Date of Award
Doctor of Philosophy (PhD)
Patrick J. Walsh
The efficient stereoselective formation of C−C and C−O bonds remains a critical challenge in organic chemistry. The level difficulty of these bond formations increases dramatically when regio-, diastereo-, and chemoselectivity issues are present. In efforts to address such challenges, this thesis summarizes three successful strategies to develop highly stereoselective C−C and C−O bond formation reactions: 1) the first strategy outlines the direct metallation and subsequent chemo- and regioselective cross-coupling of benzylic sp3-hybridized CH bonds (pKa values >34) to form C− bonds via palladium catalyzed deprotonative cross-coupling process (DCCP), 2) the second strategy outlines the application of 1,1-heterobimetallic borozinc reagents in the diastereoselective C− bond-forming reactions, and 3) the third strategy outlines the use of transition metal-catalysis in the highly chemo- and diastereoselective C−O bond formation via vanadium catalyzed directed epoxidation.
Chapters 1 and 2 summarize a program that we have recently initiated in our laboratory known as deprotonative cross-coupling process (DCCP). DCCP is the reversible in situ deprotonation of weakly acidic sp3-hybridized C-H bonds under mild conditions, which are then catalytically cross-coupled with aryl electrophiles under palladium catalysis.
Chapters 3 and 4 summarize the potential usefulness of 1-alkenyl-l,l-heterobimetallics in the stereoselective C−C bond formations in organic synthesis. Our group reported a practical generation of 1,1-heterobimetallics from air-stable B(pin)-substituted alkynylboronate esters and demonstrated their utility in a variety of one-pot transformations to provide boron-substituted allylic alcohols, dienols, α-hydroxy ketones, and α-dihydroxy ketones with high diastereoselectivity. More applications of these reagents are also explored in Chapters 3 and 4.
In Chapter 1 (Scheme 1), we have developed the first metal-catalyzed direct α-arylation of unactivated allylbenzenes (pKa ~ 34 in DMSO) with aryl bromides to afford 1,1-diarylprop-2-enes via a deprotonative cross-coupling process.
Usually the combination of aryl bromides, allylbenzene, base and a palladium catalyst results in a Heck coupling reaction. Herein we combine these same reagents, but override the Heck reaction through use of a strong base. While the base controls the chemoselectivity, the catalyst handles the regiochemistry, affording 1,1-diarylprop-2-ene products that are inaccessible via the Heck pathway (Scheme 1). The significance of this work is that it demonstrates that very weakly acidic hydrocarbon frameworks can be functionalized under DCCP conditions. The palladium-catalyzed arylation proceeded efficiently in the presence of PCy3 and produces α-arylated 1,1-diarylprop-2-enes in good to excellent yields (51-97%) with very high regioselectivity (>95:5).
Scheme 1: Overriding Heck Cross-Coupling Selectivity: Chemo- and Regioselective C(sp3)−H Activation in the α-Arylation of Unactivated Allyarenes via a Palladium-Catalyzed Deprotonative Cross-coupling Process.
Chapter 2 introduces the synthesis of diarylmethylamines via functionalization of weakly acidic sp3-hybridized C-H bonds adjacent to nitrogen in benzylic amines.
Direct deprotonation of the benzylic C−H's in secondary benzylamine derivatives under catalytic conditions is very challenging. This result is due to the weak acidity of sp3-hybridized benzylic C-H bonds adjacent to nitrogen, which requires strong organometallic bases such as alkyl lithiums for deprotonation. These strong bases, however, are impractical for cross-coupling reactions due to their limited compatibility with catalysts and coupling partners. We, therefore, envisioned the reversible in situ metallation/deprotonation and subsequent palladium catalyzed cross-coupling of the N-Boc benzylalkylamines with aryl electrophiles to form C−C bonds via deprotonative cross-coupling processes (DCCPs).
In Chapter 2 (Scheme 2), we, therefore, disclose the first direct cross-coupling of N-Boc benzylalkylamines with aryl electrophiles to provide N-Boc diarylmethylamines in moderate to high yields (5093%, 29 examples). Upon removal of Boc group, secondary diarylmethylamines are generated (7595% yields, 2 examples).
