In situ generation complexes

An excellent level of regio- and enantiocontrol in the alkylation of cinnamyl-like systems has been achieved by using the dipyridyl ligand 32 (Eq. 8E.30). The in situ generated complex... 

The corresponding hydroazidation of olefins 351 proceeded best by use of an in situ-generated complex from 6 mol% Co(BF4)2 6H20 and salicylidene... 

The same conversion is successfully catalyzed by using in-situ generated complexes of Ti(OPr )4 and tridentate Schiff bases (Stmcture 54), which are derived from substituted salicylaldehydes with chiral aminoalcohols [85]. Another similar chiral reagent is derived from reaction of titanium tetraisopropoxide and the Schiff base of 3,5-di-tert-butylsalicylaldehyde and (5)-valinol. The mechanism and stereoselectivity of these chiral Lewis acids are discussed by Corey and co-workers. Other chiral Ti Schiff base complexes have been employed in asymmetric TMSCN addition to benzaldehyde [85]. 

Scheme 5.6.8 Cascade employing an in situ generated complex vinyl stannane...

For the conversion of more challenging aryl chlorides, apincer-type nickel(ll) complex proved superior (Scheme 10.14) [38]. However, both the in situ generated complex and the air-stable well-defined nickel(ll) carbene complex 45 required 3 equiv B114NI as additive. Additionally, the protocol proved only highly effective for the conversion of electron-poor aryl chlorides. 

Despite the considerable synthetic potential of this reaction, only two NHC-con-taining catalysts have been reported to date. The in situ generated complex from the reaction between [RhCl(COD)]2 and a NHC-silver ligand transfer reagent was active in the arylation of an A-phosphinoyl aldimine with phenylboronic acid, but... 

Despite the considerable synthetic potential of this reaction, only two NHC-containing catalysts have been reported to date. The in situ generated complex from the reaction between [RhCl(COD)]2 and the NHC-silver ligand transfer reagent 20 was found to be active in the arylation of an At-phosphi-noyl aldimine with phenylboronic acid but unfortunately only one catalytic test was performed [eqn (8.9)]. In a more systematic study, Suzuki and Sato screened various azolium salts, among which the system [RhCl(COD)]2/ lAd-HCl proved to be the most active catalyst for the arylation of a series of A -sulfonyl and A -phosphinoyl arylimines. ... 

Alkenylcarbene complexes react with in situ-generated iodomethyllithium or dibromomethyllithium, at low temperature, to produce cydopropylcarbene complexes in a formal [2C+1S] cycloaddition reaction. This reaction is highly diastereoselective and the use of chiral alkenylcarbene complexes derived from (-)-8-phenylmenthol has allowed the enantioselective synthesis of highly interesting 1,2-disubstituted and 1,2,3-trisubstituted cyclopropane derivatives [31] (Scheme 9). As in the precedent example, this reaction is supposed to proceed through an initial 1,4-addition of the corresponding halomethyllithium derivative to the alkenylcarbene complex, followed by a spontaneous y-elimi-nation of lithium halide to produce the final cydopropylcarbene complexes. 

For cyclopropanation of enol ethers with in situ-generated acyloxycarbene complexes of chromium and in the absence of CO, see reference [10a]... 

Attempts to prepare related hpp complexes of tantalum were only partially successful. Reaction of 2 equivalents of either (hpplSiMes or in situ generated Li[hpp] with Tads afforded a solid that analyzed correctly for the compound (hpp)2TaCl3. X-ray structural analysis of the crystallized product identified the coordination isomer [Ta(hpp)4][TaCl6], with a distorted dodecahedral cation and a regular octahedral anion (Figure 28). °... 

Cheletropic elimination of the in situ generated phosphinidene complex 8 from the 7-phosphanorbornadiene precursor is believed to be the rate-determining step before 1,2-cycloaddition occurs to the unsaturated hydrocarbon. Without catalysts these are first-order processes that depend only on the concentration of the precursor and not on that of any substrate [47]. The configu-... 

Complexed azaphosphirenes have been used by the Streubel group to generate at 45-75 C in situ phosphinidene complex (OC)5W=PCH(SiMe3)2 (10) together with nitrilium ylid 11 [56]. 

Nolan and colleagues published in 1999 the first coupling between aryl chlorides, bromides or iodides and aryl Grignards, in which an in situ generated NHC-Pd complex served as catalyst. The reaction proceeded in most cases in excellent yields, however, very sterically hindered products formally derived from aryl halides and aryl Grignards both possessing orf/io-substituents could not be obtained [75] (Scheme 6.16). 

In situ generated Ni-IPr complexes were also active in this oxidation reaction, however higher catalyst loadings (5 mol%) and temperatures (60°C) were required to enable the reaction. A proposed mechanism for the aerobic oxidation of alcohols in presented in Scheme 10.8. 

The reaction mechanism is believed to involve in situ generation of an imi-dochlorotitanium complex (CpTi(Cl)=NR), which then undergoes a [2+2] cycloaddition. 

Cyclopropanation reactions involving ethyl diazoacetate and olefins proceed with high efficiency in aqueous media using Rh(II) carboxy-lates. Nishiyama s Ru(II) Py-box and Katsuki s Co(II) salen complexes that allow for highly enantioselective cyclopropanations in organic solvents can also be applied to aqueous cyclopropanations with similar results. In-situ generation of ethyl diazoacetate and cyclopropanation also proceeds efficiently (Eq. 3.33).135... 

The Co complexes of the o-phenylenediamine-linked dioxime Hdmg2Ph bearing a variety of monodentate ligands (halides and pseudo halides) in the axial coordination sites have been synthesized.1176 B12 model complexes [RCo(dmg2Ph)(L)]+ (R = Me, Et, Me2CH, Bz, ch L = py, H-im, or PPh3) were prepared by the oxidative addition reactions of the alkyl halide to the in situ-generated monovalent dibromo complex. 

Disilenes are generally unstable with respect to polymerization. However, if very bulky groups are attached to the silicon atoms then they can be obtained as stable compounds.54 Such disilenes react with in situ generated [PL(PR3)2] to give disilene complexes analogous to alkene complexes.50... 

Chromium(III) is a commonly-used crosslinker for preparing profile control gels with polymers having carboxylate and amide functionalities (la,b). Cr(III) is applied in many forms. For example, it can be used in the form of simple chromic salts of chloride and sulfate, or as complexed Cr(III) used in leather tanning (2), or as in situ generated Cr(III) from the redox reaction of dichromate and bisulfite or thiourea. The gelation rate and gel quality depend on which form of Cr(III) is used. 

The activity of catalytic systems based on imidazolylidene carbenes depends on many factors, among which the most important are likely to be the electronic effects of the ligand and the parameters of complexation. Therefore, the dependence of the performance of such systems on, e.g., the choice of precatalyst is not well understood, as in the following example (Equation (34)) in which two similar ligands behave in exactly the opposite way in the systems based on the presynthesized complex or in situ generation of the catalyst 454... 

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