Unveiling the Takai Olefination Reagent via Tris(

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Unveiling the Takai Olefination Reagent via Tris(tert-butoxy)siloxy Variants Daniel Werner, and Reiner Anwander J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08739 • Publication Date (Web): 14 Sep 2018 Downloaded from http://pubs.acs.org on September 14, 2018

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Journal of the American Chemical Society

Unveiling the Takai Olefination Reagent via Tris(tert-butoxy)siloxy Variants Daniel Werner and Reiner Anwander* Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der Morgenstelle 18, 72076, Germany KEYWORDS: Alkylidene, Olefination, Organochromium, Iodoform, Siloxy ABSTRACT: The elusive Takai olefination reagent, namely the iodo-methylidene Cr(III) complex [Cr2Cl4(CHI)(thf)4], has been isolated by careful handling of the reaction between CrCl2 and CHI3 in THF at -35 °C. Alternatively, treatment of [Cr(OSi(OtBu)3)2] with CHI3 gave the mixed-valent di-halido-methanide complex [CrII/III2I2(OSi(OtBu)3)2(CHI2)], featuring a Cr(III)–CHI2 moiety. In the presence of TMEDA nucleophilic attack at CHI2 occurred generating the zwitterionic species [CrIII(OSi(OtBu)3)2(tmeda-CHI)][I]. Complexes [Cr2Cl4(CHI)(thf)4] and [CrII/III2I2(OSi(OtBu)3)2(CHI2)] were screened for their ability to induce mono-halido olefination of benzaldehyde. Remarkably, both complexes promote olefination, with [Cr2Cl4(CHI)(thf)4] accomplishing the same E selectivity as Takai´s original mixture. Complex [CrII/III2I2(OSi(OtBu)3)2(CHI2)], however, gave strictly the Z isomer, confirming the mono-iodo-methylidene species Cr(III)–CHI–Cr(III) as the active olefination component of the original in situ generated Takai reagent mixture.

Introduction Over thirty years ago,1 Takai and co-workers reported the Eselective mono-halido-olefination of aldehydes (= conversion of aldehydes to vinyl iodides) by an in situ generated organochromium species (Scheme 1). Such a discovery was fundamental to the development of olefination chemistry, and due to the relative ease of synthesis (namely stirring CrCl2 and CHI3 in THF, Scheme 1),2,3 it has been a popular reagent in the total synthesis of natural products ever since.4-9 Despite its ubiquitous use in chemistry laboratories, clarification of this enigmatic organochromium reagent has remained elusive in view of its high reactivity. But it has been hypothesized that the active species is either a chromium di-halido-methanide species (Scheme 1, A) or more favorably the ostensible dichromium mono-halido-methylidene (Scheme 1, B). This uncertainty has been extensively studied over the years,10-12 substantiating species B as the likely candidate. Yet as neither types (A or B) were ever isolated, both must still be considered as the active species, especially as metal-halidomethanide complexes can initiate the olefination of aldehydes.13 Irrespective of the active species, the formation of either an A- or B-type complex is intriguing, considering the difficulty in studying the chemistry of early transition metal complexes bearing halogenated alkyl ligands.14-19 By alteration of the CrCl2/CHI3 stoichiometry, and addition of an appropriate Lewis base like TMEDA (N,N,N΄,N΄tetramethylethylenediamine), Takai and co-workers dramatically altered the behavior of the CrCl2/CHI3 reaction mixture. Now, the in-situ generated species initiates the mono-halidocyclopropanation of terminal alkenes, via reduction of CrIII (Scheme 1),20 a process which occurs in preference over ole-

fination of co-carbonyl groups. The cyclopropanation mechanism was recently elucidated by Takai et al. upon the successful isolation of a remarkably stable trimethylsilyl derivative of the B-type complex, [Cr2Cl4(CHSiMe3)(tmeda)2].21 This complex is part of a small family of structurally characterized chromium alkylidenes.22-26 Interestingly, the latter complex only induced carbonyl olefination upon the removal of the coordinating Lewis base (e.g., TMEDA). Scheme 1. Generation of the Takai Olefination Reagent (Type A or B), and Reaction with either Aldehydes Giving E-IodidoFunctionalized Olefins or Alkenes (in the Presence of TMEDA), Causing Cyclopropanation and Reduction to CrII.

x CrIICl2 + y CHI3

O

THF I

THF/DMF

I

R'

H

(x = 6 or 8; y = 2)

CH

Cl Cl

+ CrIICl2

H

R'

+ "CrIIIO"

I CrIII

I

HC

HC or

A

CrIII

CrIII

Cl Cl

Cl Cl

R

CH2 B

TMEDA (x = 4; y = 2)

R

H

H C

H

I

H2C + CrIICl2(tmeda)2

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Scheme 2. Synthesis of both A- and B-Type Organochromium Reagents (cf., Scheme 1) from Chloride (according to Takai)1 and Tris(tert-butoxy)siloxide Precursors type B

type A

I thf

thf

CX CrIII

thf Cl

CrIII Cl

Cl

thf Cl

[CrIII2Cl4(CXI)(thf)4] X = H (1), X = D

THF, -35 °C L = Cl (n = 4) - 2 [CrIIICl2I(thf)3] (2)

C6D6, rt L = OSi(OtBu)3 (n = 2)

CrIIL2 + 1/n CXI3 -

[CrIIICrIIL4I]

I

I CX

O

I

CrIII

(6a)

CrII O

tBu O

Takai protocol performed @-35 °C

Si(OtBu)2 O tBu

I

Si(OtBu)2 [CrIIICrIIL2I2(CXI2)]

(1D)

X = H (5a), X = D (5aD)

Takai Olefination Reagent

To shed further light on the identity of the Takai olefination reagent, we set out to expand the structural chemistry of such CrCHX complexes (X = halogenido), aiming at either a chromium di-iodo-methanide (A type), or the iodido-methylidene (B type) complex. It was envisioned that use of a more flexible electron-withdrawing ligand, namely tris(tert-butoxy)siloxy (OSi(OtBu)3),27 a ligand we are familiar with,28 would enable formation of more stable derivatives permitting isolation.29 In addition, this siloxy ligand would impart enhanced solubility and thus facilitate cleaner reactions of the respective chromium(II) precursor with the oxidant or carbene source. Herein, we report the isolation of the original chromium iodomethylidene complex, [Cr2Cl4(CHI)(thf)4] (1, type B), along with a siloxy-supported A-type complex, [CrII/III2I2(CHI2)(OSi(OtBu)3)2]. Both complexes are highly reactive, but more importantly, their formation allowed direct comparisons between both A- and B-type complexes in the mono-halido olefination of benzaldehyde. We also examined the role of TMEDA in such organochromium complexes, revealing that it can insert into the CHI2 moiety to generate a new bidentate CNN ligand (type C).

