Group-Transfer Reactions of a Cationic Iridium Alkoxycarbene Generated by Ether Dehydrogenation

Generated by Ether Dehydrogenation

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Group-Transfer Reactions of a Cationic Iridium Alkoxycarbene

Scott M. Chapp and Nathan D. Schley*

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sı Supporting Information

ABSTRACT: Despite broad interest in metal carbene complexes,

there remain few examples of catalytic transformations of ethers that

proceed via alkoxycarbene intermediates generated by α,α-dehydro-

genation. We demonstrate that both neutral and cationic

alkoxycarbene derivatives are accessible via ether dehydrogenation

at a PNP( Pr) pincer-supported iridium complex (PNP( Pr) = 2,6-

bis((diisopropylphosphino)methyl)pyridine). Both cationic and

neutral alkoxycarbene complexes undergo group transfer imination

with azides, with the cationic derivative serving as a more eﬃcient

catalyst for cyclopentyl ether imination. Mechanistic studies support an iridium(I)dinitrogen complex as the resting state in the dark

and a role for light-promoted N dissociation. Isoamyl nitrite and phenyl ethyl ketene are also found to engage with the cationic

alkoxycarbene complex in formal alkoxide and O atom transfer reactions, respectively. In the former case an isolable

dialkoxyalkyliridium complex is obtained, representing only the second example of a structurally characterized dialkoxyalkyl complex

of a transition metal.

INTRODUCTION

isocyanate, and adamantyl azide (Ad-N ). These trans-

■

formations were proposed to occur via initial [2 + 2]

cyclization to give a 4-membered iridacyclic intermediate.

This reactivity is distinct from that of canonical group 6

alkoxycarbenes and argues for a signiﬁcant role for the high-

The selective activation and functionalization of inert C−H

bonds by homogeneous metal complexes has been an area of

signiﬁcant interest since pioneering studies on alkane C−H

−4

oxidative addition and alkane dehydrogenation.

These

lying, ﬁlled d orbital in reactions of d alkoxycarbenes.

foundational stoichiometric systems led the way to early

examples of catalytic alkane transfer dehydrogenation.

Further development of alkane dehydrogenation over the

intervening decades has revealed design criteria that favor

neutral, electron-rich, and thermally stable pincer iridium

complexes analogous to those initially applied to alkane

−9

In this context our group has been exploring the chemistry

of cationic bis(phosphine)iridium alkoxycarbene complexes

with the aim of developing systems with reduced metal-

alkoxycarbene backbonding and enhanced electrophilicity

relative to neutral variants. Prior to our studies, a single

example of THF dehydrogenation to give a cationic iridium

alkoxycarbene over a period of days had been reported by

0−14

dehydrogenation by Kaska and Jensen in 1996.

In contrast to the large body of work on alkane

dehydrogenation, the dehydrogenation of compounds bearing

heteroatom functionality has seen considerably less attention.

Jensen and Kaska reported the transfer dehydrogenation of

THF to a mixture of furan and dihydrofurans, and Goldman

has reported the α,β-dehydrogenation of alkyl amines.

More recently, Brookhart and Huang demonstrated that

neutral PCP pincer-supported iridium complexes catalyze the

transfer dehydrogenation of alkyl ethers to vinyl ether

products. The α,β-dehydrogenation of amines and ethers is

complicated by competing α,α-dehydrogenation to give

aminocarbene and alkoxycarbene complexes, respectively.

The formation of iridium alkoxycarbene complexes via ether

dehydrogenation and their reactivity has been studied

Schneider (Figure 1). We have since demonstrated facile

formation of cationic alkoxycarbene complexes via ether

dehydrogenation in Lewis base-directed intra- and intermo-

lecular examples, with evidence for reversible α-hydride

insertion and C−O bond cleavage reactions in certain

16,17

24,25

cases.

Despite the body of work on the generation of

alkoxycarbene complexes via ether dehydrogenation, few

stoichiometric transformations and only a single catalytic

21,26,27

reaction have been reported.

