Grubbs reagent / catalyst

Green polymer, 31 Grignard reagents, 467, 484 Grubbs-type carbenes, 433 Grubbs-type catalysts, 438... 

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. 

Cross-metathesis reactions. The valuable cinnamylation reagent and other homologous allylation reagents are readily accessible from cross-metathesis using the Grubbs II catalyst ... 

Barrett and co-workers adapted ring-opening metathesis polymerization (ROMP) to prepare the ROMPgel TosMlC reagent 1621 (Scheme 1.414, p. 364). Thus copolymerization of the norbornene-derived formamide 1620 with norbor-nene in the presence of Grubbs s catalyst" gave 1621 in quantitative yield after dehydration. Aromatic aldehydes react with 1621 in the presence of a strong... 

Scheme 33 Synthesis of enf-phyllanthidine (7) by Kerr. Reagents and conditions (a) YbfOTOs, toluene, A, 86% (b) LiCI, DMSO, H20,160 °C, 85% (c) KHMDS, Davis oxaziridine, THF, 78 °C, 80% (d) LiAIH4, THF, 0 °C (e) IBX, DMSO (0 vinylmagnesium bromide, THF (g) IBX, DMSO, 20% (four steps) (h) Grubbs II catalyst 130, CHjClj, A, 74% (i) (Et0)2P0CH2C02H, DCC, CH2CI2, 71% 0) K2CO3, 18-crown-6, 100% (k) DDa CH2CI2, 98% (I) PPh3, DIAD, toluene, 98%.

Scheme 15 Beak s formal synthesis of (+)-lentiginosine (127) and (-)-2-ep/-lentigino-sine (128)J° Reagents (a) BHa-THF (b) Swern oxidation (c) Wittig reaction (d) TFA, CH2CI2, rt, 2 h (e) Fl2C=CFICOCI, K2CO3, TFIF, reflux, 24 h, (f) Grubbs II catalyst (6), CH2CI2, reflux, 24 h.

Scheme 17 Synthesis of (—flentiginosine (enM27) by Genisson and coworkers." Reagents and conditions (a) H2SO4 (3 M, 8 equiv.), dioxane, reflux (b) H2C=CH(CH2)20Tf (1.2 equiv.), proton sponge, CH2CI2, rt (c) Grubbs II catalyst (6) (8 mol%), PhMe, 70 °C (d) H2 (12 bar), 10% Pd/C, MeOH, HCI (12 M, cat.), (e) PPhs, CCI4, NEts, DMF, rt.

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