Peterson olefination reaction

Addition of a-silylcarbanions to ketones and subsequent elimination of a silanoale or silanol afford the corresponding olefins (Peterson reaction) (eq (33)) [30]. The use of a-silylcarbanions in olefination of carbonyl compounds offers some advantages over the Wittig reaction that the method can be applied to highly enolizable ketones and both cis and trans isomers are stereoselectively formed depending on the reaction conditions employed for the elimination step (eq (34)) [31]. 

Silicon-stabilized anions can be put to various uses, among which is the formation of olefins (Peterson reaction), and the 1-phenylthio-l-trimethylsilyl-alkanes required as precursors can also be used in a synthesis of aldehydes. Some of the chemistry reported this year relating to such compounds is shown in Schemes 20 and 21. ... 

The phosphorus ylides of the Wittig reaction can be replaced by trimethylsilylmethyl-carbanions (Peterson reaction). These silylated carbanions add to carbonyl groups and can easily be eliminated with base to give olefins. The only by-products are volatile silanols. They are more easily removed than the phosphine oxides or phosphates of the more conventional Wittig or Homer reactions (D.J. Peterson, 1968). 

The Peterson reaction has two more advantages over the Wittig reaction 1. it is sometimes less vulnerable to sterical hindrance, and 2. groups, which are susceptible to nucleophilic substitution, are not attacked by silylated carbanions. The introduction of a methylene group into a sterically hindered ketone (R.K. Boeckman, Jr., 1973) and the syntheses of olefins with sulfur, selenium, silicon, or tin substituents (D. Seebach, 1973 B.T. Grdbel, 1974, 1977) illustrate useful applications. The reaction is, however, more limited and time consuming than the Wittig reaction, since metallated silicon derivatives are difficult to synthesize and their reactions are rarely stereoselective (T.H. Chan, 1974 ... 

Another method that has been used to prepare phosphaalkenes is the phos-pha-Peterson reaction, a phosphorus analog of the Peterson olefination [46-49]. In this reaction a lithium silylphosphide is treated with an aldehyde or ketone to yield the phosphaalkene (9). Analogous reactions can be conducted with bis(trimethylsilyl)phosphines (10) and ketones (11) using a catalytic quantity of anhydrous base (i.e., NaOH, KOH) [50]. Generally, the reactions proceed cleanly and in high yield. Sufficiently bulky substituents must be employed to stabilize the P=C bond and prevent rapid dimerization to 1,3-diphosphetaines. 

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

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. 

The spirocompounds 34 (M = Ti or Zr) have been prepared . Studies of the thermolysis of pentacoordinate 1,2-oxasiletanides 35, potential intermediates in both the Peterson reaction and the homo-Brook rearrangement of p-hydroxyalkylsilanes with bases, in the presence of a proton source afforded the olefin, RCH=C(CF3)2 and/or the alcohol, (CF3)2CHOH <99CL1139>. 

A stereoselective route to 2-(phenylthio)-l,3-butadienes such as 327 or 328 was developed by Pearson et al. [167] with allylboranes as crucial intermediates. Addition of 9-BBN to allenyl sulfide 324 gives the allylborane intermediate 325, which subsequently adds to aldehydes (Scheme 8.89). Typical of Peterson olefinations, this reaction can also be terminated by two different work-up procedures, either acidic conditions leading to anti-elimination, which affords Z-configuration of dienes 327, or basic work-up resulting in a syn-elimination to form (E)-dienes 328. 

The stereochemical outcome of the Peterson reaction between unsymmetrically substituted a-silyl carbanions and aldehydes or unsymmetrical ketones is determined by the relative rates of formation of the threo and erythro /3-oxidosilancs. Often the rates are similar, to give a product alkene E Z ratio of 1 1, although some workers report a predominance of cis olefins in the reactions of aldehydes. 

Carbonyl olefination.1 The reaction of 1 with benzaldehyde results in a 1 1 separable mixture of the threo- and eryfAro-adducts (2a and 2b, respectively). The adducts undergo stereospecific ypn-elimination when heated to give /i-phenyl-thiostyrene (3). The (E)-isomer (3a) is formed from 2a, and the (Z)-isomer (3b) is formed from 2b. On the other hand, anfr -elimination obtains on treatment of 2 with perchloric acid in methanol. This carbonyl olefination has one advantage over the Peterson reaction in that intermediate adducts can be isolated and converted as desired to an (E)- or a (Z)-olefin. 

