Hoveyda

Venturello catalyst 197 Hoveyda 243 hydrazine 469 hydrazone, unsaturated 46 hydroboration 48 hydrogen shift 55 hydrogenation... 

We will focus on the development of ruthenium-based metathesis precatalysts with enhanced activity and applications to the metathesis of alkenes with nonstandard electronic properties. In the class of molybdenum complexes [7a,g,h] recent research was mainly directed to the development of homochi-ral precatalysts for enantioselective olefin metathesis. This aspect has recently been covered by Schrock and Hoveyda in a short review and will not be discussed here [8h]. In addition, several important special topics have recently been addressed by excellent reviews, e.g., the synthesis of medium-sized rings by RCM [8a], applications of olefin metathesis to carbohydrate chemistry [8b], cross metathesis [8c,d],enyne metathesis [8e,f], ring-rearrangement metathesis [8g], enantioselective metathesis [8h], and applications of metathesis in polymer chemistry (ADMET,ROMP) [8i,j]. Application of olefin metathesis to the total synthesis of complex natural products is covered in the contribution by Mulzer et al. in this volume. 

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

Scheme 16 Hoveyda s (left, [49a]) and Blechert s (right, [49b]) synthesis of phosphine-free complex 71a...

The unique power of Hoveyda s recyclable ruthenium catalyst D in RCM with electron-deficient and sterically demanding substrates is illustrated in Honda s total synthesis of the simple marine lactone (-)-malyngolide (54), which contains a chiral quaternary carbon center (Scheme 10) [35]. Attempted RCM of diene 52 with 5 mol% of NHC catalyst C for 15 h produced the desired... 

Scheme 10 The power of Hoveyda s catalyst D in total synthesis of malyngolide (54) [35]...

A different task was pursued by the CM of CsA with various maleates 339 [ 148]. The CM demanded in this case the highly active Hoveyda catalyst D, that exhibits potency not reached by the phosphine-containing catalysts C and E. Under the conditions given in Scheme 65, metathesis with maleates 339 led (E)-selectively to the a,/J-unsaturated ester derivatives 340 in high yield. Compounds 340 still demonstrated activity comparable to that of CsA and are thus potential soft drugs via esterase-mediated biotransformation to the corresponding inactive carboxylic acids 341. 

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

---