CâC versus CâH Bond Oxidative Addition in PCX (X=P,N,O) Ligand

Chapter 4

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C-C versus C-H Bond Oxidative Addition in PCX (X=P,N,O) Ligand Systems: Facility, Mechanism, and Control Boris Rybtchinski and David Milstein* Department of Organic Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel

A n overview of recent results regarding the activation of strong C - C and C - H bonds by Rh(I) and Ir(I) in PCX (X=P,N,O) type ligand systems is presented. Whereas both C-C and C - H oxidative addition involve non-polar 3-centered transition states and 14 electron intermediates, steric requirements differ markedly and chelation plays a much more important role in C - C than in C - H activation. Control over C - C vs C - H activation can be achieved by choice of ligand and solvent. Under optimal conditions, C - C activation can be thermodynamically and kinetically more favorable than C - H activation and proceed even at - 7 0 ° C . Activation parameters for an apparent single-step metal insertion into a C-C bond were obtained. A combination of C - H and C - C activation in conjunction with oxidative addition of other bonds has led to unique methylene transfer chemistry.

70

© 2004 American Chemical Society

In Activation and Functionalization of C—H Bonds; Goldberg, K., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

71 Whereas C - H activation by late transition metals in homogeneous media is a topic of much research effort, resulting in mechanistic insight (/) and development of catalytic systems (2), the field of C-C bond activation is much less developed (3). In general, thermodynamic and kinetic factors are expected to favor CH over C - C activation. However, although M - H bonds in solution are in many cases significantly stronger than the M - C ones (4) appropriate design can make C-C bond activation in solution thermodynamically feasible (3). For example, - C A r y l bonds are very strong in cases of rhodium and iridium (5), sometimes stronger than M - H bonds. Kinetic factors favoring C - H over C - C bond activation include (a) an easier approach of the metal center to C - H bonds (b) the statistical abundance of C - H bonds, and (c) a higher activation barrier for C C vs C - H oxidative addition due to the more directional nature of the C - C bond (3). C-C bond activation by soluble metal complexes in most of the reported systems is driven by strain relief, aromatization or the presence of a carbonyl group (3). Aiming at the activation of strong, unstrained C-C bonds, we chose to use bis-chelating PCP and P C X ligand systems (6) in order to bring the metal center to the proximity of the C - C bond and gain understanding as to whether insertion into a strong C-C bond is possible and what are the factors that might favor this process. 9


M

PCP: R=Ph, Me, *Pr, %u

PCX: X=NEt , OMe 2

C-C activation in these systems is expected to be irreversible and lead to a stable C - C activation product. The C M e " A r y l bonds of these ligands, targeted for metal insertion, are very strong (eg BDE(C6H5-CH3) = 101.8±2 kcal mol" ), stronger than the competing benzylic C - H bonds (compare BDE(C6H5CH2-H) = 88±1 kcal m o l ) (7). We demonstrated that C - C bond oxidative addition can take place in P C P ligand systems in the presence of hydrogen, the evolving methane being a significant thermodynamic driving force (8). We have also demonstrated that the process of metal insertion into a C - C bond can be direct (not requiring an additional driving force) and thermodynamically more favorable than C - H bond activation (9). Even C - C bonds as strong as those in aryl-CF3 can be cleaved (10). When both sp -sp and sp -sp C-C bonds are available for activation, only activation of the former was observed (//). Catalytic C - C bond activation in the P C P system was also c

1

-1

2

3

3


3


72 demonstrated (72). The scope of metal complexes capable of C-C activation in bis-chelating systems includes complexes of Ru(II) (13), Os(II) (14), Rh(I) (3), Ir(I) (15), Pt(II) (13,16) and Ni(II) (17). Here we present an overview of our recent results describing C - H and C-C activation in P C X (X=P,N,0) systems. We have recently found that C - C bond activation can be very facile, and preferred over C - H activation both thermodynamically and kinetically. We have also found that a significant degree of control can be achieved regarding C-C vs C - H bond activation aptitudes. The obtained mechanistic insight, including the direct measurement of the activation parameters o f the oxidative addition of a strong C - C bond as well as computational studies, reveals similarity in the electronic requirements for C - C and C - H oxidative addition but significantly different steric prerequisites. A combination of C - H and C - C bond activation, in conjunction with oxidative addition of other bonds has resulted in a unique methylene transfer reaction.

