Leading Question: Tutorial on Rh2(OAc)4-Catalyzed O-H Insertion Reactions
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The formation and reactivity of the carbene-like transition-metal complex is facilitated by the rhodium-catalyzed leaving of nitrogen gas from diazoketone compounds. This begins when the rhodium(II) complex uses one of its reactive, vacant d orbitals (on rhodium) to form a coordination with the substrate compound containing the α-diazoketone, resulting in a rhodium complex. This complex loses nitrogen gas from the central carbon atom, giving way to the formation of a rhodium-carbenoid (Fischer carbene).
This carbenoid is generally attacked by some form of a nucleophile, such as N–H, O–H, or S–H, depending on the reaction. These nucleophiles can be intermolecular, thereby introducing new functional groups to the compound, or intramolecular, as is the case for maoecrystal V. With OH insertion reactions, the lone pairs of oxygen perform the nucleophilic attack on the rhodium carbenoid, pushing a pair of electrons back onto rhodium. Rhodium(II) acetate is then released, leaving a carbanion on the reacting compound that is protonated by an arbitrary acid; in maoecrystal V, this protonation is intramolecular. This completes the reaction.
Schemes 1 and 2 give a mechanistic overview of the reaction from the α-diazoketone with a rhodium(II) catalyst to the -OH-containing compound, with intermediates. In Scheme 2, certain reaction pathways are ruled out due to improbability, leaving the most logical-and actual-reaction pathway.
Scheme 4 elaborates on the path that A would have to take to form a different compound and ultimately arrive at the desired product of D".
Rhodium(II) catalysts have the potential to completely replace Wolff rearrangements for bond insertions, Wolff rearrangements previously being the best means of substituting or inserting bonds onto diazocarbonyls. They also expand the diversity of reactions with diazocarboyls. However, rhodium(II) acetate catalysis still has particular strengths and weaknesses:
Strengths of rhodium(II) catalysis
The advantages of using rhodium as the heavy metal catalyst in N–H, O–H, or S–H insertions is high turn over numbers, broad substrate possibilities, and bimolecular modification.
Weaknesses of rhodium(II) catalysis
The disadvantages of using Rhodium as the heavy metal catalyst in such insertions is high cost, competing C–H insertions, limited stereocontrol, and susceptibility to reaction inhibition by a Lewis base.
Now, that wasn't so bad, class, was it?
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Citations
Ye, T.; McKervey, M. A. Chem. Rev. 1994, 94, 1091.
Hoye, T. R.; Dinsmore, C. J. J. Am Chem. Soc. 1991, 113, 4343-4345.
Pirrung, M. C.; Liu, H.; Morehead, A. T. J. Am. Chem. Soc. 2002, 124, 1014–1023.
Xie, Z.-Z.; Liao, W.-J.; Cao, J.; Guo, L.-P.; Verpoort, F.; Fang, W. Organometallics 2014, 33, 2448–2456.
Gillingham, D.; Fei, N. Chem. Soc. Rev. 2013, 42, 4918–4931.
Strengths of rhodium(II) catalysis
The advantages of using rhodium as the heavy metal catalyst in N–H, O–H, or S–H insertions is high turn over numbers, broad substrate possibilities, and bimolecular modification.
Weaknesses of rhodium(II) catalysis
The disadvantages of using Rhodium as the heavy metal catalyst in such insertions is high cost, competing C–H insertions, limited stereocontrol, and susceptibility to reaction inhibition by a Lewis base.
Let's continue our adventure!
Citations
Ye, T.; McKervey, M. A. Chem. Rev. 1994, 94, 1091.
Hoye, T. R.; Dinsmore, C. J. J. Am Chem. Soc. 1991, 113, 4343-4345.
Pirrung, M. C.; Liu, H.; Morehead, A. T. J. Am. Chem. Soc. 2002, 124, 1014–1023.
Xie, Z.-Z.; Liao, W.-J.; Cao, J.; Guo, L.-P.; Verpoort, F.; Fang, W. Organometallics 2014, 33, 2448–2456.
Gillingham, D.; Fei, N. Chem. Soc. Rev. 2013, 42, 4918–4931.