We now report the

Received: February 25, 2020

20,21

extensively both by Carmona and by Whited and Grubbs.

Grubbs’ work demonstrated that neutral iridium alkoxycar-

benes undergo group-transfer reactions with heterocumulenes

and certain 1,3-dipolar reagents including CO , phenyl

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

tert -butoxide gives the neutral alkoxycarbene complex 4 (eq 3),

Article

Figure 2. ORTEP diagrams of 1 (left) and 3 (right) shown at 50%

probability. Full disorder models are omitted for clarity. Selected

bond distances in 3 (Å): IrC 1.880(12), IrN 2.179(8), and

OC

1.342(16).

carbene

Figure 1. Recent ether-derived alkoxycarbenes.

Work by Milstein has shown that metal-bound bis-

phosphinomethyl)pyridines like L1 can undergo deprotona-

(

synthesis of a cationic pincer-supported Ir(I) alkoxycarbene

complex generated by α,α-dehydrogenation of cyclopentyl

methyl ether (CPME) and demonstrate its reactivity in atom-

and group-transfer reactions with an alkyl azide, a ketene, and

an alkyl nitrite. This complex serves as a catalyst in ether

imination, showing that cationic iridium complexes are

suﬃciently competent at ether dehydrogenation to support

catalytic transformations.

tion at the α-phosphino methylene position to give a formally

monoanionic PNP ligand. Treatment of 3 with potassium

RESULTS AND DISCUSSION

Treatment of commercially available [(cod) Ir]BAr with 2,6-

bis((diisopropylphosphino)methyl)pyridine (L1) gives com-

plex 1 (eq 1 ) which undergoes hydrogenation to an iridium

■

which can be distinguished by a shift of the IrCHOR 13_C

resonance from 234.7 to 221.5 ppm. The bond metrical

parameters obtained from the crystal structure of 4 show

signiﬁcant Kekule

contraction of one exocyclic C−C bond consistent with those

distortion of the pyridine moiety and

34,35

observed in other systems (Figure 3 ).

The IrC

carbene

Figure 3. ORTEP diagram of 4 shown at 50% probability. Selected

bond distances (Å): IrC

¹^.⁸⁸²⁽³⁾^,^I^r^^Npy 2.096(3), and O

dihydride complex 2 (eq 2). Dehydrogenation of 2 with tert-

carbene

Ccarbene 1.342(4).

butylethylene in CPME yields a product with a single P{ H}

NMR signal and a broad, downﬁeld-shifted H resonance at

3.5 ppm. This signal is in the range expected for an

bond length in 4 is 1.882(3) Å, which is indistinguishable from

the bond length of 1.880(12) Å in 3 and similar to that of a

neutral (PNP)Ir(I) alkoxycarbene complex reported by

28−30

alkoxymethylidene IrCHOR resonance,

which along

with the appearance of a ¹³C{ H} resonance at 234.7 ppm led

us to assign the product as cationic alkoxycarbene complex 3, a

site of CPME activation distinct from the 3,4-dehydrogenation

Grubbs (1.884(4) Å) as well as a cationic iridium(I)-

alkoxycarbene (1.912(4) Å) we have previously reported.

observed in a related nonpincer complex. Numerous

attempts at conﬁrmation of this assignment through X-ray

crystallography ultimately led to a structure solution for 3 from

samples crystallizing as 3-component merohedral twins in the

P2 space group (Figure 2). The unambiguous characterization

of 3 represents one of only a few cationic iridium

Thus, the iridium carbene bond length in 3 and 4 appear

insensitive to the net ionic charge. In contrast, these values are

shorter than those observed for a cationic iridium(III)

alkoxycarbene reported by Schneider (1.938(3) Å) as well

as a trio of cationic iridium(III) alkoxybenzylidenes we have

24,25

previously reported (1.997(3), 1.997(8), and 2.001(2)).

alkoxycarbene complexes generated by ether dehydrogen-

ation.

