Silylcarbinols

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

When silylcarbinols are treated with catalytic amounts of base or active metal, the silyl group migrates from carbon to oxygen (Equations Si 1.2 and Si 1.3). Detailed studies of this reaction have been carried out by A.G. Brook whose name is now associated with the reaction. 

The same method can be used to obtain cyclopropyl and vinyl a-lithiosilanes. The a-lithiosilanes add to aldehydes and ketones to form p-silylcarbinols, which form olefins with loss of trimethylsilanol on treatment with potassium hydride (THF or diglyme) or in some cases with sulfuric acid. 

In the case of base-induced elimination, the Peterson alkenation relies on the strong bond formed between silicon and oxygen, and the ready propensity for silicon to be attacked by alkoxide, to drive the reaction. In the original study by Peterson, the -silylcarbinols were prepared by the addition of (trimethylsilyl)methylmagnesium chloride to the carbonyl. The caibinols were subsequently eliminated by treatment with sodium or potassium hydride or with sulfuric acid to form the methylene derivatives in excellent yield. The Peterson reaction has proven to be of general utility in the synthesis of alkenes. ... 

As Peterson outlined in his preliminary communication of the method, either basic (KH, KOBu or NaH) or acidic conditions (acetic acid, sulfuric acid or boron trifluoride etherate) may be utilized to effect the elimination of the silylcarbinol. Alternatively the initial adduct may be treated in situ with thio-nyl or acetyl chloride. This procedure may be advantageous in cases where isomerization of the alkene is problematic, and is particularly useful in the synthesis of terminal alkenes. As discussed in Section 3.1.3.4.2, the Johnson group has successfully employed aqueous HF to effect the elimination and this method may also have advantages in situations complicated by base-catalyzed isomerization. ... 

An example is the preparation of allylidenecyclopropanes (Scheme 4S). The l-(trimethylsilyl)cyclo-propane (312) is reductively lithiated with lithium l-(dimethylamino)ntq)hthalenide (LDMAN) followed by addition of an aldehyde to form the 3-silylcarbinol (314). This method of anion formation is general but has seen greatest application in the synthesis of cyclopropyl compounds. The intermediate can be eliminated in situ with KOBu to form the alkene (315). Yields are good but, as discussed in the mechanistic section, in unsymmetrical cases a mixture of products results. This reaction has been extended by Halton and Stang to the synthesis of cycloproparenes. ... 

The a-silylcarbinol rearrangement, discussed in Section IV,B, was discovered by Brook and his students as early as 1958, but was not recognized as an anionic rearrangement until later (5). 

This reaction provides a new way of generating Si—C bonds and is useful in the synthesis of silylcarbinols which are otherwise difficult to prepare. 

Long before the silyl Wittig rearrangement was discovered, the base-catalyzed rearrangement of a-silylcarbinols to alkoxysilanes was known. This reaction, carefully studied by Brook and his students (4-6) takes place in exactly the opposite direction to the silyl Wittig reaction. In the... 

The proposed mechanism, involving a bridged transition state, is shown in Scheme 1. Inversion at carbon is also found for the inverse silylcarbinol to alkoxysilane rearrangement, and a closely similar mechanism has been suggested (4). 

Brook had set the basis for developing the reaction that came to bear his name, but clearly many mechanistic details remained to be elucidated. In addition, Brook did not anticipate the potential utility of the rearrangement, in fact concluding the 1959 paper with what turned out to be wholly unwarranted self-deprecation It is doubtful whether the rearrangement reported here will have much synthetic application since in general silyl ethers are much more readily prepared than are the corresponding a-silylcarbinols. ... 

These careful studies supported the mechanism shown for 8 —> 12. The first order kinetics in silylcarbinol were consistent with intramolecular C - O silyl migration. Reaction via 10 as a cyclic transition state, with the conjugate acid of the base remaining nearby, would account for the large negative entropy of activation. The Hammett studies indicated a transition state with buildup of carbanionic character in proceeding toward product. 

The retro-1,2-Brook rearrangement was used in a stereocontrolled synthesis of bis-silylated enals and enones (e.g., 152). Deprotonation of propargyl silyl ether 150 led to cr-silylcarbinol 151 via the West rearrangement. The product was best isolated upon cold quench with trifluoroacetic acid. The bis C-silylated propargyl alcohol 151 was the substrate for a stereoselective Lewis acid-catalyzed conversion to enal 152. ... 

Brook, A.G. (1986) One thing leads to another-from silylcarbinols to silaethenes. /. Organomet. Chem., 300, 21. 

Apart from problems of silicon-carbon bond cleavage, a second difficulty limiting the utility of this synthesis is that of obtaining the appropriate silylcarbinol. Whereas a-hydroxysilanes can be isolated from the reaction of silylmetallic reagents with aliphatic aldehydes 33,34), the initial adducts from reaction with aromatic aldehydes such as benzaldehyde rearrange to the isomeric silyl ethers too rapidly for any silylcarbinol to be isolated 33, 35). The mechanism of this reaction, summarized below, has been investigated in detail 30). 

In addition, since trialkylsilyllithium reagents have not been accessible 36) and arylsilyllithium reagents are not always easily prepared, this represents a further restriction to the variety of silylcarbinols readily available. 

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