Chelating-assisted

A chelation-assisted Pd-catalyzed Cope rearrangement was proposed in the reaction of phenanthroline to generate isoquinolinone derivatives (Eq. 12.78).177 The use of aqueous media and ligands enables a double-Heck reaction on a substrate favoring alkene insertion over (J>-hydride elimination. 

Other Group 10 metal complexes, such as Pd2(dba)3 and Pt(cod)2, were tested for chelate-assisted oxidative addition of 55. Use of Pd2(dba)3 in the reaction of 55... 

The groups of Loupy and Jun have presented a chelation-assisted rhodium(I)-cata-lyzed ortho-alkylation of aromatic imines with alkenes (Scheme 6.57) [119]. The use of 2 mol% of Wilkinson s catalyst, RhCl(PPh3)3, and 5 equivalents of the corresponding alkene under solvent-free conditions proved to be optimal, providing the desired ortho-alkylated ketones in high yields after acidic hydrolysis. Somewhat lower yields were obtained when the imine preparation and the ortho-alkylation were realized in a one-pot procedure. 

Structural studies on the nature of the organometallic intermediates following chelation-assisted CH additions of pincer iridium complexes have been carried out. The product was found to have an unexpected /ram-disposition of the hydride with respect to the metallated aromatic group. This is not the expected direct outcome of a chelation-assisted reaction since coordination of oxygen to iridium prior to C-H activation would be expected to afford the m-isorner (Equation (97)). 

A mechanism has been proposed for this, and related transformations, involving a chelation assisted C-H bond functionalization. Following hydride addition to the solvent, acetone, and a transmetallation reaction, reductive elimination yields the ketimine. Hydrolysis of the latter affords the ketone (Equation (131)).114 114a... 

Hydroamidations have been reported involving the ruthenium-catalyzed chelation assisted activation of formamide (Equation (133)).116... 

The alkylation of the G-H bonds occurs even in the CH=N moiety of five-membered heteroaromatic compounds such as thiazoles, benzimidazoles, and oxazoles (Equation (25)).26,26a This intermolecular alkylation without chelation assistance is very interesting. 

The alkylation of the sp3 C-H bonds adjacent to a heteroatom becomes more practical when the chelation assistance exists in the reaction system. The ruthenium-catalyzed alkylation of the sp3, C-H bond occurs in the reaction of benzyl(3-methylpyridin-2-yl)amine with 1-hexene (Equation (30)).35 The coordination of the pyridine nitrogen to the ruthenium complex assists the C-H bond cleavage. The ruthenium-catalyzed alkylation is much improved by use of 2-propanol as a solvent 36 The reaction of 2-(2-pyrrolidyl)pyridine with ethene affords the double alkylation product (Equation (31)). 

The carbonylation of the sp3 C-H bond adjacent to a nitrogen atom is also possible by means of chelation-assisted C-H bond activation.121 The carbonylation reaction of A-(2-pyridyl)pyrrolidine occurs at the a-position of the pyrrolidine ring by using [RhCl(cod)]2 as a catalyst and 2-propanol as a solvent. Cyclic amines exhibit a high reactivity (up to 84%) (Equation (93)), while acyclic amines show relatively low reactivity (18%). The use of Ru3(CO)i2 as a catalyst does not result in a carbonylation reaction, but instead the addition of the sp3 C-H bond across the olefin bond to give an alkylation product, as mentioned before (Section 10.05.4). 

Ackermann L (2007) Chelation-Assisted Arylation via C-H Bond Cleavage. 24-. 35-60 Akiyama K, see Mikami M (2005) 14-. 279-322... 

As mentioned in Section 52.4, chelate-assisted aldehyde coordination can occur with platinum(II), but the Pt—O bond in the anthranilaldehyde complex will undergo ready substitution with rotation of the C=0 functionality away from platinum.1624... 

An alternative synthetic route to platinum(II) thiolates is by the oxidative addition of the S—S bond to platinum(O). When the sulfur atom has phenyl or electron-withdrawing substituents such as CF3, this reaction is a useful one to synthesize the thiolato platinum(II) complexes (equation 503).1703-1705 Simple alkyl disulfides such as Me2S2 and Et2S2 do not form stable dithiolato complexes of platinum(II) by S—S addition to Pt(PPh3)3, but if chelation can occur, chelate-assisted oxidative addition can induce S—S cleavage (equation 504).30 An unusual cyclic thiolato complex is obtained by the decarbonylative cleavage of a C—S bond (equation 505).1707... 

Loupy, A., Chatti, S., Delamare, S., Lee, D.Y., Chung, J.H. and Jun, G.H., Solvent-free chelation-assisted hydroacylation of olefin by rhodium(I) catalyst under microwave irradiation, /. Chem. Soc., Perkin Trans. 1, 2002, 1280-1285. 

Spectrophotometrically, we could detect no reaction between 1 and AsCHO. This result demonstrates that the identity of the neutral donor component of the bifunctional substrate is crucial for the success of our methodology. This observation substantiates the mechanism outlined in Scheme 2 for the chelate assisted oxidative addition reaction. 

For catalytic C-H bond transformation, there are two types of reaction. One involves non-chelation-assisted C-H bond cleavage, the other is chelation-assisted. 

In the former reaction regioisomers are possibly formed, so regioselectivity cannot be controlled. Chelation-assisted reactions, on the other hand, occur regioselec-tively, usually at the position ortho to the directing group. 

As mentioned above, three types of reaction are used for silylation of C-H bonds. In this section, we describe the reaction pathway of chelation-assisted silylation of C-H bonds with hydrosilanes (reaction 1). 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

The first example of silylation of C-H bonds in arenes with hydrosilanes was reported by Curtis [2]. Later, silylation of C-H bonds with triethylsilane using a rhodium catalyst was reported (Scheme 3) [3, 4], The reaction of arenes with bis(hydrosilane) using a platinum catalyst involves a bis(silyl)platinum species in the coupling reaction (Scheme 3) [5]. In these non-chelation-assisted reactions possible regioisomers should be formed. 

For high regioselectivity, one of the most reliable procedures is chelation-assistance, which involves coordination of a heteroatom to a metal. A representative example of regioselective silylation is the Ru3(CO)12-catalyzed reaction of aryloxa-... 

Scheme 1. The proposed reaction pathway of the ruthenium-catalyzed C-H/olefin coupling by means of the chelation-assistance.

This chelation-assisted C-H/olefin and C-H/acetylene coupling can be applied to a variety of aromatic compounds with a directing group such as ester, aldehyde, imine, azo, oxazolyl, pyridyl, and nitrile [7]. In this section, we describe the coupling reactions of aromatic carbonyl compounds with olefins using a transition metal catalyst. 

Scheme 3. C-H bond transformation by ortho-chelating assistance (see Chapter 111.1.3.2.1).

Takahashi, A., Hirose, Y., Kusama, H., and Iwasawa, N. (2008) Chelation-assisted electrocyclic reactions of 3-alkenyl-2,2 -bipyridines an efficient method... 

Chelation-assisted C-H/olefin coupling can be applied to the atroposelective alkylation of biaryl compounds. The reaction of 2-(l-naphthyl)-3-methylpyri-dine with ethylene using [RhCl(coe)2]2 where coe is cyclooctene, and PCy3 results in the formation of an ethylation product in 92% yield (Eq. 21) [20]. In place of the PCy3 ligand, the use of (R) - (1 - [ (S) - 2- diphenylphosphino ] ferro-cenyl)ethyl methyl ether [(R),(S)-PPFOMe] leads to the atropselective alkyla-... 

---