[CrCl2I(thf)3] (2) was obtained, and if the reaction was kept at -35 °C the reaction was sluggish and 1 could not be not isolated. Although the crystals were of suitable size and apparent visual quality, attempted characterization by X-ray crystallography proved difficult. Nevertheless, the overall connectivity gave Cr(III) complex 1 as the formula (Figure 1, Figure S1), with the overall structural motif matching that of the TMEDAligated trimethylsilyl derivative [Cr2Cl4(CHSiMe3)(tmeda)2].21 Complex 1 features two terminal and two bridging chlorido ligands, four terminal thf ligands (in lieu of TMEDA), and one bridging CHI methylidene. The crystals diffracted poorly and showed significant disorder. For instance, it appeared that atom Cl2 and the CHI moiety are disordered over the same positions, and the terminal atom Cl4 ligand appeared disordered over multiple positions. To overcome this difficulty, many different crystallization attempts were tried, but 1 proved either too sensitive (e.g., in non-coordinating solvents), or the obtained crystals diffracted poorly due to large solvent voids. Due to the poor quality of the crystal data a detailed discussion of bond lengths and angles has to be ruled out. It is very likely that previous reports do not mention isolation or crystallographic characterization based on these uncertainties.

Results and Discussion Synthesis and Solid-state Structures of Organochromium Reagents The Takai Olefination Reagent. Studies concerning the Takai olefination reagent hardly discuss the isolation of the active species,1,12,20 as it is always prepared in situ. Better to comprehend the olefination process and construct strategies to improve its efficiency, we initially probed the accessibility of the active species from the reaction of CrCl2 and CHI3 in THF. Not unexpectedly this endeavor proved difficult, but the chlorido-ligated iodo-methylidene complex, [Cr2Cl4(CHI)(thf)4] (1) could be isolated from the reaction mixture. A cold (-35 °C) THF solution of CHI3 was added carefully to an equally chilled THF slurry of CrCl2 and stirred for half an hour (slowly warming to ambient temperature). Upon quick filtration followed by immediate storage at -35 °C compound 1 could be harvested as highly sensitive bright red crystals. Crystalline 1 could only be obtained when THF of high purity and recently purchased CrCl2 was used. Furthermore, the role of varying temperature was also crucial to the isolation of 1, when the reaction was performed at ambient temperature only

Figure 1. Connectivity of [Cr2Cl4(CHI)(thf)4] (1), found in the solid state. Hydrogen atoms (with the exception of H1) are omitted for clarity. Due to the poor quality of the data, a ball and stick representation visualizes the connectivity. A detailed discussion of interatomic distances and angles has to be ruled out. For more crystallographic details, see supporting information. Although 1 could not be characterized satisfyingly by X-ray crystallography, it could be dried and handled under an argon


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Journal of the American Chemical Society atmosphere as a red micro-crystalline powder, the elemental analysis of which indicated partial loss of THF, giving the formula [Cr2Cl4(CHI)(thf)3.5] (1-THF). The 1H NMR spectrum of 1 did not show any resonances,30 but the deuterated version of 1-THF, namely [Cr2Cl4(CDI)(thf)3.5] (1D-THF), obtained by performing the reaction with CDI3, gave three signals in the 2D NMR spectrum (performed in non-deuterated THF). Two signals were attributed to deuterated THF-d (due to an apparent deuteride exchange process, see ESI), and a resonance at 2.10 ppm, which disappeared within 5 h, tentatively assigned as the CDI ligand (Figure S2). Complex 1 is not stable at ambient temperature, neither in THF solution nor in the solid state, as indicated by discoloration. Such instability is in stark contrast to the stable trimethylsilyl-derivative [Cr2Cl4(CH(SiMe3))(tmeda)2] reported recently.21 Crucially, the highest obtained isolated yield of 1 was 47% (based on CHI3), while the yield of the orange co-product [CrCl2I(thf)3] (2, for crystallographic data, see ESI, Figure S3), was consistently higher than 100% when assuming the only two products of the reaction mixture are 1 and 2 in equal amounts (Scheme 2). This indicates the occurrence of a competing reaction pathway during the synthesis of 1, prior to methylidene formation. This pathway is likely either di-iodoethene elimination of the transient di-halido-methanide (see Scheme S1), or a competing halogenido exchange reaction between CrCl2 and CHI3, where the resulting CHClxI3-X/CrI2-yCly species undergoes a different reaction path, which does not result in formation of 1. Either process effects subsequent olefination reactions (vide infra), and is the reason that the reagents must be employed in excess for efficient olefination.1 Nevertheless, storage of 1 under argon at -35 °C minimized decomposition allowing 1 to be used stoichiometrically. Tris(tert-butoxy)siloxy Derivatives. Chlorido/tris(tertbutoxy)siloxy ligand exchange was initially performed by adding four equivalents of [K(OSi(OtBu)3] during the synthesis of 1, prior to filtration from 2 (not shown in Scheme 2). Although this led to a myriad of apparent species, a few red

