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Silicon-Based Blocking Agents

Gerald L. Larson Vice President, Research Products

Materials for protection of: Alcohols Phenols Diols Amines Carboxylic Acids Thiols

Materials for: Derivatization for gas chromatographic analyses Synthetic transformations

Supplement to the Gelest Catalog, “Silicon Compounds: Silanes & Silicones” which is available upon request.

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SILICON-BASED BLOCKING AGENTS

General Considerations The ideal protecting group for an active-hydrogen moiety such as an alcohol or amine would be one that would mimic the hydrogen atom itself, but be much more flexible in its reactivity. It would readily go on in high yield, be stable over as wide a variety of reaction conditions as possible and, at the same time, be selectively removable in the presence of other functional groups including other protecting groups. While no single silyl group can fulfill all of these conditions in all cases, the available range of silicon-based blocking agents can offer the synthetic chemist viable answers to nearly every protection-deprotection challenge. The ability to vary the organic groups

on silicon introduces the potential to alter the R3Si group in terms of both its steric and electronic characteristics and thereby influence the stability of the silylated species to a wide variety of reaction and deprotection condi- tions. This allows the synthetic chemist to select a silyl protecting group that can, for example, be simultaneously removed during the work-up of the reaction step(s) that required the protection, or that can be selectively removed in the presence of another silyl or other protecting group. In addition to having the ability to be fine-tuned to fit a particular need, the ease and high-yield introduction and deprotection of the silyl protecting groups contribute significantly to their popularity and utility. The commonly utilized trisubstituted silyl protecting groups and their acronyms are shown below.

Me Ph Ph Et

Me Si Me Si MeSi Et Si Si

Me Me Ph Et TMS DMPS MDPS TES TIPS

Me Me Ph Me

Si Si Si Si

Me Me Ph Me

DMIPS TBS TBDPS TDS The use of silicon-based blocking agents has been reviewed with regard to reaction/deprotection,1-7 oxidation of silyl ethers,8 and selective deprotection9. Introduction of the Silyl Groups In general smaller groups on silicon make the trisubstituted silyl group more reactive and easier to introduce. Thus, i t t a general relative reactivity scale is: Me3Si > Ph2MeSi > Et3Si > PrMe2Si > BuMe2Si > BuPh2Si > ThMe2Si (where i Th = thexyl) > Pr3Si. The leaving group on silicon also plays a role in both reactivity and byproducts. The general relative reactivity of R3Si-X as a function of X is: X = CN > OTf > I > Br > Cl > trifluoroacetamido > acetamido > R2N > RO. Others, such as perchlorates, and sulfates, though very reactive, are not very practical. These general reactivity trends will not apply to all sets of reaction conditions, but present a practical working guide. The introduction of the silyl group depends on the nature of the silyl group itself and the substrate. Silyl groups are most often used for the protection of alcohols in the form of their silyl ethers. Generally, the less sterically hindered the silyl group the easier it is to introduce (and, indeed, to remove). The introduction of the sterically unimpeded trimethylsilyl group to a primary, secondary or tertiary alcohol is a straightforward process taking place with a variety of reagents under several mild, high-yield reaction conditions. On the other hand, the intro- duction of the more sterically demanding tert-butyldimethylsilyl group requires reaction of the alcohol with tert- butyldimethylchlorosilane in the presence of imidazole as a catalyst10 and the formation of the tert-butyldimethylsi- lyl ether of tertiary alcohols is very difficult. Alternatively, a more reactive form such as tert-butyldimethylsilyl trifluoromethanesulfonate can be used.

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Consideration of Byproducts The introduction of a silyl group onto an alcohol, amine or other active hydrogen system is the result of a substitution reaction at silicon releasing the protonated form of the leaving group from silicon. This leaving group can be acidic, e.g. HCl, HBr, HI, HO3SCF3; basic, e.g. NH3, Me2NH, Et2NH; or neutral, e.g. CH3CONH2, CH3CONHMe, CF3CONH2, CF3CONHMe. The effect of the nature and physical properties of these leaving groups on the reaction product and purification must be considered when selecting a silicon- based protecting group. Stability of Silyl-Protected Functional Groups The relative stabilities of the silyl-protected functional groups, for example, alcohols as silyl ethers, is roughly the same as the relative rates for their introduction. It must be remembered, however, that the stability of the system is dependent upon the specific reaction conditions and, in particular, the pH. For example, phenyl-substituted silyl ethers are equal or more reactive than their trimethylsilyl counterparts under alkaline conditions, but less reactive under acidic conditions. In general terms, however, the relative stabilities of the silyl-protected functional groups will i t t i follow the order of: Pr3Si > ThMe2Si > BuPh2Si > BuMe2Si > PrMe2Si > Et3Si > Ph2MeSi > Me3Si. Again, this order is meant to be a general guide as the actual stabilities will depend on the pH of the medium as well as other reaction conditions. The results of a study of the stability of various silyl ethers as a function of the groups on sili- con and the nature of the alcohol towards various hydrolysis conditions and common organic reagents are shown in Table 1. The reader is referred to the excellent review by Crouch and Nelson on the selective deprotection of silyl groups in synthesis.9 The Trimethylsilyl, TMS, Group Common reagents for this reaction are trimethylchlorosilane, TMCS, and hexamethyldisilazane, HMDS, although others are used depending on the leaving group from silicon. The reaction of trimethylchlorosilane with an alcohol liberates HCl, which must be trapped. This is normally achieved with the presence of a tertiary amine. Hexamethyldisilazane reacts with alcohols to give ammonia as a byproduct. This can be simply led away from the reaction or trapped. The reactions of HMDS in the absence of a catalyst are oftentimes quite slow. Common catalysts are TMCS, lithium salts, ammonium chloride among others. Trimethylsilylation reagents which liberate neutral byproducts are bis(trimethylsilyl)trifluoroacetamide, bis(trimethylsilyl)acetamide and others as listed in Table 2. If greater reactivity is necessary one can turn to the more reactive species such as the (dimethylamino)trimethylsi- lane, cyanotrimethylsilane, and trimethylsilyl triflate. Deprotection of the TMS group is usually accomplished by mild hydrolysis in aqueous or alcohol medium. Trimethylsilyl ethers can be deprotected in the presence of TES ethers.11,12 The Triethylsilyl, TES, Group The triethylsilyl protecting group is primarily used for the protection of alcohols, although other functional groups can be and have been protected as their TES derivatives. The stability of the triethylsilyl ethers falls between those of the corresponding TMS and TBS ethers. The most common reagent for the introduction of the TES group is triethylchlorosilane and the methods used for the reaction are those that are employed for TMCS. TES ethers can be selectively removed in the presence of TBS ethers.13 The tert-Butyldimethylsilyl, TBS, Group The TBS group is used for the protection of alcohols10, amines14, thiols14, lactams14, and carboxylic acids14. The TBS group is commonly introduced via tert-butyldimethylchlorosilane, although other, more-reactive derivatives, such as the triflate, can be used. Because of its steric bulk the reaction of an alcohol with tert-butyldimethyl- chlorosilane requires the presence of imidazole as a catalyst.10 The high stability of TBS-protected groups to a variety of reaction conditions, its clean NMR characteristics, and its ease of removal with fluoride ion makes it a popular choice among the silicon-based blocking agents. TBS ethers can be removed in the presence of TIPS,15 and TBDPS,16 ethers. The Triisopropylsilyl, TIPS, Group The TIPS group is one of the most sterically hindered silyl protecting groups, being removed only slowly under standard acid- or base-catalyzed hydrolysis conditions. TIPS ethers show a stability greater than TBS ethers. Reaction with tetrabutylammonium fluoride is a common method for TIPS desilylations.17 The TIPS group is usually introduced from triisopropylchlorosilane, but triisopropylsilane15 or the triflate16 can also be used.

