Allylic 2,3,-rearrangement

Allylic uitro compounds undergo [3.3 sigmatropic rearrangement to afford rearranged idcohols, as shown in Eq. 7.43 ° and Eq. 7.44. Because the allylic uitro compounds used in these reacdons are readily prepared either by the Henry reacdon or the Michael adchdon, these rs may be usefril in orgardc synthesis. 

Bii3SnH, (PhCOlaOa benzene, 18 h MeSNa HMPA, 16 h Bii3SnH, (PhCOlaOa benzene, 18 h MeSNa DMF, 8h 

Phvj/ x COaMe NO2 Ph v./ COaMe H BujSnH, AIBN benzene, 1.5 h 64 51 

The Hetuy reacdon of uitroalkiines followed by deuitradon is a good method for the preparadon of Mcohols. This methodobgy has been apphed in carbohydrate chemistry. For 

Martin has used this strategy for the preparation of fi-d,6 and linked C-disaccha- 

The substituent X moves two atoms along the chain, and the double bond moves in the opposite direction to the position where X is still an allylic group. This migration can occur so readily that it is reasonable to regard the isomerization of allyl halides as true tautomerism. For example, equilibrium between 2-butenyl bromide and 1-methylallyl bromide is set up in a few hours at 75° 14 

It is not thus permissible to assign a structure to an allylic halide on the basis of its method of formation. 

The possibility of rearrangement must be borne in mind particularly when considering reactions of allylic compounds. Substitution of SN1 type, which dominates in polar solvents, always leads to mixtures of isomeric rearrangement products bimolecular substitution occurs without rearrangement. Thus choice of strongly nucleophilic reactants and, particularly, repression of SN1 reaction by selection of an apolar solvent such as acetone permit reactions of allylic compounds to be undertaken without rearrangement.13 

Allylic rearrangement has enormously increased the difficulty of determining the structure of many compounds of allylic type, but on the other hand there is a whole series of cases in which it has been put to preparative use. For example, Heilbron and his co-workers15 prepared the dienynediol (1) from crotonaldehyde and the Grignard derivative of acetylene and converted it jnto the isomeric diol (2) by a double allylic rearrangement  

A solution of 2,8-decadien-5-yne-4,7-diol (5 g) (1) in ether (20 ml) was shaken with 10% sulfuric acid (30 ml) under nitrogen for 24 h at 20°. The ether phase was separated, washed, and dried, and then afforded 3,7-decadien-5-yne-2,9-diol (4.3 g) as a viscous liquid that distilled at 65-70° (bath temperature)/10 4 mm. 

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Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

Various S-nucleophiles are allylated. Allylic acetates or carbonates react with thiols or trimethylsilyl sulfide (353) to give the allylic sulfide 354[222], Allyl sulfides are prepared by Pd-catalyzed allylic rearrangement of the dithio-carbonate 355 with elimination of COS under mild conditions. The benzyl alkyl sulfide 357 can be prepared from the dithiocarbonate 356 at 65 C[223,224], The allyl aryl sufide 359 is prepared by the reaction of an allylic carbonate with the aromatic thiol 358 by use of dppb under neutral condi-tions[225]. The O-allyl phosphoro- or phosphonothionate 360 undergoes the thiono thiolo allylic rearrangement (from 0-allyl to S -allyl rearrangement) to afford 361 and 362 at 130 C[226],... 

Perfluoroalkylzinc iodides, prepared in situ from iodides and ultrasonically dispersed Zn, are coupled with allylic halides via an allylic rearrangement[271]. The Pd-catalyzed homocoupling of allylic acetate in the presence of Zn to give a mixture of regioisomers 416 and 417 may proceed via in situ formation of allylzinc species[272,273]. 

Allylic Rearrangement, Claisen Rearrangement, and Related Reactions Catalyzed by Pd(0) and Pd(II)... 

Allylic ester rearrangement is catalyzed by both Pd(II) and Pd(0) compounds, but their catalyses are different mechanistically. Allylic rearrangement of allylic acetates takes place by the use of Pd(OAc>2-Ph3P [Pd(0)-phosphine] as a catalyst[492,493]. An equilibrium mixture of 796 and 797 in a ratio of 1.9 1.0 was obtained[494]. The Pd(0)-Ph3P-catalyzed rearrangement is explained by rr-allylpalladium complex formation[495]. 

The stereoselective allylic rearrangement of the allylic alcohol 798 catalyzed by PdCl2(MeCN)2 and Ph3P under Mitsunobu inversion conditions is explained as proceeding via a rr-allylpalladium intermediate[496]. The smooth rearrangement of the allylic p-tolylsulfone 799 via a rr-allylpalladium intermediate is catalyzed by a Pd(0) catalyst[497]. 

Carboxylic acids react with butadiene as alkali metal carboxylates. A mixture of isomeric 1- and 3-acetoxyoctadienes (39 and 40) is formed by the reaction of acetic acid[13]. The reaction is very slow in acetic acid alone. It is accelerated by forming acetate by the addition of a base[40]. Addition of an equal amount of triethylamine achieved complete conversion at 80 C after 2 h. AcONa or AcOK also can be used as a base. Trimethylolpropane phosphite (TMPP) completely eliminates the formation of 1,3,7-octatriene, and the acetoxyocta-dienes 39 and 40 are obtained in 81% and 9% yields by using N.N.N M -tetramethyl-l,3-diaminobutane at 50 in a 2 h reaction. These two isomers undergo Pd-catalyzed allylic rearrangement with each other. 