crystals of a bis(siloxy) derivative to 1, namely [Cr2Cl2(OSi(OtBu)3)2(CHI)(thf)4] (3, Figure 2 left), were hand-picked from the reaction mixture. The structural data of 3 were of sufficient quality and confirmed the same dimeric structural motif as found for 1 and [Cr2Cl4(CHSiMe3)(tmeda)2].21 The iodo-methylidene is bridging at an almost equal distance between the two chromium centers, with bond lengths (av. 2.041(3) which are closer to those of [Cr2(C5Me5)2(µ-CH3)2(µ-CH2)] (2.033(7) & 2.050(8) Å)22 or [Cr2(µ-Cl)2(µ-CHMe)(η5-Py’)] (2.048(13) Å, Py’ = 2,5dibutylpyrrol-1-ide),25 rather than that of [Cr2Cl4(CHSiMe3)(tmeda)2] (2.082(3) Å).21 This can be rationalized by the differences in steric constraints exerted by the iodo and trimethylsilyl functionalities. The flanking terminal OSi(OtBu)3 ligands in 3 are transoid to a bridging chloride ligand (Cl2), and are both cisoid to the methylidene. Although complex 3 features only a fortunate structural snapshot, to the best of our knowledge there are currently no other structurally characterized examples of any metal complexes bearing a mono-halogenated methylidene ligand, though there are some mid transition metal complexes bearing di-fluorinated methylidenes.14,15 Treatment of dimeric [Cr(OSi(OtBu)3)2]231 with only 1/3 equivalents of CHI3 in THF led to the instant formation of trivalent [Cr(OSi(OtBu)3)2I(thf)3] (4, not shown in Scheme 2), as identified by X-ray crystallography (Figure S4). Any iodomethylidene or di-iodo-methanide species could not be obtained in THF. However, treatment of [Cr(OSi(OtBu)3)2]2 with either one or half an equivalent of CHI3 in C6D6 led to a deep brown solution (Scheme 2), which still included the presence of CHI3 as indicated by 1H NMR spectroscopy (Figure S5). Crystallization of the reaction mixture from n-hexane gave crystals of CHI3 and purple/yellow dichroic crystals of a mixed valent di-iodo-methanide species [CrII/III2I2(OSi(OtBu)3)2(CHI2)] (5a, Figure 2 right), along with a mixed-valent iodide species [CrII/III2(OSi(OtBu)3)4I] (6a, Figure S6).

Figure 2. Crystal structures of [Cr2Cl2(OSi(OtBu)3)2(CHI)(thf)4] (3, left) and [Cr2I2(OSi(OtBu)3)2(CHI2)] (5a, right). Ellipsoids shown at 50% probability, hydrogen atoms (except H1) are omitted for clarity. Selected bond lengths (Å) and angles (°) for 3: Cr1-C1: 2.042(3), Cr2-C1:2.040(3), Cr1-Cr2: 2.9707(6), Cr1-Cl1: 2.3920(7), Cr1-Cl2: 2.4428(8), Cr1-O1: 1.881(2), Cr1-O9: 2.055 (2), Cr1-O10: 2.196(2), Cr2-Cl1: 2.3774(8), Cr2-Cl2: 2.4425(8), Cr2-O5: 1.880(2), Cr2-O11: 2.048(2), Cr2-O12: 2.212(2), C1-I1: 2.147(3); Cr1-C1-Cr2: 93.4(1), Cr1-Cl1-Cr2: 77.05(3), Cr1-Cl2-Cr2: 74.90(2), O1-Cr1-Cl2: 177.19(6), O1-Cr1-Cl1: 95.19(6), O1-Cr1-C1: 98.65(1), O5-Cr2-Cl2: 176.52(7), O5-Cr2-Cl1: 94.25(7), O5-Cr2-C1: 98.2(1). Selected bond lengths (Å) and angles (°) for 5a: Cr1-I1: 2.657(1), Cr2-I2: 2.637(1), Cr2-C1: 2.05(1), Cr1---Cr2: 2.974(2), Cr1-O1: 2.031(5), Cr1-O2: 2.114(5), Cr1-O5: 1.999(5), Cr2-O1: 1.915(6), Cr2-O5: 1.999(5), Cr2-



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O6: 2.116(6), C1-I3: 2.100(11), C1-I4: 2.148(11); C1-Cr2-Cr1: 89.4(3), Cr1-O1-Cr2: 97.8(2), Cr1-O5-Cr2: 96.3(2), O1-Cr1-O2: 71.1(2), O5-Cr2-O6: 71.3(2).

Complex 5a is the first reported structurally characterized Atype complex (Scheme 1), and persistently co-crystallized with 6a. Continued attempts at separating 5a led to additional formation of 6a, likely due to a ligand redistribution process. When [Cr(OSi(OtBu)3)2]2 was treated with 1/3 equivalents of CHI3 (not shown in Scheme 2), crystals of 6a co-crystallized along with a tetra(siloxy) variant of 5a, namely mixed-valent [Cr2(OSi(OtBu)3)4(CHI2)] (5b, Figure 3, left). Use of 1/4th

equivalents of CHI3 did not yield any di-iodido-methanide species but divalent [Cr2I2(OSi(OtBu)3)2] (6b, Figure 3 right) along with crystals of mixed-valent [CrII/III2I(OSi(OtBu)3)4] (6a). It is remarkable that mixed-valent complexes (5, 6a) form in the presence of excess CHI3, as this suggests that the divalent Cr center within both 5 or 6a becomes resilient against both inter (CHI3) and intra ({CHI2}-) redox chemistry. Synthesis of the deuterated derivative of [Cr2I2(OSi(OtBu)3)2(CDI2)] (5aD) and analysis by 2D NMR spectroscopy (in toluene) did not give any observable signal for the methanide ligand, and only resonances attributable to toluene-d (due to deuteride exchange, see ESI), and CDI3, were observed. The two chromium atoms in 5a exhibit two different coordination environments, namely distorted squareplanar (Cr1), and distorted square-planar pyramidal (Cr2), due to the selective κ(C)-CHI2 coordination exclusively to Cr2. For 5b, the CHI2 unit coordinates in a µ-1κ(I):2η2(C,I′) across both chromium atoms, giving Cr2 a distorted octahedral geometry, and Cr1 a square pyramidal geometry. In 5a, analysis of the bond lengths between the chromium atoms and their respective ligands are essentially equal, but as Cr2 has one additional ligand, ergo increasing the respective bond lengths, it is likely the trivalent metal center. The difference in bond lengths in 5b are much more defined, where Cr1 is the likely trivalent species with overall shorter bond lengths. The Cr2– C1 bond lengths in 5a (2.05(1) Å)and 5b (2.046(2) Å) are