Scheme 2: Palladium Catalyzed DCCP of N-Boc Benzylmethylamine derivatives followed by Deprotection to generate Diarylmethylamines.
In Chapter 3 (Scheme 3), we disclose the vinylation of N-(2-pyridylsulfonyl) aldimines with versatile alkenyl-1,1-borozinc heterobimetallic reagents to furnish B(pin)-substituted allylic amines with high stereoselectivity in 60-93% yield in a one-pot procedure. The addition step can be followed by either C bond oxidation to provide α-amino ketones (71‒98% yields) or Suzuki cross-coupling to provide densely functionalized trisubstituted (E)-allylic amines (51-73% yields).
Scheme 3: Addition of Alkenyl-1,1-heterobimetallics to N-Pyridyl Sulfonyl Imines: Stereoselective Synthesis of B(pin)-substituted Allylic Amines, α-Amino Ketones, and Trisubstituted (E)-Allylic Amines.
As part of our program in developing stereoselective C-C bond forming reactions, we have reported the generation of 1-alkenyl-1,1-heterobimetallic reagents based on boron and zinc from readily available, air-stable B(pin)-substituted alkynes. Thus, regioselective hydroboration of B(pin)-alkynes generates the 1,1-bis (boro) intermediates. Chemoselective transmetallation of the more reactive vinyl-BCy2 bond generates 1-alkenyl-1,1-heterobimetallic reagents. The difference in reactivity between Zn-C vs. BC bonds allows for selective reaction at the Zn-C bond with aldehydes to yield B(pin)-substituted allylic zinc alkoxide intermediates. The alkoxide intermediates were then employed in various tandem reactions to form an array of compounds such as B(pin)-substituted allylic amines, α-amino ketones and trisubstituted (E)-allylic amines.
In Chapter 4 (Scheme 4), a retrosynthetic disconnection for the stereoselective preparation of αα′-dioxygenated carbonyl compounds is disclosed. Herein we report a method to divert the oxidation of vinyl boronate esters from the B-C bond to the C=C bond, resulting in a new stereoselective class of oxidation products from vinyl boronate esters. Treatment of 2-B(pin)-substituted allylic alcohols with catalytic OV(acac)2 and TBHP resulted in a highly chemo- and diastereoselective directed epoxidation to provide B(pin)-substituted epoxy alcohols (55-96% yield, dr > 20:1). In the case of B(pin)-substituted bis-allylic alcohols, highly substituted bis-epoxy alcohols with five contiguous stereocenters were obtained (dr >20:1). Furthermore, the difference in reactivity between allylic alcohols and 2-B(pin)-substituted allylic alcohols towards epoxidation enabled the selective oxidation of the allylic alcohol in the presence of TBHP and VO(acac)2. The reactivity difference between the two allylic alcohols suggests C=CB(pin) to be more electron deficient than C=C(alkyl). The B(pin)-substituted epoxy alcohols are also useful synthetic intermediates. Tandem vanadium catalyzed epoxidation of the 2-B(pin)-substituted allylic and bis-allylic alcohols with excess TBHP generated the intermediate epoxides and bis-epoxides, respectively. Subsequent addition of NaOH resulted in the oxidation of the B-C bond of the B(pin)-substituted epoxides to afford 2-keto-anti-1,3-diols (30-83% yield) and epoxide-substituted 2-keto-anti-1,3-diols (61-78% yield, dr >20:1). The latter underwent a novel facile acid-mediated cyclization to furnish fully substituted dihydroxy-tetrahydrofuran-3-ones (65-92% yield, dr >20:1). Such compounds are difficult to efficiently access via conventional synthetic methods.
Scheme 4: Diastereo- and Chemoselective Dual Oxidation of B(pin)-substituted Allylic Alcohols: Synthesis of Epoxy Alcohols, 2-Keto-anti-1,3-diols and Dihydroxy-tetrafuran-3-ones.
Hussain, Nusrah, "Chemo-, Diastero-, and Regioselective C−c and C−o Bond formation via Transition Metal Catalysis" (2015). Publicly Accessible Penn Dissertations. 1066.