Comparative reactivities of hydrocarbon carbon-hydrogen bonds with a transition-metal complex (original) (raw)

Kinetics and thermodynamics of intra- and intermolecular carbon-hydrogen bond activation

Journal of the American Chemical Society, 1985

The preference for intra-and intermolecular C-H bond activation has been determined by equilibration of the complex (C5Me5)Rh(PMe2CH2C6H5)(C6H5)H and its cyclometalated analogue (C5Me5)Rh(PMe,CH2C6H4)H in neat benzene at 51.2 OC ( K , = 36.7, AGO = -2.32 kcal/mol). By monitoring the approach to equilibrium over a 40 O C temperature range, the difference between the activation parameters for intra-and intermolecular activation by the 16-electron intermediate [(C5Me5)Rh(PMe2CH2C6HS)1 can be obtained (intra-inter): AAH' = 1.7 f 0.8 kcal/mol; AAS' = 4.5 f 2.5 eu. At 25 OC, this corresponds to a 1.86:l kinetic preference for intermolecular activation of the neat benzene solvent by the coordinatively unsaturated intermediate [(C5Me5)Rh(PMe2CH2C6H5)] over intramolecular cycloaddition. The effect of solvent concentration on activation selectivity is discussed. A comparison with intra-and intermolecular alkane activation is made by equilibrating the complex (C5Me5)Rh(PMezCH2CH2CH2)H with benzene and by examining the kinetics of cyclometalation vs. alkane activation. These studies reveal the same general trend with regard to thermodynamic and kinetic selectivity in alkanes and arenes: while there is little kinetic selectivity between intra-and intermolecular reactions involving neat solvent, there is a moderate thermodynamic preference for the intramolecular activation. . The activation of carbon-hydrogen bonds by homogeneous transition-metal complexes is a topic that has received a great deal of attention recently. Much of this interest arises from the recent reports that indicate that even the C-H bonds of alkanes can 0002-7863/85/1507-0620$01.50/0 (1) (a) Crabtree, R. H.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. SOC. 1979, 101, 7738-7740. Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am. Chem. SOC. 1982, 104, 107-113. Crabtree, R. H.; Demou, P. C.; Eden, D.; Mihelcic, J. M.; Parnell, C. A.; Quirk, J. M.; Morris, G. E. J. Am. Chem. SOC. 1982,104,6994-7001. (b) Baudry, D.; Ephritikhine, M.; Felkin, H. J . Chem. Soc., Chem. Commun. 1980, 1243-1244. Baudry, D.; Ephritikhine, M.; Felkin, H.; Zakrzewski, J.

Carbon−Carbon Bond Activation by Rhodium(I) in Solution. Comparison of sp 2 −sp 3 vs sp 3 −sp 3 C−C, C−H vs C−C, and Ar−CH 3 vs Ar−CH 2 CH 3 Activation

Journal of the American Chemical Society, 1998

Reaction of [RhClL 2 ] 2 (L ) cyclooctene or ethylene) with 2 equiv of the phosphine {1-Et-2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } (1) in toluene results in a selective metal insertion into the strong Ar-Et bond. This reaction proceeds with no intermediacy of activation of the weaker sp 3 -sp 3 ArCH 2 -CH 3 bond. The identity of complex Rh(Et){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 }Cl (3) was confirmed by preparation of the iodide analogue 6 by reaction of the new Rh(η 1 -N 2 ){2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 } (7) with EtI. It is possible to direct the bond activation process toward the benzylic C-H bonds of the aryl-alkyl group by choice of the Rh(I) precursor, of the substituents on the phosphorus atoms ( t Bu vs Ph), and of the alkyl moiety (Me vs Et). A Rh(III) complex which is analogous to the product of insertion into the ArCH 2 -CH 3 bond (had it taken place) was prepared and shown not to be an intermediate in the Ar-CH 2 CH 3 bond activation process. Thus, aryl-C activation by Rh(I) is kinetically preferred over activation of the alkyl-C bond in this system. Moreover, cleavage of an Ar-CH 2 CH 3 bond, followed by -H elimination, may be preferred over sp 2 -sp 3 C-C activation of an Ar-CH 3 group.

Reactivity of Alkynes with M-C Bonds generated through C-H Activation

Transition metal-catalyzed C-H activation and functionalization with various coupling partners is a well-explored area of research. Among the various coupling partners used, alkynes occupy a prominent position on account of their varied reactivity. Due to their low steric demand and a high degree of unsaturation they effectively overlap with metal d-orbitals and form multiple bond-forming events giving rise to complex skeletons that are otherwise challenging to synthesize. This makes alkyne one of the most successful coupling partners in terms of the number of useful transformations. Remarkably, by changing the reaction conditions and transition-metals from 5d to 3d, the behaviors of alkynes also change. Despite enormous explorations with alkynes, there are still a lot more possible ways by which alkynes can be made to react with M-C bonds generated through C-H activation. Especially with the development of new high and low valent first-row metal catalysts, there is plenty of scope ...