Iodotrimethylsilane1

Me3SiI

[16029-98-4]  · C3H9ISi  · Iodotrimethylsilane  · (MW 200.11)

(a versatile reagent for the mild dealkylation of ethers, carboxylic esters, lactones, carbamates, acetals, phosphonate and phosphate esters; cleavage of epoxides, cyclopropyl ketones; conversion of vinyl phosphates to vinyl iodides; neutral nucleophilic reagent for halogen exchange reactions, carbonyl and conjugate addition reactions; use as a trimethylsilylating agent for formation of enol ethers, silyl imino esters, and N-silylenamines, alkyl, alkenyl and alkynyl silanes; Lewis acid catalyst for acetal formation, a-alkoxymethylation of ketones, for reactions of acetals with silyl enol ethers and allylsilanes; reducing agent for epoxides, enediones, a-ketols, sulfoxides, and sulfonyl halides; dehydrating agent for oximes)

Alternate Names: TMS-I; TMSI; trimethylsilyl iodide.

Physical Data: bp 106-109 °C; d 1.406 g cm-3; n20D 1.4710; fp -31 °C.

Solubility: sol CCl4, CHCl3, CH2Cl2, ClCH2CH2Cl, MeCN, PhMe, hexanes; reactive with THF (ethers), alcohols, and EtOAc (esters).

Form Supplied in: clear colorless liquid, packaged in ampules, stabilized with copper; widely available.

Analysis of Reagent Purity: easily characterized by 1H, 13C, or 29Si NMR spectroscopy.

Preparative Methods: although more than 20 methods have been reported1 for the preparation of TMS-I, only a few are summarized here. Chlorotrimethylsilane undergoes halogen exchange with either Lithium Iodide2 in CHCl3 or Sodium Iodide3 in MeCN, which allows in situ reagent formation (eq 1). Alternatively, Hexamethyldisilane reacts with Iodine at 25-61 °C to afford TMS-I with no byproducts (eq 2).4

Several other methods for in situ generation of the reagent have been described.5,6 It should be noted, however, that the reactivity of in situ generated reagent appears to depend upon the method of preparation.

Purification: by distillation from copper powder

Handling, Storage, and Precautions: extremely sensitive to light, air, and moisture, it fumes in air due to hydrolysis (HI), and becomes discolored upon prolonged storage due to generation of I2. It is flammable and should be stored under N2 with a small piece of copper wire. It should be handled in a well ventilated fume hood and contact with eyes and skin should be avoided.

Use as a Nucleophilic Reagent in Bond Cleavage Reactions.

Ether Cleavage.5,7

The first broad use of TMS-I was for dealkylation reactions of a wide variety of compounds containing oxygen-carbon bonds, as developed independently by the groups of Jung and Olah. Simple ethers initially afford the trimethylsilyl ether and the alkyl iodide, with further reaction giving the two iodides (eq 3).7,8 This process occurs under neutral conditions, and is generally very efficient as long as precautions to avoid hydrolysis by adventitious water are taken. Since the silyl ether can be quantitatively hydrolyzed to the alcohol, this reagent permits the use of simple ethers, e.g. methyl ethers, as protective groups in synthesis. The rate of cleavage of alkyl groups is: tertiary &AApprox; benzylic &AApprox; allylic >> methyl > secondary > primary. Benzyl and t-butyl ethers are cleaved nearly instantaneously at low temperature with TMS-I. Cyclic ethers afford the iodo silyl ethers and then the diiodide, e.g. THF gives 4-iodobutyl silyl ether and then 1,4-diiodobutane in excellent yield.7,8 Alcohols and silyl ethers are rapidly converted into the iodides as well.8a,9 Alkynic ethers produce the trimethylsilylketene via dealkylative rearrangement.4b Phenolic ethers afford the phenols after workup.5,7,10 In general, ethers are cleaved faster than esters. Selective cleavage of methyl aryl ethers in the presence of other oxygenated functionality has also been accomplished in quinoline.11 g-Alkoxyl enones undergo deoxygenation with excess TMS-I (2 equiv), with the first step being conjugate addition of TMS-I.12

Cleavage of Epoxides.

Reaction of epoxides with 1 equiv of TMS-I gives the vicinal silyloxy iodide.8e With 2 equiv of TMS-I, however, epoxides are deoxygenated to afford the corresponding alkene (eq 4).13a,b However, allylic alcohols are efficiently prepared by reaction of the intermediate iodosilane with base.13c,d Furthermore, acyclic 2-ene-1,4-diols react with TMS-I to undergo dehydration, affording the corresponding diene.13e

Ester Dealkylation.14

Among the widest uses for TMS-I involves the mild cleavage of carboxylic esters under neutral conditions. The ester is treated with TMS-I to form an initial oxonium intermediate which suffers attack by iodide (eq 5). The trimethylsilyl ester is cleaved with H2O during workup. Although the reaction is general and efficient, it is possible to accomplish selective cleavage according to the reactivity trend: benzyl, t-butyl > methyl, ethyl, i-propyl. Neutral transesterification is also possible via the silyl ester intermediate.15 Aryl esters are not cleaved by TMS-I, however, since the mechanism involves displacement of R2 by I-. Upon prolonged exposure (75 °C, 3 d) of simple esters to excess TMS-I (2.5 equiv), the corresponding acid iodides are formed.14b,16 b-Keto esters undergo decarboalkoxylation when treated with TMS-I.17 An interesting rearrangement reaction provides a-methylene lactones from 1-(dimethylaminomethyl)cyclopropanecarboxylates (eq 6).18

