Allylation of aldehydes and ketones

Allylation of aldehydes and ketones by reagents, electrochemically generated at the cadmium anode from allyl bromides, are also known30. 

Later, Araki et al. found that the allylation of aldehydes and ketones can be carried out by using catalytic amounts of indium(III) chloride in combination with aluminum or zinc metal.109 This reaction was typically performed in a THF-water (5 2) mixture at room temperature, although the conversion was much slower compared to the same reaction mediated by use of a stoichiometric amount of indium and it required days to complete. When the reaction was carried out in anhydrous THF alone, the yield dropped considerably and side-reactions such as reduction to alcohol increased. The combinations of Al-InCL or Zn-InCl3 gave comparable results. 

Pb. Homoallylic alcohols can be obtained from allylation of aldehydes and ketones with allyl bromide promoted by metallic lead in aqueous media.176... 

Table 2 Allylation of aldehydes and ketones with allylytterbium bromide...

Removal of the iodine atom from aryl iodides proceeds smoothly in a DMF-Bu4NCl04-(Hg) system [561, 562]. A Cd-modified electrode-assisted allylation of aldehydes and ketones has been attained in a DMF-Et4NCl04 system, in which the... 

Homoallylk ethers. lodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Catalytic asymmetric allylation of aldehydes and ketones with allylsilanes can be achieved by using chiral Lewis acids, transition metal complexes, and Lewis bases. In recent years, much attention has been paid for the chiral Lewis base-catalyzed system using allyltrichlorosilanes. Advances in catalytic asymmetric carbonyl allylation have been described in detail in recent reviews.116,117,117a... 

In indium-mediated reactions, a catalytic use of expensive indium is desirable from an economical standpoint, and it has been achieved in combination with more electron-positive metals. Thus, allylation of aldehydes and ketones, as well as prenylation of 2-chlorobenzoquinone, has been performed by using metallic aluminum or zinc in combination with a catalytic amount of indium(m) chloride.63 Also, allylation of aldehydes and ketones with allyl bromide is carried out with a catalytic amount of indium powder (from 0.01 to 0.1 equiv.) in THF in the presence of manganese and chlorotrimethylsilane as the reducing and oxophilic agents, respectively (Scheme 2). 64,65... 

Tietze et al. emphasized the usefulness of chiral trimethylsilyl ethers of readily accessible amino alcohol derivatives in allylation of aldehydes and ketones [43]. As a consequence, careful design of the norpseudoephedrine derivatives and proper choice of silicon Lewis acids have led to the convergent preparation of enantiomerically enriched secondary and tertiary homoallylic alcohols in high yields (Sch. 12) [43a], It should be noted that the configuration of the newly formed stereogenic center of the secondary homoallylic alcohols is the opposite of that in the allylation of ketones [43c], They also described in detail mechanistic studies of the above allylation reaction by use of and NMR. 

Bis(allyldibutylstannyl)naphthalene is highly effective for the selective allylation of aldehydes and ketones under neutral conditions (Eq. 112) [151]. Such a transformation is not realized with monostannane. The former reaction proceeds without any catalyst by taking advantage of the chelation-induced Lewis acidity of bidentate bis(stannyl) compounds. 

The achiral allylation of aldehydes has also been achieved in recyclable ionic liquids" and in water." Greener still is the corresponding Barbier allylation" of aldehydes and ketones with allylic bromides in water mediated by tin metal. The atom efficiency of this reaction is actually less than the corresponding tetraallyltin allylation of (say) benzaldehyde (65% and 83%, respectively) because of the loss of the heavy bromine atom, but this neglects the synthesis of tetraallyltin, which is prepared from allyl bromide or chloride. A particularly intriguing recent advance with this thoroughly studied reaction is the use of nano-tin " (Scheme 5.8.17). 

The indium-mediated Barbier reaction has certainly become one of the most popular reactions for creating a carbon-carbon bond under aqueous conditions and has led to spectacular developments in recent years. Compared to other metals, indium is resistant to oxidation, hydrolysis, and has a very low first ionization potential (5.79 eV, in contrast to the second one which is quite normal) which confers on it a remarkable reactivity in Barbier-type reactions. In 1991, Li and co-workers reported the first allylation of aldehydes and ketones mediated by indium in water without any additives or special activation [133]. In particular, the use of indium allowed reactions with acid-sensitive compounds [134] or the preparation of complex carbohydrates such as deaminated... 