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The Thexyldimethylsilyl, TDS, Group The TDS group was proposed as a silyl blocking agent that would have more stability than the TBS group and whose reagent, TDS-Cl, would be less expensive to produce commercially.18 TDS-Cl is a liquid making it some- what easier to handle this air-sensitive material than the corresponding solid TBS-Cl. The TDS group provides protection for alcohols, amines, carboxylic acids, and thiols. In the even of low reactivity the more reactive triflate derivative, TDS-OTf, can be used. Reaction of TDS-Cl with alcohols is carried out under conditions like those for the TBS-Cl silylations. The tert-Butyldiphenylsilyl, TBDPS, Group The TBDPS group has greater steric demands than the TBS group and, therefore, tends to form more stable silylated species than their corresponding TBS analogs.19 Alcohols, amines,20 amides,21 hydroxamic acids,22 and carboxylic acids23 are among the groups that have been blocked by the TBDPS group. Deprotection is usually through the reaction with tetrabutylammonium fluoride, TBAF. Phenyl-Substituted Silyl Groups Phenyldimethylsilyl, diphenylmethylsilyl, and triphenylsilyl groups are also used to protect alcohols. While these reagents bring no special steric advantages to the blocking agent picture they do bring an electronic one. Phenylsilyl ethers are more stable towards acid with an increase in stability following an increase in phenyl substitu- tion.24 A similar stability towards alkaline hydrolysis is not seen. The stability of the diphenylmethylsilyl ethers has been carefully studied.25 Introduction of the phenyl-substituted silyl groups is through their respective chlorides. Other Trialkylsilyl Groups In principle any trialkylsilyl group can be considered a silyl blocking agent, though not all are, of course, used in that manner. The more notable ones can be found in the Tables along with pertinent information concerning reactivity and stability. Bridging Blocking Agents The commonly employed silicon-based agents for the protection of diols and related species such as diamines and hydroxyacids are shown below.

Me Me Et Et Ph Ph

Si Si Si Si

DMS DES DPS DIPS

Me Me Me O Me Si Si Si Si Me Me Si Me Me

TMDS DTBS STABASE

Si iPr O iPr Si Si iPr iPr Si

TIPDS BSB The use of various silylene units such as the dimethyl-(DMS)26, diethyl-(DES)27, diisopropyl-(DIPS)28, di-tert-butyl- (DTBS)28,29 and diphenylsilylene(DPS)30 groups have been employed for the protection of diols, hydroxyacids, diamines, and similar difunctional systems. Here again, as expected, the more hindered the silicon center the more stable the silylated species becomes. Thus, the DTBS ethers of diols are hydrolytically stable between pH 4 and pH 10. The 6- and 7-membered ring systems from 1,3- and 1,4-diols, respectively, are more stable than the

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5-membered rings that result from the reaction of 1,2-diols. The disilylethane derivative, tetramethyldisi- lylethane (STABASE) is used for the protection of primary amines, including those of the esters of aminoacids.31 The tetraisopropyldisiloxanyl (from TIPDS) unit is highly useful for the protection of the 3’,5’-dihydroxy units of nucleosides.32 The benzostabase group can be used to protect primary aliphatic and aromatic amines.33,34 Special Silicon-Based Blocking Agents Diisopropylchlorosilane has been shown to provide both protective and stereoselective reactive properties in the sily- lation/reduction of β-hydroxyketones.35 The chloromethyldimethylchlorosilane has been very effectively used in the analytical arena taking advantage of the greater electron capture detection of the chlorine atom.36,37 Suitably substituted alcohols can be reacted with bromomethyldimethylchlorosilane to silylate the hydroxyl, followed by a radical cyclization and functionalization of the silicon-carbon bond to provide diols, or α- methyl alcohols.38, 39 The (pentafluorophenyl)dimethylsilyl group has been successfully used in the derivatiza- tion of hydroxysteroids, which can be detected at femtogram levels.40 Carboxylic acids are protected as their trimethylsilylethyl esters with the advantage of being readily cleaved with the elimination of ethylene.41 Deprotection The clean deprotection or desilylation of a silyl-protected functional group is essential to its utility. The relative sta- bility correlation of trisubstituted silyl ethers towards hydrolysis under acid catalysis is TMS ≅ DMPS ≅ MDPS < TES ≅ DMIPS < TPS < TBS < TDS < TIPS < TBDPS < DTBMS.42 The relative stabilities towards hydrolysis in base is TMS ≅ DMPS ≅ MDPS < DMIPS ≅ TES < TBDPS ≅ TBS < TDS < TIPS < DTBMS.42 Denmark and coworkers25 have looked at the steric and electronic effects on the desilylation of silyl ethers in acid and base. In addition the long-range structural effects on the desilylation of silyl ethers has been investigated.43

Table 1 RESISTANCE OF SILYLATED ALCOHOLS TO CHEMICAL TRANSFORMATIONS t1/2 for Si-OR bond scission at room temperature reagents

HCl - KF - CH3MgBr n-Butyl LAH - Pyridinium Blocking group Substrate THF methanol in ether lithium THF Chlorochromate

n-butanol <15 min 2 min 48 hr 2 hr 30 min <30 min

(CH3)3Si- cyclohexanol <15 min 2 min >48 hr 3 hr 1 hr <30 min t-butanol <15 min 24 hr >48 hr 50 hr 24 hr <30 min

n-butanol <15 min 2 hr no reaction 24 hr 1 hr <30 min

(C2H5)3Si- cyclohexanol <15 min 20 hr no reaction >48 hr 2 hr <30 min t-butanol <15 min no reaction no reaction no reaction no reaction 1 hr

cyclohexylMe2Si- cyclohexanol < 15 min 10 hr no reaction 36 hr 2 hr <30 min i Pr(CH3)2Si- cyclohexanol 10-30 min 24-30 hr no reaction >60 hr 3 hr <30 min

n-butanol >3 hr no reaction no reaction no reaction 25 hr 10 hr t BuMe2Si- cyclohexanol >3 hr no reaction no reaction no reaction >50 hr >20 hr t-butanol no reaction no reaction no reaction no reaction no reaction >20 hr

n-butanol 16 hr no reaction no reaction no reaction >30 hr 22 hr t HexylMe2Si- cyclohexanol 30 hr no reaction no reaction no reaction no reaction 50 hr t-butanol no reaction no reaction no reaction no reaction no reaction no reaction i Pr3Si- cyclohexanol no reaction no reaction no reaction no reaction no reaction >72 hr

n-butanol no reaction 100 hr no reaction no reaction no reaction no reaction t BuPh2Si- cyclohexanol no reaction no reaction no reaction no reaction no reaction no reaction t-butanol no reaction no reaction no reaction no reaction no reaction no reaction +44 161 226 9843 www.gelest.com (215) 547-1015FAX: (215) 547-2484 www.gelest.com AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 5 101 ᮣᮣᮣᮣ Gelest,Gelest, Inc. Inc.

TRIMETHYLSILYLTableBLOCKING 2 AGENTS Table 2 TRIMETHYLSILYL BLOCKING AGENTS COMMENTS

SIT8510.0Me Reacts in presence of HCl acceptor.6 Will 44 Trimethylchorosilane, TMCS MeSi Cl silylate strong acids with expulsion of HCl. [75-77-4] High purity grade available, SIT8510.1 Me Protects hindered alcohols w/ Mg/DMF.45

Me SIT8430.0 More reactive than SIT8510.0 Less reactive, Trimethylbromosilane, TMBS MeSi Br but more Less reactive, photolytically stable than 46 [2857-97-8]Me SIT8564.0. Me SIT8564.0 Extremely reactive silylating agent.46 Trimethyliodosilane, TMISMeSi I Used with HMDS for hindered alcohols.47 48 [16029-98-4] Me Forms enol silyl ethers with ketones and SIT8620.0. SIT8620.0Me Strong silylating agent for C- or O-silylations.48,49 Trimethylsilyltrifluoro- Reacts w/ nitroalkanes to give N, N-bis MeSi O3SCF3 methanesulfonate TMSOTf (trimethylsiloxy)enamines.50,51 [27607-77-8]Me These are useful reagents.52

SIT8585.0Me Releases toxic HCN upon reaction. Extremely Trimethysilyl Cyanide reactive silylating agent for acids and alcohols. TMSCN MeSi CN Amines and thiols react more slowly than acids [7677-24-9]Me and alcohols. Does not react with amides, ureas or carbonates.27 Silylates amino acids.53 SIH6110.0 Releases ammonia upon reaction. Both trimethylsilyl Me3Si SiMe3 Hexamethyldisilazane, HMDSN groups used. Silylations catalyzed by SIT8510.0 H [999-97-3] and other reagents.6

SID3605.0Me Similar to SID6110.0 and SID3398.0 Liberates Dimethylaminotrimethylsilane, Me NH upon reaction. Liberates Me NH MeSi NMe2 2 2 TMSDMA upon reaction. Silylates urea-formaldehyde [2083-91-2]Me polycondensates.54 Silylates phosphorous acids.55

