Reduction of Allyl Acetates

The Et3SiH/TFA reduction of a 3-acetoxy enol ether is reported. The diastereo-selectivity is high for the Z isomer, but much lower for the E isomer (Eq. 132)276 

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Another means involves reduction of allylic acetals of aromatic aldehydes by Sml2.296... 

Polymethylhydrosiloxane (PMHS) has been reported to be a more selective reducing agent when coupled with Pd° catalysts than R3SnH, permitting, for example, the reduction of allylic acetates in the presence of enones and acyl halides (equation 104).300... 

Reduction of allylic acetates. This three-component system is effective for reduction of simple allylic acetates and even of 3-acetoxyglycals, particularly if Pd[P(C6H5)3]4 is replaced by tetrakis(tri-p-tolylphosphine)palladium(0). 

Palladium-catalyzed reductive silylation has also been reported. The cathodic reduction of allylic acetates in the presence of silylating agents and a catalytic amount of (Ph3P)4Pd [53] gives the corresponding allylsilanes. The initially formed rr-allyl-palladium(II) complex seems to be reduced at the cathode to generate the allyl anion intermediate, which reacts with chlorosilane to give the final product. 

The reduction of allylic acetates to saturated hydrocarbons with NaBH4-NiCl2 (Section 2.2) ... 

Basavaiah [75] reported in 1992 that allylic reduction of allyl acetates with a a-CN substituent yielded the vinyl cyanide products in complete Z-selectivity (Fig. 36). One decade later, KabaUca reported that Pd-catalyzed allylic substitution of similar substrates using P-T0IBF3K also led to the products as an exclusive Z-isomer (Fig. 37) [76]. The CN group was necessary for Z-selectivity and the substrate scope was limited in both systems. 

It is known that tr-allylpalladium acetate is converted into allyl acetate by reductive elimination when it is treated with CO[242,243]. For this reason, the carbonylation of allylic acetates themselves is difficult. The allylic acetate 386 is carbonylated in the presence of NaBr (20-50 mol%) under severe conditions, probably via allylic bromides[244]. However, the carbonylation of 5-phenyl-2,4-pentadienyl acetate (387) was carried out in the presence of EtiN without using NaBr at 100 °C to yield methyl 6-phenyl-3,5-hexadienoate (388)[245J. The dicarbonylation of l,4-diacetoxy-2-butene to form the 3-hexenedioate also proceeds by using tetrabutylphosphonium chloride as a ligand in 49% yield[246]. 

The allylstannane 474 is prepared by the reaction of allylic acetates or phosphates with tributyltin chloride and Sml2[286,308] or electroreduction[309]. Bu-iSnAlEt2 prepared in situ is used for the preparation of the allylstannane 475. These reactions correspond to inversion of an allyl cation to an allyl anion[3l0. 311], The reaction has been applied to the reductive cyclization of the alkenyl bromide in 476 with the allylic acetate to yield 477[312]. Intramolecular coupling of the allylic acetate in 478 with aryl bromide proceeds using BuiSnAlEti (479) by in situ formation of the allylstannane 480 and its reaction with the aryl bromide via transmetallation. (Another mechanistic possibility is the formation of an arylstannane and its coupling with allylic... 

Two convenient methods have been developed for the preparation of trifluoro-methyl-substituted alkoxyallenes. Reductive elimination of allylic acetates 30 with samarium diiodide leads to 31 (Scheme 8.11) [38], whereas reaction of Wittig cumu-lene 32 with phenyl trifluoromethyl ketone (33) and thermolysis of the intermediate 34 provides 35 (Scheme 8.12) [39]. 

The Pd(0)-catalyzed displacement of allylic acetates (297) with various nucleophiles via the allylic Pd(II) complex (298) is a well-established procedure (Scheme 114). Through attack of electrons (+2e ) in place of nucleophiles, (298) is expected to undergo a reductive cleavage providing allylic carbanions (299) and the acetate anion along with Pd(0) complexes. The latter can then be captured by various electrophiles (polarity inversion. Scheme 114) leading to (300) [434]. This procedure is useful for the deprotection of allyl esters under neutral conditions. Recently, a mechanistic study of the Pd-catalyzed reaction of allylic acetate (297), using carbonyl compounds as an electrophile, has been reported [435]. 

Reduction of unsaturated halides 0-78 Reduction of allylic alcohols 0-82 Reductive cleavage of enamines 0-86 Coupling of vinylic halides 0-87 Coupling of unsaturated halides with organometallic reagents 0-88 Coupling of allylic halides, tosylates, or acetates... 

