenes and carbenoids bearing two different substituents is made by giving priority to the functional group which will appear in the later... 

Alkyl- and l-aryl-5-hydroxytriazolines with two different substituents at C-4 exist in solution as an equilibrium mixture of two diastereomers,261,262 apparently as a result of the conversion of the triazoline to open-chain... 

Some of the l-aryl-5-hydroxy-A2-l,2,3-triazolines with two different substituent groups at C-4 exist in solution as a mixture of the diastereomers they are readily interconvertible via a ketotriazene (59-62).259 The compositions of the equilibrium mixtures for a number of 1-methyl- and l-benzyl-5-hydroxy-A2-l,2,3-triazolines in solution were determined by NMR spectroscopy. The 8 values of the methyl groups at C-4 fall into two regions, namely at 81.41-1.57 or 1.05-1.21. They can be oriented either cis or trans relative to the hydroxy group at C-5. The compounds with bulky substituents at C-5 exist only in one form in which the bulky C-5 group is trans to the methyl group at C-4. The... 

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