Reduction of acid chlorides and esters

The mechanism for this addition reaction resembles the mechanism for the metal hydride reduction of acid chlorides and esters discussed in Section 20.7A. The mechanism is conceptually divided into two parts nucleophilic substitution to form a ketone, followed by nucleophilic addition to form a 3° alcohol, as shown in Mechanism 20.7. 

Kinetic studies established that tetra-n-butylammonium borohydride in dichloromethane was a very effective reducing agent and that, by using stoichiometric amounts of the ammonium salt under homogeneous conditions, the relative case of reduction of various classes of carbonyl compounds was the same as that recorded for the sodium salt in a hydroxylic solvent, i.e. acid chlorides aldehydes > ketones esters. However, the reactivities, ranging from rapid reduction of acid chlorides at -780 C to incomplete reduction of esters at four days at 250 C, indicated the greater selectivity of the ammonium salts, compared with sodium borohydride [9], particularly as, under these conditions, conjugated C=C double bonds are not reduced. 

However, the most important methods for preparing alcohols are catalytic hydrogenation (H2/Pd-C) or metal hydride (NaBH4 or LiAlH4) reduction of aldehydes, ketones, carboxylic acids, acid chlorides and esters (see Sections 5.7.15 and 5.7.16), and nucleophilic addition of organometalhc reagents (RLi and RMgX) to aldehydes, ketones, acid chlorides and esters (see Sections 5.3.2 and 5.5.5). 

Aldehydes are prepared by the hydroboration-oxidation of alkynes (see Section 5.3.1) or selective oxidation of primary alcohols (see Section 5.7.9), and partial reduction of acid chlorides (see Section 5.7.21) and esters (see Section 5.7.22) or nitriles (see Section 5.7.23) with lithium tri-terr-butox-yaluminium hydride [LiAlH(0- Bu)3] and diisobutylaluminium hydride (DIBAH), respectively. 

Fig.M. Reduction of acid chlorides, acid anhydrides, and esters with lithium aluminium hydride.

The selective reduction of acid chlorides in the presence of esters by 9-BBN in cold THF is possible because esters are reduced only under reflux in this solvent [PSl], Reduction by Zn(BH4)2 TMEDA in Et20 leaves Cl, NO2, ester groups, and conjugated double bonds unchanged [KU3]. 

Selective reductions. The reducing properties of 9-BBN in THF have been nviewed in detail. Aldehydes and ketones are reduced rapidly and in high yield th some bias, in the case of unsymmetrical ketones, for attack from the less hindered side. a,)3-Unsaturated aldehydes and ketones are reduced selectively to tilylic alcohols. Anthraquinone (1) is reduced cleanly to 9,10-dihydro-9,10- nthracenediol (2). Carboxylic acids, acid chlorides, and esters are reduced I Afflciently. Epoxides are reduced only slowly by 9-BBN, but are reduced readily... 

Lithium aluminum hydride reduces acids, acid chlorides, and esters to primary alcohols. (The reduction of acids was covered in Section 20-13.)... 

Hydrogenation reduction of acid chloride to aldehyde using BaS04-poisoned palladium catalyst. Without this poisoning, the resulting aldehyde may be further reduced to the corresponding alcohol. The possible by-products are alcohol, ester and alkane. 

Alexander Mikhaylovich Saytzev (Saytzeff) (Kazan 20 June 1841 (O.S.)-2 September 1910) studied with Kolbe in Marburg and Leipzig, and was professor in the University of Kazan. He discovered the synthesis of primary and secondary alcohols from esters, ketones, and aldehydes by the action of zinc and alkyl iodides (see Reformatsky, p. 858). He also discovered aliphatic sulphoxides. His brother Mikhayl Mikhaylovich (b. Kazan, 30 August 1845), at first his assistant and later manager of a chemical works in Kazan, discovered the reduction of acid chlorides to aldehydes by hydrogen gas in presence of palladium. ... 

Louis Bouveault (Nevers, ii February 1864-Paris, 6 September 1909), assistant professor in the Paris Faculty of Sciences, worked out methods for the conversion of nitriles or amides to acids, the synthesis of aromatic aldehydes and acids by the use of aluminium chloride, the synthesis of aldehydes from nitro-olefins, and the reduction of aldehydes, ketones, and esters to alcohols by boiling with alcohol and sodium. ... 

