A Room Temperature Direct Metal Insertion into a Nonstrained Carbon−Carbon Bond in Solution. C−C vs C−H Bond Activation

Abstract

The diphosphine 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethylbenzene (1a) upon reacting with the rhodium and iridium olefin complexes M2(olefin)4Cl2 (M = Rh, Ir) undergoes rapid, selective metal insertion into the strong unstrained aryl−methyl bond under very mild conditions (room temperature), yielding ClM(CH3)[C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (4a), Ir (7a)). The carbon−carbon bond activation is competitive with a parallel C−H activation process, which results in formation of complexes ClMH(L)[CH2C6H(CH3)2(CH2P(t-Bu)2)2] (M = Rh (3a), Ir (6a); L = cyclooctene in the case of 6a and is absent in 3a). Complexes 3a and 6a undergo facile C−H reductive elimination (at room temperature (3a) or upon moderate heating (6a)), followed by C−C oxidative addition, resulting in clean formation of 4a and 7a, respectively. The C−C bond activation products are stable under the reaction conditions, demonstrating that this process is the thermodynamically favorable one. X-ray single-crystal analysis of 4a demonstrates that the rhodium atom is located in the center of a square pyramid, with the methyl group occupying the position trans to the vacant coordination site. Direct kinetic comparison of the C−C and C−H activation processes shows thatin contrast to theoretical calculationsmetal insertion into the carbon−carbon bond in this system is not only thermodynamically but also kinetically preferred over the competing insertion into the carbon−hydrogen bond. When the ligand 1,3-bis[(di-tert-butylphosphino)methyl]-2,4,6-trimethyl-5-methoxybenzene (1b), bearing the strong electron-donating methoxy group in the position trans to the Ar−CH3 bond to be cleaved, was used instead of 1a, no effect on the reaction rate or on the ratio between the C−H and C−C activation products was observed. Our observations indicate that the C−C oxidative addition proceeds via a three-centered mechanism involving a nonpolar transition state, similar to the one proposed for C−H activation of hydrocarbons. An η2-arene complex is not involved in the C−C activation process.

Cyclometallated nitrogen heterocycles

“…Conventionally, cyclometallation has involved one ligating group holding a metal centre close to a C Á/H bond and the subsequent closure of the ring via the formation of a carbon to metal bond [1]. Whilst some have adapted this reaction to activate bonds other than C Á/H (for example C Á/Si [2] or C Á/C bonds [3]), we have been investigating bringing about double cyclometallations with a single metal (generating Ĉ N C tridentate complexes) [4,5]. Others have followed Trofimenko's pioneering work of thirty years ago [6,7] using two ligating groups to hold two metal centres close to a single benzene ring, thus forming two metal to carbon bonds on the same aromatic ring.…”

Rhodium(I) and Rhodium(III) Complexes Formed by Coordination and CH Activation of Bulky Functionalized Phosphanes

“…The two RhÀP bond lengths are slightly longer than in the related, more symmetrical chelate complex [RhHCl-(tBu 2 PCH 2 C 6 H 3 CH 2 PtBu 2 -k 3 P,C,P)] (18) obtained from RhCl 3 ¥ 3 H 2 O and 1.5-C 6 H 4 (CH 2 PtBu 2 ) 2 in 2-propanol/water under reflux. [16] In contrast, the RhÀC31 bond length of 17 (1.967(5) ä) is slightly shorter than in 18 (1.999(7) ä) and in the related hydrido and methyl complexes [RhHCl{CH(CH 2 CH 2 PtBu 2 ) 2 -k 3 P,C,P}] (2.082(2) ä) [17] and [RhCH 3 Cl{tBu 2 PCH 2 C 6 H-3.5-(CH 3 ) 2 CH 2 PtBu 2 -k 3 P,C,P}] (2.02(2) ä), [18] respectively. The P1-Rh-P2 axis of 17 is significantly bent (160.18 (5)8), which could be due both to steric hindrance between the phosphane substituents and the strain of the chelate ring.…”

Activation of C–O and C–F Bonds by Pincer–Iridium Complexes

“…4.8); this gives the isolated (PCP-CH 2 )Ir(H)(OAc) intermediate. As is well precedented in the work by Milstein et al [64,65], such species may undergo C(PCP-CH 2 )-H elimination followed by C-C cleavage; this results, in this case, in the formation of (PCP)Ir(CH 3 )(κ 2 -OAc), the C-O oxidative addition product.…”