similar to the bridging methylidene in 1, but are shorter than those in [Cr(acac)2(CHCl2)(py)] (2.13(1)/2.129(8) Å),32,33 representing the only other di-halido-methanide chromium complex structurally characterized. In fact, [Ti(Cp)(acac)(µ3S)2Ir2I(CHI2)(CO)4] seems to be the only other complex containing a di-iodo-methanide ligand. This heterobimetallic species featuring a Ir–CHI2 moiety was also obtained from reactions with iodoform and is highly reactive, readily reverting to a di-iodide species.34 Considering the difficulty in isolating complexes 5, the dearth of data on such complexes is not surprising. Role of TMEDA in Enhancing the Stability of Organochromium Compounds Takai and co-workers observed that in the absence of TMEDA (or another appropriate Lewis base) their trimethylsilylmethylidene complex [Cr2Cl4(CH(SiMe3))(tmeda)2] (Scheme 3) is unstable.21 This is another striking difference when compared to iodo-methylidene complex 1, as addition of stoichiometric amounts of TMEDA to 1 accelerated its decomposition, causing a quick color change from red to yellow after two hours at ambient temperature, and occurring even when stored at -35 °C. Crystallization gave colorless poorly diffracting crystals, but when an excess of TMEDA was added at -35 °C, crystallization at the same temperature yielded [CrCl2(tmeda)2][I] (7, Figure S7, Scheme 3). The reaction progress between TMEDA and 1D (in a 2 : 1 ratio) in nondeuterated THF was monitored by 2D NMR spectroscopy, producing a spectrum similar to that of neat 1D. The resonance at 2.15 ppm (assigned as the “CDI” ligand) decreased rapidly and vanished after two hours, leaving only resonances attributable to THF-d (Figure S8).

Figure 3. Crystal structures of [Cr2(OSi(OtBu)3)4(CHI)] (5b, left) and [Cr2I2(OSi(OtBu)3)2] (6b, right). Ellipsoids shown at 50% probability, lattice solvent hydrogen atoms (except H1) are omitted for clarity. Selected bond lengths (Å) and angles (°) for 5b: Cr1-I1: 3.0433(3), Cr1-C1: 2.046(2), Cr1-O1: 1.8692(15), Cr1-O5: 2.058(1), Cr1-O6: 2.233(2), Cr1-O9: 1.949(2), Cr2-I2: 3.0589(3), Cr2-O5: 2.046(2), Cr2O9: 2.103(2), Cr2-O10: 2.138(2), Cr2-O13: 1.936(2), C1-I1: 2.165(2), C1-I2: 2.108(3); Cr1-I1-C1: 42.20(6), Cr1-C1-I2: 118.62, Cr1-O5Cr2: 95.05(6), Cr1-O9-Cr2: 96.95(6), Cr2-I2-C1: 83.77(5), O5-Cr2-O10: 148.59(6), O9-Cr2-O13: 174.98(7); Selected bond lengths (Å)


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Journal of the American Chemical Society and angles (°) for 6b: Cr1-I1: 2.6360(9), Cr1-O1: 1.974(3), Cr1-O2: 2.084(3), Cr1-O1′: 2.015(3); I1-Cr1-O1′: 174.12(8), I1-Cr1-O2: 107.94(9), O2-Cr1-O1: 153.85(12), O2-Cr1-O1′: 71.73(11).

Scheme 3. Effect of TMEDA on the Stability of Various Organochromium Reagents THF, 25 °C 4 TMEDA L = Cl, X = SiMe3

Me3Si N

(n = 4)

Takai´s work

- 2[CrIIICl2I(tmeda)]

(ref. 21)

CrIIL

2

+

1/

THF, -35 °C L = Cl, X = I (n = 4) n CHXI2

H C

N

CrIII N

Cl

CrIII Cl

Cl

N

Cl

[CrIII2Cl4(CHI)(thf)4] (1)

[CrIII2Cl4(CHSiMe3)(tmeda)2]

exc. TMEDA

[CrIIICl2(tmeda)2][I] (7) type C

n-hexane, 25 °C exc TMEDA L = OSi(OtBu)3, X = I (1/n = 0)

I (tBuO)3SiO

N CrII

(tBuO)3SiO

C6D6, 25 °C 1/ CHI 2 3

N

[CrII(OSi(OtBu)3)2(tmeda)] (8)

On the other hand initial addition of TMEDA to [Cr(OSi(OtBu)3)2]2 in n-hexane gave quantitatively monomeric [Cr(OSi(OtBu)3)2(tmeda)] (8, Figure S9, Scheme 3). Upon dissolution of crystallized 8 in C6D6 and addition of half an equivalent of CHI3, a dramatic color change from colorless to dark purple occurred (Scheme 3). Unlike the reaction of unsolvated [Cr(OSi(OtBu)3)2]2 with CHI3 (vide supra) the 1H NMR spectrum indicated the complete consumption of CHI3 (Figure S10). It was initially suspected that TMEDA assists in monomerization of the dimeric form (e.g., in 5 or 6), and allows the CrII center to re-engage in redox chemistry, giving putative complexes “[Cr(OSi(OtBu)3)2(CHI2)(tmeda)x]” and “[Cr(OSi(OtBu)3)2I(tmeda)x]”. However, it wasn’t until the fortuitous formation of blue/purple crystals from a reaction mixture, also containing 4-tert-butyl-cyclohexanone (vide infra), that we gained insight into the role of TMEDA. In fact, as revealed by X-ray crystallography trivalent [Cr(OSi(OtBu)3)2(tmeda-CHI)](I) (9, Figure 4, Scheme 3) had formed as a monomeric organochromium complex along with trivalent [Cr(OSi(OtBu)3 )2 I(tmeda)] (10, Figure S11). Apparently, the CHI2 methanide moiety of putative complex “[Cr(OSi(OtBu)3)2(CHI2)(tmeda)x]” had suffered nucleophilic attack by the neighboring tmeda ligand, ejecting an iodido ligand and giving a zwitterionic ((N,N-dimethylamino)ethyl)N´,N´(dimethylammonium)iodomethanide ligand (tmedaCHI). Complex 9 turned out to be highly reactive and readily underwent ligand rearrangement to give crystalline 10. Monitoring the reaction of [Cr(OSi(OtBu)3)2]2 with a slight excess of CDI3 by 2D NMR spectroscopy (in toluene) indicated only resonances for toluene-dx (x=1-8) and CDI3. The Cr–C bond length in 9 (2.046(3) Å) is similar to those in 5, and also the bridging methylidene in 1/3. Although the ability of TMEDA to act as a nucleophile is known, there are only two other structurally authenticated complexes containing a “tmeda-CHX” ligand, the indium complexes [InBr3(tmeda-CH2)] (In–C: 2.21(1) Å) and a similar