Carbonyl Group Reduction. The flow of new methods for reduction of acid derivatives and aldehydes or ketones to alcohols continues unabated. The Report last year (4,134) featured the sodium borohydride reduction of carboxylic acid derivatives, originally thought to be 2-thiazoline-2-thiol esters (14), to give alcohols in good yields. Full details of the method have now appeared (Scheme 8), and it seems that the acid derivatives are in fact the 3-acyl thiazolidine-2-thiones (IS) dissappearance of their yellow colour is an easy way to monitor the reduction. Carboxylic acids or their chlorides can also be reduced to primary alcohols in good yields at room temperature using a titanium tetrachloride-sodium borohydride combination. ... 

Other Preparations.—Carboxylic acids have been converted into aldehydes through di-isobutylaluminium hydride reduction of 3-acylthiazolidine or 2-thiazoline-2-thiol ester intermediates. Bis(triphenylphosphine)copper(l) tetrahydroborate, (Ph3P)2CuBH4, shows promise as a new reagent for the reduction of acid chlorides to aldehydes. The same conversion can be accomplished using sodium borohydride in a mixture of acetonitrile and hexamethyl-phosphoramide containing a cadmium(il) chloride-dimethylformamide complex. ... 

The mechanism of ester (and lactone) reduction is similar to that of acid chloride reduction in that a hydride ion first adds to the carbonyl group, followed by elimination of alkoxide ion to yield an aldehyde. Further reduction of the aldehyde gives the primary alcohol. 

The reaction tolerates ketone, chloride, internal C=C bonds, esters, nitriles, and ether functional groups. Given that the DIBAL-H reduction of acid derivatives often suffers from over-reduction to alcohols, these catalytic procedures are of synthetic value for laboratory-scale syntheses. However, it is likely that the requirement for excess (tBuCO)20 will prevent this reaction from ever being used in commercial production. 

The hrst step in the preparation of the antidepressant maprotiline (33-5) takes advantage of the acidity of anthrone protons for incorporation of the side chain. Thus treatment of (30-1) with ethyl acrylate and a relatively mild base leads to the Michael adduct saponihcation of the ester group gives the corresponding acid (33-1). The ketone group is then reduced by means of zinc and ammonium hydroxide. Dehydration of the hrst-formed alcohol under acidic conditions leads to the formation of fully aromatic anthracene (33-2). Diels-Alder addition of ethylene under high pressure leads to the addition across the 9,10 positions and the formation of the central 2,2,2-bicyclooctyl moiety (33-3). The hnal steps involve the construction of the typical antidepressant side chain. The acid in (33-3) is thus converted to an acid chloride and that function reacted with methylamine to form the amide (33-4). Reduction to a secondary amine completes the synthesis of (33-5) [33]. 

An approach to the synthesis of a prostaglandin intermediate began with 2-furanacetonitrile (71JOC3191). Friedel-Crafts acylation with pimelic half-ester acid chloride and Wolff-Kishner reduction of the product with concomitant hydrolysis of the nitrile group to acid yielded the diester (78) on diazomethane treatment. Ring opening of the furan by a standard procedure yielded a diketo diester (79) which on refluxing in aqueous methanolic potassium carbonate underwent hydrolysis and cyclization to the diacid (80 Scheme 19). 

In the second method (Scheme 26) a 3-oxo ester is synthesized from an acid chloride and 2,2-dimethyl-l,3-dioxane-4,6-dione. Then, reductive amination of the 3-oxo ester with a-methyl(phenyl)methylamine provides a 3-amino- 3-alkyl propionic acid ester. This compound is then converted into the corresponding aldehyde, which is condensed with an eno-late to afford the final product. A representative synthetic procedure of this method is given in detail. 

An attempted synthesis of 9-methylnaphtho[crf]oxepine-2-one 494 by heterocyclization of 8-acetyl-1-naphthoic acid 491 (R = Me, R = H) has failed. In acidic medium or on heating acid 491 to 150°C, as well as under formation conditions for the acid chloride or ester from acid 491, 2-acetylacenaphthene-l-one 495 is obtained (79ZOR1562). A synthesis of tribenzo[c]oxepine derivatives 499 and 501 has been described as resulting from heterocyclization of the products of reduction (498) or oxidation (500) of 4-formyl-5-carboxyphenanthrene 497. The latter compound was obtained on ozonolysis of pyrene 496 [71JCS(C)729]. 

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