These observations suggest that iridium alkoxycarbene bond

lengths are most sensitive to the formal oxidation state.

3−25,32

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

corresponding Ir −N complexes 6 and 7 , respectively (eqs 4

Article

The structural similarity of complex 4 to the family of

neutral PNP(Ir) alkoxycarbenes prepared by Whited and

Grubbs via ether dehydrogenation encouraged a closer

comparison of the two systems. In particular, we were

interested in whether the reactivity of the neutral complex 4

toward alkyl azides would match that observed by Grubbs for

the analogous complex shown in Figure 1. Additionally,

cationic derivative 3 could be compared directly to 4 in order

to ascertain whether the net ionic charge modulates the

reactivity of alkoxycarbenes toward group-transfer reactions.

improved through the slow addition of Ad-N over 30 h,

21,27

leading to a catalytic system capable of 10 turnovers.

By comparison, cationic complex 3 serves as a competent

catalyst for the group-transfer imination of CPME under batch

conditions without requirement for slow azide addition. Under

optimized conditions under blue light irradiation we observe

8.3 turnovers to give 83% yield of formimidate 5 (Table 1,

Table 1. Evaluation of Reaction Conditions

Indeed, both 3 and 4 react rapidly with Ad-N at room

temperature to give N-adamantyl formimidate 5 and the

entry

catalyst (mol %)

T (°C)

t (h)

light source

yield

3 (10%)

3 (5%)

3 (10%)

4 (10%)

3 (10%)

blue

none

blue

0.4%

0.8%

83%

39%

70%

29%

68%

84%

1 (10%)

NMR yield. Average of two experiments.

entry 3). The majority of turnovers occur within the ﬁrst hour

entry 7), demonstrating rapid catalysis without the sensitivity

(

to excess azide observed in the neutral Grubbs system. In a

and 5). N binding can be conﬁrmed by analysis of their

series of experiments we found that 3 reacts productively with

−

infrared spectra, which show an Ir−N band at 2141 cm for 6

Ad-N to give 5 at 23 °C, but that a reaction temperature of 90

−

and 2076 cm for complex 7. For comparison, a neutral

PNP)IrN complex reported by Grubbs absorbs at 2067

cm , demonstrating a clear decrease in Ir−N backbonding

for the cationic variant 6. The assignment of 6 and 7 was

conﬁrmed by single-crystal X-ray diﬀraction of both species

°C under blue light irradiation is required for productive

(

catalysis. When a catalytic reaction is examined in the dark by

−

P{ H} NMR after exposure to blue light at 90 °C for 5 or 15

min, complex 6 appears to be the major metal-containing

species in solution. This observation supports a role for light in

N₂dissociation and is consistent with 6 being on-path

intermediate in catalysis (Scheme 1). Although 3 closely

matches the total turnover number achieved by the Grubbs

system, the tolerance of batch conditions rather than a

requirement for azide slow addition is a marked improvement.

In contrast, the neutral complex 4 achieves only 2.9 turnovers

under comparable batch conditions (entry 7). The neutral N₂

(

Figure 4) and by conversion of 6 to 7 on treatment with

potassium tert -butoxide.

Scheme 1. Proposed Catalytic Cycle for Group Transfer

Imination of CPME

Figure 4. ORTEP diagrams of 6 (left) and 7 (right) shown at 50%

probability. Selected bond distances (Å) for complex 6 (IrN

.915(5), IrN 1.994(5), NN 1.105(8)) and complex 7 (IrN

.881(5), IrN 2.053(4), NN 1.083(7)).

An analogous azide-alkoxycarbene group-transfer reaction

has been used previously as part of a synthetic cycle for the

imination of ethers to formimidates. The incompatibility of the

system with excess azide reagent initially precluded the

development of a true catalytic reaction, but through stepwise

addition under photolysis, four turnovers could be obtained

Both [2 + 2] (B) and [2 + 3] azide-carbene cycloaddition

mechanisms have been previously proposed.

over several days. These conditions were subsequently

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

complex 7 shows sensitivity to excess Ad-N , degrading to

several unidentiﬁed species over minutes to hours under

irradiation. In the case of catalysis by the cationic complex 3,

total turnover numbers appear to be limited by catalyst

degradation, as P{ H} NMR analyses show numerous

unidentiﬁed products after cessation of catalysis.