SID3398.0Me Releases diethylamine upon reaction. Moderately Diethylaminotrimethylsilane, strong silylating agent. Selectively silylates MeSi NEt TMSDMEA2 equatorial over axial hydroxyls.56 [996-50-9] Me SIB1846.0 More reactive than SIH6110.0. Releases neutral Bis(Trimethylsilyl)acetamide, O SiMe3 acetamide upon re-action. Both silyl groups used. BSAMe Used for silylation in analytical applications.57 [10416-59-8]N SiMe3 Reactions catalyzed by acid. Forms enol silyl ethers in ionic liquids.58 SIB1876.0 More reactive than BSA (SIB1846.0). O SiMe3 Bis(Trimethylsilyl)- Commonly used for analytical purposes. CF trifluoroacetamide, 3 Reacts very well in DMF or acetonitrile.57 N SiMe BSTFA 3 [25561-30-2]

SIB1878.0HN SiMe3 By-product is urea. 59 Bis(Trimethylsilyl)urea,O Used for alcohols and acids. BSU Used in synthesis of penicillins and HN SiMe3 [18297-63-7] cephalosporins.60

SIM6576.0O Silylation reagent similar to SIB1846.0, N-METHYL-N-TRIMETHYLSILYL- Si(CH3)3 but with liquid, volatile byproduct CF3C N TRIFLUOROACETAMIDE MSTFA CH3 [24589-78-4]

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TRIMETHYLSILYLTable 2BLOCKING (continued) AGENTS Table 2 (continued) TRIMETHYLSILYL BLOCKING AGENTS COMMENTS SIT8590.0 Powerful silylating agent for alcohols. N SiMe 3 6 Trimethylsilylimidazole, TMSIMN Does not react with aliphatic amines. [7449-74-3] SIT8572.0 Reacts with 1°, 2° and 3° alcohols.63

2-Trimethylsiloxypent- O OSiMe3 2-en-4-one [13257-81-3] SII6460.0 By-product is acetone.Good for alcohols Isopropenoxytrimethylsilane, and acids.64 Provides acetonides with diols.65 Me3Si O IPOTMS [1833-53-0]

SIM6496.0Me OSiMe3 Used for silylation of acids, alcohols, thiols, 1-Methoxy-1-trimethysiloxy- amides and ketones.66,67

2-methyl-1-propene Me OMe [31469-15-5]

SIM6571.5O Used for 3° alcohols and enolizable ketones.68,69

Methyl Trimethylsilylacetate Me3Si [2916-76-9] OMe

SIE4901.6O Silylation of ketones, alcohols, acetylenes, 70-73 Ethyl TrimethylsilylacetateMe3Si thiols under the influence of fluoride ion. [4071-88-9] OEt SIA0555.0 Neutral propylene by-product. Acid-catalyzed 74 75 AllytrimethylsilaneMe Si silylations. Used for acids, and thiols . [762-72-1]3 Employed in the synthesis of N-trimethylsilylpyridinium triflate an active trimethylsilylating agent.76 SIT8588.5 2-(Trimethylsilyl)ethoxymethyl chloride, 95% [76513-69-4] SIT8580.0 Trimethylsilyl Azide [4648-54-8] Table 3 TRIALKYLSILYL BLOCKING AGENTS COMMENTS

SIT8250.0Et Stability of ethers intermediate between TMS 77 TriethylchlorosilaneEtSi Cl and TBS ethers. Good for 1°, 2°, 3° alcohols. [994-30-9] Can be cleaved in presence of TBS, Et TIPS and TBDPS ethers.9 SID3603.0Et Very reactive triethylsilyl protecting group.

N,N-DimethylaminotriethylsilaneEtSi NMe2 Dimethylamine by-product produced. [3550-35-4] Et

SIT8335.0Et More reactive than SIT8250.0 Useful for more Triethylsilyl- hindered alcohols.78 trifluoromethanesulfonate EtSi O3SCF 3 [79271-56-0] Et

SII6462.0Me Ethers comparable in stability to those of TES- 79 Isopropyldimethyl-i-PrSi Cl protected ethers. chlorosilane [3634-56-8] Me

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TRIALKYLSILYLTable BLOCKING3 (continued) AGENTS TRIALKYLSILYL BLOCKING AGENTS Table 3 (continued) COMMENTS COMMENTS SIB1935.0 and Me Excellent for 1° and 2° alcohols. Silylation catalyzed 10 SIB1935.2Me CClSi by imidazole. Stable to many reagents. tert-Butyldimethylchlorosilane3 Can be selectively cleaved in presence of acetate, [18162-48-6]Me THP and benzyl ethers80 among others.81 SIB1935.4 Me tert-Butyldimethylchlorosilane, Me 3CSi Cl 1M in THF [18162-48-6] Me

SIB1935.5 Me tert-Butyldimethylchlorosilane, Me CSi Cl 2.85M in toluene 3 [18162-48-6] Me

82 SIB1967.0Me More reactive than SIB1935.0 Converts acetates tert-Butyldimethylsilyltri- to TBS ethers.83 fluoromethanesulfonate Me3COSi 3SCF3 [69739-34-0] Me

Me 84,85 SIB1966.0O Employed in silylations for analytical purposes. 84,85 N-(tert-Butyldimethylsilyl)-N-Me3C Si purposes. trifluoroacetamide N CF3 [77377-52-7] Me H i-Pr SIT8384.0 Ethers more stable than TBS ethers.15 Protects Triisopropylchlorosilanei-PrSi Cl carboxylic acids.86 Used in synthesis of 87 [13154-24-0]i-Pr free-4-hydroxylhexopyranoses. SIT8387.0i-Pr More reactive than SIT8384.017 Used to make Tsoc, Triisopropylsilyl- (triisopropylsilyloxycarbonyl) protecting groups for i-Pr Si O SCF trifluoromethanesulfonate3 3 amines.88 [80522-42-5] i-Pr SIT8385.0i-Pr Silylates strong acids with loss of hydrogen.17 89 Triisopropylsilanei-Pr Si H Silylates 1° alcohols selectively. [6485-79-6] i-Pr SIA0535.0i-Pr Reaction w/ triflic acid and then pyridine gives Allyltriisopropylsilanei-Pr Si the active triisopropylsilylating agent, 76 [24400-84-8]i-Pr triisopropylsilylpyridinium triflate. SIT7906.0Me Me Me Ethers show stability similar to or greater than the ThexyldimethylchlorosilaneSi Cl TBS ethers.Used for 1° and 2° amines.18,90 90 [67373-56-2]Me Me Me Selective for 1° alcohols. SIB1933.5.Et Blocking agent w/ potential stability tert-ButyldiethylchlorosilaneMe 3CSi Cl approaching that of TIPS. [107149-55-3] Et PHENYL-CONTAININGTable BLOCKING 4 AGENTS Table 4 PHENYL-CONTAINING BLOCKING AGENTS COMMENTS

SIB1968.0Ph Forms more stable ethers than TBS ethers.91 92 20 tert-Butyldiphenylchloro-Me3CClSi Used to protect phenols, carboxylic acids, silane amines,93 and amides.94,95 [58479-61-1] Ph

Me SIP6728.0 Used in analytical procedures.96 Phenyldimethylchlorosilane PhSi Cl

[768-33-2] Me

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PHENYL-CONTTableAINING 4 (continued BLOCKING) AGENTS Table 4 (continued) PHENYL-CONTAINING BLOCKING AGENTS COMMENTS

SID4586.0 H Similar to SIP6728.0. Emits ammonia upon reaction. Ph N Ph 97 Diphenyltetramethyl-Si Si Used for silylation of capillary columns. disilazane Me Me [3449-26-1] Me Me

Ph SIP6729.0 Reacts with alcohols in presence of Wilkinson’s PhenyldimethylsilaneMeSi H catalyst.98

[766-77-8] Me

SID4552.0Ph Stability versus other silyl ethers studied.25 Diphenylmethylchloro- studied.25 PhSi Cl silane [144-79-6] Me

Ph SIT8645.0 Ethers hydrolyze comparably to TMS ethers in TriphenylchlorosilanePhSi Cl base and 4 times slower in acid.99 Can lead to 100 [76-86-8]Ph solid products.

