Alkoxyalkyl complexe

A further synthetic approach to carbon-metal double bonds is based on the acid-catalyzed abstraction of alkoxy groups from a-alkoxyalkyl complexes [436 -439] (Figure 3.11). These carbene complex precursors can be prepared from alk-oxycarbene complexes (Fischer-type carbene complexes) either by reduction with borohydrides or alanates [23,55,63,104,439-445] or by addition of organolithium compounds (nucleophilic addition to the carbene carbon atom) [391,446-452]. 

Hydride abstraction in alkoxyalkyl complexes to the corresponding alkoxy carbene complex has also been shown (Equation (17)) [50]. 

Gold catalyzed reactions are currently enjoying considerable interest, and have also found applications in thiophene ring synthesis. A series of (a-alkoxyalkyl)(o-alkynylphenyl) sulfides 14 was subjected to treatment with catalytic amounts of AuCl, giving the benzo[fe]thiophenes 15 in excellent yields. Some additional, more complex examples were also provided <06AG(E)4473>. 

Because a-alkoxyalkyl iron complexes are thermally unstable [467] they cannot be stored for long periods of time. More suitable carbene precursors are the corresponding a-(dimethylsulfonium)alkyl complexes, which can be stored indefinitely under ambient conditions [468-473], These complexes are prepared by S-alkylation of a-(methylthio)alkyl complexes, which can be prepared by alkylation of metallates with a-halothioethers, by addition of C-nucleophiles to (alkylthio)carbene complexes, or by addition of thiols to carbene complexes. 

ALkoxyalkyl-substituted arenechromium complexes have been lithiated enantioselec-tively in the benzylic position without control over planar chirahty and the tendency for lithium amides to lithiate at a benzyhc position rather than on the ring itself poses a problem with some substrates. In the lithiation of 433, an element of planar stereochemistry is introduced during a benzyhc lithiation (Scheme 176) . Exploiting... 

When the same reaction was performed with the corresponding lithium carbenoid, a complex mixture of products was obtained and the yield of 111 was less than 10%51. 13C-labeling experiments revealed that the alkoxyalkyl moiety migrated exclusively (equation 53)58. 

We have shown, however, that a significant number of monohalogen-oalkyl transition metal compounds have now been prepared, many of them in the last decade. A variety of routes have been used to synthesize these compounds, but the most common ones are (1) the reaction of an alkoxyalkyl compound with anhydrous HX (X = Cl, Br, or I), (2) the reaction of a nucleophilic transition metal compound with a dihaloalkane, and (3) the insertion of a methylene group into a metal-halogen bond. The most common route to the co-halogenoalkyl complexes is by reaction of an anionic metal species with the a,co-dihaloalkane. These monohalo-... 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

The rearrangement reaction of a variety of alkyl phenyl ethers over a dealumi-nated HY zeolite has been shown to involve both intramolecular and intermolecular processes to afford phenol, (alkoxyalkyl)benzenes and alkylphenols as the main products. o-Benzylphenol has been obtained as the exclusive product in the rearrangement of benzyl phenyl ether in the presence of montmorillonite. The mechanism for a novel zeolite /3-catalysed rearrangement of alkoxybenzyl allyl ethers to aldehydes and ketones has been investigated by the use of cross-over reactions and deuterium labelling. The reaction was found to be mainly intramolecular and has been described as a nucleophilic attack of the double bond on the electrophilic benzylic carbon of the ether-Lewis acid complex, followed by a... 

Alkoxyalkyl-substituted arenechromium complexes have been lithiated enan-tioselectively in the benzylic position without control over planar chirality [97,... 

The insertion of olefins into the metal-oxygen bonds of isolated alkoxo, phenoxo, or hydroxo compounds has been observed directly in a few cases. As will be noted in Chapter 11, a hydroxy or alkoxyalkyl group can be formed by insertion of an olefin into a metal-oxygen bond or by attack of hydroxide or an alkoxide on a coordinated olefin. Many studies described in Chapter 11 imply that this type of compound is formed by nucleophilic attack on a coordinatively saturated olefin complex, and this reaction has been proposed as the C-0 bond-forming step during oxidations of olefins catalyzed by palladium complexes. However, Henry provided some of the first evidence that the C-0 bond forms by insertion of olefins into metal-hydroxo and -alkoxo complexes under certain reaction conditions. ... 

This divergent oxidative reactivity of alcohols and carboxylic acids with olefins is illustrated in Equation 16.103. Bofli reactions generate products from p-hydrogen elimination of an alkoxyalkyl or acetoxyalkyl complex. However, the vinyl ether product generated by p-hy-drogen elimination undergoes reaction with a second equivalent of alcohol to form the acetal in a process catalyzed by the acidic medium or the action of palladium(II) as Levris add. 

P-Alkoxyalkyl-Fp complexes are rather labile. Treatment with tetrafluoroboric acid etherate leads to elimination of the alcohol to give cationic alkene-Fp complexes. These can then be readily demetalated either by reaction with sodium iodide in acetone at room temperature or by heating in acetonitrile providing the corresponding olefins (Scheme 4-21). " ... 

Scheme 4-21. Elimination of alcohol from P-alkoxyalkyl-Fp complexes and demetalation of the cationic (Ti -alkene)iron complexes.

Hayashi T, Hayashizaki K, Kiyoi T, Ito Y. Asymmetric synthesis catalyzed by chiral ferrocenylphosphine-transition-metal complexes. 6. Practical asymmetric synthesis of 1,1 -binaphthyls via asymmetric cross-coupling with a chiral [(alkoxyalkyl)ferrocenyl] monophosphine/nickel catalyst. J. Am. Chem. Soc. 1988 110(24) 8153-8156. 

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