The success of complex 3 as an ether imination catalyst

inspired us to examine the precursor, complex 1, for the same

transformation. Dissociation of 1,5-cyclooctadiene would

−

provide access to a three coordinate 14 e Ir(I) fragment A

(

Scheme 1) analogous to those proposed as reactive

intermediates in both α−CH activation of CPME and

12−14,23,36

CH activation of alkanes.

Under our optimized

conditions complex 1 shows comparable activity to 3 with 8.4

TON (84%) (Table 1, entry 8). This observation should

simplify future reaction development since derivatives of

ligands related to L1 can be accessed in one step from

commercially available iridium starting materials. Indeed the

Bu analogue of 1 has been previously reported.

A proposed mechanism for catalytic CPME imination by 3 is

given in Scheme 1. Reaction of 3 with Ad-N could proceed via

Figure 5. ORTEP diagram of 8 shown at 50% probability. The full

disorder model and anion are omitted for clarity. Selected bond

distances (Å): IrC

1.828(5), C

C

1.304(6), and IrNpy

.124(4).

either initial [2 + 2] (B) or [2 + 3] cycloaddition, after

which extrusion of formimidate 5 would give N complex 6.

Thus, phenyl ethyl ketene appears to serve as an oxygen atom

donor rather than a carbene equivalent. This outcome is

interesting when considered alongside the reported reactivity

Light-promoted dissociation of dinitrogen would give the 14

−

e Ir(I) fragment A, which is likely the species responsible for

CPME activation to regenerate 3.

In total, the stoichiometric reactivity of alkoxycarbene

of phenyl isocyanate with a related neutral alkoxycarbene,

which serves as a nitrene source despite the electronic

complexes 3 and 4 with Ad-N closely mirrors observations

similarity of ketenes and isocyanates. Ketenes undergo

thermal [2 + 2] reactions with a variety of substrates including

simple oleﬁns, but such reactivity engages the CC fragment

made by Whited and Grubbs. In our case, complex 3 was

found to serve as a catalyst for group transfer imination of

CPME without requirement for portionwise or slow addition

of azide, while the neutral complex 4 appears to share the

of the ketene moiety. In our case the observed group transfer

reaction of phenyl ethyl ketene likely requires that initial [2 +

2] cycloaddition occur via the CO fragment, implicating the

pair of stepwise or concerted, asynchronous nucleophilic

additions shown in Scheme 2 rather than a concerted,

Grubbs system’s reported sensitivity to excess azide.

As complexes of square planar d metal ions, 3 and 4 bear a

ﬁlled high-lying d_z²orbital which has been implicated in so-

called Roper-type carbene chemistry of which group-transfer

1,22,39,40

reactions of azides represent one example.

Other

Scheme 2. Proposal for Ketene O-atom Transfer

electrophiles including CO , carbonyl sulﬁde, and phenyl

isocyanate have been demonstrated to give formate esters,

22,30

thioformates, and formimidates, respectively.

We sus-

pected that other substrates might undergo similar group-

transfer reactions via what has been proposed as an initial [2 +

] cycloaddition

to alkoxycarbene 3 and identiﬁed

diazoalkanes, ketenes, and alkyl nitrites as possible candidates

for CC or CO bond-forming chemistry. Just as alkyl

azides are observed to transfer a formal nitrene equivalent, we

41−45

hypothesized that diazoalkanes and alkyl aryl ketenes

might serve as carbene equivalents to give the products of

formal carbene−carbene cross-coupling.

