Lithium Diisopropylamide1,2

i-Pr2NLi

[4111-54-0]  · C6H14LiN  · Lithium Diisopropylamide  · (MW 107.15)

(hindered nonnucleophilic strong base used for carbanion generation, especially kinetic enolates,4 a-heteroatom, allylic,5,6 and aromatic and heteroaromatic carbanions;7-9 unavoidable in organic synthesis)

Alternate Name: LDA.

Physical Data: powder, melts with decomposition; pKa = 35.7 in THF.3

Solubility: sol Et2O, THF, DME, HMPA; unstable above 0 °C in these solvents; stable in hexane or pentane (0.5-0.6 M) at rt for weeks, when not cooled or concentrated;10 the complex with one molecule of THF is soluble in alkanes like cyclohexane and heptane.

Form Supplied in: solid; 2.0 M solution in heptane/THF/ethylbenzene stabilized with Mg(i-Pr2N)2; 10 wt % suspension in hexanes; LDA.THF complex, 1.5 M solution in cyclohexane and 2.0 M in heptane.

Analysis of Reagent Purity: several titration methods have been described.120

Preparative Methods: generally prepared directly before use from anhydrous Diisopropylamine and commercially available solutions of n-Butyllithium. Another process, especially useful for the preparation of large quantities, is the reaction of styrene with 2 equiv of Lithium and 2 equiv of i-Pr2NH in Et2O.11

Handling, Storage, and Precautions: very moisture- and air-sensitive, and should always be kept in an inert atmosphere; irritating to the skin and mucous membranes; therefore contact with skin, eyes, and other tissues and organs should be avoided; proper protection is necessary. Use in a fume hood.

Enolates.

LDA is undeniably the first-choice base for the quantitative formation of enolates in general and kinetic (usually less substituted) enolates in particular. It was first used for this purpose by Hamell and Levine;12 now it is a rare day (or night) that a chemist does not use or a graduate student does not propose LDA for a carbanionic transformation. Its advantages (fast, complete, and regiospecific enolate generation, unreactivity to alkyl halides, lack of interfering products) over earlier developed bases (e.g. Sodium Hydride, Triphenylmethyllithium)13 are no longer noted. Except for enolates of aldehydes, which are very reactive and undergo self-aldol condensation reactions,14 the derived lithio enolates can be used for a broad spectrum of reactions including O- and C-alkylations and acylations, transmetalation to other synthetically useful metallo enolates,15 and a multitude of carbanion-based condensations and rearrangements. The trapping of kinetically derived lithio enolates with Chlorotrimethylsilane as silyl enol ethers16 (e.g. eqs 1 and 2),17 originally used for separation from minor regioisomers and regeneration (with Methyllithium or Lithium Amide) for the purpose of regiospecific alkylation, has been superseded in general by methods of their direct alkylation under Lewis acid catalysis.18

Consonant with its pKa, LDA is used to derive a variety of a-stabilized carbanions, e.g. ketones (eqs 1 and 3),19,20 a-amino ketones,21 o-bromo ketones (eq 4),10 imines (eqs 6 and 7),22-24 imino ethers (eq 8),25 carboxylic acids (eq 9),26 carboxylic esters,27 lactones,28 amides,29 lactams,30 imides,31 and nitriles32 which may be alkylated or used in aldol condensations. Enolates of methyl ketones may be O-alkylated with Diethyl Phosphorochloridate to give enol phosphates which, upon b-elimination, afford terminal alkynes in high yields (eq 5).33

The (Z)/(E) stereoselectivity of enolate formation is dictated by the structure of the starting carbonyl compound and the base used for deprotonation. Compared to LDA, Lithium 2,2,6,6-Tetramethylpiperidide usually favors (E)-enolates whereas Lithium Hexamethyldisilazide preferentially leads to (Z)-enolates (eq 10).34 With a caveat for any generalization, enolate configuration usually determines the stereochemical result in the product; for example, using a hindered ester and a bulky aldehyde combination, excellent stereoselectivities in aldol reactions are observed (eq 11).27

A useful reaction of ketone enolates is their oxidative coupling,35 e.g. in the formation of a tricyclic intermediate towards the synthesis of the diterpene cerorubenic acid-III (eq 12).36

Alkylation of b-lactone enolates proceeds with high stereoselectivity dictated by strong stereocontrol from the C-4 R substituent (eq 13).28 A similar result is obtained with the lithium enolates of diarylazetidinones which, on reaction with aldehydes, alkylate with high cis diastereoselectivity (eq 14).30

Sarcosine (N-methylglycine)-containing tripeptides and hexapeptides are poly-deprotonated in the presence of excess Lithium Chloride to give amide enolates which give C-alkylated sarcosine products with Iodomethane, Allyl Bromide, and Benzyl Bromide (eq 15).29 With (S) configuration amino acids, the newly formed stereogenic center tends to have the (R) configuration. a-Nitrile carbanions are generally useful in alkylation and other reactions with electrophiles, e.g. eq 16.32

a,b-Unsaturated Carbonyl Compounds.

