Diene stereochemistry

Catalysts which lead to cis polymer show a significantly higher stereoselectivity in the bond forming reaction. When (ir-allyl nickel iodide)2 modified with TiCl is employed, the 1, 2 deuterium stereochemistry is 70% dl, and 30% meso as revealed by analysis of succinic anhydride. In addition, monomer isomerization is extensive, and could account for a large fraction of the meso structures which are formed, j ke use of (ir-allyl nickel trifluoroacetate)2 as catalyst led to a similar result (32% meso), accompanied by little if any monomer isomerization. Thus, it appears that in reactions to form cis polymer, some, but not always all, of the stereochemical information present in the starting diene is preserved in the polymer. In contrast, none of the initial diene stereochemistry can be detected in the trans polymer. 

The stereoregularity of polystyrenes prepared by anionic polymerization is predominantly syndiotactic (racemic diad fraction P = 0.53-0.74) and the stereoregularity is surprisingly independent of the nature of the cation, the solvent, and the temperature, in contrast to the sensitivity of diene stereochemistry to these variables [3, 156]. The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., atactic) microstructure for example, was 0.53 for the butyllithium/toluene system [3, 191, 192]. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium f-butoxide to 58% isotactic triads with potassium f-butoxide [193]. 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

When the diene stereochemistry is changed, the stereochemistry of the adduct differs... 

Maintaining an eye for ciarity With every edition we improve the presentation of topics, reactions, and diagrams where the opportunity arises. In this edition some examples include improved discussion and diagrams regarding endo and exo Diels-Alder transition states, the effect of diene stereochemistry in Diels-Alder reactions (Section 13.1 OB), and improved mechanism depictions for aromatic sulfonation and thionyl chloride substitution. 

Note that the stereochemistry comes out right. H s a and b are cis because they were cis in the starting quinone and the Diels-Alder reaction is stereospecific in this respect. H is also cis to and H " because the Diels-Alder reaction is stereoselectively endo. These points are described in more detail in Norman p.284-6 and explained in Ian Fleming Frontier Orbitals and Organic Chemical Reactions, Wiley 1976, p. 106-109. How would you make diene A ... 

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). 

The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

Diacetoxylation of various conjugated dienes including cyclic dienes has been extensively studied. 1,3-Cyclohexadiene was converted into a mixture of isomeric l,4-diacetoxy-2-cyclohexenes of unknown stereochemistry[303]. The stereoselective Pd-catalyzed 1,4-diacetoxylation of dienes is carried out in AcOH in the presence of LiOAc and /or LiCI and beiizoquinone[304.305]. In the presence of acetate ion and in the absence of chloride ion, /rau.v-diacetox-ylation occurs, whereas addition of a catalytic amount of LiCl changes the stereochemistry to cis addition. The coordination of a chloride ion to Pd makes the cis migration of the acetate from Pd impossible. From 1,3-cyclohexadiene, trans- and ci j-l,4-diacetoxy-2-cyclohexenes (346 and 347) can be prepared stereoselectively. For the 6-substituted 1,3-cycloheptadiene 348, a high diaster-eoselectivity is observed. The stereoselective cij-diacetoxylation of 5-carbo-methoxy-1,3-cyclohexadiene (349) has been applied to the synthesis of dl-shikimic acid (350). 

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

It was claimed that the Z-form of the allylic acetate 430 was retained in homoallylic ketone 431 obtained by reaction with the potassium enolate of 3-vinylcyclopentanone (429), after treatment with triethylborane[282]. Usually this is not possible. The reaction of a (Z)-allylic chloride with an alkenylaluminum reagent to give 1,4-dienes proceeds with retention of the stereochemistry to a considerable extent when it is carried out at -70 C[283]. 

The reaction of the allylic acetate with a diene system 784 affords the poly-fused ring system 785 by three repeated alkene insertions[487]. An even more strained molecule of the [5.5.5.5] fenestrane 788 has been constructed by a one-pot reaction in a satisfactory yield by the Pd-catalyzed carbonylation-cycliza-tion of 786 without undergoing elimination of /3-hydrogen in the cr-alkylpalla-dium intermediate 787 owing to unfavorable stereochemistry for syn elimination[488]. 

Another example of a reaction in which the stereochemistry of the process provides some valuable information about the mechanism is the thermal rearrangement of 1,5-dienes and substituted analogs ... 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

Most dienones that have been reduced have structures such that they cannot give epimeric products. However, reduction of 17 -hydroxy-7,17a-dimethyl-androsta-4,6-dien-3-one (63) affords 17 -hydroxy-7j9,17a-dimethylandrost-4-en-3-one (64), the thermodynamically most stable product, albeit in only 16% yield. The remainder of the reduction product was not identified. Presumably the same stereoelectronic factors that control protonation of the / -carbon of the allyl carbanion formed from an enone control the stereochemistry of the protonation of the (5-carbon of the dienyl carbanion formed from a linear dienone. The formation of the 7 -methyl compound from compound (63) would be expected on this basis. 

The FMO approach shown in Figure 15.19 again indicates that the 4s - - 2s interaction should lead directly to formation of two new bonding a-bonds, i.e. this is an allowed reaction. The preference for a concerted 4s + 2s reaction is experimentally supported by observations which show that the stereochemistry of the diene and dieneophile is carried over to the product, e.g. a trans,frans-l,4-disubstituted diene... 

The ring closure of a diene to a cyclobutene can occur with rotation of the two termini in the same conrotatory) or opposite disrotatory) directions. For suitable substituted compounds, these two reaction modes lead to products with different stereochemistry. 

The validity of the model was demonstrated by reacting 35 under the same reaction conditions as expected, only one diastereoisomer 41 was formed, the structure of which was confirmed by X-ray analysis. When the vinylation was carried out on the isothiazolinone 42 followed by oxidation to 40, the dimeric compound 43 was obtained, showing that the endo-anti transition state is the preferred one. To confirm the result, the vinyl derivative 42 was oxidized and the intermediate 40 trapped in situ with N-phenylmaleimide. The reaction appeared to be completely diastereoselective and a single diastereomer endo-anti 44 was obtained. In addition, calculations modelling the reactivity of the dienes indicated that the stereochemistry of the cycloaddition may be altered by variation of the reaction solvent. 

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

Unlike polyethylene and other simple aikene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

Another stereochemical feature of the Diels-Alder reaction is that the diene and dienophile partners orient so that the endo product, rather than the alternative exo product, is formed. The words endo and exo are used to indicate relative stereochemistry when referring to bicyclic structures like substituted norbornanes (Section 4.9). A substituent on one bridge is said to be exo if it is anti (trans) to the larger of the other two bridges and is said to be endo if it is syn (cis) to the larger of the other two bridges. 

Show the product of the Diels-Alder reaction of the following diene with 3-buten-2-one, H2C=CHCOCH3. Make sure you show the full stereochemistry of the reaction product. 

Note that a conjugated diene and a conjugated triene react with, opposite stereochemistry. The diene opens and doses by a conrotatory path, whereas the triene opens and closes by a disrotatory path. The difference is due to the different symmetries of the diene and triene HOMOs. 

Figure 30.7 Photochemical cyclizations of conjugated dienes and trienes. The two processes occur with different stereochemistry because of their different orbital symmetries.

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

Diels-Alder reaction, 492 characteristics of, 492-497 dienes in, 496-497 dienophiles in. 493-494 electrostatic potential map of. 493 endo stereochemistry of, 495 HOMO in. 1188-1189 LUMO in. 1188-1189 mechanism of. 493 s-cis diene conformation in, 496-497... 

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