SIA0575.0Ph Reaction w/triflic acid and then pyridine gives

AllyltriphenylsilanePh Si triphenylsilylpyridinium triflate, an active [18752-21-1] triphenylsilylating agent.76 Ph

SID4552.5Ph More reactive than SID4552.0. Liberates

Diphenylmethyl PhSi NMe 2 dimethylamine upon reaction. (dimethylamino)silane Me [68733-63-1]

H SIB1026.4p-Ph-Ph N Ph-Ph-p Reactivity and stability similar to that of SID4586.0 Si Si 1,3-Bis(4-biphenyl)- Me Me 1,1,3,3-tetramethyldisilazane Me Me

Table 5 SPECIALTY SILICON-BASED BLOCKING AGENTS Table 5 COMMENTS

FF

SIP6716.1Me Named Flophemesyl group. Ethers detectable at Pentafluorophenyldi- femtogram level by ECD. Forms excellent derivatives F Si Cl methylchlorosilane for mass spectral analysis.102,103 [20082-71-7] Me F F SIT8589.2 Used for protection of acids.104,105 OH Trimethylsilylethanol Me Si [2916-68-9] 3

SIC2285.0Me Can form cyclic products with appropriate Chloromethyldimethyl- 1,2-difunctional substrates.36 Used in analytical ClCH2 Si Cl chlorosilane applications for greater ECD detectability.37 [1719-57-9] Me

SIB1890.0Me Has been applied to the synthesis of diols and Bromomethyldimethyl- β-methyl alcohols.38,39 BrCH2 Si Cl chlorosilane [16532-02-8] Me

SID3535.0i-Pr H Silylates and reduces β-hydroxy DiisopropylchlorosilaneSi ketones selectively.35 [2227-29-4]i-Pr Cl Photochemically removable365

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Table 5 (continued) SPECIALTY SILICON-BASED BLOCKING AGENTS Table 5 (continued) COMMENTS SIH5840.4 Potential blocking agent for fluorous phase (Heptadecafluoro-1,1,2-2- synthesis.106 tetrahydradecyl)- CF3(CF27 ) CH 2 CH 2 SiMe 2 Cl dimethylchlorosilane [74612-30-9]

n SIB1815.5C613F O Me Potential blocking agent for fluorous phase Si Si 106 Bis(tridecafluoro-1,1,2,2-2 Cl synthesis. tetrahydrooctyldimethylsiloxy)- Me Me methylchlorosilane SIB1837.0 Sterically-hindered for the Bis(trimethylsiloxy) protection of diols dichlorosilane [2750-44-9]

BRIDGING SILICON-BASEDTable 6 BLOCKING AGENTS BRIDGING SILICON-BASED BLOCKING AGENTS Table 6 COMMENTS

107 108 SID4120.0 and Me Cl Reacts with alcohols, diols, and 109 SID4120.1Si hydroxy carboxylic acids . Dimethyldichlorosilane Employed as a protecting group/template Me Cl [75-78-5] in C-glycoside synthesis.110

SIB1072.0Me NMe2 More reactive than SIB4120.0. 111 112 Bis(Dimethylamino)-Si Reacted with diols, diamines, and 113 dimethylsilaneMe NMe2 treatment for glass . [3768-58-9]

SIB1068.0Me NEt2 Similar to SIB1072.0. Bis(Diethylamino)- Si dimethylsilane Me NEt2 [4669-59-4]

Me2 SIH6102.0Si Silylates diols with loss of ammonia. Similar in Hexamethylcyclo-HN NH reactivity to HMDS.114,115 trisilazane Me2Si SiMe2 N [1009-93-4] H SID4510.1 Reacts with alcohols,116 diols,111 2- hydroxybenzoic Diphenyldichloro-Ph Cl acids117. silane Si [80-10-4] Ph Cl

SID3402.0Et Cl Similar to, but more stable derivatives than DiethyldichlorosilaneSi dimethylsilylenes. [996-50-9] Et Cl

118-120 SID3537.0i-Pr Cl Forms tethered silyl ethers from diols.

Diisopropyldichloro-Si Protects 3’,5’ hydroxyls of nucleosides, but less silane effectively than SIT7273.0121 i-Pr Cl [7751-38-4]

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Table 6 (continued) BRIDGING SILICON-BASED BLOCKING AGENTS BRIDGING SILICON-BASED BLOCKING AGENTS Table 6 (continued) COMMENTS

29 122 SID3205.0Me3C Cl Used to protect 1,2-diols, and 1,3- diols . Di-tert-butyldichlorosilaneSi Forms 4,6-cyclic di-tert-butylsilylenediyl ethers 123 [18395-90-9]Me3C Cl w/glycopyranosides.

SID3345.0 More reactive than SID3205.0.28 Converts Di-tert-butylsilylbis(tri-Me3C O3SCF3 1,3-diols to cyclic protected 1,3-diols.124 fluoromethanesulfonate)Si Reacts w/1,3-diols in preference to 1,2-diols.125 [85272-31-7] Me3C O3SCF3 SID3534.0 More reactive than SID3345.0. Di-isopropylbis(tri- Protects diols366 fluoromethanesulfonyl) silane [85272-30-6]

126,127 SIB1042.0Me Protection for 1° amines, including amino Me 128 Bis(dimethylchlorosilyl)-Si Si acid esters . Me ethane Me [13528-93-3] Cl Cl

SIT7273.0i-Pr O i-Pr Highly useful for protection of 3’,5’- Tetraisopropyldichloro-Si Si dihydroxynucleosides.32 disiloxanei-Pr i-Pr Protects 1,2-diequatorial diols.129 Cl Cl [69304-37-6]

Me SIB1084.0Me Used to protect anilines,34 amines,35 and 1,2-Bis(dimethylsilyl)-Si H amino acids130. benzene [17985-72-7] Si H Me Me

SID1048.2 Potentially useful silylating agent for diols and 1,3-Bis(chlorodimethylsilyl)- related systems. propane Me2Si SiMe2

[2295-06-9] Cl Cl SIT7087.0 Used in the protection of 3’,5’- O 1,1,3,3-Tetracyclopentyl-Si Si dihydroxynucleosides. dichlorodisiloxane Cl Cl

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Table 7

DEPROTECTION OF 1° SILYL ETHERS IN THE PRESENCE OF OTHER 1° SILYL ETHERS9 Deprotection of: In the Presence of: 1° TMS 1° TES 1° TIPS 1° TBS 1° TBDPS ° 367 132 132 132 1 TMS NaHCO3 NaOH/EtOH NaOH/EtOH NaOH/EtOH 368 133 133 MCM-41 Cu(NO3)2 Cu(NO3)2 133 133 Ce(NO3)3 Ce(NO3)3 134 132,135 [Bu2(NCS)Sn]2O HCl 369 369 BiCl3 Bi(O2CCF3)3 370 368 K2CO3 , MCM-41

° 371 136 139 1 TES TFA HF/py SiF4/CH2Cl2 372 137 137,138 H2/Pd-C TCNQ/MeCN/H2O DDQ/MeCN/H2O 368 137,138 140 MCM-41 DDQ/MeCN/H2O MCM-41/MeOH 373 375,376 372,377 CSA CSA , H2/Pd-C 374 378 IBX/DMSO MSOTf/HCO2DPM/SiO2 368 379 MCM-41 ZnBr2/H2O 372 H2/Pd-C

° 378 1 TIPS TMSOTf/HCO2DPM, SiO2

° 132 145 132,150,394,395 1 TBS HCl/EtOH TsOH/THF/H2O HCl 141,142 146,147 19,151-154 H2SiF6/tBuOH Cl2CHCO2H HOAc/THF/H2O NaOH/EtOH15 TBAF148 CSA/MeOH155,156,164 Cyclohexene/PdO143 TBSOTf149 PPTS157,158,399-403, TsOH159 144 388 160 Alumina HOAC MeOH/CCl4 380 389 133 H2SO4 PPTS Cu(NO3)2 381,382 390 133 CSA TBAF Ce(NO3)3 PPTS383 DDQ391 Cyclohexene/PdO143 384 392 145 H2SiF6 MnO2/AlCl3 Alumina 372,377 393 139 H2/Pd-C DMSO/H2O SiF4/CH2Cl2 385 372 137,138 TMSOTf/Et3N/MeOH H2/Pd-C DDQ/MeCN/H2O 386 161 162 CeCl3•7H2O/NaI AcBr/CH2Cl2 , TMSOTf Decaborane387 HF-py/THF163 PPTS/EtOH164 396 397.398 H2SO4 , TFA TsOH404, 405, CSA406-408 409 LL-ALPS-SO3H 410 411 AcCl/MeOH , Zn(BF4)2 412 413 Cu(OTf)2/Ac2O , InCl3 386 CeCl3•7H2O/NaI 414 Ce(OTf)4/THF/H2O 415 416 PdCl2(MeCN)2 , I2/MeOH 417 418 Br2/MeOH , IBr 419 420 LiCl/DMF , CCl4/MeOH 421 379 CeCl3•7H2O , ZnBr2/H2O 422 423,424 ZrCl4/Ac2O , HF-pyr 400 372,377 I2/KOH , H2/Pd-C 425 426 Bu4NOH/MeOH , TBAF 378 TMSOTf/HCO2DPM/SiO2 Decaborane387