Of particular synthetic value is the generation of kinetic enolates of cyclic enones which may be alkylated (e.g. eq 17)37 or undergo more demanding processes, e.g. double Michael addition (eq 18).38 Imines of a,b-unsaturated aldehydes and ketones with d-acidic hydrogens undergo clean deconjugative a-alkylation (eq 19).23,39

Copper dienolates of a,b-unsaturated acids, prepared by Copper(I) Iodide transmetalation, can be selectively g-alkylated with allylic halides; the use of nonallylic electrophiles leads mainly to a-alkylation (eq 20).40 Copper enolates of a,b-unsaturated esters41 and amides42 show similar behavior.

Enolates with Chiral Auxiliaries.

Enantioselective alkylation of carbonyl derivatives encompassing chiral auxiliaries constitutes an important synthetic process. The anions derived from aldehydes,43 acyclic ketones,44,45 and cyclic ketones with (S)-1-Amino-2-methoxymethylpyrrolidine (SAMP)46 are used to obtain alkylated products in good to excellent yields and high enantioselectivity (e.g. eq 21).46

Metalation47,48 of chiral oxazolines, derived from (1S,2S)-1-phenyl-2-amino-1,3-propanediol, followed by alkylation and hydrolysis, leads to optically active dialkylacetic acids, e.g. eq 22,49 2-substituted butyrolactones and valerolactones,50 b-hydroxy and b-methoxy acids,51 2-hydroxy carboxylic acids,52 and 3-substituted alkanoic acids (eq 23).53

Imide enolates derived from (S)-valinol and (1S,2R)-norephedrine and obtained by either LDA or Sodium Hexamethyldisilazide deprotonation (eq 24)54 exhibit complementary and highly diastereoselective alkylation properties. Mild and nondestructive removal of the chiral auxiliary to yield carboxylic acids, esters, or alcohols contributes to the significance of this protocol in small- and large-scale synthesis.55,56

Metalated t-butyl 2-t-butyl-2,5-dihydro-4-methoxyimidazole-1-carboxylate is alkylated in good yields and with high trans diastereoselectivity (>99:1); hydrolysis of the resulting adducts liberates the a-amino acid methyl esters in high yields (eq 25).57 Using this method, the (S)-t-butyl 2-t-butylimidazole derivative gave, upon isopropylation and hydrolysis, the L-valine methyl ester with 81% ee.

Alkylations and aldol condensations of aldehydes and ketones with enolates of chiral dioxolanes proceed generally with high diastereoselection, e.g. eq 2658,59 and eq 27.60 The magnesium enolate of S-(+)-2-acetoxy-1,1,2-triphenylethanol generated by transmetalation with Magnesium Bromide has enjoyed considerable success in aldol condensations, e.g. eq 28.61

b-Lactams with high ee values62 result from the condensation of lithium enolates of 10-diisopropylsulfonamide isobornyl esters with azadienes (eq 29).63,64

Enolate Rearrangements.

Ireland-Claisen Rearrangement.65,66

Silyl enol ethers of allyl esters undergo highly stereoselective Ireland-Claisen rearrangement to afford 4-pentenoic acids.67 The structure of the ester and the reaction conditions dictate the stereoselectivity. It has seen wide synthetic application, e.g. the construction of unnatural (-)-trichodiene (eq 30).68

Wittig Rearrangements.69

Enolates derived from chiral propargyloxyacetic acids lead to chiral allenic esters, via a rapid diastereoselective [2,3]-Wittig rearrangement followed by esterification (eq 31).70 Analogous rearrangements are feasible with carbohydrate derivatives71 and steroids (eq 32).72 In some cases, improved enantioselectivities resulted when the lithium enolate was transmetalated with Dichlorobis(cyclopentadienyl)zirconium.73 Benzylic, allylic, propargylic, and related lithio derivatives of cyanohydrin ethers undergo the [2,3]-Wittig rearrangement to afford ketones, e.g. eq 33.74,75 Lithiated N-benzyl- and N-allylazetidinones lead, via [1,2]-Wittig rearrangement exhibiting radical character, to pyrrolidones (eq 34).76

Stevens Rearrangement.

Ammonium ylides undergo a stereoselective Stevens [2,3]-rearrangement when treated with LDA to afford N-methyl-2,3-disubstituted piperidines (eq 35).77

Heteroatom-Stabilized Carbanions.

