Allylic transformation

This mechanism therefore involves interconversion between trans- and cis-2-butene in accordance with their experimental results. The activation energy for the anti- to syn-Tr-allyl conversion is about 50 kJ moP higher than that for the conversion of cis-2-butene to 1-butene. The forbidding activation energy for cis trans-2-butene conversion therefore effectively eliminates the direct anti- to syn-iT-allyl transformation at lower temperatures. 

Asymmetric Pd-catalyzed allylic alkylations and other allylic transformations can be now performed efficiently with very high enantioselectivity. There are several steps in the catalytic cycle where enantioselection can take place and this is nicely analyzed in a recent review. One of these steps is the nucleophilic attack on a Pd 77 -allyl complex bearing a chiral auxiliary ligand. Considering an equally substituted rf-zWyX and bearing in mind that the stereochemistry of the attack is trans- (exo-), the regiochemistry (Cl or C3) of the attack controls the configuration of the final product (Scheme 67). 

Rearrangements. "—Cyclopropyl-Allyl Rearrangement. This is one of the molecular rearrangements which has been investigated by calculations based upon the principle of least motion (PLM). ° In agreement with more sophisticated theoretical methods, PLM leads to the correct prediction of which of the two disrotatory openings will occur in the cyclopropyl-allyl transformation. 

The allyl radical plays an important role in many photochemical transformations, as further discussed in Section IV. 

The system provides an opportunity to test our method for finding the conical intersection and the stabilized ground-state structures that are formed by the distortion. Recall that we focus on the distinction between spin-paired structures, rather than true minima. A natural choice for anchors are the two C2v stmctures having A2 and B, symmetry shown in Figures 21 and 22 In principle, each set can serve as the anchors. The reaction converting one type-I structirre to another is phase inverting, since it transforms one allyl structure to another (Fig. 12). 

You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

Treatment of 7r-allylpalladium chloride with CO in EtOH affords ethyl 3-butenoate (321)[284]., 3, y-Unsaturated esters, obtained by the carbonylation of TT-allylpalladium complexes, are reactive compounds for 7r-allyl complex formation and undergo further facile transformation via 7r-allylpalladium complex formation. For example, ethyl 3-butenoate (321) is easily converted into 1-carboethoxy-TT-allylpalladium chloride (322) by the treatment with Na PdCL in ethanol. Then the repeated carbonylation of the complex 322 gives ethyl 2-... 

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]. 

Addition of several organomercury compounds (methyl, aryl, and benzyl) to conjugated dienes in the presence of Pd(II) salts generates the ir-allylpalladium complex 422, which is subjected to further transformations. A secondary amine reacts to give the tertiary allylic amine 423 in a modest yield along with diene 424 and reduced product 425[382,383]. Even the unconjugated diene 426 is converted into the 7r-allyllic palladium complex 427 by the reaction of PhHgCI via the elimination and reverse readdition of H—Pd—Cl[383]. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

In addition to the catalytic allylation of carbon nucleophiles, several other catalytic transformations of allylic compounds are known as illustrated. Sometimes these reactions are competitive with each other, and the chemo-selectivity depends on reactants and reaction conditions. 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

Hydrosilylation of I-vinyl-1-cyclohexene (77) proceeds stereoselectively to give the (Z)-l-ethylidene-2-silylcyclohexane 78, which is converted into (Z)-2-ethylidenecyclohe.xanol (79)[74]. Hydrosilylation of cyclopentadiene affords the 3-silylated 1-cyclopentene 80. which is an allylic silane and used for further transformations[75.75a]. Cyclization of the 1,3,8, lO-undecatetraene system in the di(2.4-pentadienyl)malonate 69 via hydrosilylation gives the cyclopentane derivative 81. which corresponds to 2.6-octadienylsilanc[l8,76]. 

The employment of non-protic electrophiles for the foregoing type of cyclizations as illustrated in Scheme 8 has the particular merit of leaving a useful point of departure for further transformations. Comparable cyclizations of 2-allyl-3-aminocyclohexenones with mercury(II) acetate are preceded by dehydrogenation to the corresponding 2-allyl-3-aminophenol as shown in Scheme 9 82TL3591). The preferred direction of cyclization depends upon the nucleophilicity of the amino group. 

The title compounds also undergo the Claisen rarrangement (5-allyloxypyrazoles 4-allyl-5-pyrazolones) and are readily transformed into 5-chloropyrazoles by means of phosphorus oxychloride (8OCHE1). In the presence of aluminum chloride 5-acyloxypyrazoles (481) undergo the Fries rearrangement affording 4-acyl-5-hydroxypyrazoles (482). 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

A similar transformation results when trimethylsilyloxy-substituted allylic halides react with silver perchlorate in nitromethane. The resulting allylic cation gives cycloaddition reactions with dienes such as cyclopentadiene. The isolated products result from desilyla-tion of the initial adducts ... 

Allylic rearrangement (Section 10.2) Functional group transformation in which double-bond migration has converted one allylic structural unit to another, as in ... 

Allylic protection groups and their removal through catalytic palladium rr-allyl methodology in transformations of heterocycles 98T2967. [3- -3]Benzannelation of heteroaromatics consisting of successive nucleophilic... 

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