° 427 166,428,429 167 1 TBDPS KOH TBAF/HOAc LiAlH4 TBATB/MeOH165 HF-py432, TBAF390 NaOH429-431 428 Bu4NOH

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Table 8 DEPROTECTION OF 1° SILYL ETHERS IN THE PRESENCE OF 2° SILYL ETHERS9 Deprotection of: In the Presence of: 2° TMS 2° TES 2° TIPS 2° TBS 2° TBDPS

° 168 367 171 1 TMS Rexyn 101 NaHCO3 BF3OEt2 169 K2CO3/MeOH Swern Alumina170 conditions434 HOAc433 Swern conditions434

° 434 172 172,174 139 1 TES Swern conditions HOAc HOAc/H2O/THF SiF4/CH2Cl2 452 175 454 436 137,138 TMSOTf/i-Pr2NEt RMgX , HCl , HBr/PPh3 DDQ/MeCN/H2O HF-pyr367,453 CSA384,451,455, PPTS456, TFA457,458 TsOH448 LiOH367 HF/pyr459,460, HF447, TMSOTf/ citric acid463 461 384 461 i-Pr2EtN , TBAF , TBAF/ TMSOTf/i-Pr2NEt 439 447 462 385 HOAc , KF , Swern cond. TMSOTf/Et3N/MeOH

° 176 464 177 178 1 TIPS TBAF , CSA TFA/H2O/THF TFA/H2O/THF 453 465 469 TBAF , SiF4 CSA 466 POCl3-DMF 466 (TfO)2O-DMF 467 CBr4/MeOH 468 CAN/SiO2

° 373 15,179,470,471 187,489 172,490-492 205,206 1 TBS HF-pyr HCl HCl , HOAc HOAc/H2O/THF 180,181,473-477 188,189 207-209 HOAc TFA/H2O/THF TsOH TsOH/PPTS182 CSA/MeOH156, PPTS190 PPTS158,210,211 HF/py183,184 Acid Amberlite191 TBAF212 15 192 113 NaOH NH4F HF/py 143 193,194 214 Cyclohexene/PdO HF/MeCN HF/Et3N 185,186 t 142 195 81 TsOH H2SiF6/ BuOH , HF/py IBr/DCM 472 196 197 179 H2SO4 H2/wet Pd , AgOAc HCl HOAc87-91 NBS/DMSO198, TBAF199,200 HOAc758 381-383,470,478-481 160 15 407,408,759,760 CSA MeOH/CCl4 , NaOH CSA PPTS482 Cyclohexene/PdO201 PPTS761-764 TsOH483 Alumina141,202 TsOH764-766 484 i 203 696 NH4Cl/MeOH CAN/PrOH HF-pyr 479,480,485-487 204 469,767-769 HF-pyr POCl3/DMP HF 488 204 713 TBAF polymeric (CF3SO2)2)/DMF H2SiF6 617 381-383,401,479,480,493-502 385 DCKA CSA TMSOTf/Et3N/MeOH 384 389,401,496,497,519-544 412 TBAF•3H2O PPTS Cu(OTf)2/Ac2O 404,405,510 411 TsOH Zn(OTf)2 511 466 TsOH/Bu4NHSO4 K2CO3 398,512-516 517 752 TFA , acidic CHCl3 NaOH 412 518 770 Cu(OTf)2/Ac2O , BCl3 TBTU , QFC HF-py479,480,485-545,, HF546,547 polymeric DCKA618 548 549 413 H2SiF6 , NH4F InCl3 TBAF/HOAc452,550,564, TBAF551-555 556 ν557,558 TAS-F , CBr4-h Jones rgt.523, LiBr/18-C-6559 560 466,561 CAN/i-PrOH , POCl3-DMF 466 (TfO)2O-DMF 372 562 H2/Pd-C , QFC 425 Bu4NBr3/MeOH 392 563 MnO2/AlCl3 , Oxone 468 CAN/SiO2

1° TBDPS TBAF/ NaH/HMPA215 NaH/HMPA218 HF/py223 HOAc565,566 HF-pyr568 NaOH217 alumina144 NaOH/ KOH/18-C-6569 KOH/MeOH218 CSA578 DMPU567 TBAF199,219,220,524,529,576 HF-py432,579 193,570,571 570 NH4F NH4F NaOH/DMPU567 TBAF469,580 569 466 KOH/18-C-6 POCl3-DMF 428,519,526,565, 568,566,572-575 466 TBAF/HOAc (TfO)2O-DMF TAS-F577 NaH/propargyl alcohol395

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Table 9 DEPROTECTION OF 1° SILYL ETHERS IN THE PRESENCE OF 3° SILYL ETHERS9 Deprotection of: In the Presence of: 3° TMS 3° TES 3° TIPS 3° TBS 3° TBDPS 1° TMS PPTS224 1° TES HF/py235,479,525,533,587 Amberlyst-15583 HOAc771 CSA376,586 TBAF/HOAc440 PPTS435

° 235 224 1 TIPS TBAF SiF4 1° TBS HF584,585 CSA376,581,586 TBAF227,543,772 TBAF/HOAc588 219,228 407,408,773 HF-pyr479,525,533,587 CSA 41 NH4F 142 H2SiF6 139 SiF4/CH2Cl2 160 544 MeOH/CCl4 HF-py oxone563 1° TBDPS TBAF/HOAc566 TAS-F774

Table 10 DEPROTECTION OF 2° SILYL ETHERS IN THE PRESENCE OF 1° SILYL ETHERS9 Deprotection of: In the Presence of: 1° TMS 1° TES 1° TIPS 1° TBS 1° TBDPS 2° TMS Amberlyst 15168 TBAF464 HCl229 HCl232 230,231 233 K2CO3/MeOH TsOH 589 234 TsOH Acetone/Me2C(OMe)2/CSA 548 235 citric acid PhSeCl/K2CO3 548 236 H2SiF6 TBAF NaOH471 PPTS237,238 Alumina170 239 K2CO3/MeOH HOAc590, CSA591, TsOH589,592,594 590 531 BF3-OEt2 , TMS-OTf

° 225 177 138 135,234,520,529,603,604 2 TES HF/py TFA DDQ/MeCN/H2O HCl 240 241 245,395 H2SiF6/HF/H2O HCl/py HOAc 595 242,243 246 HCl NH4Cl Cl3CCO2H HOAc469 TBAF/THF/ TsOH247,382,602-604 380,451 242 248,249 CSA NH4Cl(s) HF/MeCN 596 370 235 PPTS HF-py PhSeCl/K2CO3 380 598,599 156,599,607 H2SO4 I2/Ag2CO3 TBAF 380 600 138 Ph3P-HBr Pd/C, MeOH DDQ/MeCN/H2O 367,384,453 372 139 HF-pyr H2, Pd/C SiF4/CH2Cl2 597 250 HF-Et3N CSA/CHCl3/MeOH CSA601, PPTS164,602 607 382,602 H2SO4 , BF3-OEt2 608 Et3N-HF HF-pyr609,2,4,4,6-tetrabromo-2,5- 611 cyclohexadienone/PPh3 612 H2, Pd/C 379 ZnBr2/H2O 2° TIPS

° 208 251 15,252 2 TBS H2SiF6/HF/H2O DIBAL-H HCl 596 392 253,254 CSA MnO2/AlCl3 HOAc/H2O/TH TsOH255,256 PPTS158,257,432,613 258 BF3OEt2 TMSOTf162 HF/MeCN188 TBAF155 139 SiF4/CH2Cl2 CSA/MeOH164 569 385 H2SiF6 , TMS-OTf/Et3N 432,614 412 TMS-OTf , Cu(OTf)2/Ac2O 413 615,616 775 InCl3 , LiAlH4 , IBr 405 419 P2O5/(MeO)2CH2 , LiCl/DMF polymeric DCKA618 379 411 ZnBr2/H2O , Zn(BF4)2 2° TBDPS NaH/HMPA259 TBATB/MeOH165

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Table 11 DEPROTECTION OF 2° SILYL ETHERS IN THE PRESENCE OF OTHER 2° SILYL ETHERS9 Deprotection of: In the Presence of:

2° TMS 2° TES 2° TIPS 2° TBS 2° TBDPS 2° TMS TBAF260 HF-pyr533,582 Citric Acid/MeOH261 HCl265 TBAF274 776 446 262,263 266,267 133 SiO2-Cl/ NaI TBAF TBAF/HOAc TsOH Cu(NO3)2 567 264 268 133 KF FeCl3 HOAc/H2O/THF Ce(NO3)3 464 269,622 628,629 TBAF PPTS K2CO3 447 270 631 KF HF/Et3N NaIO4 NaOH471 Citric Acid/MeOH271,272 548 133 H2SiF6 Cu(NO3)2 133 Ce(NO3)3 DDQ/wet EtOAc273 239 K2CO3/MeOH HOAc620,621 TsOH594 CSA623 533,582 624 HF-pyr HF/Et3N 625,626 BF3-OEt2 627-629 K2CO3 TBAF630, KF447,567

