Titanium(IV) Chloride


[7550-45-0]  · Cl4Ti  · Titanium(IV) Chloride  · (MW 189.68)

(Lewis acid catalyst;52 affects stereochemical course in cycloaddition38,39 and aldol reactions;4-17 electrophilic substitutions;55 powerful dehydrating agent; when reduced to low-valent state, effects C-C bond formation by reductive coupling;7-9 reduction of functional groups81)

Physical Data: mp -24 °C, bp 136.4 °C, d 1.726 g cm-3.

Solubility: sol THF, toluene, CH2Cl2.

Form Supplied in: neat as colorless liquid; solution in CH2Cl2; solution in toluene; TiCl4.2THF [31011-57-1].

Preparative Methods: see Perrin et al.1

Purification: reflux with mercury or a small amount of pure copper turnings and distill under N2 in an all-glass system. Organic material can be removed by adding aluminum chloride hexahydrate as a slurry with an equal amount of water (ca. 2% weight of the amount of TiCl4), refluxing the mixture for 2-6 h while bubbling in Cl2 which is subsequently removed by a stream of dry air, before the TiCl4 is distilled, refluxed with copper, and distilled again. Volatile impurities can be removed using a technique of freezing, pumping, and melting.2

Handling, Storage, and Precautions: moisture sensitive; reacts violently, almost explosively, with water; highly flammable; toxic if inhaled or swallowed; causes burns on contact with skin; use in a fume hood.

General Discussion.

TiCl4 is a strong Lewis acid and is used as such in organic reactions. It has high affinity for oxygenated organic molecules and possesses a powerful dehydrating ability. Low-valent titanium, from the reduction of TiCl4 by metal or metal hydrides, is used for reductive coupling reactions and for reduction of functional groups.

Carbon-carbon bond formation by reductive coupling of ketones by low-valent titanium leads to vicinal diols and alkenes, as does coupling using low-valent reagents from Titanium(III) Chloride. Reviews are available on the preparation and reactions of low-valent titanium.3-6 The reagent from Titanium(IV) Chloride-Zinc in THF or dioxane in the presence of pyridine transforms ketones into tetrasubstituted alkenes. Unsymmetrical ketones yield (E)/(Z)-isomer mixtures (eq 1). Strongly hindered ketones react slowly and may preferentially be reduced to alcohols.7 Reductive coupling of diketo sulfides yields 2,5-dihydrothiophenes (eq 2).8 The macrocyclic porphycene has been obtained, albeit in low yield, by McMurry coupling of diformylbipyrrole (eq 3).9 Aldimines are reductively coupled in a reaction analogous to the reaction of carbonyl compounds. The product is a 1:1 mixture of meso- and (±)-isomers (eq 4).10

Methylenation of aldehydes and ketones results from reactions with the complex from TiCl4-Zn and Diiodomethane or Dibromomethane (eq 5). The reagent can be used for methylenation of enolizable oxo compounds.6 Recommended modifications to the reagent have been reported.11 In keto aldehydes, selective methenylation of the keto group results when the aldehyde is precomplexed with Titanium Tetrakis(diethylamide). Chemoselective methenylation of the aldehyde function is possible by direct use of CH2I2/Zn/Titanium Tetraisopropoxide (eq 6).12 Cyclopropanation to gem-dihalocyclopropanes uses Lithium Aluminum Hydride-Titanium(IV) Chloride. The exclusion of a strong base, as frequently used in alternative procedures, is an advantage (eq 7).13 Allylation of imines has been effected by low-valent titanium species generated from TiCl4 and Aluminum foil (eq 8).14

TiCl4 is a powerful activator of carbonyl groups and promotes nucleophilic attack by a silyl enol ether. The product is a titanium salt of an aldol which, on hydrolysis, yields a b-hydroxy ketone. TiCl4 is generally the best catalyst for this reaction. The temperature range for reactions with ketones is normally 0-20 °C; aldehydes react even at -78 °C, which allows for chemoselectivity (eq 9).15 In a- or b-alkoxy aldehydes, the aldol reaction can proceed with high 1,2- or 1,3-asymmetric induction. With the nonchelating Lewis acid Boron Trifluoride Etherate, the diastereoselectivity may be opposite to that obtained for the chelating TiCl4 or Tin(IV) Chloride (eq 10).16

