Stereoinversion Mechanism of 1-Aryl-1-alkenyllithiums - American

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Pseudomonomolecular, Ionic sp2‑Stereoinversion Mechanism of 1‑Aryl-1-alkenyllithiums† Rudolf Knorr,* Thomas Menke, Claudia Behringer, Kathrin Ferchland, Johann Mehlstaü bl, and Ernst Lattke Department Chemie, Ludwig-Maximilians-Universität, Butenandtstrasse 5-13, 81377 München, Germany S Supporting Information *

ABSTRACT: The trans/cis stereoinversion of the trigonal carbanion centers C-α in a series of monomeric 2-(α-aryl-αlithiomethylidene)-1,1,3,3-tetramethylindanes (known to be trisolvated at Li) is rapid on the NMR time scales (400 and 100.6 MHz) in THF solution. The far-reaching redistribution of electric charge in the ground-state molecules caused by lithiation (formal replacement of α-H by α-Li) is illustrated through NMR shifts, Δδ. The transition states for stereoinversion are significantly more polar and charge-delocalized than the ground states (Hammett ρ = +5.2), pointing to a mechanism that involves heterolysis of the C−Li bond via a solventseparated ion pair (SSIP). This requires immobilization of only one additional (the fourth) THF molecule at Li+, which accounts for part of the apparent activation entropies of ca. −23 cal mol−1 K−1 and constitutes a kinetic privilege of THF depending on microsolvation at Li. Thus, the sp2-stereoinversion process is “catalyzed” by the solvent THF; its mechanism is monomolecular with respect to the ground-state species because the pseudo-first-order rate constants, measured through NMR line shape analyses, are independent of the concentrations (inclusive of decomposition) of the dissolved species (hence no associations and no dissociation to give free carbanion intermediates). In the deduced pseudomonomolecular mechanism (bimolecular through solvent participation), the angular C-α of the SSIP undergoes rehybridization (approximately in-plane inversion) through a closeto-linear transition state; this motion occurs with a concomitant “conducted tour” migration of Li+(THF)4 and is unimpaired by additional ortho-methylations at α-aryl. The synthetic route started with preparations of three α-chloro congeners through the carbenoid chain reaction, followed by vinylic substitution of α-Cl by α-SnMe3 (most efficient in THF despite steric congestion). The final Sn/Li interchange reaction afforded the new 1-aryl-1-alkenyllithium samples, initially uncontaminated by free Li+.

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−78 °C on the laboratory time scale: The pseudo-first-order rate constants became enhanced with increasing concentrations of 2 (and also of LiI) or upon dilution with the aggregationpromoting solvent Et2O, whereas a 20-fold retardation was measured for monomeric 2 when the dimerization process was inhibited.1 The stereoinversion mechanism at the quasi-sp2-hybridized carbanionic center of lithioalkenes such as α-(1,1,3,3tetramethylindan-2-ylidene)benzyllithium (3) is uncertain, but 3 has the most welcome property of being monomeric with known microsolvation numbers d at Li in preponderantly ethereal solutions and also in Me 2 N−CH 2 CH 2 −NMe 2 (TMEDA),5 whereas it is trimeric (with d = 0) in toluene6 and dimeric (with d = 1) in the presence of small amounts of ethereal donor ligands in both toluene solution and the crystals.6 The depicted angular (bent) configuration of monomeric 3 is stable at sufficiently low temperatures (exhibiting two sets of six-proton NMR singlet signals for the four methyl groups, for example). Upon warming in THF solution, 3 was averaged with the configuration 3′ by trans/cis

INTRODUCTION Knowledge of the temperature-dependent configurational stability of C−Li centers can be helpful for the use of organolithium reagents in asymmetric syntheses. The reaction rates of such configurational inversion may depend on structural properties, on molecular aggregation, on the solvent, and on the normally uncertain microsolvation of the C−Li centers by donor ligands; Reich et al.1 have published an instructive short survey of the primary literature.2 For a further example, the monomolecular3 racemization of chiral benzyllithium derivatives such as 1 (R = CH2Ph3a or Ph3b in Scheme 1) in tetrahydrofuran (THF) solution was thought3a to occur through heterolytic breaking of the bond between Li and the quasi-sp3-hybridized carbon atom of a polar ground-state CIP-1 (CIP = contact ion pair carrying d donor ligands “Don” at Li). The assumed ensuing immobilization of free THF molecules would generate the more polar solvent-separated ion pair SSIP1 with a flattened3b,4 (but not planar) C-α center, followed by an achiral transition state on the way to the enantiomers entSSIP-1 and finally ent-CIP-1. In contrast, the bimolecular axial/equatorial diastereoisomerization1 of 3eq,5eq-diphenylcyclohexyllithium (2) in THF requires aggregation (presumably1 dimerization) to occur at © 2013 American Chemical Society

Received: January 30, 2013 Published: July 29, 2013 4070

dx.doi.org/10.1021/om4000852 | Organometallics 2013, 32, 4070−4081

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(5c), an excess of 4 furnished only 29% of the almost pure chloroalkene 6c. Halodesilylation was the method of choice for transforming 6a with N-chlorosuccinimide into 6d as the only product or with bromine in acetic acid into 92% of pure 6e. Despite steric shielding, the chlorine atoms at C-α of 6a (in tBuOMe), 6b (in THF), and 6c (in Et2O) are sufficiently reactive for a quick Cl/Li interchange reaction with nbutyllithium (n-BuLi). However, the emerging α-lithioalkenes 8a, 8b, and 8c, respectively (Scheme 3), deteriorated quickly

Scheme 1

Scheme 3

diastereotopomerization7 with a roughly estimated8 preinversion half-life of ca. 0.014 s at −45 °C, as recognized by the NMR coalescences (one 12-proton Me4 singlet, for instance) of the signal pairs of diastereotopic protons and 13C nuclei, because the 3/3′ positional interchange became fast on the NMR time scale. Having synthesized further α-aryl analogues of 3 with similar quasi-benzyllithium properties, we are now able to present evidence for the pseudomonomolecular, ionic inversion mechanism of the quasi-sp2 carbanion derivative 3 in THF, including an elucidation of the transient increase of microsolvation at lithium.