° 13,275 177,277,278 278 293 2 TES HOAc TFA HOAc/H2O/THF HOAc/H2O/THF TBAF276,436,498 HOAc279-281 TFA177,277,278,633,634,652, CSA294 TsOH632 PPTS11,479,636, PPTS11,285,286,613,623,646,651,683 TBAF155,295,665,666 633,634 272,283 287 139 TFA MoO5/HMPA HF/MeCN SiF4/CH2Cl2 533,579 282,283 287,288,384,437,454,533,609,582,606,653,654 138 HF-py WO5/HMPA HF/py DDQ/MeCN/H2O 439 284 138 287 TBAF/ HOAc HF-py/py-THF DDQ/MeCN/H2O HF/MeCN 635 380 637 282,283 286 (NH4)HF2 H2SO4 TFA MoO5/HMPA HF/py 447 367,638,639 282,283 662 KF HF-py WO5/HMPA HCl 567 656 289 469,663 NaOH/DMPU Zn(OTf)2/EtSH PTSA/MeOH HOAc 637 290 664 NH4F TfOH/H2O-THF HF 641 291 667 Amberlyst-15 TBAF/THF K2CO3 292 631 PPTS/MePH NaIO4 MCM-41/MeOH140 HCl642,643, HOAc569,644,645 CSA373,596 TsOH460 435,655 HF-Et3N HF781 656 654 Zn(OTf)2/EtSH , TiCl3(O-iPr) TBAF/HOAc439,658 TBAF395,436,498,659,660, KF447 NaOH/DMPU567MCM-41368 661 PdCl2/CuCl/H2O

° 536,674 297,668,669 631 2 TIPS HF TBAF NaIO4 PTSA/MeOH298 TsOH480 639, 670 HF/Et3N LiAlH4 2° TBS HCl299 TBAF654 HCl15,671,672 HCl15 TsOH312-314 142 302,439,677 315 H2SiF6(aq) CSA HCO2H/THF/H2O HOAc280,281 TsOH303 CSA316 HF/MeCN188 HF/MeCN188,304-306 PPTS317,318,694,695 263,300,383 167 319 TBAF LiAlH4 HF/MeCN 479,673 307 200,320,321 CSA KF/H2O TBAF 536,674 274,308-311,499,619,534,535,616,651,683- 139 HF TBAF SiF4/CH2Cl2 675 688,777 133 Et3N-3HF Cu(NO3)2 676 678 679,680 133 H2SiF6/Et3N H2SO4 , HF-py Ce(NO3)3 681 138 HF DDQ/MeCN/H2O 682 689,690 BF3-OEt2 HCl 405 691-693 P2O5/(MeO)2CH2 HOAc 392 696,697 MnO2/AlCl3 HF-pyr TMS-OTf432,614 698 BF3-OEt2 699 Sc(OTf)3 631 NaIO4 2° TBDPS TBAF/THF179

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Table 12 DEPROTECTION OF 2° SILYL ETHERS IN THE PRESENCE OF 3° SILYL ETHERS9

Deprotection of: In the Presence of: 3° TMS 3° TES 3° TIPS 3° TBS 3° TBDPS 2° TMS TsOH267,322 HOAc268 CSA700 KF447,567 HF-py533,582

2° TES TsOH323 HOAc324 HF-pyr531,579 CSA199 HCl642,643, PPTS701 HCl440 HF-py384,533,582 TfOH703 TBAF436,702 2° TIPS

° t 142 325 2 TBS H2SiF6/ BuOH/H2O HOAc CSA326,327 TBAF328,329 TfOH703 HF704 2° TBDPS

Table 13 DEPROTECTION OF 3° SILYL ETHERS IN THE PRESENCE OF 1° SILYL ETHERS9

Deprotection of: In the Presence of: 1° TMS 1° TES 1° TIPS 1° TBS 1° TBDPS

° 330 146 398 3 TMS PPTS/MeOH ClCH2CO2H/MeOH HCl PPTS778 HCl705 HOAc771 706 707,708 398 595 BH -SMe BF -OEt , K CO HCl 3 2 3 2 2 3

° 597 331 709 3 TES HF-Et3N SiO2 TBAF 709 709 TBAF TBAF/ NH4Cl 3° TIPS

° 326,332 3 TBS LiAlH4 3° TBDPS

Table 14 DEPROTECTION OF 3° SILYL ETHERS IN THE PRESENCE OF 2° SILYL ETHERS9

Deprotection of: In the Presence of:

2° TMS 2° TES 2° TIPS 2° TBS 2° TBDPS 3° TMS TBAF/HOAc263,711 HCl333,398,705 HCl224 334 713,714 LiAlH4 H2SiF6 335 715 TBAF K2CO3 336 FeCl3 681 712 HF , BH3-THF TBAF/HOAc711 398 K2CO3

° 711 337,338 3 TES TBAF/HOAc Et3NHF 435,655 HF-Et3N TBAF/HOAc711

3° TIPS

3° TBS TBAF/HOAc711 TBAF324 TBAF/HOAc711

3° TBDPS

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Table 15 DEPROTECTION OF 3° SILYL ETHERS IN THE PRESENCE OF 3° SILYL ETHERS9

Deprotection of: In the Presence of:

3° TMS 3° TES 3° TIPS 3° TBS 3° TBDPS 3° TMS HCl/THF339

3° TES

3° TIPS

3° TBS CSA340

3° TBDPS

Table 16 DEPROTECTION OF PHENOLIC SILYL ETHERS IN THE PRESENCE OF ALKYL SILYL ETHERS9

Deprotection of: In the Presence of: 1° TMS 1° TBS 2° TBS 3° TBS 1° TBDPS 2° TBDPS ArOTMS Dowex 1-X8 TBAF342,343 TBAF344 - 341 369 718 (HO form) BiCl3 SiO2 369 Bi(O2CCF3)3 369 Bi(OTf)3 PIFA-MK10717 ArOTES NaOH132

132 349 165 ArOTBS NaOH K2CO3/Kryptofi x 10% HCl TBATB/MeOH K CO /Kryptofi x 345 411 2 3 222/MeCN Zn(BH4)2 222/MeCN345 TBAF344,346 TBAF750,751 TBAF346,347,429,717,719-721 CSA732 NaOH/TBAH728 347 KF/18-crown-6 TBAF719,733,734 348 KF/Al O /MeCN 724 2 3 K CO 722 2 3 KF-Al2O3 735 728 CsF/CH3CN CsF/RX/DMF 726 724 725 Et3NO K2CO3 , CsCO3 728 726 727 NaOH/TBAH Et3NO , LiOH 736 NaOH/ TBAH728 KOH 730 KOH729 LiOH/RX/DMF 737 730 LiOH/RX/ DMF KF-Al2O3 TMG731 PIFA-MK10717 393 DMSO/H2O ArOTIPS PIFA-MK10717 NaOH754 TBAF753 NaOH754 ArOTBDPS NaOH132 TMG731 TMG731 PIFA-MK10717

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Table 17 DEPROTECTION OF ALKYL SILYL ETHERS IN THE PRESENCE OF PHENOLIC SILYL ETHERS9

Deprotection of: In the Presence of: ArOTMS ArOTES ArOTBS ArOTIPS ArOTBDPS 1° TMS Dowex CCR-2 (H+ HCl132 HCl132 HCl132 form)341 Amberlite IR-120 (H+ form)351 2° TMS Amberlite IR-120 (H+ form)351

600 379 379 379 1° TES Pd/C/MeOH ZnBr2/H2O ZnBr2/H2O ZnBr2/H2O

738 379 379 2° TES PPTS ZnBr2/H2O ZnBr2/H2O 779 BiBr3/Et3SiH 379 ZnBr2/H2O

1° TBS HCl132,352 HCl132 HCl132,476 353 345,735 + 379 TFA , PPTS Amberlite IR-120 (H ZnBr2/H2O 345,354,355 351 413 HF/MeCN form) InCl3 345,356 746 745 BF3OEt2 HF-pyr BiOClO4 139 747 SiF4/CH2Cl2 BF3-OEt2 160 379 MeOH/CCl4 ZnBr2/H2O 357 422 TMSCl/H2O/CH3CN ZrCl4/Ac2O 358,563 Oxone/MeOHaq Nafi on-H/NaI739 409 LL-ALPS-SO3H AcCl/MeOH410 740 TMS-Cl/H2O 740 TMS-Cl/NaI/H2O 739 Me2SBr2 TBSOTf/THPOAc741 742 BiBr3/H2O/MeCN 743 BiCl3/NaI 727 CeCl3•7H2O 412 CuOTf, Ac2O 699 Sc(OTf)3/H2O 379 413 ZnBr2/H2O , InCl3 422 ZrCl4/Ac2O decaborane387 414 Ce(OTf)4/THF/H2O 467 CBr4/MeOH 744 I2/MeOH 468 CAN/SiO2 Oxone/MeOH563 780 H2/Pd-C

