Lewis acids iodotrimethylsilane

In the presence of proton and/or Lewis acid and strong nucleophiles bicyclo[3.2.0]heptan-6-ones are converted to 3-substituted cycloheptanones (Table 15). Bicyclo[3.2.0]heptan-6-ones rearrange to give 3-iodocycloheptanones on treatment with iodotrimethylsilane. Zinc(II) iodide or mercury(II) halides as catalysts enhance the rate and the selectivity of the reaction.31 If a second, enolizable carbonyl group is present, an intramolecular alkylation may follow the ring enlargement under these reaction conditions.32 Consecutive treatment with tributyltin hydride/ 2,2 -azobisisobutyronitrile affords reduced, iodo-free cycloheptanones, whilst treatment with l,8-diazabicyclo[5.4.0]undecene yields cycloheptenones.33 Similarly, benzenethiol adds to the central bond of bicyclo[3.2.0]heptan-6-ones in the presence of zinc(II) chloride and hydrochloric acid under anhydrous conditions to form 3-(phenylsulfanyl)cycloheptanones.34... 

In all these reactions, an equimolar amount of Lewis acid [8-10] was used. In 1981 [11], a catalytic version was developed, using 10 mol% of iodotrimethylsilane 17 (TMSI) as the Lewis acid. A few examples are shown in Scheme 13.7. The postulated mechanism is depicted in Scheme 13.8. 

Simple organosilicon Lewis acids, for example trimethylsilyl triflate (la) [5], trimethylsilyl nonaflate (2) [6], iodotrimethylsilane [7], fert-butyldimethylsilyl triflate (Ic) [8], triisopropylsilyl triflate (Id) [8], etc., are commercially available, but because of their easy hydrolysis and the appearance of traces of a protic acid during storage. 

Protodesilylation of allyl- or aryl-substituted trialkylsilanes with an acid partner is a cleaner method, producing propene or an aromatic hydrocarbon as a volatile inert byproduct. Although some methods of isolation have been reported, they can usually be prepared in situ in aprotic media and employed without further purification. Methods of preparation of representative organosilicon Lewis acids, la, trimethylsilyl bis(trifluoro-methanesulfone)imide (7) [9], trimethylsilyl perchlorate (6a) [10], iodotrimethylsilane, and trimethylsilyl tetrakis(trifluoromethanesulfonyl)borate [11] are described below. In principle, these analogs are also accessible in a similar manner (Table 1) [2d]. 

Some of the low-temperature rearrangements listed in this section take place under Lewis acid catalysis (iron(III) chloride, iodotrimethylsilane, zinc(II) bromide, 010.]. °... 

The C 0 bonds of ethers, acetals, and esters are cleaved with iodotrimethylsilane (eq (39)) [36]. Various halosilanes, silylamines, and N,0-bis(trimethylsilyl)acetamide are used not only for the protection of alcohols, amines, and carboxylic acids but also for the conversion of them to the volatilizable compounds for GC analysis (eq (40)) [37]. Silyltriflates act as silating agents as well as Lewis acids (eq (41)) [38]. 

Another mild method for deacetaiisation requires treatment of the substrate with iron(III) chloride hexahydrate in refluxing dichloromethane or iron(lll) chloride adsorbed on silica. The conditions are sufficiently mild to release a ketone in the presence of an easily epimerisable centre [Scheme 2.10]. For more rugged substrates, a greater latitude of Lewis acidity can be employed using reagents such as bromodimethylborane or iodotrimethylsilane. 

The Morita-Baylis-Hillman reaction of imines with ynones in the presence of iodotrimethylsilane was efficiently catalyzed by ZrCU (Equation 15) [20]. Compared with the other Lewis acids, ZrCU showed much higher activity for the reaction. The newly formed olefin moiety has (E) configuration exclusively. 

The reaction of allylsilanes with acetals to give homoallylic ethers is usually conducted with stoicheiometric amounts of Lewis acids it has now been shown that a catalytic quantity of iodotrimethylsilane is equally effective. ... 

General Discussion. 2,4-Pentadienyltrimethylsilane is used as a stable (storable) nucleophilic substitute for pentadienyllithium. This compound reacts with Lewis acid (boron trifluoride etherate, iodotrimethylsilane, titanium(IV) chloride) activated electrophiles in the same manner as allylsilanes (eq 1). Generally only e-substitution is observed (cf. Table 1). ... 

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