Other stereochemistries

Most Fe complexes are octahedral but several other stereochemistries are known (Table 25.3). [FeX4] (X = Cl, Br, I, NCS) are tetrahedral. A single absorption around 4000 cm due to the T2 E transition is as expected, but magnetic moments of these and other apparently tetrahedral complexes are reported to lie in the range 5.0-5.4BM, and the higher values are difficult to explain. 

We promised in chapter 1 that a synthesis of the elm bark beetle would appear and here it is. It has four chiral centres but one of them (marked as a hidden carbonyl group) is unimportant. Disconnecting the acetal reveals keto-diol 33. If we make 33 it must cyclise to 3—-no other stereochemistry is possible. Further C-C disconnection with alkylation of an enolate in mind reveals symmetrical ketone 34 and a diol 35 with a leaving group (X) at one end and the two chiral centres (marked with circles) adjacent. 

Although not nearly as mainly results are available for other stereochemistries, regioselectivities seem to be in accord with those already noted. Thus, allopyranosides react at 0-2 when 0-4 is protected,231 and talopyrano-sides have a reference for reaction at 0-3.9,205-233... 

Catalytic codimerizations between cyclopropenes and alkenes (or alkynes) to give four-membered rings have only been achieved with 3,3-dimethyl- and 3-cyclopropyl-3-methylcyclopropene on the one hand and norbomene or norbomadiene on the other (stereochemistry not determined)77> ... 

Whereas chirality in tetrahedral compounds of carbon requires four different groups to be bonded around the tetrahedral carbon atom, this is not necessarily the case for other central atoms with other stereochemistries. For example, octahedral complexes have more relaxed rules. Whereas a chiral tetrahedral organic compound is, as a consequence of the rule for chirality, asymmetric (or totally lacking in symmetry), chiral octahedral complexes need not be asymmetric, but may have axes of rotation (they are then dissymmetric). The common rule for chirality is simple - a compound must have non-superimposable mirror images. For an octahedral complex, this can occur even when three different pairs of monodentate ligands are coordinated, as discussed later. We shall look a little more closely at four- and six-coordinate complexes below. 

Irradiation of the C25 equatorial methyl protons (8 0.89) of the stereoisomer 155 gave positive NOEs for the H10 and H24 protons. In addition to the H4, Hs, H25 protons, the H2 proton was enhanced instead of the ester methyl proton, when the C24 axial methyl protons (8 1.23) were irradiated. Therefore, the methoxycarbonyl group of 155 had to be attached equatorially to the cyclohexanone ring, and the other stereochemistries were identical with those of 154. 

In Chapter 1 we discussed the synthesis of multistriatin (20), a pheromone of the elm bark beetle. The time has come for a stereochemical analysis of this problem. The molecule has four chiral centres ( in 20). One of them (a) turns out to be unimportant as disconnection of the acetal reveals (21) as the true-target. If (21) cyclises to form an acetal it must give (20)—no other stereochemistry is possible. 

Stereochemistry at 13-follows from rotation. Other stereochemistry is probable but rigid evidence seems to be lacking. 

Tabic 2-6 gives an overview on the most common file formats for chemical structure information and their respective possibilities of representing or coding the constitution, the configuration, i.c., the stereochemistry, and the 3D structure or conformation (see also Sections 2..3 and 2.4). Except for the Z-matrix, all the other file formats in Table 2-6 which are able to code 3D structure information arc using Cartesian coordinates to represent a compound in 3D space. 

Comments The diene A is symmetrical so it doesn t matter which double bond is attacked by the carbene. On the other hand, it may be difficult to stop carbene addition to the second double bond. The only control over the stereochemistry will be that the trans compound we want is more stable. Japanese chemists have recently synthesised optically active trans chrysanthemic acid by this route (Tetrahedron Letters. 1977, 2599). 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively[5,l8-20]. In the first step, 7r-allylpalladium complex formation by the attack of Pd(0) on an allylic part proceeds by inversion (anti attack). Then subsequent reaction of soft carbon nucleophiles, N- and 0-nucleophiles proceeds by inversion to give 1. Thus overall retention is observed. On the other hand, the reaction of hard carbon nucleophiles of organometallic compounds proceeds via transmetallation, which affords 2 by retention, and reductive elimination affords the final product 3. Thus the overall inversion is observed in this case[21,22]. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

The regioselective and stereospecific construction of C-20 stereochemistry is explained by the following mechanism. The Pd(0) species attacks the ( )-/3-carbonate 616 from the a-side by inversion to form the Tr-allylpalladium species 620, which has a stable syn structure[392]. Then concerted decarboxylation-hydride transfer as in 621 takes place from the a-side to give the unnatural configuration in 617. On the other hand, the Tr-allylpalladium complex 622... 

In this case the relationship between stability and stereochemistry is easily explained on the basis of van der Waals strain The methyl groups on the same side of the ring m cis 1 2 dimethylcyclopropane crowd each other and increase the potential energy of this stereoisomer Steric hindrance between methyl groups is absent m trans 1 2 dimethylcyclopropane... 

The Cahn-Ingold-Prelog R-S notation has been extended to chiral allenes and other molecules that have a chiral ity axis Such compounds are so infrequently encountered however we will not cover the rules for specifying their stereochemistry in this text... 

Maltose obtained by the hydrolysis of starch and cellobiose by the hydrolysis of cellulose are isomenc disaccharides In both maltose and cellobiose two d glucopyra nose units are joined by a glycosidic bond between C 1 of one unit and C 4 of the other The two are diastereomers differing only m the stereochemistry at the anomeric carbon of the glycoside bond maltose is an a glycoside cellobiose is a (3 glycoside... 

The probabilities of the various dyad, triad, and other sequences that we have examined have all been described by a single probability parameter p. When we used the same kind of statistics for copolymers, we called the situation one of terminal control. We are considering similar statistics here, but the idea that the stereochemistry is controlled by the terminal unit is inappropriate. The active center of the chain end governs the chemistry of the addition, but not the stereochemistry. Neither the terminal unit nor any other repeat unit considered alone has any stereochemistry. Equations (7.62) and (7.63) merely state that an addition must be of one kind or another, but that the rates are not necessarily identical. 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

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