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rough oxidations or reductions in organic chemistry carbonyl reduction is the conversion of any carbonyl group usually to an alcohol it is a common transformation that is practiced in many ways 1 ketones aldehydes carboxylic acids esters amides and acid halides some of the most pervasive functional groups comprise carbonyl compounds carboxylic acids esters and acid halides can be reduced to either aldehydes or a step further to primary alcohols depending on the strength of the reducing agent aldehydes and ketones can be reduced respectively to primary and secondary alcohols in deoxygenation the alcohol group can be further reduced and removed altogether by replacement with hydrogen two broad strategies exist for carbonyl reduction one method which is favored in industry uses hydrogen as the reductant this approach is called hydrogenation and requires metal catalysts the other broad approach employs stoichiometric reagents that deliver h and h separately this article focuses on the use of these reagents prominent among these reagents are the alkali metal salts of borohydrides and aluminium hydrides general considerations edit resonance structures of different carbonyls ordered from most stable least reactive to least stable most reactive in terms of reaction mechanism metal hydrides effect nucleophilic addition of hydride to the carbonyl carbon the ease of addition of hydride to the carbonyl is affected by electrophilicity and bulk of the carbonyl as well as the corresponding electronic and steric properties of the hydride reagent the result of these trends is that acid halides ketones and aldehydes are usually the most readily reduced compounds while acids and esters require stronger reducing agents importantly and characteristically these hydride reagents generally do not attack c c bonds 2 several factors contribute to the strength of metal hydride reducing agents the reducing power of borohydride reagents is affected by the counter ion such as na vs li which can activate carbonyls by coordinating to the carbonyl oxygen li binds to carbonyl oxygen more strongly than does na 3 in the case of tetrahydroaluminates however naalh 4 and lialh 4 behave similarly 2 many metal additives have been investigated for example zinc borohydride nominally zn bh 4 2 is used for mild selective reduction of aldehydes and ketones in the presence of other reducible groups 4 the central metal usually b vs al strongly influences reducing agent s strength aluminum hydrides are more nucleophilic and better reducing agents relative to borohydrides 5 the relatively weak reducer sodium borohydride is typically used for reducing ketones and aldehydes it tolerates many functional groups nitro group nitrile ester 6 in their handling properties lithium aluminium hydride and sodium borohydride and their derivatives strongly differ nabh 4 is far easier to handle than lialh 4 being air stable for weeks it can be used with water or ethanol as solvents whereas lialh 4 reacts explosively with protic solvents substituents on the boron or aluminium modulate the power selectivity and handling properties of these reducing agents electron withdrawing groups such as acetoxy and cyano lower the reducing power such that nabh oac 3 and nabh 3 cn are weak reducing agents electron donating groups such as alkyl groups enhance the reducing strength superhydride lithium triethylborohydride and l selectride are strong reductant they are correspondingly hazardous to handle the following table 7 illustrates which carbonyl functional groups can be reduced by which reducing agents some of these reagents vary in efficacy depending on reaction conditions table of possible reactions between carbonyl groups and reducing agents substrates edit ketones are less reactive than aldehydes because of greater steric effects and because the extra alkyl group contributes electron density to the c o bond making it less electrophilic 8 since aldehydes reduce more easily than ketones they require milder reagents and milder conditions at the other extreme carboxylic acids amides and esters are poorly electrophilic and require strong reducing agents 9 carboxylic acid and esters edit relative to aldehydes and ketones carboxylic acid are difficult to reduce lithium aluminium hydride is typically effective the first step involves deprotonation of the carboxylic acid the final step in the reduction of carboxylic acids and esters is hydrolysis of the aluminium alcoxide 10 esters and amides are more easily reduced than the parent carboxylic acids their reduction affords alcohols and amines respectively 11 the idealized equation for the reduction of an ester by lithium aluminium hydride is 2 rco 2 r lialh 4 lial och 2 r 2 or lial och 2 r 2 or 4 h 2 o lial oh 4 2 hoch 2 r 2 hor sodium borohydride can under some circumstances be used for ester reduction especially with additives 1 forming aldehydes from carboxylic acid derivatives is challenging because