Unsaturated carbon atoms

I Ozonolysis of alkenes yields ketones if one of the unsaturated carbon atoms is disubstituted (Section 7.9). 

Hydrosilylation turned out to be a unique method in organosilicone chemistry, but in some cases it suffers from severe side reactions. An explanation is provided by the generally accepted reaction mechanism known as "Chalk-Harrod mechanism" described elsewhere [7]. Included in this series of reaction steps is an insertion of olefmic ligands into a platinum-hydrogen bond. Since the metal may be bonded to either of the unsaturated carbon atoms and the reaction is also an equilibrium, alkenes may result which are in fact isomerized starting material. Isomeric silanes are to be expected as well (Eq. 1), along with 1-hexylsilane, which is by far, the main compound produced. 

Analysis of possible structures and reaction pathways in reactions 1-4 led to various model structures for these complexes (9t25). Some of these involved C-H activation of the substituents attached to the unsaturated carbon atoms. To test the validity of these models, two additional types of metal vapor reactions were examined. In one case, reactions with simpler unsubstituted hydrocarbons were examined. In another case, substrates ideally set up for oxidative addition of C-H to the metal center were examined. As described in the following paragraphs, both of these approaches expanded the horizons of organolanthanide chemistry. 

Compounds in which a hydroxyl group is attached to an unsaturated carbon atom of a double bond (i.e., C=C-OH) are called enols. 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

The l-arylpyrazol-5-ones (4.9), prepared by the two-step condensation of an arylhydrazine with ethyl acetoacetate, are the most commonly used coupling components for the synthesis of greenish yellow azo dyes. Coupling occurs at the 4-position of the pyrazolone ring which, as in the case of the acetoacetarylamides discussed above, is activated towards electrophilic attack by the two flanking unsaturated carbon atoms (Scheme 4.14). 

Note that during the cyclization process, the bonds between the unsaturated carbon atoms and their nearest substituent remain intact and on this basis the configurations have been determined. If, however this center, also called steric center, is disturbed, then the results of configurations may become erroneous. 

The selectivity in the hydrogenation of isolated double bonds depends on the type of substitution of the unsaturated carbon atoms, as in the reaction in equation 55144. 

Table 8.17 shows the scope of the reaction of acetylcobalt tetracarbonyl with polyenes. The reactions are regiospecific with the acetyl group adding to the terminal unsaturated carbon atom of the ir-electron system to produce the E-a,p-unsaturated ketones [9]. In the reaction with fulvenes [10], only the 1-acetyl and 1,4-diacetyl derivatives are formed, with no evidence of the 2-isomer. This is an indication of the relative stabilities of the cyclic it-allyl complexes, compared with the exocyclic complex. It has been postulated that, in the reactions of conjugated systems, the initial o-allyl adduct proceeds to the products via the it-allyl complex (cf Scheme 8.1), whereas in the case of unconjugated tt-systems, the initial o-adduct is more stable and tends to undergo a further carbonylation reaction. 

Hadler 26) employed conformational analysis to explain the difference in the proportion of cholestane to coprostane derivatives resulting from the reduction of A and d steroids. He suggested that the hydrogenation process involved the formation of a quasi-ring structure between the unsaturated carbon atoms and two hydrogens originally dissolved in the metal, a mechanism which is similar to one proposed by Beeck (27) and by Jenkins and Rideal 28). He assumed, in effect, that the saturated struc-... 

Figure 3. The conformational sphere for pyranoid rings. The perfect chairs are at the north and south poles (0=0 and 180 , respectively). The boat and skew (B and S designations) at the equator permit pseudorotation that is slightly hindered, at least for cyclohexane. The envelopes, E (also called sofas and half-boats), and half-chairs, H, are not observed for rings coiqposed of saturated carbon and oxygen atoms, but are iiqportant forms for rings with unsaturated carbon atoms. The aiqplitude of puckering corresponds to the radius of the sphere.

Increasing separation of radical and unsaturated carbon atom —> ... 

Fig. 6-11 Reaction coordination diagram for the reaction of a polymer radical wth a monomer. The dependence of the potential energy of the system (radical + monomer) on the separation between the radical and the unsaturated carbon atom of the monomer is shown. The subscript. indicates the presence of a substituent that is capable of resonance stabilization. Activation energies are represented by the solid-line arrows heats of reaction, by the broken-line arrows. After Walling [1957] (by permission of Wiley, New York).

This result is similar to that obtained by Widing and Levitt for the normal alkanes and to that observed for alkyl-substituted ethylenes [171] in the latter case, linear correlations were obtained between the IPs and the sum of charges of the unsaturated carbon atoms, whereby any increase of their electron density due to substituent effects leads to a lowering of the molecular ionization potential. 

When bonded to an unsaturated carbon atom or to an arene, the fluorine atom exerts an inductive electron-withdrawing effect (cti>0) and an electron-donating effect through resonance (ctr < 0), both being very superior to the effects of the other halogens (Figure 1.1). The values of the Hammet parameters crj and fluorinated substituents are reported in Table 1.13. " ... 

Reactions with an unsaturated carbon atom bonded to a heteroatom have been less intensively studied. However, transformations of Schiff bases and imino ethers have been reported the course of the reaction depends on the structure of the substrate and the reaction conditions,22 32,4950 e.g. formation of 26. 

Hz, with larger J values for unsaturated carbons atoms owing to the shorter bonds in these systems. 

