Aldehydes cross-addition

The criss-cross addition of azines of aromatic aldehydes with various electron-deficient olefins in which the double bond is terminal, for example, methyl acrylate, acrylonitrile, or in which allylic substituents do not sterically hinder the reaction, for example, maleic anhydride, is well known and was duly covered in CHEC-II(1996)<1996CHEC-II(8)747>, as well as in a review <1997ALD97>. Recently, the reaction has been used for the preparation of hyperbranched polymers <1998MI2655, 2002MAC712>. 

Few examples of what might be described as an intermolecular coupling reaction on inactivated alkenes has appeared [62], Thus ketyl radicals generated from aromatic aldehydes and ketones underwent intermolecular addition to the para position of another aldehyde. Cross-coupling reactions are not feasible in these systems and typically yields are quite low. 

Additionally, a nickel-catalyzed Tishchenko reaction has been developed, where the selective crossed addition of two different aldehydes to generate a single ester product is possible (Scheme 3-76). By employing an aromatic aldehyde with a nonaromatic aldehyde, a metallacycle derived from heterocoupling is formed, followed by its decomposition to the ester product. 

Check your work.) In one of the boxes above, an aldol reaction (leading to Product A) will occur even before addition of the second aldehyde. Cross out this box. 

In the same year, Hayashi and coworkers [39] also reported the use of an amphiphihc L-proHne derivative bearing a long alkyl chain on the 4-position via an ether bond for the enantioselective aldehyde cross-aldol reaction without the need for an additional co-solvent or additives (Scheme 8.14). Probably, emulsions offer an ideal reaction environment in which organic molecules can be assembled through hydrophobic interactions, thus enabling the aldol reaction to proceed efficiently. As a result, the corresponding products, chiral 1,3-diols, could be obtained with high diastereo- and enantioselectivity. 

If only one of the two aldehydes has an a-hydrogen, only two aldols can be formed and numerous examples have been reported, where the crossed aldol reaction is the major pathway. For two different ketones, similar considerations do apply in addition to the unfavorable equilibrium mentioned above, which is why such reactions are seldom attempted. 

The polymer-bound catalysts A-C. (Table 31) are prepared by reaction of the corresponding amino alcohols with partially chloromethylated 1 -2% cross-linked polystyrene. In the case of A, the enantioselectivity of the addition of dialkylzincs to aldehydes is higher than with the corresponding monomeric ephedrine derivatives (vide supra). Interesting insights into the mechanism of the alkylation of aldehydes by dialkylzinc reagents can be obtained from the experi-... 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

The coupling of a secondary alcohol 1 with a primary alcohol 2 is achieved by the temporary removal of from each substrate which generates the ketone 3 and aldehyde 4 intermediates. A crossed aldol condensation occurs under the reaction conditions by the enolate derived from ketone 3 undergoing nucleophilic addition... 

Titanium enolates can also be used under conditions in which the titanium exists as an ate species. Crossed aldehyde-aldehyde additions have been accomplished starting with trimethylsilyl enol ethers, which are converted to lithium enolates and then to ate species by addition of Ti(0- -Bu)4.26 These conditions show only modest stereoselectivity. 

Crossed aldol condensations, where both aldehydes (or other suitable carbonyl compounds) have a-H atoms, are not normally of any preparative value as a mixture of four different products can result. Crossed aldol reactions can be of synthetic utility, where one aldehyde has no a-H, however, and can thus act only as a carbanion acceptor. An example is the Claisen-Schmidt condensation of aromatic aldehydes (98) with simple aliphatic aldehydes or (usually methyl) ketones in the presence of 10% aqueous KOH (dehydration always takes place subsequent to the initial carbanion addition under these conditions) ... 

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