10 APTER 1 TE EMITRY F ARBXYLI AID DERIVATIVE TUDY GUIDE LIK 1.5 Esters and ucleophiles 1.17 Give the structure of the product in the reaction of each of the following esters with isotopically labeled sodium hydroxide, a 18. Ph L L L 3 Ph L L L 3 B A 1.18 ow would you synthesize each of the following compounds from an acid chloride? (a) Ph (b) 3 L L 3 3 L L L L (c) A (d) ( 3 ) 3 L L L L L L( 3 ) 3 1.9 REDUTI F ARBXYLI AID DERIVATIVE A. Reduction of Esters to Primary Alcohols thium aluminum hydride reduces all carboxylic acid derivatives. Reduction of esters with this reagent, like the reduction of carboxylic acids, gives primary alcohols. 3 L L L 5 Al 4 3 lithium aluminum ethyl -methylbutanoate hydride ether 3 3 L L L 5, Al 3 salts 3 ethanol -methyl-1-butanol (91 yield) (1.47) Two alcohols are formed in this reaction, one derived from the acyl group of the ester (- methyl-1-butanol in Eq. 1.47), and one derived from the alkoxy group (ethanol in Eq. 1.47). In most cases, a methyl or ethyl ester is used in this reaction, and the by-product methanol or ethanol is discarded; the alcohol derived from the acyl portion of the ester is typically the product of interest. As noted several times (ec. 0.10), the active nucleophile in Al 4 reductions is the hydride ion (3 ) delivered from Al 4, and this reduction is no exception. ydride replaces alkoxide at the carbonyl group of the ester to give an aldehyde. (Write the mechanism of this reaction, another example of nucleophilic acyl substitution.) L L R 5 RL L 5 Al 3 an aldehyde (1.48a)
1.9 REDUTI F ARBXYLI AID DERIVATIVE 103 The aldehyde reacts rapidly with Al 4 to give the alcohol after protonolysis (ec. 19.8). R L L Al 4 3 RLL (1.48b) The reduction of esters to alcohols thus involves a nucleophilic acyl substitution reaction followed by a carbonyl addition reaction. odium borohydride (ab 4 ), another useful hydride reducing agent, is much less reactive than lithium aluminum hydride. It reduces aldehydes and ketones, but it reacts very sluggishly with most esters; in fact, ab 4 can be used to reduce aldehydes and ketones selectively in the presence of esters. Acid chlorides and anhydrides also react with Al 4 to give primary alcohols. owever, because acid chlorides and anhydrides are usually prepared from carboxylic acids, and because carboxylic acids themselves can be reduced to alcohols with Al 4 (ec. 0.10), the reduction of acid chlorides and anhydrides is seldom used. B. Reduction of Amides to Amines Amines are formed when amides are reduced with Al 4. 1) 3 ) Al 4 Ph L L Ph L L, Al 3 salts lithium aluminum hydride benzamide benzylamine (80 yield) (1.49) In the workup conditions, 3 is followed by. An aqueous acidic solution is often used to carry out the protonolysis step that follows the Al 4 reduction (as shown in the following mechanism). The excess of acid that is typically used converts the amine, which is a base, into its conjugate-acid ammonium ion. ydroxide is then required to neutralize this ammonium salt and thus give the neutral amine. R 3 conjugate-base ammonium ion (typical pk a = 8 11) R amine (pk a = 15.7) (1.50) Although water itself rather than acid can be used in the protonolysis step, for practical reasons the acidic workup is more convenient. Thus, the extra neutralization step is required. Amide reduction can be used not only to prepare primary amines from primary amides, but also to prepare secondary and tertiary amines from secondary and tertiary amides, respectively. 1) 3 ) Al 4 0L ( 3 ) L 0L (, Al 3 L 3 ) salts (88 yield) (1.51)
104 APTER 1 TE EMITRY F ARBXYLI AID DERIVATIVE The reaction of Al 4 with an amide differs from its reaction with an ester. In the reduction of an ester, the carboxylate oxygen is lost as a leaving group. If amide reduction were strictly analogous to ester reduction, the nitrogen would be lost, and a primary alcohol would be formed. Instead, it is the carbonyl oxygen that is lost in amide reduction. Ester reduction: RLLR Al 4 the carbonyl oxygen is retained 3 R L R (1.5a) Amide reduction: RLL 1) 3 Al 4 ) the carbonyl oxygen is lost L R (1.5b) Let s consider the reason for this difference, using as a case study the reduction of a secondary amide. (The mechanisms of reduction of primary and tertiary amides are somewhat different, but they have the same result.) In the first step of the mechanism, the weakly acidic amide proton reacts with an equivalent of hydride, a strong base, to give hydrogen gas, Al 3, and the lithium salt of the amide. 33 R 33 3 3 L Al 3 Al 3 (1.53a) The lithium salt of the amide, a Lewis base, reacts with the Lewis acid Al 3. 3 3 Al 3 (1.53b) The resulting species is an active hydride reagent, conceptually much like Al 4, and it can deliver hydride to the A double bond. reactive hydride Al Al 3 3 L L L The LAl group is subsequently lost from the tetrahedral intermediate because it is less basic than the other possible leaving group, R. The resulting product is an imine (ec. 19.11)..... 3 L Al 3 (1.53c).... R
