Reactions of allylic systems

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... 

The palladium catalysed substitution reaction of allylic systems has also been utilised in the formation of five membered rings. Intramolecular nucleophilic attack of the amide nitrogen atom on the allylpalladium complex formed in the oxidative addition of the allyl acetate moiety on the catalyst led to the formation of the five membered ring (3.63.). In the presence of a copper(II) salt the intermediate pyrroline derivative oxidized to pyrrole.80... 

All products from the nickel-catalyzed reaction of allylic systems with alkynes can be derived from two main intermediates. The course of the reaction is dependent on the solvent, however, by appropriate choice of the reaction conditions, good yields of cyclopentenones can be attained in some cases38-39. 

Figure 6.33 1,3-Allylic strain, in combination with stereoelectronic preferences, can be used for the design of stereoselective reactions of allylic systems.

Chiral sulfinyl sulfur atoms can coordinate to palladium in Pd-catalyzed reactions of allylic systems bearing chiral sulfinyl groups at the appropriate site, if the steric environment allows the sulfinyl sulfur atoms to access the n -allylpalladium complexes generated for participation by the coordination. 

PROBLEM 12.59 Here are two reactions of allyl systems that are related to the Diels-Alder reaction. Use a HOMO-LUMO molecular orbital analysis to show why reaction a succeeds whereas reaction b fails. 

Similarly, the coupling reactions of allyl systems in intermolecular or intramolecular (cyclization) ways is also best performed with nickel(O) reagents (a Corey reaction) [2]. 

From Allyl Alcohol. The reaction of allyl alcohol [107-18-6] with chlorine and water gives a mixture of glycerol m on ochl orohydrin s consisting of 73% 3-chloropropane-l,2-diol and 27% of 2-chloropropane-l,3-diol (57). In a recycle reaction system in which allyl alcohol is fed as a 4.5—5.5 wt % solution, chlorine is added at a rate of 7—9 moles per hour. The reaction time is about five seconds, the reaction temperature 50—60°C and the recycle ratio is 10—20 1. Under these conditions m on ochl orohydrin s have been obtained in 88% yield with 9% dichlorohydrins (58) (see Allyl ALCOHOL AND DERIVATIVES). 

Intramolecular cycloaddition reactions of allylic cations with participation and/ or formation of heterocycles, mainly [4+3]-cycloaddition to furan system 97T6235. 

Some examples of conjugate addition reactions of allylic silanes are given in Scheme 9.5. Entries 1 to 3 illustrate the synthesis of several (3-allyl ketones. Note that Entry 2 involves the creation of a quaternary carbon. Entry 4 was used in the synthesis of a terpenoid ketone, (+)-nootkatone. Entry 5 illustrates fluoride-mediated addition using tetrabutylammonium fluoride. These conditions were found to be especially effective for unsaturated esters. In Entry 6, the addition is from the convex face of the ring system. Entry 7 illustrates a ring closure by intramolecular conjugate addition. 

The spectroscopy, structure, photochemistry, and unimolecular reactions of allyl radical have been studied extensively and reviewed recently.145 Possible dissociation channels of allyl radical, their energetics, and the potential energy barriers of the C3H5 system are shown in Figs. 20 and 21.145,146... 

The selectivity in the Heck reaction of allylic alcohol 111 is interesting, and the factors that lead to the observed preference for (3-hydride elimination toward nitrogen in this system are unclear, although a combination of steric effects and stereoelectronic factors (i.e., alignment of C-H and C-Pd bonds, nN a c H interactions) is likely involved. Examination of related examples from the literature (Scheme 4.20) reveals no clear trend. Rawal and Michoud examined substrate 115, which lacks the influence of both the amine and hydroxyl substituents and also seems to favor (3-hydride elimination within the six-membered ring over formation of the exocyclic olefin under standard Heck conditions [18a]. However, under... 

The above examples represent Jl-heteroaromatic annulation involving the reaction of allyl anions whose double bond is a part of the heterocyclic ring system (Scheme 1). The corresponding a-oxoketene dithioacetals (1,3-electrophilic component) were generally derived from nonheterocyclic carbonyl precursors. Alternatively the Jl-heteroaromatic annulation can also be employed to a-oxoketene dithioacetals derived from heterocyclic ketones (1,3-bielectrophile) and hetero/nonheteroallyl anions (1,3-binucleophile). These reactions are described below. 

The electrocyclic reactions of n systems containing an impaired electron are difficult to interpret using the above simple theories. The symmetry of the HOMO of the radical system corresponds to that of the corresponding anion. Thus the allyl radical would be expected to cyclize in the same manner as the alkyl anion i.e., in a conrotatory manner. In fact the interconversion takes place in a disrotatory manner. Theoretical calculations based on Huckets theory also give ambiguous or incorrect predictions. And therefore more sophisticated calculations are required to obtain reliable results. 

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. 

The Pd(0)-catalyzed electrophilic reaction of allyl acetates (301) with elec-trochemically induced carbanions leading to (302) has been performed (Scheme 115) [436], The electrolysis is carried out in a DM F-Et4NCl04-(Pt) system in the presence of the active hydrogen compound, Ph3P, and Pd(II)(PhCN)2Cl2 at a current density of 0.26 A dm and the allyl acetate (301). 

Compared to the reaction of benzylic systems, the situation with allylic organometallic compounds is much more complicated. 

When either an alcohol or an amine function is present in the alkene, the possibility for lactone or lactam formation exists. Cobalt or rhodium catalysts convert 2,2-dimethyl-3-buten-l-ol to 2,3,3-trimethyl- y-butyrolactone, with minor amounts of the 8-lactone being formed (equation 51).2 In this case, isomerization of the double bond is not possible. The reaction of allyl alcohols catalyzed by cobalt or rhodium is carried out under reaction conditions that are severe, so isomerization to propanal occurs rapidly. Running the reaction in acetonitrile provides a 60% yield of lactone, while a rhodium carbonyl catalyst in the presence of an amine gives butane-1,4-diol in 60-70% (equation 52).8 A mild method of converting allyl and homoallyl alcohols to lactones utilizes the palladium chloride/copper chloride catalyst system (Table 6).79,82 83... 

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

A further subtlety is the cyclization of the initial 2,5-hexadienoyl complex to form a six-membered ring, as an alternative to the cyclopentene system. In the reaction of allyl chloride with acetylene and carbon monoxide in the presence of Ni(CO)4, only traces of six-membered ring systems... 

The most synthetically useful Claisen rearrangement, the ester enolate reaction of allyl esters, requires only relatively mild reaction conditions and is most amenable to scale up.297-332 The geometry of the initially formed enol ether substrate is controlled by the choice of solvent system (Scheme 26.12).298-333 335 Thus, the methodology provides a useful alternative to an aldol approach. 

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