Precatalysts activation

A combination of the Lewis acid zinc triflate and the bases NEt, or pyridine acted as an achiral catalyst for this reaction. Instead, using a chiral base which incorporates a bipy ligand to bind zinc gave 26% ee of the product (Scheme 5-42a). Alternatively, diethylzinc was an active precatalyst, but attempts to use chiral amino alcohols as ligands in this system gave low ees (Scheme 5-42b) [31]. 

Pyridine-functionalized N-heterocyclic carbene Rh and Ir complexes have also been described as active precatalysts for C=0 bond TH. For example, Peris and coworkers observed the formation of metal hydrides by C—H oxidative addition of a pyridine-N-substituted imidazolium salt such as N-"Bu-N -(2-pyridylmethyl-imidazolium) hexafluorophosphate in the reaction leading to M-pyNHC complexes, that is [lr(cod)H(pyNHC)Cl] (58) [54]. Transmetallation from silver carbene... 

In several recent studies on organoyttrium-catalyzed reactions the methyl complex (C5Me5)2 YMe(THF) has been employed as a highly active precatalyst (cf. Sect 4.1,4.2. and 5.3). The main advantage of this material is that it is much more readily available than the corresponding bis(trimethylsilyl)methyl derivative. (C5 Me5)2 YMe(THF) can be synthesized in one pot by the reaction of anhydrous YC13 with two equivalents of Li(C5Me5) followed by addition of MeLi [27],... 

The mechanism of thermal borylation of alkanes seems to be similar to that of photochemical borylation of alkanes. The most active precatalysts for thermal alkane borylation, Cp Rh( 4-C6Me6) and Cp Rh(j/2-C2H4), are converted to mixtures of Cp Rh(Bpin)3(H) and Cp Rh(Bpin)2(H)2 in the catalytic reactions [48], Reaction of HBpin or B2pin2 with Cp Rh(Bpin)2(H)2 yields Cp Rh(Bpin)3(H). Furthermore, reaction of Cp Rh(Bpin)3(H) with alkanes and arenes as solvent yields Cp Rh(Bpin)2(H)2. Both species are kinetically capable of being catalysts of the borylation of alkanes. 

It was found that substituted cyclopentadienyltitanium trichloride, in a combination with methylaluminoxane, exhibited higher catalytic activity for syndiospecific polymerisation of styrene than CpTiCl3 [52,53]. The efficiency of half-sandwich titanocenes as methylaluminoxane-activated precatalysts for the syndiospecific polymerisation of styrene increases in the following order CpTi(OMe)3 < Me4(Me3Si)CpTi(OMe)3 < Cp Ti(OMe)3. Thus, electron-donating substituents on the cyclopentadienyl ligand lead to increased catalyst activity and stability, stereospecificity and polymer Mw. 

Catalyst Preparation Two palladium sources are generally used to form the active precatalysts (1) and (2) in situ, the allylpalladium chloride dimer [12012095-2] and the tris(dibenzylideneacetone)dipalladium(0)-chloroform adduct... 

Bis(pyridonate) zirconium complex 14, bearing sterically demanding pyridonate ligands, can be synthesized via protonolysis (Scheme 15), and is an active precatalyst for the intramolecular hydroamination of aminoalkenes to generate pyrrolidine and... 

Palladium(O) complexes bearing NHC ligands have been used as active precatalysts in the Suzuki-Miyaura reaction. Homoleptic (NHC)2Pd complexes were prepared by phosphine displacement from Pd(P(o-tol)3)2 with free carbene... 

Under related reaction conditions bis (aryUmino) pyridine ferric halides can also serve as active precatalysts for ethylene polymerization [6]. While MAO or MMAO continues to be the preferred co-catalyst/activator, a range of alternative activators have also been screened (Table 5.1). 

Among this new family of helicene-based gold complexes, the Au(I) complex bearing the phosphathiahelicene hgand Rp-P)-69 has proved to be a highly active precatalyst for the cycloisomerizations of various N-tethered 1,6-dien-ynes 70 and 72 as shown in Scheme 19 (14CEJ12373). 

In polymerization tests, 35b was activated with MAO, 36b was activated with [CPh3]+[B(C6F5)4] , and 36a was activated with [CPh3]+[B(C6F5)4] or BfC Fsls. The authors carried out these experiments to investigate whether fluorinated borane or borate activators would provide comparable stereoselectivity and productivity as compared to MAO as a cocatalyst. All three compounds were found to be active precatalysts for 1-pentene polymerization. Polymerizations were... 

Complexes containing the fragment Cp Ir (Cp is pentamethylcyclopentadienyl) are active precatalysts in the C-H oxidation of cw-decalin and cyclooctane. Ceric ammonium nitrate was a sacrificial oxidant and water was the oxygen source... 

In the meantime, we had found two additional reactions which more directly indicated the involvement of gold vinyhdene species, the formation of benzofulvenes by activation of an alkyl C-H bond by this new intermediate (Scheme 17) and the formation of dibenzopentalenes by electrophilic attack at an aryl group (Scheme 18). In both cases, the aurated and the ge/n-diaurated organometallic species could be isolated and characterized by X-ray crystal structure analyses. And it could be shown that the gem-diaurate species are ideal instant dual activation precatalysts for this new type of reaction, immediately providing an ideal 1 1 ratio of both the cationic LAu" species and the species for the catalyst transfer to the gold acetylide, two species needed for the dual activation process (Scheme 17). 

Scheme 17 The benzofulvene products more directly reflect the gold vinylidene intermediate. The em-diaurated species as an instant dual activation precatalyst shows three times the activity of other gold catalysts...

As mentioned in the phosphorus chapter, A-heterocycUc carbenes are efficient catalysts for the phospfia-Michad additions of secondary phosphine oxides to activated alkenes. Instead of using the sensitive free carbenes directly, masked carbenes were used as precatalysts for the addition reaction. These precatalysts were comprised of the carbene bound to carbon dioxide and were synthesized by simply bubbling CO through a solution of the carbene. hi solid form, these adducts were quite stable and could be handled in air. These adducts were used to promote addition reactions involving thiophenols (Scheme 5.10) [11]. After a series of screening experiments, THF was selected as the solvent for the reactions, and the most active precatalyst... 

Substitution of noble metals in catalysts for cheap and abundant materials is of obvious importance, and many works concern the preparation and study of the latter systems. Thus, cobalt(II) alkyl complexes of aliphatic PNP pincer ligands (Fig. 12) are active precatalysts for the hydrogenation of ketones and the acceptorless dehydrogenation of alcohols under mild conditions in this case the alcohol dehydrogenation likely proceeds through a cobalt(I)/(III) redox cycle. ... 

Trimerization of alkynes to form aromatic compounds has been achieved first by Reppe and Schweckendiek using nickelcarbonyl complexes as catalysts. The applicability of carbonyliron complexes for this purpose was reported as early as 1960. Unsymmetric acetylenes give exclusively the arenes with symmetric substitution pattern under these conditions (Scheme 4-309). Alternatively, (Ti -arene)iron complexes such as bis(ethene)(toluene)iron can be used for the cyclotrimerization of unsymmetric alkynes. A variety of symmetrically substituted benzene derivatives has been obtained. Another very active precatalyst for this reaction is (Ti -cyclohepta-l,3,5-triene)(T -cycloocta-l, 5-diene)iron(0). ... 

Note that the double bonds of the ligand 1,5-COD in 5.51 are notably resistant toward hydrosilylation. From the reaction of 5.51 and HSiMeCl, complex 5.52 has been isolated. This complex has been shown to be an active precatalyst for the hydrosilylation reaction (5.5.4). 

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