Nomenclature system . The prefix 17 indicates that all unsaturated carbon atoms of the ligand indicated are ir-bonded to the metal atom for complexes in which the metal atom is bonded only to particular carbon atom(s) of a ligand, the total (x) of such metal-bonded atoms is indicated by a superscript (tj ) or, for ambiguous cases, by preceding locant designators, e.g. (tj -allyl), (1-4-Tj-cyclooctatetraene). 

The effect of lithiating various unsaturated ethers, including 2,3-dihydrofuran, has been examined by means of 13C NMR spectroscopy and from the results (Table 24) the degree of s character in the unsaturated carbon atoms has been estimated. It differs but little amongst vinyl ethers and is somewhat more than the 33.3% of a formal sp2 hybrid (80JOC4959). 

Also, hydrogen atoms that are bonded directly to the unsaturated carbon atoms of a double bond often are called vinyl hydrogens, although the term alkenic hydrogens is more accurate and therefore preferable. 

Insertion into CH bonds may occur both at saturated and at unsaturated carbon atoms. 

If steric interactions are the main factors in determining the different free energies for the four possible transition states, the substituent would occupy preferentially that quadrant in which more space is available. If the quadrant preferred by the substituent is known, i.e., if the relative positions of L, S, Z, and H are known, we can predict the enantiomer that is formed prevailingly. The same model also allows us to predict which of the two unsaturated carbon atoms of the substrate will be bound to the metal and carbonylated eventually. For instance, in the case of a monosubstituted olefin (see Scheme VII), Isomer b will prevail over the sum of the antipodes of the other isomer (a + c) and Antipode a will prevail over Antipode c. 

Table VI. Comparison Between Experiments and Forecast for the Prevailingly Carbonylated Unsaturated Carbon Atom and for Absolute Configuration in Asymmetric Hydrocarbalkoxylation by PdCl2/(-)DIOP on the...

Reaction (15) actually involves electrophilic attack at an unsaturated carbon atom and leads, as might be expected, to rates of substitution very different to those for the corresponding saturated compounds. 

Optically active aldehydes can be obtained by asymmetric hydroformylation of olefinic substrates when at least one asymmetric carbon atom is formed either by addition of a formyl group or of a hydrogen atom to an unsaturated carbon atom (Scheme 1, reactions (1) and (2)). In the case of trisubstituted olefins, two new asymmetric carbon atoms can form due to the cis stereochemistry of the reaction10), in the absence of isomerization, the formation of only one epimer is expected. 

In Table 6 the results concerning the asymmetric hydroformylation of 1-butene and of styrene with different catalytic systems are reported. When rhodium-containing catalytic systems are used in the presence of several diphosphine ligands, the face of the prochiral unsaturated carbon atom which is preferentially formylated is the same in both substrates for each chiral ligand. 

The same is also true for the Co/( + )-R -Sal and for the [(—)-DIOP-DBP]PtCl2/ SnCl2 48> catalytic systems. For [(-)-DIOP]Pt(SnCl3) Cl43) and [(-)-CHIRAPHOS] Pt(SnCl3) Cl52), however, opposite faces of the unsaturated carbon atom are mainly formylated in the two substrates. 

On the whole, with each catalytic system, the same face of the substituted unsaturated carbon atoms is attacked for different substrates. There are 2 exceptions out of 32 cases for rhodium- and 2 exceptions out of 8 cases for platinum-containing catalytic systems. 

The results obtained are listed in Table 8. For identical catalytic systems, faces of the disubstituted unsaturated carbon atom having the same geometric requirements (although named differently, Fig. 4) are preferentially hydroformylated 15) in aromatic substrates. This is also true for aliphatic olefins. However, with aromatic... 

The results of the hydroformylation of internal olefins are reported in Table 9. In the case of (Z)- and (E)-2-butene, the same fare of the unsaturated carbon atom is formylated with either a rhodium- or platinum (—)-DIOP-containing catalytic system. With the rhodium catalyst, when an acyclic olefin is used as the substrate, the same fare is always attacked, and it is only the notation but not the geometric requirement that is different for (E)-l-phenyl-1-propene. The only exception is represented by bicyclo[2,2,l]heptene. However, using (—)-CHIRAPHOS instead of (—)-DIOP, also bieyelo[2,2,l]heptene behaves like internal butenes. No regularity is observed for the cobalt or ruthenium (—)-DIOP catalytic systems. With the same system, only in 3 cases out of 15 the face of the prochiral atom preferentially formylated has different geometric requirements. 

Table 9. Face of the Unsaturated Carbon Atom Prevailingly Attacked by CO in Internal Olefins Hydroformylation... 

The relative position of L with respect to S and of H with respect to Z can be decided, for instance, by hydroformylation of (Z)-2-butene, which yields only one aldehyde that is chiral. When the catalyst is optically active, the predominating antipode in the reaction product indicates the face of the unsaturated carbon atoms preferentially attacked by CO and therefore the more stable transition state (Fig. 8) (that is, on the assumption that the difference in the free energy of the transition state mainly depends on steric interactions, the transition state in which such steric interactions are smaller). 

In the case of internal Cs olefins, for which 4 transition states must exist (Fig. 9), the model also predicts the carbon atom preferentially formylated (corresponding to the unsaturated carbon atom nearer to the metal atom than to the hydrogen atom of the catalyst in the transition state). 

---