1.9 REDUTI F ARBXYLI AID DERIVATIVE 105 Al 3 L L L L Al 3 3 an imine (1.53d) The A of the imine, like the A of an aldehyde, undergoes nucleophilic addition with 3 from Al 4 or from one of the other hydride-containing species in the reaction mixture. Addition of acid to the reaction mixture converts the addition intermediate into an amine by protonolysis and then into its conjugate-acid ammonium ion. L L L Al L L Al 3 L L L L 3 3 L L (1.53e) The ammonium ion is neutralized to the free amine when is added in a subsequent step (Eq. 1.5b).. Reduction of itriles to Primary Amines itriles are reduced to primary amines by reaction with Al 4, followed by the usual protonolysis step. ' 1) 3 ) y Al 4 y, Al 3 salts -(1-cyclohexenyl)ethanenitrile lithium aluminum hydride -(1-cyclohexenyl)ethanamine (74 yield) (1.54) As in amide reduction, isolation of the neutral amine requires addition of at the conclusion of the reaction. The mechanism of this reaction illustrates again how the ' and A bonds react in similar ways. This reaction probably occurs as two successive nucleophilic additions. RL ' 3 RL A 3 Al 3 L Al 3 imine salt (1.55a) In the second addition, the imine salt reacts in a similar manner with Al 3 (or another equivalent of Al 4 ). RL A 3 L Al RL L3 Al RL L Al (1.55b)
106 APTER 1 TE EMITRY F ARBXYLI AID DERIVATIVE In the resulting derivative, both the L and the LAl bonds are very polar, and the nitrogen has a great deal of anionic character. Both bonds are susceptible to protonolysis. ence, an amine, and then an ammonium ion, is formed when aqueous acid is added to the reaction mixture. R L Al 3 3 R L R L, Al 3 salts 3 (neutralization with gives the amine) (1.55c) itriles are also reduced to primary amines by catalytic hydrogenation using Raney nickel, a type of nickel aluminum alloy. 3 ( ) 4 ' 3 ( ) 4 hexanenitrile Raney i 000 psi 10 130 1-hexanamine (1.55d) An intermediate in the reaction is the imine, which is not isolated but is hydrogenated to the amine product. (ee also Problem 1., p. 108.), catalyst, catalyst RL ' RL A RL L imine (1.56) The reductions discussed in this and the previous section allow the formation of the amine functional group from amides and nitriles, the nitrogen-containing carboxylic acid derivatives. ence, any synthesis of a carboxylic acid can be used as part of an amine synthesis, but it is important to notice that the amine prepared by these methods must have the following form: ) RL L $ A or ' carbon of the carboxylic acid derivative As this diagram shows, the carbon of the carbonyl group or cyano group in the carboxylic acid derivative ends up as a L L group adjacent to the amine nitrogen. tudy Problem 1.1 utline a synthesis of (cyclohexylmethyl)methylamine from cyclohexanecarboxylic acid.? L 3 cyclohexanecarboxylic acid (cyclohexylmethyl)methylamine olution Any carboxylic acid derivative used to prepare the amine must contain nitrogen; the two such derivatives are amides and nitriles. owever, the only type of amine that can be prepared directly by nitrile reduction is a primary amine of the form L. Because the desired product is not a primary amine, the reduction of nitriles must be rejected as an approach to this target.
1.9 REDUTI F ARBXYLI AID DERIVATIVE 107 The amide that could be reduced to the desired amine is -methylcyclohexanecarboxamide: 1) Al 4 ) 3 3 3) L 3 -methylcyclohexanecarboxamide This amide can be prepared, in turn, by reaction of the appropriate amine, in this case methylamine, with an acid chloride: l 3 methylamine (excess) 3 3 3 l cyclohexanecarbonyl chloride Finally, the acid chloride is prepared from the carboxylic acid (ec. 0.9A). D. Reduction of Acid hlorides to Aldehydes Acid chlorides can be reduced to aldehydes by either of two procedures. In the first, the acid chloride is hydrogenated over a catalyst that has been deactivated, or poisoned, with an amine, such as quinoline, that has been heated with sulfur. (Amines and sulfides are catalyst poisons.) This reaction is called the Rosenmund reduction. 3 L l 3 3 3,4,5-trimethoxybenzoyl chloride Pd/ 50 psi quinoline sulfur 3 3 3 L 3,4,5-trimethoxybenzaldehyde (54 83 yield) l (1.57) The poisoning of the catalyst prevents further reduction of the aldehyde product. A second method of converting acid chlorides into aldehydes is the reaction of an acid chloride at low temperature with a cousin of Al 4, lithium tri(tert-butoxy)aluminum hydride. ( 3 ) 3 L Ll,-dimethylpropanoyl chloride L Al[( 3 ) 3 ] 3 lithium tri(tert-butoxy)aluminum hydride,-dimethylpropanal 78 diglyme 3 ( 3 ) 3 L L 3 ( l Al 3 3 ) 3 salts (1.58)