(tBuO)2Si tBu

O

(tBuO)3SiO

O

CH

I N

CrIII N

[CrIII(OSi(OtBu)3)2I(tmeda)] (10)

[CrIII(OSi(OtBu)3)2(tmeda-CHI)][I] (9)

I complex [InI(tmeda-CH2)2][I]2 (In–C: 2.193(9), 2.187(9) Å), both obtained through treatment of [InX2CH2X] (X = Br or I) with TMEDA.35 Otherwise, “TMEDA-X” derivatives were observed as counter cations (or zwitterions), not being involved in bonding to the metal center.14 Although we could not isolate and further characterize 9, due to its inherent instability, we were able to repeatedly obtain crystals from similar reaction mixtures, indicating it is the species (denoted as type C) formed in situ when 8 is treated CHI3. Furthermore, the formation of this ligand indicates that perhaps TMEDA has an alternative role in the reaction mixture of CrCl2 and CHI3, and could explain the rapid decomposition of 1D, likely induced by TMEDA insertion into the CH–I bond.

Figure 4. Crystal structure of [Cr(OSi(OtBu)3)2(tmeda-CHI)][I] (9), ellipsoids shown at 50% probability, lattice solvent, hydrogen atoms (except H1), and I- are omitted for clarity. Selected bond lengths (Å) and angles (°) for 9: Cr1-C1: 2.046(3), Cr1-N2: 2.045(2), Cr1-O1: 1.877(2), Cr1-O5: 1.926(2), Cr1-O6: 2.320(2),

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C1-I1: 2.153(3), C1-N1: 1.509(4), N1-C4: 1.510(4), N2-C5: 1.498(4); Cr1-C1-I1: 100.1(1), Cr1-C1-N1: 122.4(2), I1-C1-N1: 112.3(2), O5-Cr1-O6: 68.91(8), C1-Cr1-O1: 95.1(1), N2-Cr1-O1: 102.3(2).

I thf

Scheme 4. Olefination Performance of Complexes 1 (BType), 5 (A-Type), and 9 (C-Type) with Benzaldehyde

thf

CX CrIII

thf Cl

Performance of Organochromium Reagents in the MonoIodo-Olefination of Benzaldehyde Initially, we investigated the behavior of in situ prepared 1, to ensure comparative conditions with isolated 1. Thus, when CrCl2, CHI3 and benzaldehyde (4:1:1 ratio) were mixed in THF-d8 at -35 °C, 1H NMR spectroscopy indicated only 50% conversion of the benzaldehyde to (E/Z)-(2iodovinyl)benzene. The yield could not be increased beyond 50%, even when heat was applied. Thus, it can be assumed that 50% of the species formed in the CrCl2/CHI3/THF mixture engage in a reaction path that does not produce an active iodo-methylidene (e.g., 1), but instead yields [CrCl2I(thf)3] (2). This confirms the necessity for an excess of CrCl2 and CHI3 to obtain efficient olefination. As expected, treatment of isolated 1 with benzaldehyde immediately gave the corresponding (E/Z)-(2-iodovinyl)benzene with essentially the same E selectivity (E/Z= 94:6, yield >99%, Scheme 4), as observed for the original reaction mixture reported by Takai and co-workers.1 Furthermore, when 1 was treated with the sterically more demanding ketone 4-tert-butylcyclohexanone (TBCH), under the same conditions, 4-(tert-butyl)-1(iodomethylene)cyclohexane was obtained in 69% yield (originally reported by Takai: 74%),1 and upon warming the reaction mixture, the conversion increased to 84%. Higher conversions were not achieved due to decomposition of 1 in solution at 50 °C. Thus, on the basis of selectivities and conversions similar to the original mixture,1 complex 1 appears to be indeed the active Takai olefination reagent. Addition of benzaldehyde to in situ generated 5a gave instantly a color change to light brown. Analysis by 1H NMR spectroscopy indicated that mono-iodo-olefination of benzaldehyde occurred, but the selectivity reversed to give exclusively the Z isomer,36 with no resonance attributable to either the E isomer or benzaldehyde. Thus, the di-iodo-methanide ligand (or A-type complex, Scheme 1) is also an olefination reagent, but is not the active ligand in the original Takai reaction mixture.1

Page 6 of 9

O

X C

CrIII Cl

-35 °C, THF-d8

thf

Cl

Cl

I E isomer (94%) Z isomer (6%)

type B X = H (1), X = D (1D)

I

I CX

O

I

Si(OtBu)2 O tBu

CrIII

CX

O tBu O

I

O

CrII

rt, C6D6

I

Z isomer (>99%)

Si(OtBu)2

type A (in situ) X = H (5a), X = D (5aD)

I (tBuO)2Si tBu

O

O

O

CH N

CrIII

(tBuO)3SiO

O

I

CrIII rt, C6D6

N

type C (9, in situ) + other de-activated species

N

(tBuO)3SiO (tBuO)3SiO I

N

11 no olefination reaction!

