Allyl reaction with carbonyl compounds

Scheme 9.6. Reactions of Allylic Stannanes with Carbonyl Compounds...

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

T. Akiyama, J. Iwai, Scandium Trifluoromethane-sulfonate-Catalyzed Chemoselective Allylation Reactions of Carbonyl Compounds with Tetraallylgermane in Aqueous Media Tetrahedron Lett. 1997,38, 853-856. 

Scheme 9.6 gives some other examples of Lewis acid-catalyzed reactions of allylic stannanes with carbonyl compounds. 

Allylation of carbonyl compounds. The coupling reaction of allylic bromides with carbonyl compounds such as aromatic and aliphatic aldehydes as well as ketones leads to the corresponding allylic alcohol in good yields (54-85%) and with high regioselectivity (see, for instance, equation 16)25. 

A partially soluble polyallylscandium triflamide ditriflate 45 was prepared and used to catalyze a three-component coupling reaction.67 An aldehyde, an aromatic amine, and an alkene were mixed in the presence of the catalyst to afford tetrahydroquinolines (equation 17). The catalyst was recovered from the reaction mixtures by precipitation with hexane and could be recycled without loss of activity. Another polymer-supported scandium catalyst was prepared by treating Nafion with scandium chloride to afford the Nafion-scandium catalyst 46.68 This catalyst was used in allylation reactions of carbonyl compounds by tetraallyltin (equation 18). It could be easily recovered by filtration and reused without appreciable loss of activity. 

The reaction of allylic alcohols with carbonyl compounds was tested on a particular type of allylic alcohol, such as 2-furylmethanol derivatives. While the reaction of 2-furylmethanol with benzophenone showed low regioselectivity, the presence of larger substituents on the carbon bearing the alcoholic function allows a high regioselectivity (Scheme 3.42) [86]. Furthermore, when 2-furyl-ethanol (38) was used as substrate, a 1 1 mixture of stereoisomers was obtained ... 

The control of diastereoselectivity in the allylation reaction of carbonyl compounds with allylic indium reagents has been an important issue since the discovery of the indium-mediated carbonyl allylation. As earlier discussions have been summarized in the precedent reviews,6-24 only relatively recent references are cited below. 

The reaction of allylindium sesquibromide with 4 equiv. of bulky lithium alkoxide results in modified reagents 7, which show unusual degrees of chemo- and diastereoselectivity in the reactions with carbonyl compounds for example, allylation of 2-methylcyclohexanone gives a high ratio of axial alcohol 8/equatorial alcohol 9 (Scheme 15).128... 

The class of ene reactions with carbonyl compounds as the enophile, which we denote the carbonyl-ene reaction [45], is an efficient alternative to the carbonyl addition reaction of allylic metals (Sch. 11). 

Sc(OTf)3 promoted allylation reactions of carbonyl compounds were reported, (a) Tetraallylger-manium T. Akiyama, J. Iwai, Tetrahedron Lett. 1997, 38, 853-856. (b) Allyltrimethylsilane V. K. Aggarwal, G. P. Vennall, Tetrahedron Lett. 1996,37, 3745-3746. Sc(OTf)3 is also an effective catalyst for the allylation of imines with allyltributyltin. (c) C. Bellucci, P. G. Cozzi, A. Umani-Ronchi, Tetrahedron Lett. 1995,36,7289-1292. 

The triethylaluminum or triethylborane ate complexes (12) of the (isopropylthio)allyl carbanion react with carbonyl compounds at the a-position (equation 10). In the reactions with carbonyl compounds, very high regioselectivity (for example, butanal 95 5, 3-methylbutanal 99 1, cyclohexanone 92 8 and acetophenone 95 5) was achieved by using the aluminum ate complex. On the other hand, the a-regio-selectivity with ketones decreases if the boron ate complex is used (cyclohexanone 72 28, acetophenone 45 55). It is noteworthy that the stereoselectivity of the a-adduct from an aldehyde is low. Ihesumably the geometry of the double bond in the ate complex (12) is not homogeneous. ... 

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