Titanium enolates are generally prepared by transmetalation of alkali-metal enolates but may also arise as structural parts of intermediates in TiCl4-promoted reactions. This is illustrated in a stereoselective alkylation using an oxazolidinone as a chiral auxiliary (eq 11). The enolate, or its ate complex, may be the intermediate in the reaction.17

The stereoselectivity in TiCl4-promoted reaction of silyl ketene acetals with aldehydes may be improved by addition of Triphenylphosphine (eq 12).18 Enol ethers, as well as enol acetates, can be the nucleophile (eqs 13 and 14).19 2-Acetoxyfuran, in analogy to vinyl acetates, reacts with aldehydes to furnish 4-substituted butenolides under the influence of TiCl4 (eq 15).20

Silyl enol ethers react with acetals at -78 °C to form b-alkoxy ketones.21 In intramolecular reactions, six-, seven-, and eight-membered rings are formed.22 With 1,3-dioxolanes (acetals), 1-2 equiv of TiCl4 leads to pyranone formation (eq 16), whereas no cyclization products are obtained with SnCl4 or ZnCl2.23

Alkynyltributyltin compounds react with steroidal aldehydes. In the presence of TiCl4 the reaction gives 9:1 diastereoselectivity (eq 17). Reactions of alkynylmetals with chiral aldehydes generally show only slight diastereoselectivity.24 With silyloxyacetylenes and aldehydes, a,b-unsaturated carboxylic acid esters are formed with high (E) selectivity (eq 18).25

Allylsilanes are regiospecific in their Lewis acid-catalyzed reactions, the electrophile bonding to the terminus of the allyl unit remote from the silyl group (eq 19).26,27 Allylstannanes are less reliable in this respect.26,28 The example in eq 19 illustrates regioselectivity.29 On extended conjugation the reaction takes place at the terminus of the extended system (eq 20).30

Intramolecular reactions work well, both for allylstannanes and allylsilanes. An epoxide can be the electrophile. Opening of the epoxide with allylic attack gives the cyclic product (eq 21). TiCl4 is superior to Ti(O-i-Pr)4, SnCl4, and Aluminum Chloride, the other Lewis acids tested in the cyclization reaction. The reaction is stereospecific.31

Stereochemical aspects in these TiCl4-promoted reactions have been covered in reviews.26,27 Acetals are very good electrophiles for allylsilanes, and may be better than the parent oxo compounds because the products are less prone to further reaction; intramolecular reactions are facile.26,32 In the unsymmetrical acetal in eq 22, the methoxyethoxy group departs selectively because of chelation to the Lewis acid.33

The TiCl4-mediated addition of allenylsilanes to aldehydes and ketones provides a general, regiocontrolled route to a wide variety of substituted homopropargylic alcohols. With acetals, corresponding ethers are formed (eq 23).34 Allenylation of acetals results from the reaction of propargylsilanes with acetals. The products are a-allenyl ethers (eq 24).35

Homoallylamines are formed in TiCl4- or BF3.OEt2-mediated reactions between allylstannanes and aldimines. Crotyltributyltin gives mainly syn-b-methyl homoallylamines under optimal conditions (eq 25).36

Diastereoselective Mannich reactions may result with TiCl4 as adjuvant (eq 26); transmetalation of the initial lithium alkoxide adduct and displacement by a lithium enolate gives a diastereoselectivity of 78%.37

Lewis acid catalysis increases the reactivity of dienophiles in Diels-Alder reactions by complexing to basic sites on the dienophile. In aldehydes, complexation takes place via the lone pair on the carbonyl oxygen. The stereochemistry is strongly influenced by the Lewis acids.38 Under chelating conditions, when a-alkoxy aldehydes are used, the prevalent products from TiCl4 catalysis have cis configuration (eqs 27 and 28). Stereochemical aspects of pericyclic pathways or passage through aldol type intermediates have been summarized and discussed.38,39