Upon treatment of in situ prepared 9 (type C, Scheme 3) with benzaldehyde, an immediate color change to light green occurred. Analysis by 1H NMR spectroscopy indicated only broadened features (Figure S12), similar to those observed when benzaldehyde was added to [Cr(OSi(OtBu)3)2]2 (Figure S13). Crucially, it appeared that olefination did not occur in this reaction mixture. Although this might be due to steric constraints at the chromium center, it is more likely that CHI2/TMEDA insertion products are generated and desensitized toward olefination by following an alternative reaction path. This is further emphasized as crystallization from this reaction mixture led to the formation of light green twinned crystals, the molecular connectivity of which could be determined as [Cr(OSi(OtBu)3)2I(benzaldehyde)] (11, Figure S14) by X-ray crystallography. Thus, benzaldehyde only coordinates to 9, likely inducing displacement of the “tmedaCHI” unit, and hence hindering olefination (Scheme 4). Although 5a initiated the olefination of benzaldehyde, treatment of 5a (or 9) with TBCH did not promote olefination. In both reactions an alternative reaction path occurred, where the products remained unidentified. For 5a, the inability to undergo olefination with TBCH could possibly be due to either steric hindrance at the chromium center, or the reduced reactivity of the ketone over the aldehyde. In a preliminary study to explore the conversion of terminal alkenes to iodocyclopropanes,19 reactions were probed between allyl benzyl ether and deuterated complexes 1D (with two equivalents of TMEDA, vide supra), 5aD and 9D. Upon treatment of each complex with one equivalent of allyl benzyl ether, analysis by 2 D NMR indicated that only 1D initiated cyclopropanation, with a trans:cis ratio of (77:23), a ratio similar to that reported in the original publication (85:15).20 Complexes 5a and 9 failed to induce cyclopropanation, likely due to either steric crowding at the chromium center, or the weaker CHI2/[(tmedaCHI)(I)] nucleophile.

Conclusion


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Journal of the American Chemical Society After 30 years, the active species of the Takai olefination reagent has been conclusively determined to be the iodomethylidene complex [Cr2Cl4(CHI)(thf)4]. The dichromium(III) complex was obtained in maximum 47% yield, while higher yields appeared to be impeded by a competitive reaction pathway, which instead involves the formation of [CrCl2I(thf)3]. Such observations emphasize the necessity for an excess of CrCl2 and CHI3 to achieve efficient olefination. In the original publication by Takai, it was hypothesized that the only other plausible olefination reagent could be a di-iodomethanide species “[CrCl2(CHI2)]” coexisting with “CrCl2”. Through utilization of the tris(tert-butoxy)siloxy ligand, we obtained mixed-valent complex [Cr2I2(OSi(OtBu)3)2(CHI2)] featuring such a Cr(III)–CHI2 moiety. Curiously, treatment of [Cr2I2(OSi(OtBu)3)2(CHI2)] with benzaldehyde did induce olefination but gave the Z isomer exclusively, which is in stark contrast to the predominant formation of the E isomer, when employing pre-isolated [Cr2Cl4(CHI)(thf)4]. Thus, “CHI2“ indeed initiates olefination but it is not the active species in the original Takai reagent mixture. Moreover, unlike previously observed for trimethylsilyl methylidene derivative [Cr2Cl4(CHSiMe3)(tmeda)2], TMEDA destabilizes [Cr2Cl4(CHI)(thf)4], and for “[Cr(OSi(OtBu)3)2(CHI2)]” it inserts into the CH–I bond giving [Cr(OSi(OtBu)3)2 (tmedaCHI)][I], a complex which does not engage in olefination of benzaldehyde. Formation of the latter zwitterionic complex, along with its inability to engage in olefination reactions, could provide an alternative explanation as to how addition of TMEDA to [Cr2Cl4(CHI)(thf)4] induces cyclopropanation instead of olefination in the CrCl2/CHI3 mixture, as undoubtably such nucleophilic attack would occur.34 Finally, the finding that [CrII/III2I2(OSi(OtBu)3)2(CHI2)] engages in olefination and not in cyclopropanation reactions is in stark contrast to the Simmons-Smith CH2I ligand at ZnII,18,37,38 which does not promote Wittig-type reactions.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on the ACS Publications website at DOI xxxx Supporting figures S1–S21, Scheme S1, Table S1, detailed crystallographic data, spectroscopic data (NMR), analytical details. X-ray crystallographic data compounds 2, 3, 4, 5a, 5b,

6a, 6b, 7, 8, 9, and 10 (CIF)

AUTHOR INFORMATION Corresponding Author * [email protected]

ORCID Reiner Anwander: 0000-0002-1543-3787 Daniel Werner: 0000-0001-6573-9226

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Dedicated to Prof. Dr. Dr. h.c. mult. W. A. Herrmann on the occasion of his 70th birthday. We thank the German Science Foundation (Grant AN 238/15-2) for funding.

