Bystanding F+ oxidants enable selective reductive elimination from high-valent metal centers in catalysis

Abstract

Reductive elimination from partially or completely oxidized metal centers is a vital step in a myriad of carbon-carbon and carbon-heteroatom bond-forming reactions. One strategy for promoting otherwise challenging reductive elimination reactions is to oxidize the metal center using a two-electron oxidant (that is, from M((n)) to M((n+2))). However, many of the commonly used oxidants for this type of transformation contain oxygen, nitrogen, or halogen moieties that are subsequently capable of participating in reductive elimination, thus leading to a mixture of products. In this Minireview, we examine the use of bystanding F(+) oxidants for addressing this widespread problem in organometallic chemistry and describe recent applications in Pd(II) /Pd(IV) and Au(I) /Au(III) catalysis. We then briefly discuss a rare example in which one-electron oxidants have been shown to promote selective reductive elimination in palladium(II)-catalyzed C-H functionalization, which we view as a promising future direction in the field.

Scheme 1

BQ-promoted reductive elimination from a Pd(II) intermediate in a Pd(II)-catalyzed C–H activation/C–C cross-coupling reaction (Yu et al., 2006).^[6d]

Scheme 2

CuCl₂-promoted reductive amination in a Pd(II)-catalyzed olefin diamination reaction (Muñiz et al., 2008).^[7] DMF = N,N-dimethylformamide.

Scheme 3

Possible outcomes of reductive elimination from an octahedral Pd(IV) intermediate.

Scheme 4

The first report of diastereoselective Pd(II)-catalyzed C(sp³)–H iodination via Pd(II)/Pd(IV) catalysis (Yu et al., 2005).^[13b] DCM = dichloromethane. RT = room temperature.

Scheme 5

The first report of Pd(II)-catalyzed C–H fluorination via Pd(II)/Pd(IV) catalysis (Sanford et al., 2006).^[28a]

Scheme 6

Selective C–F reductive elimination using catalytic Pd(OTf)₂ with benzyltriflamide substrates (Yu et al., 2009).^[28b] DCE = 1,2-dichloroethane. NMP = N-methyl-2-pyrrolidone.

Scheme 7

Possible outcomes of reductive elimination from an octahedral Pd(IV) intermediate in our studies of Pd(II)-catalyzed C–H fluorination.^[28b]

Scheme 8

C–F reductive elimination from a Pd(IV) complex (Furuya and Ritter, 2008).^[18b] o-Nos = 2-nitrobenzenesulfonyl. DMSO = dimethylsulfoxide.

Scheme 9

C–F reductive elimination from a Pd(IV) complex (Ball and Sanford, 2009).^[18c] tBu-bpy = 4,4′-di-tert-butyl-2,2′-bipyridine.

Scheme 10

Heterocycle formation via C–H activation/C–Y cyclization.

Scheme 11

Challenges in selective C–Y reductive elimination from Pd(IV) intermediates. “Ox¹–Ox²” represents a generic oxidant.

Scheme 12

Pd(II)-catalyzed C–H amination (Buchwald et al., 2005).^[30]

Scheme 13

Pd(II)-catalyzed intramolecular C–H amination using a bystanding F⁺ oxidant (Yu et al., 2009).^[16]

Scheme 14

Pd(II)-catalyzed intramolecular C–H etherification using a bystanding F⁺ oxidant (Yu et al., 2010).^[34]

Scheme 15

Selective C–C reductive elimination induced by XeF₂ (Vigalok et al., 2003).^[27]

Scheme 16

Pd(II)-catalyzed olefin carboamidation using a bystanding F⁺ oxidant for selective reductive elimination from a Pd(IV) intermediate (Michael et al., 2009).^[41,42]

Scheme 17

The first example of Pd(II)-catalyzed C–H trifluoromethylation (Yu et al., 2010).^[45] TFA = trifluoroacetic acid.

Scheme 18

Selective C–CF₃ reductive elimination from a Pd(IV) complex induced by a bystanding electrophilic N–F reagent (Sanford et al., 2010).^[44b]

Scheme 19

Electrophilic iodination of a [Au(I)–vinyl] intermediate in an Au(I)-catalyzed cyclization reaction (Buzas and Gagosz, 2006).^[51a]

Scheme 20

Electrophilic fluorination of a [Au(I)–vinyl] intermediate in an Au(I)-catalyzed cyclization reaction (Gouverneur et al., 2008).^[52a]

Scheme 21

Selectfluor (62) as a bystanding oxidant in C–O bond–forming Au(I)/Au(III) catalysis (Zhang et al., 2009).^[59]

Scheme 22

Selectfluor (62) as a bystanding oxidant in C–C bond–forming Au(I)/Au(III) catalysis (Zhang et al., 2009).^[60]

Scheme 23

Selectfluor (66) as a bystanding oxidant in olefin oxyarylation and aminoarylation via Au(I)/Au(III) catalysis (Zhang et al., 2010).^[61] Ts = p-toluenesolfonyl.

Scheme 24

Selectfluor (66) as a bystanding oxidant in olefin aminoarylation via Au(I)/Au(III) catalysis (Toste et al., 2010).^[62] dppm = 1,1,-bis(diphenylphosphino)methane.

Scheme 25

A three component coupling reaction via Au(I)/Au(III) catalysis using Selectfluor (66) (Toste et al., 2010).^[63] Phth = phthaloyl.

Scheme 26

Intramolecular oxidative arylation of a [Au(I)–vinyl] intermediate via Au(I)/Au(III) catalysis using Selectfluor (66) as a bystanding oxidant (Gouverneur et al., 2010).^[64]

Scheme 27

Oxidation of Au(I) complex 97 with an F⁺ reagent (Mankad and Toste, 2010).^[65]

Scheme 28

Reactivity of 98 and 100 complexes with PhB(OH)₂ (Mankad and Toste, 2010).^[65]

Scheme 29

Depiction of one-electron oxidation events that can take place with a [Pd(II)–R] intermediate.

Scheme 30

Ce(IV) as a one-electron oxidant for selective C–N reductive elimination in an intramolecular Pd(II)-catalyzed C–H amination reaction (Yu et al., 2009).^[16]

Scheme 31

C–C reductive elimination promoted by Cp₂Fe⁺, a one-electron oxidant (Mayer and Sanford et at., 2009).^[6g]

Scheme 32

Proposed mechanism for C–C reductive elimination (Mayer and Sanford et at., 2009) ^[6g]