108 APTER 1 TE EMITRY F ARBXYLI AID DERIVATIVE The hydride reagent used in this reduction is derived by the replacement of three hydrogens of lithium aluminum hydride by tert-butoxy groups. As the hydrides of Al 4 are replaced successively with alkoxy groups, less reactive reagents are obtained. In fact, the preparation of Al[( 3 ) 3 ] 3 owes its success to the poor reactivity of its hydride: the reaction of Al 4 with tert-butyl alcohol stops after three moles of alcohol have been consumed. Al 4 3( 3 ) 3 LL L Al L ( 3 ) 3 3 3 L (1.59) The one remaining hydride reduces only the most reactive functional groups. Because acid chlorides are more reactive than aldehydes toward nucleophiles, the reagent reacts preferentially with the acid chloride reactant rather than with the product aldehyde. In contrast, lithium aluminum hydride is so reactive that it fails to discriminate to a useful degree between the aldehyde and acid chloride groups, and it thus reduces acid chlorides to primary alcohols. The reduction of acid chlorides adds another synthesis of aldehydes and ketones to those given in ec. 19.4. A complete list of methods for preparing aldehydes and ketones is given in Appendix V. PRBLEM 1.19 how how benzoyl chloride can be converted into each of the following compounds. (a) benzaldehyde (b) Ph L 1.0 omplete the following reactions by giving the principal organic product(s). (a) Raney i Ph ' heat (b) 5 L L L Al 4 (excess) (c) L L 3 Ph L L 5 Al 4 (excess) 1) 3 ) 3 1.1 Give the structures of two compounds that would give the amine ( 3 ) after Al 4 reduction. 1. (a) In the catalytic hydrogenation of some nitriles to primary amines, secondary amines are obtained as by-products: R L ' (catalyst) R (R ) secondary amine uggest a mechanism for the formation of this by-product. (int: What is the intermediate in the reduction? ow can this intermediate react with an amine?) (b) Explain why ammonia added to the reaction mixture prevents the formation of this byproduct. E. Relative Reactivities of arbonyl ompounds Recall that the reaction of lithium aluminum hydride with a carboxylic acid (ec. 0.10) or ester (ec. 1.9A) involves an aldehyde intermediate. But the product of such a reaction is a
1.10 REATI F ARBXYLI AID DERIVATIVE WIT RGAMETALLI REAGET 109 primary alcohol, not an aldehyde, because the aldehyde intermediate is more reactive than the acid or ester. The instant a small amount of aldehyde is formed, it is in competition with the remaining acid or ester for the Al 4 reagent. Because it is more reactive, the aldehyde reacts faster than the remaining ester reacts. ence, the aldehyde cannot be isolated under such circumstances. n the other hand, the lithium tri(tert-butoxy)aluminum hydride reduction of acid chlorides can be stopped at the aldehyde because acid chlorides are more reactive than aldehydes. When the aldehyde is formed as a product, it is in competition with the remaining acid chloride for the hydride reagent. Because the acid chloride is more reactive, it is consumed before the aldehyde has a chance to react. These examples show that the outcomes of many reactions of carboxylic acid derivatives are determined by the relative reactivities of carbonyl compounds toward nucleophilic reagents, which can be summarized as follows. (itriles are included as honorary carbonyl compounds. ) Relative reactivities of carbonyl compounds: nitriles < amides < esters, acids << ketones < aldehydes < acid chlorides increasing reactivity (1.60) The explanation of this reactivity order is the same one used in ec. 1.7E. Relative reactivity is determined by the stability of each type of carbonyl compound relative to its transition state for addition or substitution. The more a compound is stabilized, the less reactive it is; the more a transition state for nucleophilic addition or substitution is stabilized, the more reactive the compound is (Fig. 1.6, p. 101). For example, esters are stabilized by resonance (Eq. 1.8, p. 1014) in a way that aldehydes and ketones are not. ence, esters are less reactive than aldehydes. In contrast, resonance stabilization of acid chlorides is much less important, and acid chlorides are destabilized by the electron-attracting polar effect of the chlorine. Moreover, the transition-state energies for nucleophilic substitution reactions of acid chlorides are lowered by favorable leaving-group properties of chlorine. For these reasons, acid chlorides are more reactive than aldehydes, in which these effects of the chlorine are absent. 1.10 REATI F ARBXYLI AID DERIVATIVE WIT RGAMETALLI REAGET A. Reaction of Esters with Grignard Reagents Most carboxylic acid derivatives react with Grignard or organolithium reagents. ne of the most important reactions of this type is the reaction of esters with Grignard reagents. In this reaction, a tertiary alcohol is formed after protonolysis. (econdary alcohols are formed from esters of formic acid; see Problem 1.4a, p. 103.) ( 3 ) L L 5 3 MgI 3 ( ether 3 ) L L 3 5 Mg salts ethyl -methylpropanoate methylmagnesium 3 iodide,3-dimethyl--butanol (1.61) (9 yield)