Unsaturated Aliphatic Aldehydes

Crotonaldehyde can be produced so cheaply on a commercial scale that its preparation in the laboratory is rarely undertaken. It is, however, included here as an illustration of the preparation of an unsaturated aliphatic aldehyde. 

The addition of HCN to aldehydes or ketones produces cyanohydrins. This is an equilibrium reaction. For aldehydes and aliphatic ketones the equilibrium lies to the right therefore the reaction is quite feasible, except with sterically hindered ketones such as diisopropyl ketone. However, ketones ArCOR give poor yields, and the reaction cannot be carried out with ArCOAr since the equilibrium lies too far to the left. With aromatic aldehydes the benzoin condensation (16-54) competes. With oc,p-unsaturated aldehydes and ketones, 1,4 addition competes (15-33). Ketones of low reactivity, such as ArCOR, can be converted to cyanohydrins by treatment with diethylaluminum cyanide (Et2AlCN see OS VI, 307) or, indirectly, with cyanotrimethylsilane (MesSiCN) in the presence of a Lewis acid or base, followed by hydrolysis of the resulting O-trimethylsilyl cyanohydrin (52). The use of chiral additives in this latter reaction leads to cyanohydrins with good asymmetric... 

The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

In the Diels-Alder reaction with inverse electron demand, the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group at the oxabutadiene at the 3-position lowers its LUMO dramatically, the cycloaddition as well as the condensation usually take place at room or slightly elevated temperature. There is actually no restriction for the aldehydes. Thus, aromatic, heteroaromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. For example, a-oxocarbocylic esters or 1,2-dike-tones for instance have been employed as ketones. Furthermore, 1,3-dicarbonyl compounds cyclic and acyclic substances such as Meldmm s acid, barbituric acid and derivates, coumarins, any type of cycloalkane-1,3-dione, (1-ketoesters, and 1,3-diones as well as their phosphorus, nitrogen and sulfur analogues, can also be ap-... 

The secretion of the red hartebeest is characterized by its high aldehyde content. Of an estimated 100 detectable constituents, 25 are saturated and unsaturated aliphatic aldehydes [138]. Because the aldehydes are highly susceptible to autoxidation, the secretion could therefore only be used for shortterm territorial marking. On the other hand, the conversion of the aldehydes to carboxylic acids could also be transmitting information with a date stamp . In this regard, it is debatable whether fatty acids, which are almost ubiquitous in the animal world, really are such major carriers of semiochemical information in all of the many species in which they are purported to fulfill this role. 

Claisen-Schmidt condensation org chem A reaction employed for preparation of unsaturated aldehydes and ketones by condensation of aromatic aldehydes with aliphatic aldehydes or ketones in the presence of sodium hydroxide. klas-on jshmit kand-on sa-shon ... 

A method for the conversion of unsaturated aliphatic aldehydes to saturated aldehydes is a gentle catalytic hydrogenation. Palladium is more selective than nickel. Hydrogenation over sodium borohydride-reduced palladium in methanol at room temperature and 2 atm reduced crotonaldehyde to butyralde-hyde but did not hydrogenate butyraldehyde [57]. Nickel prepared by reduction with sodium borohydride was less selective it effected reduction of crotonaldehyde to butyraldehyde but also reduction of butyraldehyde to butyl alcohol, though at a slower rate [57]. Hydrogenation of 2,2,dimethyl-... 

Unsaturated aliphatic aldehydes were selectively reduced to unsaturated alcohols by specially controlled catalytic hydrogenation. Citral treated with hydrogen over platinum dioxide in the presence of ferrous chloride or sulfate and zinc acetate at room temperature and 3.5 atm was reduced only at the carbonyl group and gave geraniol (3,7-dimethyl-2,6-octadienol) [59], and crotonaldehyde on hydrogenation over 5% osmium on charcoal gave crotyl alcohol [763]. 

Reduction of unsaturated aliphatic aldehydes to saturated alcohols was... 

In addition to the straight-chain saturated aldehydes, a number of branched-chain and unsaturated aliphatic aldehydes are important as fragrance and flavoring materials. The double unsaturated 2-trviolet leaf aldehyde (the dominant component of cucumber aroma), is one of the most potent fragrance and flavoring substances it is, therefore, only used in very small amounts. 2-frfatty odor character is indispensible in chicken meat flavor compositions. 

The major components of cardamom oil are 1,8-cineole and a-terpinyl acetate (ca. 30% each). Trace constituents like unsaturated aliphatic aldehydes may be important for the typical aroma [301-307b]. It is produced from cultivated or wild plants in the mountainous regions of southern India, Sri Lanka, Indonesia, and Guatemala. 

The main components are limonene (65-75%) and q-terpinene (16-22%). The characteristic feature of mandarin oil is its content of a-sinensal (0.2-0.5%), methyl A-methylanthranilate (0.3-0.6%, which is responsible for fluorescence), and long-chain unsaturated aliphatic aldehydes [369, 370a, 370d, 370e, 381, 394c, 414-421b]. 