REFERENCES 1. a) Takai, K.; Nitta, K.; Utimoto, K., Simple and Selelective Method for RCHO to (E)-RCH=CHX Conversion by Means of a CHX3–CrCl2 System. J. Am. Chem. Soc. 1986, 108, 7408-7410; b) Okazoe, T.; Takai, K.; Utimoto, K., (E)-Selective Olefination of Aldehydes by Means of gem-Dichromium Reagents Derived by Reduction of gem-Diiodoalkanes with Chromium(II) Chloride. J. Am. Chem. Soc. 1987, 109, 951-953. 2. Matsubara, S.; Oshima, K., Olefination of Carbonyl Compounds by Zinc and Chromium Reagents. In Modern Carbonyl Olefination, Ed. Takeda, T. Wiley-VCH Verlag GmbH & Co. KGaA: 2004; pp 200-222. 3. For comparison, the handling of the prominent Tebbe reagent is more elaborate due to the use of hazardous AlMe3: a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S., Olefin Homologation with Titanium Methylene Complexes. J. Am. Chem. Soc. 1978, 100, 3611-3613; b) Thompson, R.; Nakamaru-Ogiso, E.; Chen, C.-H.; Pink, M.; Mindiola, D. J., Structural Elucidation of the Illustrious Tebbe Reagent. Organometallics 2014, 33, 429-432. 4. Wessjohann, L. A.; Scheid, G., Recent Advances in Chromium(II)- and Chromium(III)-Mediated Organic Synthesis. Synthesis 1999, 1-36. 5. Dias, L. C.; de Lucca, E. C., Total Synthesis of (-)Marinisporolide C. J. Org. Chem. 2017, 82, 3019-3045. 6. Trost, B. M.; Knopf, J. D.; Brindle, C. S., Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products. Chem. Rev. 2016, 116, 15035-15088. 7. Matoušová, E.; Koukal, P.; Formánek, B.; Kotora, M., Enantioslective Synthesis of the Unsaturated Fragment Callyspongiolide. Org. Lett. 2016, 18, 5656-5659. 8. Fürstner, A., Carbon–Carbon Bond Formations Involving Organochromium(III) Reagents. Chem. Rev. 1999, 99, 991-1046. 9. Saikia, B.; Joymati Devi, T.; Barua, N. C., First total synthesis of Debilisone C. Org. Biomol. Chem. 2013, 11, 905-913. 10. Geddis, S. M.; Hagerman, C. E.; Galloway, W. R. J. D.; Sore, H. F.; Goodman, J. M.; Spring, D. R., (Z)-Selective Takai olefination of salicylaldehydes. Beilstein J. Org. Chem. 2017, 13, 323-328. 11. Evans, D. A.; Black, W. C., Total synthesis of (+)-A83543A [(+)-lepicidin A]. J. Am. Chem. Soc. 1993, 115, 4497-4513. 12. Takai, K., 1.06 Organochromium Reagents A2 - Knochel, Paul. In Comprehensive Organic Synthesis II (Second Edition), Elsevier: Amsterdam, 2014; pp 159-203. 13. Williams, D. R.; Nishitani, K.; Bennett, W.; Sit, S. Y., A preparation of bromoolefins from carbonyl compounds. Tetrahedron Lett. 1981, 22, 3745-3748. 14. Groom, C. R.; Bruno, I. J.; Lightfoot, M. P.; Ward, S. C., The Cambridge Structrual Database. Acta Crystallogr. B 2016, 72, 171-179. 15. Schulze, W.; Hartl, H.; Seppelt, K., CF2-Bridged Metal Complexes. Angew Chem. Int. Ed. Engl. 1986, 25, 185-187. 16. For a rare chromium halomethyl derivative see: Hubbard, J. L.; McVicar, W. K., Facile Conversion of OrganometallicsHalide Complexes to Halomethyl Derivatives: Synthesis, Structure, and Reactivity of the (η5-C5R5)Cr(NO)2CH2X Series (R = H, CH3; X = Cl, Br, I, OCH3, OCH2CH3, PPh3, CN, SO3C6H4CH3). Organometallics 1990, 9, 2683-2694. 17. Gessner, V. H., Stability and reactivity control of carbenoids: recent advances and perspectives. Chem. Commun. 2016, 52, 12011-12023.



18. For the structural elucidation of zinc halomethyl reagents, also known as “Wittig-Furukawa” cyclopropanation reagents as used in the Simmons-Smith cyclopropanation, see: a) Denmark, S. E.; Edwards, J. P.; Wilson, S. R., Solution and Solid-State Structure of the “Wittig–Furokawa” Cyclopranation Reagent. J. Am. Chem. Soc. 1991, 113, 723-725; b) Denmark, S. E.; Edwards, J. P.; Wilson, S. R., Solution- and Solid-State Structural Studies of (Halomethyl)zinc Reagents. J. Am. Chem. Soc. 1992, 114, 25922602; c) Charette, A. B.; Marcoux, J.-F., Spectroscopic Characterization of (Iodomethyl)zinc Reagents Involved in Stereoselctive Reactions: Spectroscopic Evidence That IZnCH2I Is Not Zn(CH2I)2 + ZnI2 in the Presence of an Ether. J. Am. Chem. Soc. 1996, 118, 4539-4549; d) Charette, A. B.; Marcoux, J.-F.; Bélanger-Gariépy, F., X-ray Crystal Structure of a Zinc Carbenoid Cyclopropanating Reagent: The IZnCH2I.18-crown-6 and Benzo18-crown-6 Complexes. J. Am. Chem. Soc. 1996, 118, 67926793 ; e) Charette, A. B.; Marcoux, J.-F.; Molinaro, C.; Beauchemin, A.; Brochu, C.; Isabel, E., Preparation, Solid-State Structure, and Synthetic Applications of Isolable and Storable Haloalkylzinc Reagents. J. Am. Chem. Soc. 2000, 122, 45084509. 19. For an early review on “µ-CH2” metal complexes, see: Herrmann, W. A., The Methylene Bridge. Adv. Organomet. Chem. 1982, 20, 159-263. 20. Takai, K.; Toshikawa, S.; Inoue, A.; Kokumai, R., Stereoselective Iodocyclopropanation of Terminal Alkenes with Iodoform, Chromium(II) Chloride, and N,N,N’,N’Tetraethylethylenediamine. J. Am. Chem. Soc. 2003, 125, 1299012991. 21. Murai, M.; Taniguchi, R.; Hosokawa, N.; Nishida, Y.; Mimachi, H.; Oshiki, T.; Takai, K., Structrual Characterization and Unique Catalytic Performance of Silyl-Group-Substituted Geminal Dichromiomethane Complexes Stabilized with a Diamine Ligand. J. Am. Chem. Soc. 2017, 139, 13184-13192. 22. Noh, S. K.; Heintz, R. A.; Janiak, C.; Sendlinger, S. C.; Theopold, K. H., A Paramagentic µ-Methylene Complex with a Schort CrIII–CrIII Bond. Angew Chem. Int. Ed. Engl. 1990, 29, 775-777. 23. a) Wei, P.; Stephan, D. W., Cationic and Neutral PhosphidoBridged Pentamethylcyclopentadienyl–Chromium Dimers. Organometallics 2003, 22, 1712-1717; b) Wei, P.; Stephan, D. W., Organometallics 2003, 22, 1992-1994. 24. Noor, A.; Schwarz, S.; Kempe, R., Low-Valent Aminopyridinato Chromium Methyl Complexes via Reductive Alkylation and via Oxidative Addition of Iodomethane by a Cr– Cr Quintuple Bond. Organometallics 2015, 34, 2122-2125. 25. Licciulli, S.; Albahily, K.; Fomitcheva, V.; Korobkov, I.; Gambarotta, S.; Duchateau, R., A Chromium Ethylidene Complex as a Potent Catalyst for Selective Ethylene Trimerization. Angew. Chem. Int. Ed. 2011, 50, 2346-2349. 26. a) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H., Structure and Reactivity of Trimethylsilylmethyl Complexes of Chromium, Including the 13Electron Alkyl Cp*Cr(CH2SiMe3)2. Organometallics 1998, 17, 5477-5385; b) Wu, P.; Yap, G. P. A.; Theopold, K. H., Structure and Reactivity of Chromium(VI) Alkylidenes. J. Am. Chem. Soc. 2018, 140, 7088-7091. 27. Krempner, C., Role of Siloxides in Transition Metal Chemistry and Homogeneous Catalysis. Eur. J. Inorg. Chem. 2011, 1689-1698. 28. For selected references on complexes of redox-active metal centers, see: a) Le Roux, E.; Michel, O.; Sommerfeldt, H.-M.;

