Aldehydes Tebbe reagent

Titanacyclobutenes, prepared readily from Tebbe reagent and alkynes, react with aldehydes and ketones to form insertion products which undergo facile retro-Diels-Alder reaction to afford substituted 1,3-dienes (equation 107)185. 

CPv. / Ti Al Tebbe reagent methylenation of aldehydes, ketones, esters, thioesters, amides, carbonates can also induce olefin metathesis in situ preparation [711-713]... 

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

McMurry coupling, Tebbe reagent, Petasis reagent and Takeda reagent) and catalytic alke-nation of aldehydes and ketones plus the related reactions in each case. 

The Tebbe reagent (4.85) converts aldehydes and ketones to alkenes. The reaction of the Tebbe reagent (4.85) with cyclohexanones in toluene produces the corresponding methylenecyclohexanes in >65% yields. For example (4-methylenecyclohex-l-yl)benzene (4.88) was prepared jfrom 4-phenylcyclohexanone (4.87) in 96% crude yield on reaction with the Tebbe reagent (4.85). ... 

A reaction that appears to be mechanistically similar to the Tebbe reaction was developed by Oshima in 1978. Diiodomethane or dibromomethane in the presence of zinc is treated with a Lewis acid to form, presumably, a divalent complex (72), which reacts with aldehydes and ketones to produce the corresponding methylene derivative (73 Scheme 18). This reagent complements the reactivity of the Tebbe reagent, in that the zinc methylenation is not reactive towards esters or lactones. Because it is an electrophilic reagent, it is suitable for the methylenation of enolizable ketones and aldehydes. 

The synthesis of dihydrofurans with an additional ester moiety and one or two quaternary centers are prepared from titanium enolates formed by reactions of 3,4-dienoates with Cp2TiCl2. Titanium enolate derivatives TiCl3[CH2CH2C(=0)0Et] and the Tebbe reagent compound have been applied in the synthesis of pumilio-toxin.1915 Chiral allyl and mono-Gp chloro enolato titanium compounds add with high enantioface discrimination to aldehyde.973... 

The Petasis reagent reacts with aldehydes, ketones, and esters to afford terminal olefins. In addition, tliis reagent enjoys greater air stability than the Tebbe reagent. 

To a solution of the aldehyde (2.56 g, 4.40 mmol) in THF (50 mL) cooled to 0 °C was added the Tebbe reagent (0.5 M, 13.2 mL, 6.60 mmol). After being stirred at 0 °C for 30 min, the reaction mixture was treated with 0.1 M aqueous NaOH (30 mL) and diluted with ether. The resulting mixture was stirred at room temperature for 30 min, filtered through a pad of Celite, and the filtrate was concentrated under reduced pressure. The residue was purified by column chromatography (silica gel, 10% EtOAc/hexanes) to give 2.30 g (90%) of the terminal olefin as a colorless oil. 

Other titanium-based olefination reagents have been developed. Eisch used a zinc analog of the Tebbe reagent (688) in a reaction with benzophenone to give 1,1-diphenylethene in 78% yield. Similarly, Clawson et al. used 689 in olefination reactions with ketones and aldehydes. Alkoxytitanium reagents such as 690 have been employed, as in the conversion of cyclohexane carboxaldehyde to 1-cyclohexyl-1,3-butadiene (691), in 86% yield.In this olefination reaction, the (Z)-isomer predominated over the ( ) (96 4 Z/E). 

The necessary 2-(ethylthio)allyl silyl ether 414 is prepared in good yield from 402 by conversion to thioester 413 followed by treatment with Tebbe reagent. Reaction of 414 with a variety of aldehydes in the presence of Me2AlCl produces adducts 415 in high yield. To... 

Dienes Reaction of the Tebbe reagent with 2-butyne provides the dimethylti-tanacyclobutene 1. Aldehydes react with 1 to provide a complex (a) formed by insertion into the titanium-vinyl bond. This complex is unstable and at 60° decomposes to a 1,3-diene (2) and Cp2Ti=0. Ketones react with 1 to give two insertion products. One is formed by insertion into the titanium-vinyl bond and corresponds to (a) the other (b) is formed by insertion into the titanium-alkyl bond. This latter complex does not decompose into a diene when heated gently. The trend to formation of the undesired... 

A number of hexopyranose epoxides have been treated with UBr to furnish ring-contracted dihydrofuran aldehydes. Epoxides (58) and (59) afforded unsaturated aldehydes (60) and (61), respectively. Conjugate addition of LiMeCuCN to aldehyde (61) with m situ trapping of the enolate produced the enol silyl ethers (62). The branched nitro compound (63) has been converted separately into the aldehyde (64) and the thioglycoside (65). Whereas previous applications of the Tebbe reagent have used specially purified material, apparently the crude reagent system can be employed on sugar esters and lactones CHoOR... 

A few years later, Tebbe and co-workers found that the methylene-bridged metallacycle 3, which has become known as the Tebbe reagent, is useful for the methylenation of ketones and aldehydes [5]. Titanocene-methylidene 4, the active species of this olefination, also transforms carboxylic acid derivatives into heteroatom-substituted olefins. Because the carbene complex 4 is much less basic than conventional olefination reagents such as phosphorus ylides, it can be employed for the olefination of carbonyl compounds possessing highly acidic a-protons or of highly hindered ketones, and has become an indispensable tool in organic synthesis. Various methods for the preparation of titaniumcarbonyl olefination. This chapter focuses on the use of metal-carbene complexes and some related species in carbonyl olefination (Scheme 4.2). 

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