Lactone Cleavage.14,19

Analogous to esters, lactones are also efficiently cleaved with TMS-I to provide o-iodocarboxylic acids, which may be further functionalized to afford bifunctional building blocks for organic synthesis (eq 7). Diketene reacts with TMS-I to provide a new reagent for acetoacylation.20

Cleavage of Carbamates.21

Since strongly acidic conditions are typically required for the deprotection of carbamates, use of TMS-I provides a very mild alternative. Benzyl and t-butyl carbamates are readily cleaved at rt,22 whereas complete cleavage of methyl or ethyl carbamates may require higher temperatures (reflux). The intermediate silyl carbamate is decomposed by the addition of methanol or water (eq 8). Since amides are stable to TMS-I-promoted hydrolysis,7a this procedure can be used to deprotect carbamates of amino acids and peptides.21d

A recent example used TMS-I to deprotect three different protecting groups (carbamate, ester, and orthoester) in the same molecule in excellent yield (eq 9).23

Cleavage of Acetals.24

Acetals can be cleaved in analogy to ethers, providing a newly functionalized product (eq 10), or simply the parent ketone (eq 11). Glycals have also been converted to the iodopyrans with TMS-I,25 and glycosidation reactions have been conducted with this reagent.26

Orthoesters are converted into esters with TMS-I. The dimethyl acetal of formaldehyde, methylal, affords iodomethyl methyl ether in good yield (eq 12)27a (in the presence of alcohols, MOM ethers are formed).27b a-Acyloxy ethers also furnish the iodo ethers,28 e.g. the protected b-acetyl ribofuranoside gave the a-iodide which was used in the synthesis of various nucleosides in good yield (eq 13).28b Aminals are similarly converted into immonium salts, e.g. Eschenmoser's reagent, Dimethyl(methylene)ammonium Iodide, in good yield.29

Cleavage of Phosphonate and Phosphate Esters.30

Phosphonate and phosphate esters are cleaved even more readily with TMS-I than carboxylic esters. The reaction of phosphonate esters proceeds via the silyl ester, which is subsequently hydrolyzed with MeOH or H2O (eq 14).

Conversion of Vinyl Phosphates to Vinyl Iodides.31

Ketones can be converted to the corresponding vinyl phosphates which react with TMS-I (3 equiv) at rt to afford vinyl iodides (eq 15).

Cleavage of Cyclopropyl Ketones.32

Cyclopropyl ketones undergo ring opening with TMS-I, via the silyl enol ether (eq 16). Cyclobutanones react analogously under these conditions.33

Halogen Exchange Reactions.34

Halogen exchange can be accomplished with reactive alkyl halides, such as Benzyl Chloride or Benzyl Bromide, and even with certain alkyl fluorides, by using TMS-I in the presence of (n-Bu)4NCl as catalyst (eq 17).

Use of TMS-I in Nucleophilic Addition Reactions.

Carbonyl Addition Reactions.35

a-Iodo trimethylsilyl ethers are produced in the reaction of aldehydes and TMS-I (eq 18). These compounds may react further to provide the diiodo derivative or may be used in subsequent synthesis.

An example of a reaction of an iodohydrin silyl ether with a cuprate reagent is summarized in eq 19.36 An interesting reaction of TMS-I with phenylacetaldehydes gives a quantitative yield of the oxygen-bridged dibenzocyclooctadiene, which was then converted in a few steps to the natural product isopavine (eq 20).35,37

b-Iodo ketones have been produced from reactions of TMS-I and ketones with a-hydrogens.38 This reaction presumably involves a TMS-I catalyzed aldol reaction followed by 1,4-addition of iodide.

Conjugate Addition Reactions.39

a,b-Unsaturated ketones undergo conjugate addition with TMS-I to afford the b-iodo adducts in high yield (eq 21). The reaction also works well with the corresponding alkynic substrate.40

TMS-I has also been extensively utilized in conjunction with organocopper reagents to effect highly stereoselective conjugate additions of alkyl nucleophiles.41

Use of TMS-I as a Silylating Agent.

Formation of Silyl Enol Ethers.42

TMS-I in combination with Triethylamine is a reactive silylating reagent for the formation of silyl enol ethers from ketones (eq 22). TMS-I with Hexamethyldisilazane has also been used as an effective silylation agent, affording the thermodynamic silyl enol ethers. For example, 2-methylcyclohexanone gives a 90:10 mixture in favor of the tetrasubstituted enol ether product.42a The reaction of TMS-I with 1,3-diketones is a convenient route to 1,3-bis(trimethylsiloxy)-1,3-dienes.42c

In an analogous process, TMS-I reacts with lactams in the presence of Et3N to yield silyl imino ethers (eq 23).43a

Halogenation of Lactams.43b

Selective and high yielding iodination and bromination of lactams occurs with Iodine or Bromine, respectively, in the presence of TMS-I and a tertiary amine base (eq 24). The proposed reaction mechanism involves intermediacy of the silyl imino ether.

