Chapter 7
Hydrocarbon Functionalization by C-H Activation
C-C bond formation by C-H activation of aromatic molecules
There are a number of examples of C-C bond forming reactions involving C-H activation of aromatic ring C-H bonds. These reactions can even be found in classic organic reactions (Friedel-Crafts alkylation). For a review see: Chem. Rev. 2002, 102, 1731. The metal catalyzed versions of these reactions that couple arenes and alkenes are very attractive because they represent perfectly atom economical reactions (every atom in the starting materials is found in the product).
cat. + 60 + 40
Cat. =
Alkane Functionalization Alkane functionalization reviews: Acc. Chem. Res. 1995, 164. Chem. Rev. 2001, 101, 953-996 J. Chem. Soc., Dalton Trans. 2001, 2437-2450 J. Mol. Cat. A.: Chem. 2004, 230, 7-25
J. Chem. Soc., Chem. Commun. 1982, 1235
J. Am. Chem. Soc. 1987, 109, 8025 t-BuCH=CH2 converts the Ir dihydride to a coordinatively unsaturated Ir(I) complex. The driving force for this reaction is the very exothermic hydrogenation of tBuCH=CH2. The unsaturated Ir(I) species can then oxidatively add the substrate.
• The reaction is not inhibited by CO • Photolysis under 13CO does not result in incorporation of 13CO into the complex • Reaction in pentane and pentane-d12 showed indistinguishable conversion rates, but a large isotope effect was observed in a 1:1 mixture of C5H12 and C5D12 • Photolysis of the W-Bcat' complex in the presence of PMe3 in pentane led to formation of the PMe3 adduct in addition to pentyl-Bcat'. The ratio of PMe3 complex: pentylBcat' was dependent [PMe3] Formation of a strong B-C bond drives this reaction (B-C is 10-15 kcal/mol stronger than C-H, while M-B and M-H have similar bond strengths).
Reactivity of H-CH3 100 X higher than that of HCH2OH Very strong preference for oxidation of C-H bonds on sterically unhindered methyl groups.
For original Shilov chemistry, the Pt(0)/Pt(II) and Pt(II)/Pt(IV) redox couples have similar potentials. Thus Pt(II) can also disproportionate into Pt(0) and Pt(IV). This leads to precipitation of Pt metal and undesired side reactions.
Alkane Dehydrogenation: Alkene hydrogenation is well precedented. Would it be possible to identify systems where the reverse reaction (alkane dehydrogenation) would be favored?
Angew. Chem. Int. Ed. 2001, 40, 3596-3600
Borylation of Alkanes:
1. Stoichiometric Functionalization
(Science, 1997, 277, 211)
Possible Mechanisms:
Observations:
Ph3P
PPh3
Ph3P Ph3P H
Ru PPh3 O H
PPh3
PPh3
R
There are a large number of examples of this type of transformation in which an electrophilic metal center is used to activate a C-H bond on an aromatic center. Typically this occurs by an electrophilic aromatic substitution mechanism.
The Bergman systems are very efficient at oxidatively adding C-H bonds. However efforts to transform the resulting metal-alkyl to a functionalized organic product have met with limited success.
The dehydration can also be carried out thermally without an H2 acceptor if hydrogen can be purged from the reaction system. Catalyst stability becomes an important issue under these conditions (150-230 C). In particular, ligand metallation and other decomposition pathways become important.
H LnM H
R LnM
H H R
H2 H H R + LnMn-2 LnM H H R
The first catalytic system: In order to drive the reaction towards dehydrogenation, it is usually necessary to remove H2 from the reaction system. Often a reactive alkene is used as a hydrogen acceptor. 3,3Dimethyl-1-butene is used as a hydrogen acceptor because of it's high heat of hydrogenation.
Examples where the C-H activation is promoted by coordination to a Lewis-basic functional group are very common.
O + (Ph3P)3(CO)RuH2 Si(OEt)3 toluene, 135 C
Zh. Fiz. Khim. 1969, 43, 2174
CH3OH + HCl
[PtCl4]2-
CH4
H2O
Cl Cl Pt Cl Cl CH3 Cl 2-
HCl 2-
Cl Pt ]2-
[PtCl6]2-
the Shilov system represented a very selective method for alkane oxidation.
C-H activation by transition metals has been demonstrated for numerous systems. Therefore, C-H activation is not such a challenge. However making use of C-H activation to produce functionalized alkanes catalytically still represents a formidable challenge.
J. Am. Chem. Soc. 1982, 104, 352 J. Am. Chem. Soc. 1983, 105, 3929