Enolate compounds carbonyl allylation

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

A very extensive and detailed study of the cationic rhodium(i)-catalyzed isomerization of allylic alcohols demonstrated that mono- and disubstituted allylic alcohols can be efficiently isomerized to the corresponding carbonyl compounds through the corresponding enol compounds (Scheme 20).45 The isomerization using cationic rhodium(l)... 

Asymmetric Coiyugate Addition of Allyl- and Crotylphosphonamides/ The asymmetric C-allylation of a, 3-unsaturated carbonyl compounds is a powerful tool for the functionalization of a carbonyl compound in the P-position. Since such a process normally leads to the corresponding enolate derivative when anionic reagents are used, there exists the possibility of trapping with an electrophile. Thus sequential addition and trapping can lead to vicinally substituted carbonyl compounds. Asymmetric allylation has been achieved previously with simple cycloalkenones using phosphorus and sulfur based reagents that must be prepared in diastereomerically pure form. 

Thermal [1,3] H shifts such as the conceited rearrangement of enols to carbonyl compounds are disallowed. The allylic C-C-O unit itself can only react suprafa-cially, as it is geometrically impossible for the H(ls) orbital to bond simultaneously to a top lobe on one terminus and a bottom lobe at the other terminus, and the H atom itself must also react suprafacially, as the H(ls) orbital has only one lobe. The Woodward-Hoffmann rules, though, say that one of the two components of this four-electron rearrangement must react antarafacially for it to be allowed. Therefore this rearrangement reaction always proceeds through a nonconcerted mechanism and requires acidic or basic catalysis. 

Since the resulting silyl enol ethers and allylic silyl ethers can readily be converted by hydrolysis to saturated carbonyl compounds and allylic alcohols, respectively, these... 

Since the resulting silyl enol ethers and allylic silyl ethers can readily be converted by hydrolysis to saturated carbonyl compounds and allylic alcohols, respectively, these reactions furnish a unique method for the selective reduction of a,0-unsaturated carbonyl compounds125,126,128. a,0-Unsaturated carbonyl compounds containing an isolated double bond in the same molecule also undergo the selective reduction without isomerization or reduction at the isolated double bond125,126,128 (see, e.g., equations 68 and 69). 

Cross-coupling of silyl enol ethers and allylic silanes. The formation of y,S-unsaturated carbonyl compounds involves one-electron oxidation. 

Finally, a heterocyclic derivative of tartaric acid should be mentioned, w hich has been used for the construction of boron enolates useful for allyl additions to carbonyl compounds (Section D. 1,3.3.3.). The eight-membered ring is formed from 2,3-O-benzylidenetartaric acid and lV,./V -dibenzyl-l,2-ethanediamine with the help of. V-methyl-2-chloropyridinium iodide as the condensing agent43. 

Cyclohexadienes. The reagent reacts with enolates of carbonyl compounds in THF (25°, 6-18 hr.) to form 1,3-cyclohexadienes in moderate yields (35-60%). It may react in the i-c -form to give a cw-allyl ylide, which... 

Titanium tetrachloride is a moisture-sensitive, highly flammable liquid reacting violently with water (34). It is a strong Lewis acid capable of promoting Diels-Alder reactions (35) and induces the addition of silyl enol ethers and allyl silanes to carbonyl compounds and derivatives (34r-36). It is a less commonly used catalyst in Friedel-Crafts reactions but very useful for the acylation of activated alkenes and in the Fries rearrangement. 

Although it is mechanistically different from the Tsuji-Trost allylation, indirect allyla-tions of ketones, aldehydes, and esters via their enolates are briefly summarized here. Related reactions are treated in Sect V.2.1.4. Pd-catalyzed allylation of aldehydes, ketones, and esters with aUyhc carbonates is possible via the Tr-allylpaUadium enolates of these carbonyl compounds. Tr-AUylpalladium enolates can be generated by the treatment of silyl and stannyl enol ethers of carbonyl compounds with allyl carbonates, and the allylated products are obtained by the reductive elimination of the Tr-allylpalladium enolates. 

Allyl anion synthons A and C, bearing one or two electronegative hetero-substituents in the y-position are widely used for the combination of the homoenolate (or / -enolate) moiety B or D with carbonyl compounds by means of allylmetal reagents 1 or 4, since hydrolysis of the addition products 2 or 5 leads to 4-hydroxy-substituted aldehydes or ketones 3, or carboxylic acids, respectively. At present, 1-hetero-substituted allylmetal reagents of type 1, rather than 4, offer the widest opportunity for the variation of the substitution pattern and for the control of the different levels of stereoselectivity. The resulting aldehydes of type 3 (R1 = H) are easily oxidized to form carboxylic acids 6 (or their derivatives). 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Recently, Grubbs138 demonstrated that olefin isomerization of allyl-lic ethers and alcohols is catalyzed by Ru(II)(H20)6(tos)2 (tos = p-toluenesulfonate) in aqueous medium. The olefin migration products, enols, and enol ethers thus generated are unstable and are hydrolyzed instantly to yield the corresponding carbonyl compounds (Eq. 3.34). 

The isomerization of allylic alcohols provides an enol (or enolate) intermediate, which tautomerizes to afford the saturated carbonyl compound (Equation (8)). The isomerization of allylic alcohols to saturated carbonyl compounds is a useful synthetic process with high atom economy, which eliminates conventional two-step sequential oxidation and reduction.25,26 A catalytic one-step transformation, which is equivalent to an internal reduction/oxidation process, is a conceptually attractive strategy due to easy access to allylic alcohols.27-29 A variety of transition metal complexes have been employed for the isomerization of allylic alcohols, as shown below. 

The mechanistic study revealed that the isomerization of allylic alcohols to the corresponding enols proceeds through a 7r-allyl intermediate (Scheme 21), and the isomerization of the enols to the corresponding carbonyl compounds proceeds through an oxy-7r-allyl intermediate (Scheme 22).45... 

The prime functional group for constructing C-C bonds may be the carbonyl group, functioning as either an electrophile (Eq. 1) or via its enolate derivative as a nucleophile (Eqs. 2 and 3). The objective of this chapter is to survey the issue of asymmetric inductions involving the reaction between enolates derived from carbonyl compounds and alkyl halide electrophiles. The addition of a nucleophile toward a carbonyl group, especially in the catalytic manner, is presented as well. Asymmetric aldol reactions and the related allylation reactions (Eq. 3) are the topics of Chapter 3. Reduction of carbonyl groups is discussed in Chapter 4. 

It is only since the early 1980s that significant progress has been made with aldol reactions. This chapter introduces some of the most important developments on the addition of metallic enolates and the more important of the related allylic metal derivatives to carbonyl compounds. These processes are depicted as paths A and B in Scheme 3-1. 

Besides the allylation reactions, imines can also undergo enol silyl ether addition as with carbonyl compounds. Carbon-carbon bond formation involving the addition of resonance-stabilized nucleophiles such as enols and enolates or enol ethers to iminium salt or imine can be referred to as a Mannich reaction, and this is one of the most important classes of reactions in organic synthesis.104... 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

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