Heteroatom-stabilized and allylic carbanions serve as homoenolate anions and acyl anion equivalents,5,78 e.g. a-anions of protected cyanohydrins of aldehydes and a,b-unsaturated aldehydes are intermediates in general syntheses of ketones and a,b-unsaturated ketones (eq 36).79 Allylic anions of cyanohydrin ethers may be a-alkylated (eq 37)80,81 or, if warmed to -25 °C, may undergo 1,3-silyl migration to cyanoenolates which may be trapped with TMSCl.82 Metalated a-aminonitriles of aldehydes are used for the synthesis of ketones and enamines (eq 38).83 Similarly, allylic anions from 2-morpholino-3-alkenenitriles undergo predominantly a-C-alkylation to give, after hydrolysis, a,b-unsaturated ketones (eq 39).84

a-Sulfoxide- and sulfone-stabilized carbanions are highly useful synthetic intermediates, e.g. a-lithiated (+)-methyl p-tolyl (R)-sulfoxide reacts with a,b-unsaturated aldehydes to give, after dehydration, enantiomerically pure 1-[p-tolylsulfinyl]-1,3-butadienes (eq 40),85 metalation of phenyl 3-tosyloxybutyl sulfone leads to efficient cyclopropane ring formation (eq 41),86 and precursors for the Ramberg-Bäcklund rearrangement are prepared via a-lithiated cyclic sulfones (eq 42).87

Lithiated allylic sulfoxides may be a-alkylated and the resulting products subjected to [2,3]-sigmatropic rearrangement induced by a thiophile to give allylic alcohols (eq 43).6,88 In contrast, alkenyl aryl sulfoxides produce a-lithiated species which are alkylated with MeI or PhCHO in good yields (eq 44).89,90 LDA has also been used to metalate allylic91 and propargylic selenides92,93 as well as aryl vinyl selenides.94

Aromatic and Heteroaromatic Metalations.

Aromatics.

The utility of LDA for kinetic deprotonation of aromatic substrates is compromised by its insufficient basicity compared to the alkyllithiums.8 Selective lateral deprotonation may be achieved which complements the ortho-metalation process (eq 45).95

Of considerable synthetic utility is the LDA-induced conversion of bromobenzenes into benzynes. Thus low-temperature deprotonation of m-alkoxyaryl bromides followed by warming to rt in the presence of furan gives cycloadducts (eq 46);96 the lithio species may also be trapped with CO2 at -78 °C to give unusual aromatic substitution patterns. Treatment of aryl triflates with LDA leads to N,N-isopropylanilines in good yield (67-98%).97

Remote metalation of biaryls and m-teraryls provides a general synthesis of substituted and condensed fluorenones (eq 47).98 Similarly, biaryl O-carbamates undergo remote metalation and anionic Fries rearrangement to 2-hydroxy-2-carboxamidobiaryls, which are efficiently transformed into dibenzo[b,d]pyranones.99

Lateral Lithiation.7

Laterally metalated o-toluic acids (eq 48),100 esters (eq 49),101 and amides (eq 50)102 are important intermediates for chain extension and carbo- and hetero-ring annulation. Remote lateral lithiation of 2-methyl-2-carboxaminobiaryls constitutes a general regiospecific synthesis of 9-phenanthrols (eq 51).103-105

a-Lithiated o-tolyl isocyanides may be alkylated and, following a second metalation with LiTMP, converted into 3-substituted indoles, e.g. eq 52.106 Michael addition of a-lithiated 3-sulfonyl- and 3-cyanophthalides to a,b-unsaturated ketones and esters initiates a regioselective aromatic ring annulation process to give naphthalenes, e.g. eq 53.107 Lithiated 3-cyanophthalides also undergo reaction with in situ-generated arynes to give anthraquinones (eq 54).108 Similarly, aza-anthraquinones are obtained using 3-bromopyridine as the 3,4-pyridyne precursor.109

Heteroaromatics.9

LDA (and LiTMP) are advantageous bases for the directed ortho-metalation chemistry of pyridines110,111 and, to a much lesser extent, pyrimidines.112 Several halopyridines can be lithiated ortho to the halo substituent and trapped with various electrophiles (eq 55).113

Lithiated furan- and thiophenecarboxylic acids (eq 56),114,115 indoles (eq 57),116 indole-3-carboxylic acids (eq 58),117 benzofurancarboxylic acids,118 and thiazole- and oxazolecarboxylic acids119 serve well in heteroaromatic synthesis.

Related Reagents.

Lithium Amide; Lithium Hexamethyldisilazide; Lithium Diethylamide; Lithium Piperidide; Lithium Pyrrolidide; Lithium 2,2,6,6-Tetramethylpiperidide; Potassium Diisopropylamide.


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Wouter I. Iwema Bakker, Poh Lee Wong & Victor Snieckus

University of Waterloo, Ontario, Canada



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