weaker reducing agents nabh 4 are often very slow at reducing esters and carboxylic acids whereas stronger reducing agents lialh 4 immediately reduce the formed aldehyde to an alcohol 12 one strategy is transacylate to other acyl derivatives amenable to partial reduction as discussed in acid chlorides to aldehydes acid chlorides to aldehydes edit traditionally acid chlorides are reduced to aldehydes with sterically hindered hydride donors the reducing agent dibal h diisobutylaluminium hydride is often used for this purpose although it normally reduces any carbonyl dibal h can selectively reduce acid chlorides to the aldehyde level if only one equivalent is used at low temperatures 13 lialh otbu 3 formed from lialh 4 and tbuoh in situ behaves similarly 14 the idealized equation for the reduction of an acid chloride to an aldehyde by lithium aluminium hydride is rcocl lialh otbu 3 licl al otbu 3 rcho the traditional method hindered hydrides and reactive carbonyls is limited by narrow substrate scope and great dependence on reaction conditions one workaround is the rosenmund reaction which reduces acyl chlorides using hydrogen gas and a catalyst of palladium on barium sulfate the small catalyst surface area prevents over reduction 15 alternatively there are many multi step reductions to aldehydes the simplest reduces the acyl chloride all the way down to an alcohol then oxidizes it back up to an aldehyde other techniques use the acyl chloride as an acylant one can form a thioester weinreb amide or reissert compound then reduce the new species to an aldehyde through respectively the fukuyama reduction weinreb reaction or reissert workup in the weinreb reduction a weinreb amide is treated with lithium aluminum hydride 16 reduction halts at the aldehyde because the hemiacetal intermediate is a stable chelate and much less electrophilic than the collapsed carbonyl in the fukuyama reduction the thioester is then reduced to an aldehyde by a silyl hydride with a palladium catalyst 17 conversion to thioester followed by fukuyama reduction aldehydes and ketones edit the idealized equation for the reduction of a ketone by sodium borohydride is 4 rcor nabh 4 nab ochrr 4 nab ochrr 4 4 h 2 o nab oh 4 4 hochrr 4 hor complete idealized mechanism for the reduction of ketone with sodium borohydride reductive amination edit main article reductive amination in addition to their reduction to alcohols aldehydes and ketones can be converted to amines i e reductive amination 18 because of its cyano substituent nabh 3 cn is a weak reducer at moderate ph 4 so it preferentially reduces iminium cations that exist in the presence of carbonyls α β unsaturated carbonyls edit when an α β unsaturated carbonyl is reduced three products can result an allyl alcohol from simple carbonyl reduction a saturated ketone or aldehyde resulting from 1 4 reduction also called conjugate reduction or the saturated alcohol from double reduction 19 use of nabh 4 can give any of these results but incl 3 or nicl 2 catalyze specifically 1 4 reductions 1 potassium or lithium tri sec butyl borohydride sometimes selects 1 4 reductions but can be stymied by steric hindrance 20 triphenylphosphino copper hydride clusters directs catalytic hydrogenation to perform specifically conjugate reduction to selectively form the allyl alcohol and avoid the 1 4 product the luche reduction uses cerium borohydride generated in situ from nabh 4 and cecl 3 h 2 o 7 21 22 the hydride source zn bh 4 2 also shows 1 2 selectivity as well as greater diastereoselectivity it does so by coordinating not only to the carbonyl oxygen but also to adjacent atoms 23 hydrogenolysis edit a special case of carbonyl reduction entails complete deoxygenation i e hydrogenolysis this result is often undesirable because it involves defunctionalization some reactions for this transformation include the clemmensen reduction in strongly acidic conditions and the wolff kishner reduction in strongly basic conditions as well as the various modifications of wolff kishner reaction the caglioti modification for instance uses tosylhydrazone with a hydride donor in milder conditions with no base 24 the myers modification substitutes hydrazine with bis tert butyldimethylsilyl hydrazine uses milder conditions at room temperature and is rapid and efficient 25 mechanism of wolff kishner reduction aromatic carbonyls are more readily reduced to their respective alkanes than aliphatic compounds 26 for example ketones are reduced to their respective alkyl benzenes by catalytic hydrogenation 27 28 or by birch reduction 29 under mild conditions stereoselectivity edit main article asymmetric catalytic reduction diastereoselective reduction edit in the reduction of cyclohexanones the hydride source can attack axially to produce an equatorial alcohol or equatorially to produce an axial alcohol in axial attack shown in red the hydride encounters 1 3 diaxial strain in equatorial attack shown in blue the hydride avoids the 1 3 diaxial interaction but