359,749 749 379 2° TBS TFA TFA ZnBr2/H2O 345,356 BF3OEt2 HF/MeCN346 379 ZnBr2/H2O PPTS345 139 SiF4/CH2Cl2 Nafi on-H/NaI739 742 BiBr3/H2O/ MeCN 743 BiCl3/NaI 727 CeCl3•7H2O 379 ZnBr2/H2O 413 InCl3 739 Me2SBr2 1° TIPS HCl132 HCl132 467 696 CBr4/MeOH Sc(OTf)3/H2O 744 467 I2/MeOH CBr4/MeOH 744 I2/MeOH 1°TBDPS HCl132 HCl132 699 Sc(OTf)3/H2O 467 CBr4/MeOH 744 I2/MeOH

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Table 18 DEPROTECTION OF SILYLENE- AND DISILOXANE-PROTECTED DIOLS IN THE PRESENCE OF 2° ALKYL SILYL ETHERS9 Deprotection of: In the Presence of:

2°TMS 2°TES 2°TIPS 2°TBS 2°TBDPS

DTBS silylene HF/py29,122,360,361 KF/MeOH307,362,363

TIPDS silioxane HF/MeCN364

Table 19 DEPROTECTION OF ARYL SILYL ETHERS IN THE PRESENCE OF ANOTHER ARYL SILYL ETHER9 Deprotection of: In the Presence of:

ArOTMS ArOTES AROTBS ArOTIPS ArOTBDPS ArOTMS

ArOTES

754 754 ArOTBS SbCl3 HCl 737 KF-Al2O3 755 CCl4/MeOH 756 NaClO2/NaH2PO4/py ArOTIPS ArOTBDPS

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GENERIC SILYLATION PROCEDURES

1. Hexamethyldisilazane, SIH6110.0, in the trimethylsilylation of alcohols. One equivalent of the alcohol to be silylated is mixed with 0.5 equivalents of hexamethyldi-silazane, SIH6110.0, in an inert solvent or without solvent. Warming the reaction to 40 - 50 °C or adding a drop of trimethylchlorosilane can significantly accelerate the rate of reaction. The reaction is allowed to continue until no further evidence of the evolution of ammonia is observed. For primary and secondary alcohols the reaction is quite rapid and nearly quantitative. For tertiary alcohols the reaction may be slower.

2. Trimethylchlorosilane, SIT8510.0 and SIT8510.1, in the trimethylsilylation of alcohols. One equivalent of the alcohol, 1.1 equivalent of pyridine or triethylamine are mixed in an inert solvent and one equivalent of trimethylchlorosilane, SIT8510.0 or SIT8510.1, is added. The amine can also be used as the solvent of the reaction. The reaction can be followed by any of the standard techniques including thin layer and gas chromatography. The reaction is quite fast with primary and secondary alcohols and slower with tertiary alcohols. The trimethylsilylation of amides and amines can be accomplished by a modification of this procedure wherein the reaction mixture is heated to reflux for 16 h.

3. Trimethylbromosilane, SIT8430.0, or Trimethyliodosilane, SIT8564.0, in the trimethylsilyla- tion of alcohols. One equivalent of the alcohol, 1.1 equivalent of a suitable amine base are mixed in an inert solvent and the trimethylbromosilane, SIT8430.0, or trimethyliodosilane, SIT8564.0, is added. These reagents are more reactive towards hindered alcohols than the trimethylchlorosilane.

4. Trimethyliodosilane, SIT8564.0, Hexamethyldisilazane, SIH6110.0 combination in the trimethylsilylation of hindered alcohols. The alcohol (1 equiv.), trimethyliodosilane, SIT8564.0, (2.2 equiv.) and hexamethyldisilazane (1.1 equiv.) are mixed in pyridine as solvent and the reaction mixture is stirred at room temperature.

5. Trimethylsilyltrifluoromethanesulfonate, SIT8620.0, or Trimethylsilyl Cyanide, SIT8585.1, in the trimethylsilylation of hindered alcohols. Trimethylsilylation with these very reactive silanes is carried out by the simple mixing of the alcohol and the silane in an inert solvent and allowing the reaction to occur, usually at room temperature. CAUTION: Hydrogen cyanide is generated in the reaction with trimethylsilyl cyanide.

6. Allyltrimethylsilane, SIA0555.0, in the silylation of carboxylic acids. The acid (1 equiv.) is dissolved in carbon tetrachloride (other solvents such as dichloromethane can probably be used as well) and allyltrimethylsilane, SIA0555.0, is added. To this reaction mixture is added about 3 drops (for 10 mmol reaction) of trifluoromethanesulfonic acid. The reaction is very fast and is complete when the evolution of propylene ceases.

7. Triethylchlorosilane, SIT8250.0, tert-Butyldimethylchlorosilane, SIB1935.0, tert- Butyldiphenylchlorosilane, SIB1968.0, Triisopropylchlorosilane, SIT8384.0, or Thexyldimethylchlorosilane, SIT7906.0, in the silylation of alcohols. The alcohol (1 equiv.) is dissolved in DMF along with the chlorosilane (1.1 equiv.) and imidazole (2.2 equiv.) or 2,6-lutidine (2.2 equiv.). The reaction is usually heated to about 40 °C for 10 - 20 h for completion.

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18. tert-Butyldimethylsilylation of an alcohol in dichloromethane. The tert-butyldimethylsilylation of an alcohol has been carried out by treating 0.89 equiv. of tert- butyldimethylchlorosilane in dichloromethane with 0.91 equiv. of the alcohol, 1.19 equiv. of triethylamine and 0.036 equiv. of 4-dimethylaminopyridine at room temperature for several hours.

19. Triisopropylsilyltrifluoromethanesulfonate, SIT8387.0, in the silylation of alcohols. The alcohol (1 equiv.) is reacted with triisopropylsilyltrifluoromethanesulfonate, SIT8387.0, in dichloromethane with 2,6-lutidine (2.2 equiv.) as catalyst. The reaction can be carried out as low as -78 °C in less than 5 h.

10. tert-Butyldimethylsilyltrifluoromethanesulfonate, SIB1967.0, in the silylation of alcohols. The alcohol (1 equiv.) is treated with tert-butyldimethylsilyltrifluoromethanesulfonate as a 1 M solution in a 50:50 by volume mixture of pyridine and dichloromethane. Additional SIB1967.0 may have to be added to finish the reaction for less reactive alcohols.

11. Di-tert-Butylsilylbis(trifluoromethanesulfonate), SID3345.0, in the silylation of diols. The diol (1 equiv.) is reacted with di-tert-butylsilylbis(trifluoromethanesulfonate), SID3345.0, (1.2 equiv.) and 2,6-lutidine (3.0 equiv.) in chloroform and allowed to react at 0 to 25 °C. 1,3-Diols and 1,4-diols are, in general, more reactive than 1,2-diols.

12. tert-Butyldiphenylchlorosilane, SIB1968.0, in the silylation of primary amines. The amine (1 equiv.) is reacted with tert-butyldiphenylchlorosilane, SIB1968.0, and triethylamine (1.5 equiv.) in acetonitrile at room temperature for 1-3 h. Secondary amines do not react.

13. 1,2-Bis(chlorodimethylsilyl)ethane, SIB1042.0, in the silylation of primary amines. The amine (1 equiv.) is reacted with the 1,2-bis(chlorodimethylsilyl)ethane (1 equiv.) and triethylamine (2 equiv.) in dichloromethane at room temperature for 2-3 h. A convenient way to isolate the product is to filter, concentrate, add pentane, filter again and concentrate to give the product in high purity without distillation.

14. 1,1,3,3-tetraisopropyl-1,3-dichlorodisiloxane, SIT7273.0, in the silylation of nucleosides. The nucleoside (1 equiv.) is reacted with the silane, SIT7273.0, and imidazole (4.4 equiv.) in DMF at room temperature. The yields are about 80 percent.

