Y-C-H activation

In this case the use of the Sm(II) "ate" complex Na[Sm N(SiMe3)2 3] as starting material afforded yet another novel C-substituted amidinate complex resulting from y C-H activation of a N(SiMe3)2 ligand (Scheme 194). All new... 

The mechanism for cyclometallation was proposed to involve a concerted heterolytic process with hydrogen atom abstraction and metallacycle formation occurring in a four-center transition state. Kinetic and labeUng studies in the cyclometallation reactions indicate that intramolecular y-C-H activation is the rate-limiting step [290]. 

Yi J, Miller JT, Zemlyanov DY. A reusable imsupported rhenium nanocrystalline catalyst for acceptorless dehydrogenation of alcohols through y-C-H activation. Angew Chem Int Ed Engl. 2014 53 833-836. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

J.-Y. Saillard, Theoretical Aspects of Alkane C-H Activation by Organometallics, in Selective Hydrocarbon Activation (J. A. Davies, P. L. Watson, J. F. Liebman, A. Greenberg, Edits.), p. 207 ff, VCH Publishers, New York 1990. 

The Claisen rearrangement of allyl vinyl ethers is a classic method for the stereoselective synthesis of y,J-unsaturated esters. The allylic C-H activation is an alternative way of generating the same products [135]. Reactions with silyl-substituted cyclohexenes 197 demonstrate how the diastereoselectivity in the formation of 198 improves (40% to 88% de) for the C-H insertion reactions as the size of the silyl group increases (TMS to TBDPS) (Tab. 14.14). Indeed, in cases where there is good size differentiation between the two substituents at a methylene site, high diastereo- and enantioselectivity is possible in the C-H activation. 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

An interesting aspect of the allylic C-H insertion is that the products are y,6-unsaturated esters. Traditionally, y,6-unsaturated esters are most commonly prepared by a Claisen rearrangement, especially if stereocontrol is required. Diastereocontrol is also possible in the C-H insertion as long as the reaction occurs at a methylene site where there is good size differentiation between the two substituents [21]. An example is the reaction between 17 and the silylcyclohex-ene 18 which forms the C-H insertion product 19 in 88% de and 97% ee [21]. Other catalysts such as Rh2(.R-BNP)4 and Rh2(S-MEPY)4 have been explored for allylic C-H activation of cyclohexene but none were was as effective as Rh2(S-DOSP)4 [22]. 

K. Furuta, T. Nagata, and H. Yamamoto, A direct synthesis of cyclic acetals from f - or y-hydroxy ethers by means of C-H activation, Tetrahedron Lett., 29 (1988) 2215-2218. 

The allylic activation of 1,3-dienes by Ru(COD)(COT) makes possible their hydroacylation to form /5,y-unsaturated ketones via C-H activation of aldehydes at the same metal center [117], and their selective coupling with acrylic compounds [18] (Eq. 87). 

Jin, Y. and Lipscomb, J.D. (2000) Mechanistic insights C-H activation from radical clock chemistry oxidation of substituted methylcyclopropanes catalyzed by soluble methane monooxygenase from Methylosynus trichosporium OB3b, Biochim. Biophys. Acta 1543, 47-59. 

In addition to a- and p-C-H activation, the possibility occasionally arises for y- or even 8-functionalization. This is particularly common for aryl phosphine and phosphite ligands that may undergo metallation of the ortho-C-H bond of an aryl substituent. This process may be reversible however, if a suitable co-ligand is present which can undergo subsequent reductive elimination of the hydride, stable metallacyclic organyls are obtained (Figure 4.31). The formation of metallacyclic alkyls may confer some stability, as does the possibility of increased hapticity, e.g. in the case of xylyene ligands (see also Chapter 6). 

Reactions involving spl hybridized carbon atoms of alkyl ligands. These may involve y- or S-C—H bonds the C—H activation reaction may be followed by reductive elimination ... 

Negishi, E.-i., Kondakov, D. Y., Choueiry, D., Kasai, K., Takahashi, T. Multiple Mechanistic Pathways for Zirconium-Catalyzed Carboalumination of Alkynes. Requirements for Cyclic Carbometalation Processes Involving C-H Activation. J. Am. Chem. Soc. 1996,118, 9577-9588. 

Dialkylplatinum(II) complexes bearing P-hydrogens undergo exclusive P-hydrogen elimination on thermolysis [179,180,182]. On the other hand, the di-neopentyl complex, which has no P-hydrogens. undergoes y-C—H bond activation to provide a platinacyclobutane (eq (140)) [183]. 

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