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Introduction.  Through the invention of synthetic reactions differential from currently existing technologies, we may further enable novel, highly efficient approaches to desired molecular structures.  Our funded proposal outlined a general approach to the site-selective functionalization of relatively unreactive bonds by transition metal catalysts.  This proposal represented a unification of two distinct concepts:  the metal-catalyzed functionalization of unreactive bonds, and the reversible installation of a ligating species that can render a process intramolecular.  In this approach, the ligand will act as the link between substrate and catalyst that allows and directs the desired reaction to occur (Figure 1).  It is envisioned that an equilibrating transacetalization process between the substrate and the organic molecule catalyst will form an intermediate in situ.  This covalently attached substrate-ligand adduct will then direct (via a ligating group LG) the transition metal to functionalize the substrate in a specific fashion, which upon hydrolytic release of the organic catalyst will afford the product.  It is anticipated that this versatile strategy to functionalization will lead to a variety of transformations currently unrealized by either transition metal catalysis or organocatalysis approaches.

Figure 1.  Metal-Catalyzed Functionalization of Aldehyde-Based Substrates via Transient Covalent Attachment.

Preliminary progress.  Thoughtful experimental design and execution will be necessary in order to achieve the processes we envision.  Specifically, we will need to evaluate the following: A) the molecular architecture of the ligand and its intrinsic directing group capability, both spatially and electronically, B) the transacetalization component and its inherent ability to facilitate exchange processes with organic substrates, and C) the compatibility and synchronization of these two aspects.  To start systematically addressing these issues, we have approached this problem by generating our putative linked intermediates stoichiometrically, and studied their capacity in remote functionalizations.

To that end, we chose to utilize a proline-based scaffold for our ligand as a starting point for these investigations (Scheme 1).  Ligands based on the amino amide acetalization fragment showed significant promise.  N-t-Boc-proline can be converted to 2 in two steps in good yield.  This amino amide can be condensed with isobutyraldehyde to form 4.  This compound exists as a single diastereomer, dictated by the isopropyl group preferentially residing on the convex face of the 5,5-ring system to avoid steric interactions.  The directing group pyridyl ring is then installed via enolization and diastereoselective alkylation with 2-fluoropyridine.

Scheme 1

With 6 in hand, we evaluated this molecule in a directed oxidative transformation (Scheme 2).  When aminal 6 is treated with catalytic Pd(OAc)₂ and PhI(OAc)₂ as the stoichiometric oxidant, according to the protocol of Sanford, acetate 7 is predominantly formed.  This preliminary result is exceptionally significant for several reasons.  First and foremost, sp³-hybridized C-H bond functionalization occurs, highlighting the compatibility of this substrate with the reaction conditions.  Moreover, the oxidation occurs with nearly singular site-selectivity.  Although there are a number of C-H bonds that could be oxidized under these conditions, the molecular framework directs the functionalization to almost exclusively one carbon, indicating that this oxidation process is highly diastereoselective.

Scheme 2

As part of developing our understanding of the ligand framework and its ability to direct functionalization, we have also synthesized 9 for oxidative study (Scheme 3).  This compound features an additional methylene group between the directing pyridyl and the pyrrolidine ring, which we anticipated would impart increased conformational flexibility.  This compound, when treated with the same oxidative conditions, afforded 10 and 11, the products of mono- and diacetoxylation respectively.  Importantly, this reaction displayed much greater reactivity relative to the oxidation of 6.  Current efforts are directed toward further modifications on this framework, in anticipation of understanding the effects of substitution on overall reactivity.

Scheme 3

In addition to the functionalization of sp³-hybridized C-H bonds, we are also investigating sp²-hybridized C-H bonds.  The same ligand framework of 9 can be attached to benzaldehyde to form 12 (Scheme 4).  Again using palladium oxidative catalysis, acetate 13 was formed.  We were also able to isolate a cyclometallated complex by treating 12 with stoichiometric Pd(OAc)₂, and obtain a crystal structure of the complex.  As can be seen in this structure, both the pyridyl and the pyrrolidine nitrogen atoms are bound to the metal center, and they appropriately position the metal to engage the aromatic ring.  It should also be noted that the aromatic ring is on the convex face of the bicyclic system defined by the amino amide-based aminal, consistent with our scaffold design.

Scheme 4

Lastly, we have started to see evidence of synchronized oxidation with ligand exchangability.  We have treated aminal 15, derived from p-tolualdehyde, and benzaldehyde under the oxidative conditions with added water to assist in exchange (Scheme 5).  Although 17 is the predominant oxidative product, we have observed small amounts of 13, arising from the ligand framework hydrolyzing off tolualdehyde, condensing with benzaldehyde, and then directing oxidative functionalization.  We anticipate that further evaluation of substrate and conditions will lead to a fully optimized process, and open the door for much greater generality in these types of transformations.

Scheme 5

Summary.  We anticipate that this type of transformation, once fully developed, could be widely applicable in oxidative functionalization reactions.  We plan to investigate the potential of this reactivity in a number of different oxidative manifolds.  We also anticipate this concept to be applicable in hydroarylation and hydrovinylation reactions, as well as in C-C bond insertion processes.  Our current plan is to publish our aforementioned efforts in the oxidative functionalization as a communication, and then further explore the reactivity in these different arenas.  The innovative nature of this work is likely to garner attention in the organic and organometallic chemistry communities.  I have presented posters detailing our efforts at two Gordon Conferences and a DOE contractors meeting that have been very well-received.  My graduate student Erin Stache, who has been the main contributor to these efforts, had an incredibly successful second-year oral examination, which speaks to her progressing development as a scientist due to her participation in this project.  Funding of this proposal has been instrumental in the launching of this project.