C-C Bond Activation: Facility Bulky phosphines are advantageous ligands for the study of oxidative addition processes, since upon coordination to a metal center they generate a species having a sterically shielded vacant coordination site, favoring cyclometallation. Upon reaction of the tBu-PCP ligand with rhodium olefin dimers at room temperature, direct concurrent rhodium insertion into C - H and C-C bonds took place (Scheme 1) (15). {

Bu

2

Scheme 1



73 Interestingly, it was found that associative displacement of the alkene by the phosphine takes place and that the initial coordination of the diphosphine ligand to the rhodium olefin complex is the rate determining step for the entire process rather than the C-C or C - H activation steps. In the case of Ir competitive formation of the C - H and C - C activation products also took place at room temperature (Scheme 2) but, contrary to the rhodium case, the product of C - H activation is stable under the reaction conditions (it converts to the C - C activation product at 100°C), facilitating mechanistic investigation of the process (vide infra). It is remarkable that in both tBu-PCP-Rh and tBu-PCP-Ir systems the insertion step into a very strong C-C bond in solution is not rate determining, although the overall process proceeds even at 25°C (15).

*Bu

2

Scheme 2

A further significant observation regarding the facility of C - C activation was the fact that C - C bond activation can take place even at -70 "C in the case of the P C N ligand (here ligand coordination is not rate determining), with exclusive formation of the C - C bond activated product (Scheme 3). The reaction was shown to proceed through a 14e intermediate (see below) (18).



74

Scheme 3

It should be noted that formation of 14e intermediates takes place also in the case of the Ρλ-PCP-Rh system (Scheme 4) (19). C - C bond activation is very facile here as well: it is not a rate determining step and takes place at room temperature, indicating the general importance of 14e intermediates for facile CC bond activation. The presence of the bulky iodide ligand results very likely in favorable formation of the unsaturated 14e intermediate in this case.

Scheme 4


75


C-H vs C-C Bond Activation in Neutral PCP and PCN-Rh Systems: Mechanistic Insight The tBu-PCP-iridium and rhodium systems described above (Schemes 1,2) allowed a direct comparison of C-C and C - H activation processes (Fig. 1) (15). C - C activation was found to be thermodynamically and, importantly, kinetically preferred over C - H . The two processes proceed through similar transition states, most probably through 3-centered non-polar ones, as suggested by the small difference in the activation parameters between them, and by the fact that variations of the solvent and substituent in the aromatic ring of the ligand do not influence the AAG*. This is in agreement with computational studies (20,21). However, direct measurement of the C - C insertion activation parameters was not possible in this system.

MG*(293)

Figure 1. Reaction profile for C-C vs C-H oxidative addition in the tBu-PCP system (Reproduced from reference 5. Copyright 1996 American Chemical Society.)

Further insight into the C - C bond activation mechanism was obtained by kinetic evaluation of a single step metal insertion into a carbon-carbon bond


76


in solution in the PCN-Rh system (18). The 14e intermediate Y (Scheme 3), a frozen intermediate in the process, was formed and fully characterized at low temperature. It undergoes clean oxidative addition of the C-C bond, allowing the direct measurement of the activation parameters, that are as follows: Δ Η * = 15.0(±0.4) kcal/mol, A S * = -7.5(±2.0) e. u., AG*(298) = 17.2(±1.0) kcal/mol. As expected for a concerted oxidative addition process, the activation entropy is negative. The fact that it is only moderately negative indicates that the intermediate is already significantly ordered towards the insertion step. The P C N system was also studied computationally (22).