Allylation of aldehydes and ketones and related electrophiles using allyldialkylboranes continues to stimulate interest, especially within the context of asymmetric synthesis. All four possible diastereomeric 3-methylhomoallyl alcohols corresponding to (149) and (150) may be prepared in a highly selective manner using either Z- or E- crotyldiisopinocampheylboranes (147) and (148) and by controlling... 

More recently, the Pd-catalyzed allylation of enamines and imines has been applied to the enantioselective a-allylation of aldehydes and ketones in ee values up to Until several years ago, the results obtained in this investigation represented some of the most favorable cases of the Pd-catalyzed enantioselective allylation. Several representative results are summarized in Scheme 17 U8H40] p j. (jjg pd-catalyzed allylation of 2-phenylpropionaldehyde with (5)-proline allyl ester, a mechanism involving an organopaUadium complex shown in Scheme 18 has been proposed, and it does appear very plausible. 

Barbier-type allylation of aldehydes and ketones with allyl bromides and metallic lead in an aqueous reaction medium has been developed to give the corresponding homoallylic alcohol (Scheme 22.3a). Although the process is very effective for aromatic aldehydes, including ot,p-unsaturated ones, it is less good for aliphatic aldehydes and ketones. For crolyl substrates, some syn- [erythro-) selectivity was observed (Scheme 22.3b). 

Compared to other metals, indium has proved attractive, owing to three major properties (i) indium is unaffected by boiling water (ii) it does not form oxides readily in the air and (iii) its first ionization potential is low (5.79 eV). Li and Chan (1991b) took advantage of these properties to undertake the first allylation of aldehydes and ketones mediated by indium in water, without the help of an additive, such as a salt or a cosolvent, as well as the help of ultrasound. 

Recently, Sarangi et al. (1995) discovered that the Barbier-type allylation of carbonyl compounds could be mediated by zero-valent copper. In the experimental procedure copper(ll) chloride in its hydrated form was mixed with magnesium powder. This reaction did not proceed with anhydrous CuCU, but addition of water to this anhydrous salt in the presence of magnesium allowed the allylation of aldehydes and ketones with allylic bromides. In the absence of halides, the hydrated reagent (CuCl2 2H20/Mg) turned out to be efficient in the reduction of aldehydes. However, the water of crystallization could not suffice and addition of water promoted the reduction process (Sarangi eta/., 1995). 

Nokami, J. Otera, J. Sudo, T. Okawara, R. (1983) Allylation of aldehydes and ketones in the presence of water by allylic bromides, metallic tin, and aluminium, Organometallics, 2,191-3. 

Sjoholm, R. Rairama, R. Ahonen, M. (1994) Zinc mediated allylation of aldehydes and ketones with cinnamyl chloride in aqueous medium,/. Chem. Soc., Chem. Commm., 1217-18. 

In the pioneering works by Hosomi and Sakurai, a stoichiometric or substoichio-metric amount of TiCU, a conventional Lewis acid, was used for the carbonyl allylation with allylsilanes [106]. Davis and coworkers found that TMS borates such as Me3Si[BX(OTf)3] (X = OTf, Cl) enables an efficient, catalytic Hosomi-Sakurai allylation of aldehydes although (la) and Me3SiI show low catalytic activities (Scheme 9.41) [18, 107]. Ishihara and Yamamoto have demonstrated the utility of (lb) as Lewis acid catalyst of the carbonyl allylation as well as the Mukaiyama aldol reaction [15]. The silicon Lewis acid (lb), generated in situ by the reaction of allyltrimethylsilane with HNTf2, efficiently promotes the allylation of aldehydes and ketones with a loading of 0.5 mol% (Scheme 9.41). 

Kuroboshi M, Goto K, Mochizuki M, Tanaka H. Tetrakis (dimethylamino)ethylene (TDAE) as a potent electron source for Cr-mediated allylation of aldehydes and ketones. Synlett 1999 12 1930-1932. 

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