Biocatalytic, Intermolecular C−H Bond Functionalization for the Synthesis of Enantioenriched Amides

Abstract

Supporting information for this article is given via a link at the end of the document.

Enzymatic Nitrogen Insertion into Unactivated C–H Bonds

“…Since the first report of P450 nitrene transferase activity, the accumulated in-lab collection of evolved enzymes has been used to discover new catalysts with promiscuous reactivity towards increasingly challenging reactions. 9 The collection includes recently engineered enzymes for allylic C-H amination (with Opivaloyl-hydroxylammonium triflic salt (1)) 10,36,37 and benzylic amidation (with N-Acetyl-O-pivaloyl hydroxylamine (2)) 11 of benzene-containing substrates. To detect promiscuous activity at unactivated C-H bonds, the enzyme libraries were challenged with methylcyclohexane (3) and ethylcyclohexane (4), which were chosen as test substrates that might be accepted in the enzyme active site in place of the benzene ring and elicit functionalization at some position(s).…”

“…Studies were focused on the amidation (uAMD) system to allow comparison with earlier findings (see Section 12 of SI for a discussion of uPA reactions). 11 A free energy surface for the C-H amidation of 3 was calculated using density functional theory (DFT) on a simplified iron-porphyrin model system.…”

“…4A, blue reaction coordinate); the lowest energy transition structure (TS) for hydrogen atom transfer to the iron-nitrenoid ( 3 TSHAT 29.7 kcal/mol) was located on the triplet surface. 11 As the irreversible step that desymmetrizes the substrate, the regio-and enantioselectivity for the amidation of 3 is determined at 3 TSHAT. Unsurprisingly, DFT calculations of the model system did not exhibit significant energy differences between regioisomeric hydrogen atom transfer (HAT) TSs (see SI, Figure S12.2), indicating that the experimentally observed site-selectivity is induced through interactions within the binding pocket.…”

“…A competitive KIE of ~9.8 is consistent with irreversible HAT and a higher degree of tunneling than previously observed in these systems. 11 A non-competitive KIE of 4.8 indicates that HAT has at least partial turnover-limiting influence in the reaction of 3 (Figure 4A, blue). This contrasts with previously measured KIEs for the benzylic C-H amidation of ethyl benzene (Figure 4A, red, competitive KIE ~ 8.5 and non-competitive KIE ~1.0) -values consistent with irreversible 3 TS´HAT (Figure 4A, red reaction coordinate) preceded by nitrene formation as the sole rate-limiting step (non-competitive KIE ~1.0 since nitrene formation does not involve ethylbenzene).…”

“…9 Such new-to-nature activity has, however, been limited to insertions at electronically activated benzylic, allylic or propargylic positions (bond dissociation energy, BDE < 85 kcal/mol), restricting catalysis to only a minority of chemical structures. [10][11][12] The general heme-nitrene transferase activity for modification at the most abundant sites in organic molecules, unactivated C(sp 3 )-H bonds (BDE > 95 kcal/mol), which are representative of canonical P450 chemistry, remains unsolved. Both the increased bond strength and site-and stereo-differentiation among competing locations with highly similar chemical environments pose significant challenges, as can be appreciated through an examination of the synthetic C-H functionalization literature.…”