Allenylidene ruthenium catalysts

Scheme 8.3 Macrocycle syntheses with aikene metathesis allenylidene-ruthenium catalyst la.

Bruneau C (2004) Ruthenium Vinylidenes and Allenylidenes in Catalysis. 11 125-153 Bruneau C, D4rien S, Dixneuf PH (2006) Cascade and Sequential Catalytic Transformations Initiated by Ruthenium Catalysts. 19 295-326 Brutchey RL, see Fujdala KL (2005) 16 69-115... 

Among the R2C(=C) =Ru homologs promoting alkene metathesis the most recent discoveries deal vhth the allenylidene-ruthenium and related pre-catalysts. This chapter is devoted to the class of ruthenium-allenylidene metathesis precatalysts, their intramolecularly rearranged indenylidene catalysts, and their use in... 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

Allenylidene-Ruthenium Complexes as Alkene Metathesis Catalyst Precursors the First Evidence... 

Scheme 8.1 Initial pathways for the synthesis of allenylidene-ruthenium alkene metathesis catalysts.

Scheme 8.2 Transformation of allenylidene-ruthenium into active indenylidene-ruthenium catalyst.

The allenylidene-ruthenium(arene) catalyst precursors I have been used for the synthesis of macrocycles by the RCM reaction and were revealed as active as the first generation Grubbs catalyst RuCl2(=CHPh)(PCy3)2 [35], depending on the nature of the diene functional groups and macrocyde size [32] (Scheme 8.3).These macrocyde syntheses show that the allenylidene mthenium catalysts I offer functional group tolerance. 

Allenylidene-ruthenium complex Ib readily promotes the ROMP of norbornene, much faster than the precursor RuCl2(PCy3)(p-cymene) [39] (Table 8.1, entry 1). The ROMP of cyclooctene requires heating at 80 °C (5 min), however a pre-activation of the catalyst allows the polymerization to take place at room temperature. The activation consists, for example, in a preliminary heating at 80 °C or UV irradiation of the catalyst before addition of the cyclic aikene, conditions under which rearrangement into indenylidene and arene displacement take place [39] (Table 8.1, entries 2,3). The arene-free allenylidene complexes, the neutral RuCl2(=C=C=CPh2)... 

Le Gendre and Moise [50] produced an allenylidene ruthenium complex analogous to I but with a titanium(IV)-containing phosphine such as XI (Scheme 8.9). Its rearrangement into indenylidene was not observed and its catalyst activity remained moderate. 

Kinetic studies of diallyltosylamide RCM reaction monitored by NMR and UV/VIS spectroscopy showed that thermal activation of the catalyst precursors la and Ib (25-80 °C) led to the in situ formation of a new species which could not be identified but appeared to be the active catalytic species [52]. Attempts to identify this thermally generated species were made in parallel by protonation of the catalysts I. Indeed, the protonation of allenylidene-ruthenium complex la by HBF4 revealed a significant increase in catalyst activity in the RCM reaction [31,32]. The influence of the addition of triflic acid to catalyst Ib in the ROMP of cyclooctene at room temperature (Table 8.2, entries 1,3) was even more dramatic. For a cyclooctene/ruthenium ratio of 1000 the TOF of ROMP with Ib was 1 min and with Ib and Sequiv. of TfOH it reached 950min [33]. 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

C. Bruneau, Ruthenium vinylidenes and allenylidenes in catalysis in Ruthenium catalysts and fine chemistry, Topics in Organomet. Chem., C. Bruneau,... 

Fig. 3 Metathesis-active vinylidene, allenylidene, and indenylidene ruthenium catalysts...

Ring-opening metathesis polymerization (ROMP) of norbornene by a cationic ruthenium allenylidene pre-catalyst was carried out in a biphasic medium employing [G4GiGiIm]PF6 (Scheme 33). ... 

The indenylidene-ruthenium complexes were shown to be the actual alkene metathesis catalysts arising from the addition of propargylic alcohols [15-18]. The Dixneuf group [19, 20] later revealed that the intramolecular rearrangement of allenylidene-ruthenium complexes into indenylidene-ruthenium complexes was... 

From these studies, it was demonstrated that the alkene metathesis activity was not due to the allenylidene precursor, but due to the indenylidene ruthenium catalyst 6, which has a structure analogous to the Grubbs I catalyst [15, 17]. Both complexes generate the same RuCl2(=CFl2) intermediate upon reaction with a terminal alkene. 

Allenylidene-, indenylidene-, and alkenyl carbene-ruthenium catalysts... 

As early as 1998 it was revealed that allenylidene-ruthenium complexes eould behave as alkene metathesis precursors (Scheme 18) [10]. They are easy to prepare from simple propargyUc alcohols and constitute the first well-defined ionic 18-electron catalj precursors with respect to the neutral 16-electron Grubbs or Hoveyda catalysts [42]. 

The allenylidene-ruthenium system also appeared as effieient catalyst precursors for enyne metathesis and they were applied to the preparation of fluorinated cyclic amino esters with a 1,3-diene structure allowing Diels-Alder reactions (Scheme 20) [43],... 

Previous studies on allenylidene-ruthenium complexes as alkene metathesis catalysts revealed that on thermal reaetion they produced a new active species that was also evidenced by kinetic studies and spectroscopic observations [44]. This species was identified arising from another observation the profitable influence of strong acid addition [11], Thus the RCM of A,A-diallyltosylamide led to a TOF of 10.5/h with complex 7a (80 C, 3 h, 70 %) and to a TOF of 53/h (room temperature, 1 h, 75 %) when five equivalent of TfOH or HBF4 were added to complex 7a. More drammatically, the ROMP of eyclooctene with 7a was achieved at room temperature in 15 h (TOF = 63/h), whereas only 1 min was necessary when five equivalent of TfOH were added to 7a (TOF=57.200/h) (Table 5). It was clear that strong acid addition promoted the generation of a new very active species. 

Another common class of metal-carbene complexes is that of the vinylidene complexes (whose structure compares to that of allenes), isomers of metal-alkyne complexes. A well-known example is the vinylidene complex [Ru(PPh3)2Cl2( =C=CHPh)], the first unimolecular ruthenium catalyst of olefin metathesis discovered by Grubbs in 1992. - This class is extended to the allenylidenes and cumulenylidenes. ... 

PropargyUc alcohol 124 was fragmented into the alkene 128 and CO in the presence of a ruthenium catalyst (Scheme 7.47) [66]. Oxidative addition of the C-H bond to ruthenium affords alkynylruthenium 125, which is converted into the allenylidene complex 126 with elimination of hydroxide anion. The hydroxide then adds to the allenylidene carbon directly connected to the ruthenium center, leading to the formation of acylruthenium 127 via tautomerization. Subsequent migratory deinsertion of CO followed by reductive elimination gives 128. 

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