Reaction with Carbanions.44

TMS-I has seen limited use in the silylation of carbanions, with different regioselectivity compared to other silylating reagents in the example provided in eq 25.

Silylation of Alkynes and Alkenes.45

A Heck-type reaction of TMS-I with alkenes in the presence of Pd0 and Et3N affords alkenyltrimethylsilanes (eq 26).

Oxidative addition of TMS-I to alkynes can also be accomplished with a three-component coupling reaction to provide the enyne product (eq 27).

Use of TMS-I as a Lewis Acid.

Acetalization Catalyst.46

TMS-I used in conjunction with (MeO)4Si is an effective catalyst for acetal formation (eq 28).

Catalyst for a-Alkoxymethylation of Ketones.

Silyl enol ethers react with a-chloro ethers in the presence of TMS-I to afford a-alkoxymethyl ketones (eq 29).47

Catalyst for Reactions of Acetals with Silyl Enol Ethers and Allylsilanes.

TMS-I catalyzes the condensation of silyl enol ethers with various acetals (eq 30)48 and imines,49 and of allylsilanes with acetals.50

Use of TMS-I as a Reducing Agent.

TMS-I reduces enediones to 1,4-diketones,51 while both epoxides and 1,2-diols are reduced to the alkenes.13a,b,52 The Diels-Alder products of benzynes and furans are converted in high yield to the corresponding naphthalene (or higher aromatic derivative) with TMS-I (eq 31).53

Styrenes and benzylic alcohols are reduced to the alkanes with TMS-I (presumably via formation of HI).54 Ketones produce the symmetrical ethers when treated with trimethylsilane as a reducing agent in the presence of catalytic TMS-I.55

Reduction of a-Ketols.56,57

Carbonyl compounds containing a-hydroxy, a-acetoxy, or a-halo groups react with excess TMS-I to give the parent ketone. a-Hydroxy ketone reductions proceed via the iodide, which is then reduced with iodide ion to form the parent ketone (eq 32).

Sulfoxide Deoxygenation.58

The reduction of sulfoxides occurs under very mild conditions with TMS-I to afford the corresponding sulfide and iodine (eq 33). Addition of I2 to the reaction mixture accelerates the second step. The deoxygenation occurs faster in pyridine solution than the reactions with a methyl ester or alcohol.59

Pummerer reactions of sulfoxides can be accomplished in the presence of TMS-I and an amine base, leading to vinyl sulfides.60 An efficient synthesis of dithioles was accomplished with TMS-I and Hünig's base (Diisopropylethylamine) (eq 34).61

Reaction with Sulfonyl Halides.62

Arylsulfonyl halides undergo reductive dimerization to form the corresponding disulfides (eq 35). Alkylsulfonyl halides, however, undergo this process under somewhat more vigorous conditions. Although sulfones generally do not react with TMS-I, certain cyclic sulfones are cleaved in a manner analogous to lactones.63

Other Reactions of TMS-I.

Reaction with Phosphine Oxides.64

Phosphine oxides react with TMS-I to form stable adducts (eq 36). These O-silylated products can undergo further thermolytic reactions such as alkyl group cleavage.

Chlorophosphines undergo halogen exchange reactions with TMS-I.65

Reaction with Imines.

Imines react with TMS-I to form N-silylenamines, in a process analogous to the formation of silyl enol ethers from ketones.66

Reaction with Oximes.67

Oximes are activated for dehydration (aldoximes, with hexamethylsilazane) or Beckmann rearrangement (ketoximes) with TMS-I (eq 37).

Reactions with Nitro and Nitroso Compounds.68

Primary nitro derivatives react with TMS-I to form the oximino intermediate via deoxygenation, which then undergoes dehydration as discussed for the oximes (eq 38). Secondary nitro compounds afford the silyl oxime ethers, and tertiary nitro compounds afford the corresponding iodide. Nitroalkenes, however, react with TMS-I at 0 °C to afford the ketone as the major product (eq 39).69

An interesting analogy to this dehydration process is found in the reductive fragmentation of a bromoisoxazoline with TMS-I, which yields the nitrile (eq 40).70

Rearrangement Reactions.

An interesting rearrangement occurs on treatment of a b-alkoxy ketone with TMS-I which effects dealkylation and retro-aldol reaction to give the eight-membered diketone after reductive dehalogenation (eq 41).71 Tertiary allylic silyl ethers a to epoxides undergo a stereocontrolled rearrangement to give the b-hydroxy ketones on treatment with catalytic TMS-I (eq 42).72

Related Reagents.

Bromotrimethylsilane; Chlorotrimethylsilane; Trimethylsilyl Trifluoromethanesulfonate.


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Michael E. Jung

University of California, Los Angeles, CA, USA

Michael J. Martinelli

Lilly Research Laboratories, Indianapolis, IN, USA



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