A general approach to precursors of p-quinonemethides has been developed that involved the olefinations of p-quinoneketals as 615 with suitable nucleophiles. For example, the reaction of 615 with the anion derived from 613 led directly to the p-quinonemethide ketal 616 via a Peterson reaction (Scheme 57) (225). Alternatively, 616 was prepared by the initial reaction of 615 with the anion of 614 followed by mild dehydration of the intermediate tertiary alcohol... 

The Peterson Reaction allows the preparation of alkenes from a-silylcarbanions. The intermediate p-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric p-hydroxy silanes and subsequently performing two different eliminations. 

Fig. 4.43. Stereoselective HetyHet2-eliminations in the Peterson olefination. The base-induced Peterson olefination (top reaction) takes place as a syn-elimination, and the acid-catalyzed Peterson olefination takes place as an anti-elimination.

Among such migrations, the Peterson reactions to give olefins have been shown to proceed via intramolecular 1,3-silyl migrations from C to O (equation 129). 

Ketones are rarely used as electrophiles in the enantioselective aldolization while they find application to enantioselective olefination reactions such as the Horner-Wadsworth-Emmons or the Peterson reaction. For instance, the deprotonation of an achiral phos-phonoacetate by a set of chiral 2-aminoalkoxides led to the corresponding enolate that... 

Peterson olefination. This reaction usually shows little or no selectivity. In contrast, the reaction of ethyl lithio(trimethyIsilyl)acctate with cn-substituted cyclohexanones can show moderate to high (Z)-selectivity. (Z)-Sciectivity is somewhat higher in reaction with ethyl potassio(trimethylsilyl)acetate or t-butyi lithio(trimethylsilyl)acetate, but yields... 

P-Elimination of a hetero-substituted C-C single bond. The best-known reactions of this type are the heteroatom -heteroatom (Het -Het ) elimination (Het = O, Hal, S,...), heteroatom -hydrogen (Het -H ) elimination, Peterson olefination, Tschugajew reaction and Corey-Winter olefination. 

Related reactions , Horner-Wadsworth-Emmons olefination - Still-Gennari modification, Julia-Lithgoe olefination, Peterson... 

Peterson olefination, Takai-Utimoto olefination, Tebbe olefination, Wittig reaction, Wittig reaction - Schlosser modification ... 

Peterson reaction (olefination). Reaction of a-silylated carbanions with carbonyl compounds, yielding (3-hydroxyl-alkyl silanes, which undergo instantaneous elimination to afford olefins. 

Bis(phenylsulfanyl)](trimethylsilyl)methyllithium and trimethylsilyloxirane do not afford a homo-Peterson reaction product, but a cyclopropane 4 a with a shifted phenylsulfanyl group. [Bis(phenylsulfanyl)](trimethylsilyl)methyllithium may be looked on as a carbenoid species which is in equilibrium with carbene and phenylsulfonate. This equilibrium may lie towards the carbanion. On addition of trimethylsilyloxirane, phenylsulfonate is trapped with formation of an alkoxide, which corresponds to the intermediate of a Peterson olefination of formaldehyde, and leads to phenyl vinyl sulfide. This provides a reaction partner for the liberated carbene giving cw-l,2-bis(phenylsulfanyl)-l-trimethylsilylcyclopropane (4a) in a stereospecific [2-1-1]... 

Two interesting processes, typical for a-silylcarbanions, are worth mentioning. One concerns the reaction of 328 with carbonyl compounds (for example benzaldehyde), in the course of which 330 is formed. This reaction is the gas-phase analogue of the Peterson olefin synthesis (reaction 135)159. 

The stereochemistry of the Peterson reaction has been investigated. When unsymmet-rically substituted a-silylcarbanions react with aldehydes or unsymmetric ketones, E or Z olefins are produced. In many cases the E Z ratio is 1 1, however, some workers have reported a predominance of cis olefins when aldehydes are employed. Typical results are given in Table 15. Unlike the Wittig reaction the stereochemical outcome of the Peterson reaction is insensitive to counterion, solvent, added salts and temperature255. Stereochemical control of the Wittig reaction usually depends upon the reversibility of the first step. However, as discussed earlier, the first step of the Peterson reaction is irreversible. Thus the stereochemical outcome is determined solely by the relative rates of formation of threo and erythro 0-silyl alkoxides (/ct and k in Scheme 5). 

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