Allylation and Related Reactions

Chiral (salen)chromium(III) complexes catalyse the asymmetric allylation of a range of aldehyde types, using allylstannane reagents.171 

BINOL-derived phosphoramidites are versatile ligands in palladium-catalysed umpol-ung allylation of aryl aldehydes mediated by diethylzinc.172 The possible roles of allyl-zinc and -palladium species in the mechanism are discussed in detail. 

A homoallyl alcohol has been used to generate an allylrhodium species via retro-allylation.173 Subsequent reaction with an aldehyde (RCHO) yields the corresponding secondary alcohol, R-CH(OH)CH2-C(Me)=CH2, in situ. This can be isomerized in the same pot to yield saturated ketone, R-C(=0)CH2-CHMe2. 

Pinacols (61) derived from a variety of aromatic aldehydes have been employed in enantio- and diastereo-selective allylations of aliphatic aldehydes.174 Their allyl-boronate derivatives react under Lewis acid conditions (SnCU) with a variety of aldehyde types, in good yield and ee. Even better results are obtained by addition of (61) as a I irons led acid (auto)catalyst, via coordination/activation of the tin catalyst. 

A combined experimental and computational approach has been undertaken to identify the origin of syn/anti diastereoselectivity in two types of crotylation reactions of aldehydes and ketones (i) multi-component crotylations of simple aldehydes/ketones and (ii) acetal substitution reactions of aldehyde dimethyl acetals with E- and Z-configured crotyltrimethylsilane.175 The stereochemical outcome is nearly identical in the two reactions, and the computational results suggest that this is due to near identical mechanisms an SN1 process involving attack of O-methyl-substituted carboxenium ions by crotylsilane. 

DFT has been employed to probe the mechanisms of chiral BINOL-phosphoric-acid-catalysed allylboration and propargylation reactions, with a particular focus on whether the catalyst interacts with the pseudo-axial or pseudo-equatorial oxygen of the boronate. ° 

The role of boron trifluoride etherate in the allylboration of aldehydes using (R)-pinanediol has been studied computationally, considering the alternatives of attachment of BF3 to the chiral ligand or to the aldehyde.  

Highly substituted enantiomerically pure allylboronic esters have been synthesized and added to aldehydes to give R-configured homoallylic alcohols exclusively.  

DFT has been used to probe the mechanism of allylation of ketones by allylboronates in the presence of diethylzinc. Results favour a double y-addition stepwise route, rather than concerted Lewis acid. Diethylzinc is found to be weakly catalytic, with the addition of ethanol substantially accelerating the reaction via Zn(OEt)2 catalysis, with Zn(OH)2 and ZnF2 efficiencies also being calculated. 

An allylation of aldehydes, (R-CHO, R = aryl, alkyl), with allylboronates in aqueous media reveals features unique to this solvent. Using zinc(II) hydroxide catalysis. 

The first example of enantioselective ee 92%) allylation of aldehydes (by allyl-SnBu3) using a chiral B(III) complex has been developed using Bi(OTf)3 with Trust s (/ ,R)-ProPhenol ligand.  

Allylborations of aldehydes have included enantioselective reactions with pinacol allylboronates, catalysed by l,l -spirobiindane-7,7 -diol (SPlNOL)-based phosphoric acids, and with chiral fi-(3,3-difluoroallyl)diisopinocampheylborane to form 1,1-difluorinated homoallylic alcohols with 91-97% ee  

Asymmetric allylations of ArCHO with allyltrichlorosilane in CH2CI2 to form homoallylic alcohols has been Lewis base catalysed by chiral bisformamide-type catalysts (with ee 83%) and by (f )-methyl p-tolyl sulfoxide. Mechanistic study of the latter reaction supports a dissociative pathway via an octahedral cationic complex with two sulfoxides. The greater stereoselectivity of A-oxide-catalysed allylations, compared to propargylations, has been explained by a simple electrostatic model that should enable design of suitable catalysts for both reactions.  

Vinyl silacyclopropanes generated in situ add to aldehydes with formation of seven-membered-ring traw -oxasilacycloheptenes with high diastereoselectivity. High diastereoselectivity has been reported for the formation of tertiary homoallylic alcohols on addition of allyltitanocenes to a-chiral ketones.  

An important advancement in highly regioselective and enantioselective allylation of j0-diketones has been enabled using their enol form to provide the necessary Brpnsted- 

Lewis acid-catalysed allylboronate additions to aldehydes have been reviewed.181 

An allylboronate derivative of tartaric acid developed for enantioselective allylation of aldehydes is readily recyclable without losing selectivity.182 

A chiral pyridine-bisoxazoline ( PYBOX ) ligand has been combined with indium (III) triflate to produce an enantioselective catalyst for allylation of a wide variety of aldehydes in ionic liquids 183 ees of 90% were obtained, and extraction and reuse of the catalyst-ionic liquid combination saw this figure hold up to 80% on the fourth recycle. 

A chiral BINOL-indium(in) complex has been used to catalyse the addition of allyltributylstannane to aldehydes in high ee.184 

Aldehydes can be allylated with allyltributylstannane using cerium(III) chloride in acetonitrile, a method particularly suitable for substrates bearing acid-sensitive groups.185 

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Aldol-Type Reactions, Allylation and Related Reactions... 

Palladium-catalyzed allylation and related reactions of enolates. 19... 

Acemoglu, L., Williams, J. M. J. Palladium-catalyzed asymmetric allylation and related reactions. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 2, 1945-1979. 

The reaction of methallyltri-n-butylstannane 117 with achiral aldehydes is also effectively promoted by the binol-Ti complex [89 c]. In all but one case (cyclo-hexanecarboxaldehyde), the yields and enantioselectivities observed with the methallylstannane are identical or higher than those obtained in the reactions with allyltributylstannane with only 10 mol% of the binol-Ti complex (Scheme 10-50). Insight into the nature of the titanium catalyst is provided by the observation of asymmetric amplification [89 b] and chiral poisoning [89 g]. An intruiging hypothesis on the origin of enantioselection in allylation and related reactions [89 h]. 

Various methods which use a stoichiometric amount of chiral Lewis acids or chiral Lewis bases are available for enantioselective allylation and related reactions... 

The Pd-catalyzed reactions of allylic electrophiles with metal nucleophiles can produce the corresponding allylmetal derivatives (Sect. V.233), which can, in turn, serve as allylic nucleophiles. This protocol provides a means of utilizing allylpalladium and related derivatives as nucleophiles rather than electrophiles (Sect. V.23.4). Many of the substitution reactions mentioned above can be asymmetric. Because of their special significance in organic synthesis, Pd-catalyzed asymmetric allylation and related reactions are discussed in Sect. V.2.4. 

The advent of the catalytic Tsuji-Trost reaction in the early 1970s " (Sects. V.2.1.1 and V.2.1.2) has significantly expanded the scope of enolate allylation and related reactions. Noteworthy among others is that the range of allylic electrophiles was vastly expanded from allylic halides, such as chlorides and bromides, to a much wider range of derivatives including not only halides but also O, S, N, and other heteroatom-containing derivatives. Curiously, however, the scope of the Tsuji-Trost reaction had essentially been limited to the Pd-catalyzed allylation of extra-stabilized enolates, such as those derived from malonate and acetoacetate esters, until around 1980. 

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