TS (PCN) CC

Thus, our kinetic study supports a 3-centered, non-polar transition state, TScc(PCN), for C - C oxidative addition to Rh(I), in agreement with our postulate in the case the tBu-PCP-Rh/Ir system described above. The obtained activation parameters are the first data for an apparent single step carbon-carbon bond activation by a metal complex. A r

PCN-Rh and Cationic PCP-Rh System: Control over C-C vs C-H Activation Aptitudes In addition to the great facility of C-C oxidative addition, the PCN-Rh system demonstrates very high selectivity towards C - C vs C - H bond activation: no C - H activated products are observed in this system (Scheme 3) (18,23). Such high preference for C - C bond activation is most probably due to the optimal metal positioning for the insertion into the C - C bond. Thus, as both C - H and C C activation are observed in the PCP systems mentioned above, changing of one phosphine arm for an amine results in preferential C - C activation, demonstrating that the C - C vs C - H reaction aptitudes can be controlled by appropriate ligand choice.



Having investigated the reactivity of neutral Rh and Ir complexes, we were interested in extending the scope to cationic metal centers. A cationic metal center was expected to exhibit different reactivity and selectivity due to lower electron density and possibly different interactions with the arene ring and the ligands. We found that in a cationic tBu-PCP rhodium system the reaction can be driven towards the exclusive activation of C - C or C - H bonds at room temperature by solvent choice (24). Thus, in acetonitrile exclusive C - H activation took place (Scheme 5), while in THF the C-C bond activation product was quantitatively formed (although C - H bond oxidative addition was also observed in THF at low temperature). This unique degree of control over metal insertion into strong C - H vs C-C bonds seems to be a consequence of solvent coordination. In T H F an unsaturated intermediate, possessing a vacant coordination site capable of both C - H and C-C activation, may be formed. In the case of the better coordinating acetonitrile, solvent coordination blocks vacant coordination sites and generates an intermediate too bulky for insertion into the sterically more hindered C-C bond. Thus, unlike its neutral counterpart, which is almost insensitive to the solvent (due to lower tendency to bind solvent molecules), the cationic rhodium center appears to possess very good selectivity in C-C vs C - H activation, conveniently regulated by the reaction solvent.

Me

(Tj^NCMe MeCN

Scheme 5


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C-H vs C-C Activation in a Cationic PCO-Rh System. Facility, Mechanism and Control Our attempts to extend the scope of C-C bond activation to a monochelating PC system resulted in exclusive C - H bond activation (Scheme 6) (25). No C - C bond activation was observed using various conditions and reagents. Following this observation, a PCO-type ligand was designed in order to probe the role of the chelating effect (Scheme 7). One of its chelating moieties being an oxygen donor, this system mimics the electron density environment and general structure of the mono-chelating P C system, enabling both mono- and bis-chelating binding modes.

r.t./1h

+

[Rh(coe) (soIv) | BF4 + 2

n

coe=cyclooctene 2: Solv=(a) acetone, (b) THF, (c) MeOH

Scheme 6

The P C O - R h system undergoes C - H bond activation at room temperature, forming products with an open and closed methoxy arm (Scheme 7) (26). C-C bond activation takes place upon heating. Significantly, whereas C H activation of the two methyl groups is observed, only the C - C bond between the chelating arms is activated, demonstrating that the chelating methoxy ligand is both essential and sufficient for C - C activation in this system. A n attempt to isolate the C - H activation products led to an intriguing observation: removal of the solvent from a solution of the C - H activation complexes under vacuum at room temperature resulted in conversion of the C - H to the C - C activation product. Thus, C - H vs C - C bond activation can be controlled just by the presence or absence of the solvent. This effect appears to be due to the preferential coordination of the B F counter anion to the metal center of the C - C activation product upon solvent evaporation. The exclusive activation o f the C - C bond situated between the phosphine and methoxy arm and the absence of C-C activation in the P C system suggest that chelation is crucialfor C-C bond activation. 4


79 open arm ·Ρ*Βϋ2 r.t./1h +

[Rh{coe)(solv)JBF 2

n

4

+

closed arm

Μθ

CâC versus CâH Bond Oxidative Addition in PCX (X=P,N,O) Ligand

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