Ketene silyl acetals can be dimerized to succinates on treatment with TiCl4 in CH2Cl2 (eq 29). No reaction occurs with TiCl3, nor are other metal salts efficient.40 b-Amino esters are formed in the presence of Schiff bases (eq 30).41

Cycloaddition of alkenes to quinones is effected by TiIV derivatives. The composition of the TiIV adjuvant largely controls the type of cycloadduct. TiCl4 favors [3 + 2] cycloaddition; [2 + 2] cycloaddition requires a mixed TiCl4-Ti(O-i-Pr)4 catalyst (eq 31).42

The TiCl4-promoted Michael reaction proceeds under very mild conditions (-78 °C); this suppresses side-reactions and 1,5-dicarbonyl compounds are formed in good yields.3 For TiCl4-sensitive compounds, a mixture of TiCl4 and Ti(O-i-Pr)4 is used. From silyl enol ethers and a,b-unsaturated ketones, 1,5-dicarbonyl compounds are formed (eq 32).3 The reaction also proceeds for a,b-unsaturated acetals.43 Silylketene acetals react with a,b-unsaturated ketones or their acetals to form d-oxo esters (eq 33).44

Conjugate addition of allylsilanes to enones results in regiospecific introduction of the allyl group (eq 34).45 The reaction can be intramolecular (eq 35).46 TiCl4 or SnCl4 activates nitro alkenes for Michael addition with silyl enol ethers or ketene silyl acetals. The silyl nitronate product is hydrolyzed to a 1,4-diketone or g-keto ester (eq 36).47

Conjugate propynylation of enones results from TiCl4-mediated addition of allenylstannanes. Other Lewis acids are ineffective in this reaction (eq 37).48

The 3,4-dichloro derivative of squaric acid, on TiCl4-catalysis, reacts with silyl enol ethers or allylsilanes to form 1,2- or 1,4-adducts depending on the substitution. The 3,4-diethoxy derivative, however, adds silyl enol ethers in a 1,4-fashion; the adduct subsequently eliminates the ethoxy group (eq 38).49

The Knoevenagel reaction with TiCl4 and a tertiary base (eq 39) is recommended over methods which rely on strongly basic conditions.50 The TiCl4-procedure at low temperature is suitable for base-sensitive substrates.51

TiCl4, as a Lewis acid, is used as a catalyst in Friedel-Crafts reactions. AlCl3, SnCl4, and BF3.OEt2 are more commonly used Friedel-Crafts catalysts for reactions with arenes.52 Use of TiCl4 as catalyst in the preparation of aromatic aldehydes is shown in eq 40.53

The formylation reaction can be used to prepare (E)-a,b-unsaturated aldehydes from vinylsilanes by ipso substitution (eq 41).54 Regioselectivity in alkylation reactions may depend on the Lewis catalyst. In a fluorene, regioselective 1,8-chloromethylation results with TiCl4 (eq 42).55 TiCl4 activates nitroalkenes for electrophilic substitution into arenes. The intermediate is hydrolyzed to the oxoalkylated product (eq 43).56

TiCl4 is generally a good catalyst for Friedel-Crafts acylation of activated alkenes. In the acylation of pyrrolidine-2,4-diones, particularly with unsaturated acyl substrates, TiCl4 is superior to SnCl4 and BF3.OEt2 (eq 44).57

The ready ipso substitution of a silyl group favors substitution rather than addition of electrophiles in alkenylsilanes.58 Direct acylation of isobutene is not satisfactory, but the acylation is successful on silyl derivatives (eq 45).59 Intramolecular acylation of alkenylsilanes leads to cyclic products. Even (E)-silylalkenes have been cyclized to enones (eq 46).60

In analogy to the acylation reactions, vinylsilanes can be alkylated by ipso substitution. In the example in eq 47, the MEM group is activated as leaving group by metal complexation61 The silyl group in allylsilanes directs the incoming electrophile to the allylic g-carbon (eq 48). TiCl4 is one of several Lewis acids used for catalysis in acylations.58 The same reaction is seen in allylsilanes with extended conjugation.62 Eq 49 shows the TiCl4-catalyzed alkylation of an allylsilane.63