Surprisingly, complex 3 is largely unreactive toward either

one equivalent or an excess of trimethylsilyldiazomethane at

room temperature. While elevated temperatures or irradiation

with blue light did lead to partial consumption of 3, the N₂

adduct 6 is observed only in trace quantities and no organic

product of carbene transfer was detected. In contrast,

treatment of 3 with 25 equiv of phenyl ethyl ketene gives

cyclopentyl formate and two new iridium-containing species in

a ratio of 95:5 by ³¹P{ H}NMR (eq 6).

synchronous [2 + 2] process. A closely related mechanism

has been proposed for a series of oxygen atom transfer

9,50

reactions of an isolable niobocene ketene complex,

demonstrating the O-nucleophilicity of α-metalloketenes.

The structure of a (diphenylketene-κ O,C )iridium(I) complex

reported by Grotjahn is also consistent with this proposal,

We have characterized the minor product as the Ir(I)−CO

though O atom transfer from a diphenylketene-κ C ,C adduct

complex 9 by doping with an authentic sample of 9 generated

may also be possible.

independently (see the Supporting Information). The major

product was separable by crystallization and found to be the

unexpected cationic iridium vinylidene complex 8 (Figure 5).

Previous studies by Whited and Grubbs were limited to

group- or atom-transfer reactions of heterocumulenes, but the

possibility that 3 might be capable of nonconcerted group

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

transfer reactions encouraged us to explore other electrophiles.

a single crystallographically characterized example on Pd.

One promising reagent−isobutyl nitrite was found to react

Subsequent alkoxide transfer to the alkoxycarbene would give

10. A related reverse reaction, the formation of an N-

coordinated alkyl nitrite via alkoxide attack at a metal nitrosyl,

with 3 to give a single new complex with a major P{ H}

NMR signal at 34 ppm in 92% yield. We have characterized

this species as the iridium dialkoxyalkyl nitrosyl complex 10

resulting from formal alkoxide group-transfer and scission of

the N−O bond. Transition-metal dialkoxyalkyls are proposed

as tetrahedral intermediates in the alkoxide exchange of

alkoxycarbene complexes; however, there is only a single

report of a transition-metal dialkoxyalkyl complex in the

Cambridge crystallographic database. In that case, nucleophilic

has been previously observed at Ir.

CONCLUSION

■

In summary, we report the synthesis of cationic and neutral

PNP( Pr) iridium alkoxycarbene complexes via transfer

dehydrogenation of CPME. Both alkoxycarbene complexes

react with Ad-N to form the corresponding Ir−N complex

attack of NaMn(CO) on di(phenoxy)chloromethane gave the

and a formimidate resulting from formal nitrene transfer to the

alkoxycarbene. This reactivity mirrors observations by Whited

and Grubbs on a related neutral iridium alkoxycarbene system.

We have translated this stoichiometric reactivity to catalysis in

both the cationic and neutral alkoxycarbene cases; however,

the cationic complex 3 shows superior performance. Under

optimized batch conditions 3 displays 7 TON within the ﬁrst

hour, comparing favorably against the previously reported

system which requires slow addition of azide over more than

corresponding manganese diphenoxyalkoxyalkyl, making 10

the only isolable dialkoxyalkyl generated by alkoxide transfer to

an alkoxycarbene.

4 h for comparable turnover numbers. The precursor complex

[(PNP( Pr) )Ir(η -cod)]BAr (1) is also found to serve as an

4 4

active catalyst with similar performance.

In addition, we have further expanded the scope of reagents

which undergo atom- or group-transfer reactions with iridium

alkoxycarbenes to include phenyl ethyl ketene and isobutyl

nitrite. These experiments have led to the isolation of a ketene-

derived iridium vinylidene complex and the ﬁrst late transition-

metal dialkoxyalkyl complex to be structurally characterized.

The reactivity of 3 with phenyl ethyl ketene and isobutyl nitrite

is suggestive of stepwise processes rather than a concerted [2 +

The solid-state structure of 10 shows a square pyramidal

complex containing a bent nitrosyl with an IrNO angle of

23.0(5)° and a vacant site trans to the nitrosyl ligand, an

arrangement shared by all other reported bent, 5-coordinate

3−58

iridium nitrosyls.