Reactions of allylic systems that yield products m which double bond migration has occurred are said to have proceeded with allylic rearrangement, or by way of an allylic shift... 

Clearly the temperature at which the reaction occurs exerts a major influence on the product composition To understand why an important fact must be added The 1 2 and 1 4 addition products interconvert rapidly by allylic rearrangement at elevated tempera ture m the presence of hydrogen bromide Heating the product mixture to 45°C m the presence of hydrogen bromide leads to a mixture m which the ratio of 3 bromo 1 butene to 1 bromo 2 butene is 15 85... 

The carbocations formed as intermediates when allylic halides undergo Stvfl reactions have their positive charge shared by the two end carbons of the allylic system and may be attacked by nucleophiles at either site Products may be formed with the same pattern of bonds as the starting allylic halide or with allylic rearrangement... 

Allylic rearrangement (Section 10 2) Functional group trans formation in which double bond migration has converted one allylic structural unit to another as in... 

F E R R I E R Cartx>hydrate Synthesis Synthesis of unsaturated cartx>hydrates by allylic rearrangement. 

The reaction of phenyllithium and alfyl chloride labeled with C reveals that allylic rearrangement occurs. About three-fourths of the product results from bond formation at C-3 rather than C-1. This can be accounted for by a cyclic transition state. ... 

Reaction of 1,4 dibromohexafluoro 2 butene with sodium alkoxides gives products of allylic rearrangement by an S[,j2 process [/7] (equation 16)... 

Methyl-1,2-dihydropapaverine (175, R = OMe) rearranges to the 2-methyl-3-(3,4-dimethoxybenzyl)-6,7-dimethoxy-3,4-dihydroisoquinolin-ium salt (176, R = OMe) under very mild conditions (treatment with 2% hydrochloric acid). A similar rearrangement of l-(3,4-methyl-enedioxybenzyl) - 2 - methyl - 6,7 - dimethoxyisoquinoline (175, R, R = —O—CHj—O—) affords 3-(3,4-methylenedioxybenzyl)-2-methyl-6,7-di-methoxy-3,4-dihydroisoquinolinium chloride (176, R, R = O—CHj—O—) (256). The reaction was shown to be an allylic rearrangement with internal return (275,275). 

Substituted TMM complexes also cycloadd to aldehydes in the presence of a tin cocatalyst such as MesSnOAc and MesSnOTs [31]. Reaction of 2-heptenal with methyl precursor (6) gave a mixture of methylenetetrahydrofurans (68) and (69). This regioselectivity is reversed with 10-undecenal and methyl precursor (5), where adduct (70) now predominates over (71). As in the carbocyclic system, the phenylthio group also functions as a regiocontrol element in reaction with cyclohexyl aldehyde. The initially formed adduct (72) eliminates the element of thio-phenol on attempted allyl rearrangement, and the overall process becomes a cycloaddition approach to furans (Scheme 2.21) [20]. 

The vapor-phase chlorination reaction occurs at approximately 200-300°C. The dichlorobutene mixture is then treated with NaCN or HCN in presence of copper cyanide. The product 1,4-dicyano-2-butene is obtained in high yield because allylic rearrangement to the more thermodynamically stable isomer occurs during the cyanation reaction ... 

Nucleophilic substitution at an allylic carbon takes place either by an SN2 mechanism or by an Sn2 mechanism. No allylic rearrangement takes place in the former case, but it does occur in the latter case. 

The study by Baechler and coworkers31, cited above, also provided data on the (1,3)-allylic rearrangement in /J-methylallyl phenyl sulfoxide. Using the same approach as was used in reinterpreting the sulfone data, the activation energy is estimated to be 151 kj mol- and AfH°(PhSO) = 45kJmol-1. 

Reaction of alkynols 156 with benzenesulphenyl chloride afforded either the vinylacety-lene sulphoxides 157 or the allene sulphoxides 158 depending upon the substitution pattern of alkynols 156. Vinylacetylene sulphoxides 157 result from a [2,3]-allylic rearrangement of the sulphenate ester 159 (equation 88). In the case of the cyclic... 

Treatment of (—)-(S)-276 with allyl Grignard reagents gives optically active allylic sulphoxides 288. This reaction, however, involves an allylic rearrangement via transition state 289 as evidenced by Mislow and his collaborators362 (equation 160). 

The 1,3-allylic rearrangement has also been observed with several cyclic sulfoxides. For example, the interesting thermal interconversion of the bicyclic stereoisomeric pair 141... 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

Nucleophilic substitution at an allylic carbon can also take place by an Sn2 mechanism, in which case no allylic rearrangement usually takes place. However, allylic rearrangements can also take place under Sn2 conditions, by the following mechanism, in which the nucleophile attacks at the y carbon rather than the usual... 

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