TiCl4 is a useful catalyst for the Fries rearrangement of phenol esters to o- or p-hydroxy ketones (eq 50). TiCl4 is a cleaner catalyst than the more frequently used AlCl3, which may cause alkyl migrations.64

TiCl4 is both a strong Lewis acid and a powerful dehydrating agent, and hence useful as a water scavenger in the synthesis of enamines. It is particularly useful for the preparation of enamines of acyclic ketones. It is recommended that TiCl4 is complexed with the amine before addition of the ketone.65 Highly sterically hindered enamines are available by this method (eq 51).66 Primary amines generally react very slowly with aromatic ketones to form imines. TiCl4 is a good catalyst (eq 52).67

TiCl4 with a tertiary base provides mild conditions for dehydration of both aldoximes and primary acid amides to form nitriles.68 Vinyl sulfides are formed from oxo compounds using TiCl4 and a tertiary amine.3 TiCl4 activates carbon-carbon double bonds for thiol addition (eq 53).3

TiCl4 mediates thioacetalization of aldehydes and ketones with alkanethiols or alkanedithiols in yields >90%. The reaction is satisfactory also for readily enolizable oxo compounds.69 g-Lactols, which are generally more stable than acyclic analogs, are amenable to dithiol cleavage (eq 54).70

a-Hydroxy amides are formed in a reaction involving an isocyanide, TiCl4, and an aldehyde or a ketone (eq 55).71 Vinyl chlorides undergo ready hydrolysis on combined use of TiCl4 and MeOH-H2O (eq 56).72 TiCl4-mediated hydrolysis of vinyl sulfides is a good preparative route to ketones.3,73

Low-valent titanium can be used to reduce sulfides and haloarenes to the corresponding hydrocarbons (eq 57).3,74 Low-valent titanium, prepared from TiCl4 and Magnesium Amalgam, will reduce nitroarenes to amines in THF/t-BuOH at 0 °C without affecting halo, cyano, and ester groups.75 SnCl2.2H2O is an alternative reagent for nitro group reduction.76

Deoxygenation of sulfoxides is a rapid reaction using [TiCl2] formed in situ by reduction of TiCl4 with Zn dust in CH2Cl2 or Et2O at rt, with yields in the range 85-90%.77 For deoxygenation of N-oxides of pyridine-based heterocycles, a reagent prepared from TiCl4-NaBH4 (1:2) in DME has been described.78 Carboxylic acids can be reduced to primary alcohols by TiCl4-NaBH4 (ratio 1:3). For reduction of amides and lactams the optimum molar ratio is 1:2.79

Several low-valent metal species have been found active in reductive elimination of vicinal dibromides to the corresponding alkenes. In most cases, e.g. with TiCl4-LAH and TiCl4-Zn, the reactions proceed through predominant anti elimination to yield alkenes with high isomeric purity.80 The low-valent titanium reagent obtained from TiCl4-LAH (ca. 2:1) will saturate the double bond of enedicarboxylates in the presence of triethylamine (eq 58).81

Related Reagents.

Dibromomethane-Zinc-Titanium(IV) Chloride; Diiodomethane-Zinc-Titanium(IV) Chloride; (4R,5R)-2,2-Dimethyl-4,5-bis(hydroxydiphenylmethyl)-1,3-dioxolane-Titanium(IV) Chloride; Lithium Aluminum Hydride-Titanium(IV) Chloride; Titanium(IV) Chloride-Diazabicyclo[5.4.0]undec-7-ene; Titanium(IV) Chloride-2,2,6,6-Tetramethylpiperidine; Titanium(IV) Chloride-Triethylaluminum; Titanium(IV) Chloride-Zinc.

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Lise-Lotte Gundersen

Norwegian College of Pharmacy, Oslo, Norway

Frode Rise & Kjell Undheim

University of Oslo, Norway

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