15. 1,2-Bis(dimethylsilyl)benzene, SIB1084.0. The amine (1 equiv.) is reacted with the silane, SIB1084.0, with a catalytic amount of Wilkinson’s catalyst, tris(triphenylphosphine)rhodium (I) chloride, in toluene solution.

16. Trimethylsilylethanol, SIT8589.2, in the protection of a carboxylic acid. The acid (1 equiv.) is reacted with 2-trimethylsilylethanol, SIT8589.2, (1 equiv.), dicyclohexylcarbodiimide (1 equiv.) in ethyl acetate with a catalytic amount of DMAP added. A typical reaction is about 12 h at room temperature.

17. Triisopropylchlorosilane, SIT8384.0, in protection of a sulfonic acid. The acid (1 equiv.) is reacted with triisopropylchlorosilane, SIT8384.0, (1.4 equiv.) and triethylamine (1 equiv.) in dichloromethane for 1 h at -35 °C and 15 h at room temperature.

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18. Triisopropylsilyltrifluoromethansulfonate, SIT8387.0, as precursor to Triisopropylcarbamate, Tsoc, protecting group. The amine to be protected in the presence of triethylamine is reacted with carbon dioxide (introduced as a gas or as dry ice) at -78 °C for 30-60 min. This mixture is then treated with triisopropylsilyltrifluo- romethanesulfonate, SIT8387.0, at room temperature. The reaction mixture can be washed with water without hydrolysis of the protecting group.

19. Formation of acetonides of diols with Isopropenoxytrimethylsilane, SII6460.0. The diol (1 equiv.) is mixed with isopropenoxytrimethylsilane, SII6460.0, in an inert solvent (THF, ether, toluene) and 1 or 2 drops of concentrated HCl or trimethylchlorosilane, is added. The reaction is complete in less than 30 min.

20. Dimethylaminotrimethylsilane, TMSDMA, SID3605.0 and Diethylaminotrimethylsilane, TMSDEA, SID3398.0. The alcohol or amine (1 equiv.) is reacted with the aminotrimethylsilane and dimethylamine or diethylamine is distilled off as the reaction proceeds. These reagents are particularly useful in the silylation of amines. HMDS, SIH6110.0, is preferred for the trimethylsilylation of alcohols. An alternative procedure for the silylation with the aminosilanes is with acid catalysis. This is best achieved with ammonium sulfate, trimethylchlorosilane as well as trichloroacetic acid. 21. Trimethylsilylimidazole, TMSI, SIT8590.0. Trimethylsilylimidazole, SIT8590.0, is a very reactive trimethylsilylating agent, especially for alcohols. It is typically reacted with an equivalent amount of the hydroxyl group(s) in the presence or absence of an acid catalyst.

22. N,O-Bis(trimethylsilyl)acetamide, BSA, SIB1846.0. The alcohol (1 equiv.) is reacted with N,O-bistrimethylsilylacetamide, SIB1846.0, (0.5 equiv.) in an inert solvent. The reaction proceeds faster with a small amount of trimethylchlorosilane as catalyst.

23. N,O-Bis(trimethylsilyl)trifluoroacetamide, BSTFA, SIB1876.0. The alcohol (1 equiv.) is reacted with N, O-bis(trimethylsilyl)trifluoroacetamide, SIB1876.0, (0.5 equiv.) in an inert solvent with or without trimethylchlorosilane catalysis. This has the advantage of producing the liquid by-product, trifluoroacetamide, which is oftentimes more easily removed than the solid acetamide from SIB1846.0 or diphenylurea from SIB1878.0.

24. N, N-Bis(trimethylsilyl)urea, BSU, SIB1878.0. Two equivalents of this solid trimethylsilylating agent is reacted with the alcohol (1 equiv.) in an inert solvent. The solid, insoluble diphenylurea produced is readily removed by filtration and the product is purified.

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DEPROTECTION OF SILYL ETHERS

1. Acid-Catalyzed cleavage of trimethylsilyl ethers. The silylated alcohol (0.4 mmol) in dichloromethane (4 mL) is treated with a drop of 1N HCl and the reac- tion mixture is stirred for 30 min. In a transeterification approach a 0.5 M solution of the trimethylsilylated alcohol in methanol is treated with pyridinium p-toluenesulfonate (PPTS) at room temperature for 30 min. The lower boiling trimethyl- methoxysilane is removed by distillation.

2. Base-Catalyzed cleavage of trimethylsilyl ethers. The mildest conditions for the base-catalyzed cleavage of trimethylsilyl ethers is the treatment of a methanol solution of the silylated alcohol with an excess of potassium carbonate for 1-2 h.

3. Selective cleavage of a triethylsilyl ether with hydrogen fluoride-pyridine - representative proce- dure for the cleavage of silyl ethers with HF•pyridine. Treatment of 180 mmol of the silylated alcohol with 4 mL of the stock solution of hydrogen fluoride-pyridine solution (2 mL of HF•pyridine, 4 mL pyridine and 16 mL THF) for 2-3 h results in the cleavage of the tri- ethylsilyl ether.

4. Acid-catalyzed cleavage of triethylsilyl ethers. A cold (0 °C) methanol solution of the silyl ether is treated with p-toluenesulfonic acid (0.33 equiv.) for 1-2 h. A solution of the silyl ether in THF is treated with an aqueous solution of trifluoromethane-sulfonic acid.

5. Cleavage of a tert-butyldimethylsilyl ether with tetra-n-butylammonium fluoride - representa- tive procedure for the deprotection of silyl ethers with TBAF. A solution of the silyl ether in THF (approximately 4 M) is treated with 3 equiv. of 1 M tetra-n-butyl-ammo- nium fluoride in THF at ambient temperature until the silyl ether is converted. This usually requires from 2 - 16 h.

6. Cleavage of a tert-butyldimethylsilyl ether with tris(dimethylamino)sulfur-(trimethylsilyl)diflu- oride, SIT8715.0 - representative procedure for the deprotection of silyl ethers with TAS-F. A dilute (0.4 M) solution of the alcohol in THF is added to the SIT8715.0 at ambient temperature and the resulting solution is stirred for 1 - 2 h.

7. Cleavage of a tert-butyldimethylsilyl ether with hydrofluoric acid - representative procedure for the deprotection of silyl ethers with HF. Hydrofluoric acid (49% aqueous solution, excess) is added to the silyl ether in acetonitrile at 0 °C. After stir- ring for a short time (10 - 30 min usually) the reaction is carefully quenched by the addition of saturated aqueous sodium hydrogen carbonate (CAUTION: STRONG EVOLUTION OF CARBON DIOXIDE).

8. Cleavage of a tert-butyldiphenylsilyl ether with tetra-n-butylammonium fluoride in acetic acid. A stock solution of TBAF in acetic acid is prepared (0.15 mL of HOAc per 1.0 mL of 1 M TBAF in THF). The silyl ether is dissolved in THF and reacted with an excess of the stock solution for several h.

(215) 547-1015FAX: (215) 547-2484 www.gelest.com AZmax TEL: 035543-1630 • FAX: 03-5543-0312 • www.azmax.co.jp (215) 547-1015 • FAX: (215) 547-2484 • www.gelest.com 23 119 Gelest, Inc. ᮣᮣᮣᮣ Gelest, Inc. References 57. Blau, K.; Halket, J. “Handbook of Derivatives for Chromatography”, 2nd Ed., J. Wiley and Sons, New York, NY, 1993. 1. Greene, T.; Wuts, P.G.M.; Protecting Groups in Organic 58. Smietana, M.; Mioskowski, C. Org. Lett. 2001, 3, 1037. Synthesis; 2nd ed.; Wiley, New York, 1991. 59. Verboom, W., et al. Synthesis, 1981, 807. 2. Kocienski, P. J.; “Protecting Groups; Thieme: Stuttgart, 1994. 60. Cooper, E. E. Chem. Ind. 1978, 794. 3. Lalonde, M.; Chan, T. H. Synthesis, 1985, 817. 61. Palomo, C. Synthesis, 1981, 809. 4. Olsson, L.-I. Acta Pharm. Suec. 1986, 23, 370. 62. Ballester, A.; Palomo, C. Synthesis, 1983, 571. 5. Birkofer, L.; Ritter, A. Newer Methods in Preparative Organic 63. Veysogln, T.; Mitscher, L. A. Tetrahedron Lett. 1981, 22, 1303. 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Acknowledgements: Sincere thanks are given to Georg Thieme Verlag, Stuttgart, Germany, http://www.thieme.de/chemistry/journals.html, publishers to Synthesis, for permission to use data from the article Selective Deprotection of Silyl Ethers, T.D. Nelson and R.D. Crouch, Synthesis, 1996, 1031.

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