Alkenes from allylic acetates

Several 1,3-diene syntheses involving elimination reactions that are catalyzed by Pd(Ph3P)4 have been reported. The first involves the Et3N mediated elimination of HOAc from allylic acetates in refluxing THF. A complementary procedure involves the Pd(Ph3P)4 catalyzed decarboxylative elimination of /3-acetoxy-carboxylic acids (eq 46). The substrates are easily prepared by the condensation of enals and carboxylate enolates irrespective of the diastereomeric mixture, ( )-alkenes are formed in a highly stereocontrolled manner. The geometry of the double bond present in the enal precursor remains unaffected in the elimination and the reaction is applicable to the formation of 1,3-cyclohexadienes. 

In contrast to oxidation in water, it has been found that 1-alkenes are directly oxidized with molecular oxygen in anhydrous, aprotic solvents, when a catalyst system of PdCl2(MeCN)2 and CuCl is used together with HMPA. In the absence of HMPA, no reaction takes place(100]. In the oxidation of 1-decene, the Oj uptake correlates with the amount of 2-decanone formed, and up to 0.5 mol of O2 is consumed for the production of 1 mol of the ketone. This result shows that both O atoms of molecular oxygen are incorporated into the product, and a bimetallic Pd(II) hydroperoxide coupled with a Cu salt is involved in oxidation of this type, and that the well known redox catalysis of PdXi and CuX is not always operalive[10 ]. The oxidation under anhydrous conditions is unique in terms of the regioselective formation of aldehyde 59 from X-allyl-A -methylbenzamide (58), whereas the use of aqueous DME results in the predominant formation of the methyl ketone 60. Similar results are obtained with allylic acetates and allylic carbonates[102]. The complete reversal of the regioselectivity in PdCli-catalyzed oxidation of alkenes is remarkable. 

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. 

Blechert reported a skillful method of cross-enyne metathesis. Solid-supported alkyne 139 is reacted with alkene in the presence of Ic to give 140. For cleavage of 1,3-diene from solid-supported product 140 having an allyl acetate moiety, palladium-catalyzed allylic substitution is used. Thus, 140 is treated with Pd(PPh3)4 in the presence of methyl malonate to afford three-component coupling product 141 in good yield ... 

A second type of organopalladium intermediates are 7r-allyl complexes. These complexes can be obtained from Pd(II) salts and allylic acetates and other compounds with potential leaving groups in an allylic position.79 The same type of 7i-allyl complexes can be prepared from alkenes by reaction with PdCl2 or Pd(02CCF3)2.80 The reaction occurs by electrophilic attack on the n electrons followed by loss of a proton. The proton loss probably proceeds via an unstable species in which the hydrogen is bound to... 

Heterogeneous palladium catalysts proved to be active in the conversion of simple alkenes to the corresponding allylic acetates, carbonyl compounds, and carboxylic acids.694 704 Allyl acetate or acrylic acid from propylene was selectively produced on a palladium on charcoal catalyst depending on catalyst pretreatment and reaction conditions.694 Allylic oxidation with singlet oxygen to yield allylic hydroperoxides is discussed in Section 9.2.2. 

Anthraquinone was produced in 96% yield from the oxidation of anthracene with TBHP in the presence of RhC (PPh1)3 catalyst (equation 93).266 Oxidation of cylic alkenes, e.g. cyclopentene, cyclohexene, cycloheptene, by TBHP in the presence of Rh2(OAc)4 results in the formation of a,/3-unsaturated ketones and allylic acetates as the major products (equation 94).267... 

In the presence of bromide, the oxidation of alkenes by Mn(OAc)3 in acetic acid readily occurs it 70-80 °C and produces allylic acetates in good yields. Thus cyclohexene is oxidized to cyclo-lexenyl acetate in 83% yield,508 and a-methylstyrene to j3-phenylallyl acetate in 70% yield,509 vith a mechanism involving allylic hydrogen abstraction by bromine atoms coming from the ixidation of bromide by MniU (equation 206). 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

There has been a number of developments in the use of salicylaldehydes as precursors of both chromenes and chromans. Alkenes activated by acyl, formyl, nitrile and phenylsulfonyl groups react with 2-hydroxybenzaldehydes and 2-hydroxy-1-naphthaldehyde under Bayliss-Hillman conditions to yield 3-substituted chromenes via the in situ dehydration of the initially formed chroman-4-ol <02JCS(P1)1318>. In like manner, P-nitrostyrenes yield 2- and 2,2-substituted derivatives of 3-nitrochromenes <02H(57)1033>. A simple route to 2-phenyl-2H-chromenes starting from salicylaldehyde and utilising a Pd(0)-catalysed cyclisation of an allylic acetate has been described <02SC3667>. 

The enantioselective synthesis of azabicyclic y-lactams starting from 2-azanorbornenones after treatment of a catalytic amount of RuCl2(PCy3)2 (= CHPh) in the presence of ethylene or allyl acetate proceeds also via ring rearrangement—alkene metathesis (ROM-CM-RCM) [41] (Scheme 19). If n = 0 or 3, no RCM occurs and a cyclic dialkenyl compound is formed by cascade ROM-CM reactions. 

Other transition metal salts mediate in similar oxidations. For example, mercury(II) acetate, a milder reagent than LTA, effects a-acetoxylation through a comparable mechanism. However the corresponding yields for these processes are poor. 3,3-Dimethylcyclohexanone, for example, is oxidized to the a-acetoxy derivative in only 14% yield.The, 7-unsaturated ketone, isopugelone, exhibits no oxidation at the a- or a -positions, but affords a product derived from isomerization of the alkene and allylic oxidation. Not surprisingly therefore the reagent has found little synthetic application for this transformation. 

The overall process is illustrated by the reaction of a homochiral allylic acetate (Scheme 12). With soft anions, derived from carbon acids of pA a 8 15, the addition is very efficient, exclusively anti, and can occur at either end of the allyl system (1 1 in the symmetrical case). In contrast to the addition to alkene ligands, the primary influence on regioselectivity is steric and not electronic. The less-snbstituted end of the allyl ligand is preferentially attacked. 

An alternative approach to the commonly utilized cleavage of allylic isoprenoid compounds (Scheme 16) using attack of +2e in place of H at a Pd° ir-allyl complex also requires a catalyst addition.Electrolysis of allylic acetates with Pb cathode and Pt anode in the presence of Pd(PPh3)4 in MeCN, gave moderate yields of the inner alkenes from both terminal and inner acetates (equations 92 and 93). Control of the regioselectivity was clearly dictated by the attack of H+ from the less-hindered side of the allylic carbanions. 

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