Though we have been successful in

characterizing 10 in the solid state by both single-crystal X-ray

diﬀraction (Figure 6 ) and elemental analysis, the instability of

] process. These results add to our understanding of

alkoxycarbene complexes of d metal ions and show promise

for future development for new functionalization reactions of

ethers that proceed through alkoxycarbene intermediates.

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00609.

■

Experimental procedures, X-ray crystallographic data,

and compound characterization data (PDF )

Accession Codes

Figure 6. ORTEP diagram of 10 shown at 50% probability. The full

disorder model and anion are omitted for clarity. Selected bond

distances (Å) and angles (deg): IrC 2.040(6), IrN 2.167(5),

CCDC 1986273−1986281 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

OC O 110.2(5), IrNO 123.0(5).

0 in solution has precluded the collection of high quality H

and C{ H} NMR data. Nonetheless, the dialkoxyalkyl H

resonance can be identiﬁed as a triplet occurring at 7.18 ppm

with J ₌₄_.₇_H_z_,_w_h_i_c_h_c_o_l_l_a_p_s_e_s_t_o_a_b_r_o_a_d_s_i_n_g_l_e_t_o_n31

AUTHOR INFORMATION

Corresponding Author

■

decoupling. This is in good agreement with the reported

manganese dialkoxyalkyl which resonates at 7.38 ppm. There is

no reported C chemical shift for the manganese dialkox-

Nathan D. Schley − Department of Chemistry, Vanderbilt

University, Nashville, Tennessee 27235, United States;

orcid.org/0000-0002-1539-6031; Email: nathan.schley@

vanderbilt.edu

yalkyl; however, an HSQC experiment with 10 shows a

correlation with a C resonance at 94.2 ppm, which aligns well

with an iridium hydroxyaminoalkyl reported to resonate at 96.9

Author

ppm.

Scott M. Chapp − Department of Chemistry, Vanderbilt

University, Nashville, Tennessee 27235, United States;

orcid.org/0000-0001-6293-7948

The conversion of 3 to 10 presumably occurs via initial

binding of Ir to the electrophilic N atom of isobutyl nitrite.

This binding mode has been inferred previously on the basis of

IR and NMR data at complexes of Ru and Ir and shown in

Complete contact information is available at:

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

https://pubs.acs.org/10.1021/acs.inorgchem.0c00609

Article

(14) Kumar, A.; Bhatti, T. M.; Goldman, A. S. Dehydrogenation of

Alkanes and Aliphatic Groups by Pincer-Ligated Metal Complexes.

Chem. Rev. 2017, 117 (19), 12357−12384.

Notes

15) Gupta, M.; Kaska, W. C.; Jensen, C. M. Catalytic dehydrogen-

The authors declare no competing ﬁnancial interest.

ation of ethylbenzene and tetrahydrofuran by a dihydrido iridium P −

C −P pincer complex. Chem. Commun. 1997, No. 5, 461−462.

ACKNOWLEDGMENTS

(16) Lu, Y. J.; Zhang, X.; Malakar, S.; Krogh-Jespersen, K.;

■

Hasanayn, F.; Goldman, A. S. Formation of Enamines via Catalytic

Dehydrogenation by Pincer-Iridium Complexes. J. Org. Chem. 2020,

Funding from Vanderbilt University is gratefully acknowl-

edged. This material is based upon work supported by the

National Science Foundation under grant no. CHE-1847813.

Acknowledgment is made to the Donors of the American

Chemical Society Petroleum Research Fund for partial support

of this research.

17) Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Novel synthesis

5, 3020.

of enamines by iridium-catalyzed dehydrogenation of tertiary amines.

Chem. Commun. 2003, No. 16, 2060−2061.

18) Lyons, T. W.; Bezier, D.; Brookhart, M. Iridium Pincer-

Catalyzed Dehydrogenation of Ethers Featuring Ethylene as the

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