Grubbs’ reagent

Methylenation.1 When heated in THF at 60-65°, this reagent effects methy-lenation of ketones, even readily enolizable ones, in 60-90% yield and of aldehydes (45-60% yield). It also converts esters and lactones into enol ethers, but this reaction is generally slower. It is thus an attractive alternative to the Tebbe and Grubbs reagent. 

Tebbe reagent, Grubbs reagent, Petasis reagent, Takeda reagent and Takai reagent are... 

The mechanism for olefin metathesis is complex, and involves metal-carbene intermediates— intermediates that contain a metal-carbon double bond. The mechanism is drawn for the reaction of a terminal alkene (RCH=CH2) with Grubbs catalyst, abbreviated as Ru=CHPh, to form RCH = CHR and CH2 = CH2. To begin metathesis, Grubbs catalyst reacts with the alkene substrate to form two new metal-carbenes A and B by a two-step process addition of Ru=CHPh to the alkene to yield two different metallocyclobutanes (Step [1]), followed by elimination to form A and B (Steps [2a] and [2b]). The alkene by-products formed in this process (RCH=CHPh and PhCH=CH2) are present in only a small amount since Grubbs reagent is used catalytically. 

The mechanistic proposal can be summarized in the catalytic cycle of Scheme 2. The catalyst 5 performs so well because it undergoes reactivation by the phosphine. This is not the case with the organotitanium(IV) oxide 3 (eqs. (2a), (2b)), thus explaining the noncatalytic reactivity of the Tebbe and Grubbs reagents. 

Anon. Methylenations with Tebbe-Grubbs reagents. Nachrichten aus Chemie, Technik und Laboratorium 1986, 34, 562-565. 

Alternative precursors of titanocene-methylidene 4 are titanacydobutanes 11, known as Grubbs reagents, which are prepared by reaction of the Tebbe reagent 3 with appropriate olefins. The carbene complex 4 is highly reactive towards multiple bonds, such as those of alkenes, alkynes, and nitriles, as well as carbonyl functions [11, 40]. The reaction of 3 with a variety of olefins in the presence of a Lewis base such as pyridine affords four-membered metallacycles 11 [41]. Since thermolysis of these compounds effects the reverse reaction, the titanacydes 11 are employed as precursors of titanocene-methylidene 4 (Scheme 4.10). The mechanism of thermal cleavage of 11 to give a titanocene-methylidene has been extensively studied [42], and the temperature required for the thermal degradation of compounds 11 was found to be dependent on the substituents present. The crystalline metallacycles 11 can be handled in air for a reasonable period of time and are employed under various conditions (e.g. R, R = Me 5 °C, R = Me, R = Bu 5 °C, R = H,... 

Grubbs reagents, titanacycles 50 and 51 were prepared by the reaction of Tebbe reagent with a terminal alkene in the presence of a Lewis base. When these complexes are heated, reactive titanocene methylidene 5 is regenerated and will methylenate carbonyl group (49 to 53). ... 

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