The thiazolium salt 3-benzyl-5-(2-hydroxyethyl)-4-methyl-l,3-thiazolium chloride is an excellent catalyst for the addition of unsaturated aliphatic aldehydes to vinylketones (79CB84). The presence of a base such as sodium acetate or triethylamine is required, for the thiazolium salt must first be transformed into the ylide structure (615), which then exerts a catalytic effect resembling that of cyanide ion in the benzoin condensation (Scheme 137). Yields of 1,4-diketones (616) produced in this process were generally good. The use of thiazolium salts for other related reactions has been reviewed (76AG(E)639). 

The following tables are intended to include all the reductions with aluminum alkoxides which were reported prior to February, 1943, although some examples doubtless have been overlooked. Table I lists the reduction of aldehydes, which have been subdivided into (a) aliphatic aldehydes and (b) alicyclic and aromatic aldehydes. Table II lists the reduction of ketones, which have been classified as (a)-satu-rated and unsaturated aliphatic ketones, (b) aromatic ketones, (c) alicyclic ketones, (d) unsaturated alicyclic and aromatic ketones, (e) a- — halogen substituted ketones, (f) diketones, (g) protected diketones, (h) alcoholic and phenolic ketones (and ethers or esters of these), and (i) keto esters. 

The volatiles from cooked meat contain large numbers of aliphatic compounds including aldehydes, alcohols, ketones, hydrocarbons and acids. These are derived from lipids by thermal degradation and oxidation (J7) and many may contribute to desirable flavor. In addition, the aldehydes, unsaturated alcohols and ketones produced in these reactions, as well as the parent unsaturated fatty acids, are reactive species and under cooking conditions could be expected to interact with intermediates of the Maillard reaction to produce other flavor compounds. 

Both reactivity and selectivity decreased with increasing steric bulk of alkyl group on the vinyl ether (Et> -Pr> -Bu /-Bu), and tert-butyl vinyl ether was completely unreactive. The cycloaddition of ethyl vinyl ether with a wide variety of oc, 3-unsaturated aldehydes bearing aliphatic and aromatic P-substituents proceeded with high selectivity (>95% de, 89-98% ee). Only 5 mol% of catalyst were necessary except in the case of sterically more demanding substituents (R = i-Pr, Ar) that required 10 mol% of catalyst loading. Substitution could also be introduced in the a-position of the unsaturated aldehyde, and cycloadducts were obtained with similar high selectivities.26... 

Note The pyrazinylsulfonyl grouping has been employed as a leaving group in the formation of unsaturated aliphatic compounds and aliphatic or aromatic aldehydes.297 302 308,319... 

There is a significant difference in the toxicological effects of saturated and unsaturated aliphatic aldehydes. As can be seen in Table 6, the presence of the double bond considerably enhances toxicity. The precautions for handling reactive unsaturated aldehydes such as acrolein, metliacrolein [78-85-3]y and crotonaldeliyde should be the same as those for handling other highly active eye and pulmonary irritants, as, for example, phosgene. 

The effectiveness of the 1,1.1-trichlorotrifluoroethane/triphenylphosphane system in reactions with aldehydes, which lead to the formation of (trifluoromethyl)alkenes, has been demonstrated. The reaction with aromatic and a,) -unsaturated aliphatic aldehydes proceeds with high stereoselectivity (the Z-isomcr is formed exclusively), whereas the reaction with other aliphatic aldehydes affords a mixture of the Z- and /T-isomers in proportions ranging from 2 1 to 3 1. 

Chloro- and bromoacylatiou of carbonyl compounds. This reaction (equation I) has been known in the literature since 1900, but has attracted little attention because of poor yields. The unsatisfactory yields are a consequence of the fact that the reaction is an equilibrium and that the reactants can revert to starting materials if heated in the presence of the Lewis acid catalyst. Modern research has established that the products are favored in the case of aliphatic, a,/3-unsaturate d, and aromatic aldehydes and aliphatic ketones. Highest yields are obtained at low temperatures (----10°). By use of optimum conditions yields >95% are obtainable in some cases. 

The method cannot be applied generally to the synthesis of aliphatic aldehydes, but for aromatic aldehydes and cinnamaldehyde the yields are good (Table 13). An example, the synthesis of o-tolualdehyde, is given in the literature. A variant which has proved successful for a,3-unsaturated aliphatic aldehydes is the use of chromium(II) chloride. However, as exemplified in Table 13, the yields are only fair. Moreover, there appears to be some question as to exactly how the reagent should be prepared. ... 

Generation of Carbonyl Radical Ions by Electron Transfer 459 Table 2. Reduction potentials of some saturated and unsaturated aliphatic aldehydes and ketones. ... 

These alcohol and aldehyde compounds have been previously mentioned in their proper place as unsaturated aliphatic compounds (p. 170), but are referred to again in this place because they really belong with the terpenes. 

Finch has demonstrated that the sulfoximine approach is a viable alternative for fluoromethylenation (Scheme 12). The fluorosulfoximine (54) is deprotonated with LDA in THF and the aldehyde or ketone added to the anion. Conversion to the alkene is carried out with the standard aluminum amalgam procedure to yield a 1 1 mixture of ( )- and (Z)-alkenes (56). The reaction is very effective for aromatic and aliphatic aldehydes and aliphatic and alicyclic ketones, but, while aromatic and a,p-unsaturated ketones give good yiel of the addition adduct, the reductive elimination results in a variable amount of product formation. This method was tilled to the synthesis of prostaglandin 9-fluoromethylene (58 equation IS).- ... 

---