Reduction of attyUc acetates.3 Allylic acetates can be reduced to alkencs after activation by conversion to a 7t-allylpalladium complex with Pd(0) complexes. The complex can then be reduced by either NaBH3CN or NaBH4. The former reagent is more chcmosclective. 

Reduction of derivatives of ally lie alcohols. Nickel boride can effect reduction of allylic alcohols to alkenes, but yields are generally improved by reduction of the acetates, benzoates, or trifluoroacetates.1 Reduction of allylic benzyl ethers to alkenes is effected in higher yield with Raney nickel. Methyl ethers are not reduced by either reagent. The trimethylsilyl ethers of allylic alcohols are reduced to alkenes by nickel boride in diglyme.2... 

Coupling ofallylic acetates with ketones homoallylic alcohols In the presence of Pd[P(C6H5)3]4, Sml2 effects reductive coupling of allylic acetates with ketones to form homoallylic alcohols in 50-95% yield. 

Conversion of allylic acetates of allylstannanes by treatment with Et2AlSnBus and Pd° catalysts proceeds by addition of the tin to the metal, followed by reductive elimination (equation 204).309 The corresponding ally phosphonate, however, showed some loss of stereochemical integrity (equation 205).310 The Pd°-catalyzed reaction of allylic acetates, trialkyltin chlorides and Smh produced allylstannanes with no stereospecificity.311... 

Allylstannane 176 is formed by the reaction of allyl acetate with distannanes [97,98], In this reaction, umpolung of the electrophilic 7r-ally] palladium to the nucleophilic allylstannane occurs. Allylation of bromoindole 198 to give allylindole 199 involves the oxidative addition of 198 to Pd, transmetallation with the allylstannane 176, and final reductive elimination [99],... 

In the reaction of allyl acetate 217 with ketene silyl acetal 218 of methyl acetate, using a Pd catalyst coordinated to DPPP, cyclopropane 220 is formed in addition to the expected allylacetate 219 [104], The cyclopropanation becomes main reaction when TMEDA, as a ligand, and thallium acetate are added [105]. The cyclopropanation can be understood by the attack of the enolate ion at the central carbon of 7r-allylpalladium to form the palladacyclobutane 221, followed by reductive elimination. 

Reductive acylation of alkoxy-substituted allylic acetates, Usual procedures do not permit alkylation of allylic acetates substituted with an alkoxyl group. A new method involves treatment of a substrate such as 1 with isopropyl... 

More recently the reagent has been applied to the reduction of allylic functional groups. The reagent effectively reduces allyl alcohols, acetates, trifluoroacetates and benzoates, but leaves methyl and benzyl ethers untouched and appears to lead to the thermodynamically more stable alkene (equation... 

While in propargylic systems a vinylogous reduction is the normal reactivity, in allylic systems a double bond shift on reduction is realized more generally only when terminal alkenes are produced. Especially Interesting in this respect is the Pd-catalyzed hydrogenolysis of allylic acetates (X = OAc) with ammonium formate (equation 45). ... 

This route shows a possible intermediate from which the allylic product is obtained by reductive elimination. Also, for the formation of allyl acetate in the palladium-catalyzed gas-phase oxidation of propene, a r-allylic species on the catalyst surface has been proposed [35]. /r-Allyl complexes of various metals have been synthesized in recent years. For many catalytic reactions they are assumed to be intermediates [50-53]. 

Selective reduction of allyl sulfides implies that no scrambling of the carbon-carbon double bond occurs during the process. Effectively this has proved to be the case especially when lithium in ethylamine is used, and the method has allowed the regio- and stereo-selective synthesis of a large variety of 1,5-dienes including squalene (Scheme 25, entry a), mukapolide (Scheme 25, entry b), dendrolasin (Scheme 25, entry c), the basic nucleus of crassin acetate (Scheme 25, entry d) from 7,7-dialkylallyl sulfides and allyl halides, and also of 1,5-enynes " from propargyl sulfides and allyl halides (Scheme 34, entry b). 

The palladium-catalyzed Michaelis-Arbuzov reaction of allyl acetates with triethyl phosphite provides reasonable yield (65%) of diethyl allylphosphonates. The reaction has been employed in the synthesis of diethyl 1-formylmethylphosphonate on a preparative scale (52%) by reductive ozonolysis of diethyl allylphosphonate in CH2CI2 at low temperature (Scheme 5.17). 5°... 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

The reduction of allyl chlorides and acetates with sodium formate is efficiently catalyzed by hydrophilic phosphine complexes (Structures 1-3) under water-hep-... 

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