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Liang, Y.; Maichle-Mössmer, C.; Törnroos, K. W.; Anwander, R., Grafting of peralkylated LnIIAlIII heterobimetallic complexes onto periodic mesoporous silica KIT.6. Dalton. Trans. 2010, 39, 8552– 8559; b) König, S.; Maichle-Mössmer, C.; W. Törnroos, K.; Anwander, R., Siloxide Complexes of Chromium(II), Manganese(II), Cobalt(II), and Chromium(III) Incorporating Potassium(I). Z. Naturforsch. 2014, 69b, 1375-1383; c) Friedrich, J.; Maichle-Mössmer, C.; Anwander, R., Synthesis and derivatisation of ceric tris(tert-butoxy)siloxides. Chem. Commun. 2017, 53, 12044-12047; d) Friedrich, J.; Maichle-Mössmer, C.; Anwander, R., Redox-enhanced hemilability of a tris(tertbutoxy)siloxy ligand at cerium. Dalton Trans. 2018, 47, 1011310123. 29. a) Andrez, J.; Pécaut, J.; Bayle, P.-A.; Mazzanti, M., Tuning Lanthanide Reactivity Towards Small Molecules with ElectronRich Siloxide Ligands. Angew. Chem. Int. Ed. 2014, 53, 1044810452; b) Kelly, R. P.; Maron, L.; Scopelliti, R.; Mazzanti, M., Reduction of a Cerium(III) Siloxide Complex to Afford a Quadruple-Decker Arene-Bridged Cerium(II) Sandwich. Angew. Chem. Int. Ed. 2017, 56, 15663-15666. 30. a) Theopold, K. H., Organochromium(III) Chemistry: A Neglected Oxidation State. Acc. Chem. Res. 1990, 23, 263-270; b) Köhler, F. H.; Krüger, C.; Zeh, H. J., Ein µ2.Benzylidenkomplex des Chroms mit ungepaarten Elektronen. Organomet. Chem. 1990, 386, C13-C15; c) Enders, M., Single-Site Organochromium Catalysts: Synthesis, Characterisation by Paramagentic NMR and Olefin Polymerisation. Macromol. Symp. 2006, 236, 38-47. 31. Conley, M. P.; Delley, M. F.; Siddiqi, G.; Lapadula, G.; Norsic, S.; Monteil, V.; Safonova, O. V.; Copéret, C., Polymerization of Ethylene by Silica-Supported Dinuclear CrIII Sites through an Initiation Step Involving C–H Bond Activation. Angew. Chem. Int. Ed. 2014, 53, 1872-1876. 32. Shoji, M.; Shimura, M.; Ogino, H.; Ito, T., Synthesis of trans(Dichloromethyl)bis(2,5-pentanedionato)(Aqua, Pyridine, or Methanol) Chromium(III) and Crystal Structure of of trans(Dichloromethyl)bis(2,5-Pentanedionato)PyridineChromium(III). Chem. Lett. 1986, 15, 995-998. 33. Ogino, H.; Shoji, M.; Abe, Y.; Shimura, M.; Shimoi, M., Syntheses, X-ray Crystal Structures, and Ligand Substitution Kinetics of the Carbon-Bonded Chromium(III) Complexes trans[CrR(acac)2(L)] (R = CH2Cl, CHCl2: L = H2O, CH3OH, pyridine). Inorg. Chem. 1987, 26, 2542-2546. 34. Casado, M. A.; Pérez-Torrente, J. J.; Ciriano, M. A.; Dobrinovitch, I. T.; Lahoz, F. J.; Oro, L. A., Stereoselective Oxidative Addtions of Iodoalkanes and Activated Alkynes to a Sulfido-Bridged Heterotrinunclear Early–Late (TiIr2) Complex. Inorg. Chem. 2003, 42, 3956-3964. 35. Annan, T. A.; Tuck, D. G.; Khan, M. A.; Peppe, C., Direct Electrochemical Synthesis of X2InCH2X Compounds (X = Br, I) and a Study of Their Coordination Chemistry. Organometallics 1991, 10, 2159-2166. 36. Chen, F.; Zhang, X., Pd-catalyzed Highly Regio- and Stereocontrolled Direct Alkenylation of Electron-deficient Polyfluoroarenes. Chem. Lett. 2011, 40, 978-979. 37. Simmons, H. E.; Smith, R. D., A New Synthesisi of Cyclopropanes from Olefins. J. Am. Chem. Soc. 1958, 80, 5323-5324. 38. Denis, J. M.; Girard, C.; Conia, J. M., Improved SimmonsSmith Reactions. Synthesis 1972, 549-551.


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Journal of the American Chemical Society

Table of Contents artwork

Takai Olefination Reagent I thf CrIII thf Cl

Cl

CH L = Cl

CrIII Cl

I

I thf

CH

thf Cl

- 2 [CrIIICl2I(thf)3]

CrIIL2 + CXI3

L = OSi(OtBu)3

I

- [CrIIICrIIL4I]

O CrIII

Si(OtBu)2 O tBu CrII

O tBu O

I

Si(OtBu)2 [CrIIICrIIL2I2(CHI2)]

[CrIII2Cl4(CHI)(thf)4]


Unveiling the Takai Olefination Reagent via Tris(

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