Aldehydes, preparation from primary alcohols

The alkylation of 3-bromothiophen by allylic alcohols in the presence of Pd(OAc)2 has been studied. Thus, 2-methyl-5-(3-thienyl)pentan-3-one is the major product from reaction between 3-bromothiophen, CH2=CH-CH(OH)CHMe2, Pd(OAc)2, Nal, NaHCOa, and PhsP in HMPT at 130 °C for 8 h. Thienyl aldehydes are similarly prepared from primary alcohols. ... 

The oxidation procedure used here is also applicable to the preparation of aldehydes from primary alcohols. 

Now we only need to prepare the primary alcohol from the given starting aldehyde, which is accomplished by reduction. 

As illustrated in Figure 19.7, the reduction of an acid derivative, such as an ester, with lithium aluminum hydride produces an aldehyde as an intermediate. Reduction of the aldehyde gives a primary alcohol as the ultimate product. It would be useful to be able to stop such a reduction at the aldehyde stage so that an aldehyde could be prepared directly from a carboxylic acid derivative. 

Unsymmetrical secondary amines are readily prepared in good yields by the catalytic reduction of Schiff bases at moderate temperatures in high-or low-pressure equipment. Many examples have been cited. The intermediate imines are prepared from primary amines and aldehydes—very seldom from ketones—and may be used without isolation (cf. method 431). For the preparation of aliphatic amines, e.g., ethyl-w-propylamine and n-butylisoamylamine, a prereduced platinum oxide catalyst is preferred with alcohol as the solvent. Schiff bases from the condensation of aromatic aldehydes with either aromatic or aliphatic amines are more readily prepared and are reduced over a nickel catalyst. In this manner, a large number of N-alkylbenzylamines having halo, hydroxyl, or methoxyl groups on the nucleus have been made. Reductions by means of sodium and alcohol and lithium aluminum hydride have also been described,... 

Many of the oxidants employed to prepare aldehydes from primary alcohols may be used to further oxidize the aldehyde initially formed to the corresponding carboxylic acid. The most common oxidants for this purpose include KMn04," chromic acid, sodium chlorite,silver oxide," and PDC in DMF. °... 

Trialkyltin alkoxides prepared by evacuation of methanol from a mixture of an alcohol and trialkyltin methoxide are easily oxidized by bromine in the presence of trialkyltin alkoxide as HBr scavenger, producing the desired aldehyde and ketone efficiently [324]. This oxidation is more feasible on organotin alkoxides derived from secondary alcohols than those from primary alcohols, and enables regioselective oxidation of polyols [325]. The regioselectivity in oxidation of vicinal secondary alcohols was also examined [326], and the empirical informative results were invoked for access to namenamicin A-C disaccbarides (Scheme 12.177) [327]. 

The mild reaction conditions of the Corey-Kim oxidation reaction make it an excellent choice when the oxidation of an alcohol that is contained within a complex synthetic intermediate is needed. Rapoport has shown this oxidation method to be useful to prepare aldehydes from primary alcohols in several multi-step syntheses. For example, treatment of alcohol 26 with NCS and DMS in toluene provided aldehyde 27 that was ultimately used to construct the (-)-enantiomer of the C-9-amino acid constituent (28) of immunosuppressant drug cyclosporine as well as the naturally occurring (+)-enantiomer after a slight modification.6... 

Allyl alkyl carbonates, prepared from various alcohols except simple primary ones, are converted into aldehydes or ketones in the presence of a phosphine-free palladium catalyst. Acetonitrile as coordinating solvent is necessary for the success of this reaction. A mechanism via palladium alkoxides was proposed (Scheme 8). Ruthenium hydride complexes work similarly. A similar mechanism operates for the palladium-catalyzed decomposition of allylic carbonates. The reaction can be utilized for the mild deprotection of amines, e.g., for peptide synthesis shown in equation (20). 

The Dess-Martin periodinane ( DMP ) reagent, U,l-tris(acetyloxy)-l,l-dihydro-l,2-benziodoxol-3(l//)-one, has also been used in several complex syntheses for the oxidation of primary or secondary alcohols to aldehydes or ketones, respectively (e.g., M. Nakatsuka, 1990). It is prepared from 2-iodobenzoic add by oxidation with bromic add and acetylation (D.a Dess, 1983). 

Primary alcohols are oxidized to either aldehydes or carboxylic acids, depending on the reagents chosen and the conditions used. One of the best methods for preparing an aldehyde from a primary alcohol on a small laboratory scale, as opposed to a large industrial scale, is to use pyridinium chloro-chromate (PCC, CsH NCrO Cl) in dichloromethane solvent. 

Because the olefin geometry in compound 9 will most certainly have a bearing on the stereochemical outcome of the hydroboration step, a reliable process for the construction of the trans trisubsti-tuted olefin in 9 must be identified. A priori, the powerful and predictable Wittig reaction28 could be used to construct E u, [3-unsaturated ester 10 from aldehyde 11. Reduction of the ethoxycarbonyl grouping in 10, followed by benzylation of the resulting primary alcohol, would then complete the synthesis of 9. Aldehyde 11 is a known substance that can be prepared from 2-furylacetonitrile (12). 

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

The C(9)-C(14) segment VI was prepared by Steps D-l to D-3. The formation of the vinyl iodide in Step D-3 was difficult and proceeded in only 25-30% yield. The C(15)-C(21) segment VII was synthesized from the common intermediate 17 by Steps E-l to E-6. A DDQ oxidation led to formation of a 1,3-dioxane ring in Step E-l. The A-methoxy amide was converted to an aldehyde by LiAlH4 reduction and the chain was extended to include C(14) and C(15) using a boron enolate of an oxazo-lidinone chiral auxiliary. After reductive removal of the chiral auxiliary, the primary alcohol group was converted to a primary iodide. The overall yield for these steps was about 25%. 

For preparative purposes the method of obtaining aldehydes from the primary alcohols is preferable by far, at least in the aliphatic series. The simple aromatic aldehydes can be obtained by alkaline hydrolysis of the arylidene chlorides, R.CHC12, which are produced from the hydrocarbons by substitution with chlorine (technical method for the preparation of benzaldehyde). In addition to these methods the elegant synthesis of Gattermann and Koch should be mentioned here. This synthesis, which proceeds like that of Friedel-Crafts, consists in acting on the aromatic hydrocarbon with carbon monoxide and hydrogen chloride in the presence of aluminium chloride and cuprous chloride. 

Perhaps the most important recent discovery in catalytic oxidation of alcohols is the use of a catalyst prepared from [Pd(OAc)2] and sulfonated batophenanthroline (see Scheme 8.1 above). This catalyst was found to oxidize primary and secondary, as well as benzylic and allylic alcohols with close to quantitative yields and 90-100 % select vities to the corresponding aldehydes or ketones (Scheme 8.4) [18]. The easy oxidation of non-activated secondary alcohols is particularly noteworthy since in general these are rather unreactive towards O2. 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

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