Lead-Peterson reaction

Scheme 2.152. The lead-Peterson reaction of an a,a-diplumbyl carbanion with benzaldehyde.

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. 

Peterson reactions. Reaction of Li in THF with 1 leads to the 1,4-dianion (2) of l,l,4,4-tetrakis(trimethylsilyl)butane in 96% yield. This dianion reacts with paraformaldehyde to form 1,5-diene 3, which can be converted to 4 by bromination-bromodesilylation. 

As illustrated in Scheme 63, the intermediate 3-silyl alkoxide (138) is subject to either protonation to give the corresponding 3-silyl alcohol (139), or elimination of RsSiO to generate the alkene (140). The pathway leading to the alkene is known as the carbonyl alkenation reaction, or the Peterson reaction. ... 

An interesting variant of the Peterson reaction involves the addition of an a-silylbenzylic anion (173) to a carbon-nitrogen double bond such as an imine. In this case the loss of RN—SiMes occurs less readily than the loss of alkoxytrialkylsilane (O—SiRs). The initial addition reaction is reversible and the stereochemistry of the elimination reaction is predominantly trans (Scheme 76). Both acid- and base-catalyzed elimination reactions lead to the trans product (174). 

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]... 

The vinylic and acetylenic analogues of pyridoxal 5 -phosphatc are known. The former can be prepared as a cis-trans mixture by Peterson reaction between a ,3-O-isopropylideneisopyridoxal and diethyl l-lithio-l-(trimethylsilyl)methylphosphonate. Isolation of the cis derivative and subsequent deprotection of the alcohol followed by oxidation with MnO2 in ("HCl, leads to the diethyl... 

AlkenylsUanes. Bis(trimethylsilyl)methyllithium is formed by Cl/Li exchange from reaction of (Me3Si)2CHCl with i -BuLi. A Peterson reaction leads to ( )-alkenylsilanes. ... 

Stepwise alkylation of the diester 364, obtained conventionally using the Michaelis-Arbuzov reaction, leads to 365, from which the 0-(2-trimethylsilylethyl) protecting group may be removed with HF in MeCN. A Peterson reaction on the deprotected alcohol 3 (R = prenyl) results in the formation of the unsaturated phosphonic diester 367(R = prenyl) ... 

Compounds belonging to the 2-aryl-4-benzyltetrahydrofuran series, including dihydrosesamin (74), have been prepared by reduction of the corresponding 2-arylidene lactones (scheme 23) [76]. The starting materials are the cis and trans paraconic acid derivatives (68) and (72). These are silylated at C-2 and subjected to a Peterson reaction to give the unsaturated lactones (70) and (73). The stereochemistry at C-4 in (70) and (73) determines the outcome of the hydrogenation step leading to (71) and (74) respectively. 

In Chapter 17 you saw that anti-periplanar transition states are usually preferred for elimination reactions because this alignment provides the best opportunity for good overlap between the orbitals involved. Syn-periplanar transition states can, however, also lead to elimination—and the base-promoted Peterson reaction should remind you of the Wittig reaction, which you first met in Chapter 11, with its four-membered cyclic intermediate. It is with the Wittig reaction, and a detailed discussion of its stereoselectivity, that we now finish this chapter. 

Peterson reactions of a-sUyl carbanions and carbonyl compounds leading directly to the alkenes are generally not stereoselective since the j8-hydroxyalkylsilane or yff-silylalkoxide intermediates are usually formed as a mixture of syn and anti isomers, whereas Peterson elimination from a j8-hydroxyalkylsilane proceeds in an exclusively stereospecific manner. Furthermore, the addition of an a-silyl carbanion to a carbonyl compound proceeds in an irreversible manner [29, 30]. Therefore, when the 8-hydroxyalkylsilane intermediate can neither be isolated nor separated, the diastereomeric ratio of the alkene products of the Peterson reaction is determined in the addition step of the a-silyl carbanion and the carbonyl compound. Stereospecific preparation of the y8-hydroxyalkylsilane is required for the utilization of the Peterson reaction in organic synthesis. 

Intramolecular versions of the Peterson reaction of a-sUylbenzyl anions are utilized to construct heteroaromatic frameworks [96]. Lithiation of the ortho-substituted benzylsilanes 55 with LDA and subsequent intramolecular Peterson reaction of the derived carbanions 56 with the carbonyl moiety leads directly to intramolecular cydization giving indole derivatives 57 (Scheme 2.37) [97]. 

The Peterson reactions of a-sUylbenzyl carbamates provide a method for the preparation of aromatic alkenyl carbamates. Addition of carbonyl compounds to the carbanions derived from the a-sUylbenzyl carbamates 64 with Bu Li leads directly to the corresponding alkenyl carbamates 65 with Z-selectivity (Scheme 2.41)... 

This reaction is also used for substrates in which the nitrogen atom of the bis(trimethylsilyl)methylamine moiety forms part of a ring system. Fluoride ion catalyzed Peterson reactions of the bis(trimethylsilyl)methyl-substituted benzo-1,2,3-triazoles 166 with carbonyl compounds give the N-l-alkenyl derivatives 167 with low stereoselectivity (Scheme 2.105) [292, 293]. The reaction of a 1,2,4-triazole derivative leads to a similar result [294]. 

The protodesilylation should take place via a postulated 1,2-oxasiletanide transition state, which leads to an alkene in the absence of any proton source. Protonation of the isolated pentacoordinate 1,2-oxasiletanides 7b actually gives the desi-lylated alcohol 206 after hydrolysis in competition with the Peterson reaction (Scheme 2.131) [25, 28] (see Section 2.2.1.2). The decisive factors with regard to the... 

The Germanium, Tin, and Lead Versions of the Peterson Reaction 2.5.5.1 The Cermanium-Peterson Reaction... 

Lead-Peterson olefination is achieved similarly to the germanium and tin versions, though the lead version is not advantageous over the latter reactions in most cases. A much simpler and more gentle elimination method applicable to the lead compounds is to pass the solution of the bis(triphenylplumbyl)ethanol derivative 249, which is synthesized from triplumbylmethane 248 and benzaldehyde, through a silica gel column (Scheme 2.152) [399]. 

The Peterson reaction has been employed to synthesize various functional groups containing olefins. For example, Fiirstner and coworkers have developed a one-pot Akacylation/Peterson elimination procedure to furnish enamides. An attractive feature of the Peterson reaction utilised in this procedure is that basic conditions are employed for the elimination instead of acidic conditions which in this case would lead to the hydrolysis of the enamide. The reaction proceeds in good yields over the two steps (N-acylation/Peterson elimination) and all products were obtained as single diastereoisomers, thus, with complete stereoselective conversion of the fi-hydroxysilane 66/68 to the enamide product 67/69. 

The next step of the Peterson olefination allows for the control of the E Z-ratio of the alkene to be formed by proper choice of the reaction conditions. Treatment of /3-hydroxysilanes 5 with a base such as sodium hydride or potassium hydride leads to preferential -elimination to give alkene 3a as major... 

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 transition metal nature is not essential for this redox reaction. However, one of the reaction products, namely, the anion-radical 8O4 , can be complexed by a transition metal in a higher oxidation state. This leads to some stabilization of 8O4 and increases its reactive concentration. In other words, further reactions with organic substrates are facilitated (Fristad and Peterson 1984). 

---