the substrate undergoes unfavorable torsional strain when the newly formed alcohol and added hydrogen atom eclipse each other in the reaction intermediate as shown in the newman projection for the axial alcohol large reducing agents such as libh me 2 chchme 3 are hindered by the 1 3 axial interactions and therefore attack equatorially 6 small reducing agents such as nabh 4 preferentially attack axially in order to avoid the eclipsing interactions because the 1 3 diaxial interaction for small molecules is minimal stereoelectronic reasons have also been cited for small reducing agents axial preference 30 making the substrate bulkier and increasing 1 3 axial interactions however decreases the prevalence of axial attacks even for small hydride donors 31 enantioselective reduction edit main article enantioselective reduction of ketones when asymmetrical ketones are reduced the resulting secondary alcohol has a chiral center whose can be controlled using chiral transition states or catalysts in the evans saksena reduction a nearby alcohol directs the reduction well known carbonyl reductions in asymmetric synthesis are the noyori asymmetric hydrogenation beta ketoester reduction ru binap and the cbs reduction bh 3 proline derived chiral catalyst history and alternative methods edit the bouveault blanc reduction employing a mixture of sodium metal in the presence of alcohols was an early method for reduction of carbonyls 32 it is now largely obsolete subsequent to the discovery of the bouveault blanc reduction many methods were developed including the major breakthrough of catalytic hydrogenation where h 2 serves as the reductant 33 salts boron and aluminium hydrides discovered starting in the 1940s proved to be highly convenient reagents for carbonyl reduction in the meerwein ponndorf verley reduction aluminium isopropoxide functions as the hydride source the status of this reaction has been summarized thusly the synthetic organic chemist will rarely attempt to use such a conventional technique as the meerwein ponndorf verley mpv reaction 34 see also edit baker s yeast a biotransformation route for carbonyl reductions references edit 1 2 3 banfi luca narisano enrica riva renata stiasni nikola hiersemann martin yamada tohru tsubo tatsuyuki 2014 sodium borohydride encyclopedia of reagents for organic synthesis pp 1 13 doi 10 1002 047084289x rs052 pub3 isbn 9780470842898 1 2 brown herbert c ramachandran p veeraraghavan 1996 sixty years of hydride reductions reductions in organic synthesis acs symposium series vol 641 pp 1 30 doi 10 1021 bk 1996 0641 ch001 isbn 9780841233812 könig burkhard 2009 reduction reactions pdf modern methods in organic synthesis institut für organische chemie uni regensburg archived from the original pdf on august 24 2015 retrieved december 1 2015 oishi takeshi nakata tadashi 2001 zinc borohydride encyclopedia of reagents for organic synthesis doi 10 1002 047084289x rz004 isbn 0471936235 sweeting linda m 2001 reducing agents towson university archived from the original on november 16 2015 retrieved december 1 2015 1 2 banfi luca narisano enrica riva renata 2001 01 01 sodium borohydride john wiley sons ltd doi 10 1002 047084289x rs052 isbn 9780470842898 smith michael b march jerry 2007 advanced organic chemistry reactions mechanisms and structure 6th ed new york wiley interscience p 1790 isbn 978 0 471 72091 1 roche alex ketones and aldehydes pdf rutgers university retrieved december 1 2015 clayden jonathan 2012 organic chemistry oup oxford p 200 isbn 978 0199270293 moffett robert bruce 1953 2 1 pyrrolidyl propanol organic syntheses 33 82 doi 10 15227 orgsyn 033 0082 mcmurry john e 1973 isoxazole annelation reaction 1 methyl 4 4a 5 6 7 8 hexahydronaphthalen 2 3 h one organic syntheses 53 70 doi 10 15227 orgsyn 053 0070 gaylord norman g 1957 08 01 reduction with complex metal hydrides journal of chemical education 34 8 367 bibcode 1957jched 34 367g doi 10 1021 ed034p367 zakharkin l i khorlina i m 1962 reduction of esters of carboxylic acids into aldehydes with diisobutylaluminium hydride tetrahedron letters 3 14 619 620 doi 10 1016 s0040 4039 00 70918 x cortes sergio 2010 using hydrogen as a nucleophile in hydride reductions pdf dr sergio cortes organic chemistry page university of texas at dallas retrieved december 1 2015 mosettig erich mozingo ralph 2004 01 01 the rosenmund reduction of acid chlorides to aldehydes john wiley sons inc doi 10 1002 0471264180 or004 07 isbn 9780471264187 nahm steven weinreb steven m 1981 n methoxy n methylamides as effective acylating agents tetrahedron letters 22 39 3815 3818 doi 10 1016 s0040 4039 01 91316 4 fukuyama tohru lin shao cheng li leping 1990 09 01 facile reduction of ethyl thiol esters to aldehydes application to a total synthesis of neothramycin a methyl ether journal of the american chemical society 112 19 7050 7051 bibcode 1990jachs 112 7050f doi 10 1021 ja00175a043 issn 0002 7863 afanasyev oleg i kuchuk ekaterina usanov dmitry l chusov denis 2019 reductive aminat...