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RESULTS AND DISCUSSION A. Syntheses. The preparation of 2-(α-chloro-4′-trimethylsilylbenzylidene)-1,1,3,3-tetramethylindane (6a in Scheme 2) Scheme 2 through reactions presumably involving the byproduct 1chlorobutane; for example, the transient presence of 8c was established through carboxylation to give the acid 10 in poor yield. This situation required a detour via the α-(trimethylstannyl) derivatives 7a−e, which were obtained through treatment of 6a−e with Me3SnLi in THF by the procedure described previously12 for 7a. The stannanes 7a−d were usually accompanied by the respective “parent” alkenes 9a−d (yield up to 38% of 9c from 7c). The above-mentioned enhanced α-Cl reactivity was again encountered with the α,4′-dichloride 6d, which furnished 7d with the intact 4′-Cl substituent. However, the 4′-bromo-α-chloroalkene 6e and Me3SnLi (ca. 3 equiv) provided 85% of the pure α,4′-bis(trimethylstannyl) product 7e. A NOESY experiment with 7c suggested that the small 119 Sn couplings (J ≈ 2.9 Hz) to 3-CH3 of 7a12−e (formally over five bonds, but not observed to 1-CH3) may be interpreted as through-space spin−spin coupling. The α-lithioalkenes 8a−f were preferentially generated from 7a−e through Sn/Li interchange reactions (Scheme 3) with cyclopentane solutions of labeled n-Bu6Li (ca. 1.07−2.5 equiv) or with unlabeled n-BuLi in hexanes. This method avoids the formation of disturbing byproducts such as LiHal or alkyl

from the sterically shielded dichloroalkene 4 with p(trimethylsilyl)phenyllithium (5a) was published9 as an example of the carbenoid chain reaction mechanism10 of unactivated vinylic substitution in the absence of a transition metal catalyst. Under similar conditions, the addition of solid 4 to a THF solution of 5b, as prepared from p-bromobiphenyl with tert-butyllithium (t-BuLi, 2 equiv), afforded 66% of the purified α-chloroalkene 6b. Using the alternative variant11 with tert-butyl methyl ether (t-BuOMe) replacing THF in consideration of the higher basicity of p-methylphenyllithium 4071


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halides. The various excess amounts of n-BuLi served to compensate for losses caused by the conversion of byproduct Me3SnBu with n-BuLi into Me2SnBu2. Purification of 8a−c was easy because crystalline samples were obtained in NMR tubes at room temperature through the very slow Sn/Li interchange in the alkane solutions containing the donor ligands t-BuOMe (2.5 equiv for 8a or 8b) or, better, Et2O (2−3 equiv for 8a−c). The dimeric constitution of one of these crystals (8a) had been established by a single-crystal X-ray diffraction analysis,6 and the dimeric nature of 8b and 8c followed from the similarity of their 13C NMR data with those of the Et2O solvate 8a in [D8]toluene.6 The crystals were washed with dry pentane, dried by blowing with dry argon gas for five seconds, and dissolved in anhydrous THF with a little [D8]toluene or [D6]benzene, forming solutions of monomeric 8a−c. Whereas 8a was stable up to at least 52 °C in THF, 8c isomerized within ca. one hour at 53 °C to generate the benzyllithium derivative 11 with its typical NMR chemical shifts (Tables S24 and S25).13 The 4′chloro-α-lithioalkene 8d could not be crystallized and had to be prepared through Sn/Li interchange directly in THF at −78 °C, because it decomposed within five minutes at room temperature. The Sn/Li interchange reaction of the α,4′distannyl compound 7e in THF was selective: Only the αSnMe3 group was removed by one equivalent of n-BuLi to furnish the α-lithio-4′-SnMe3 product 8e. Generated with additional n-BuLi (1.5 equiv), the α,4′-dilithio product 8f exhibited chemical shift values (Table S23)13 very similar to those of 3 (Tables S11 and S12),13 with four exceptions: The C-4′ resonance of 3 at δ = 113.8 was missing, and C-3′/C-5′ had changed from δ = 126.3 (in 3) to 142 ppm in the direction expected for a phenyllithium derivative.14 The resonance of 4′H (δ = 6.18 ppm) in 3 had been replaced in 8f by a quasi-AB spectral system for 2′-/3′-H (and 6′-/5′-H), establishing the substitution at C-4′. Carboxylation of 8f furnished mainly the α,4′-dicarboxy derivative in very low yield. Although the known11 α-chloro-2′,6′-dimethylalkene 12 did not react with elemental Li0, n-BuLi, or Me3SnLi in Et2O, it reacted readily with Me3SnLi (2 or 4 equiv) in THF at room temperature to afford a 68:32 mixture of the α-stannyl derivative 13 and its “parent” olefin 15 (Scheme 4). Similarly, the Sn/Li interchange reaction of 13 with n-BuLi to give 14 was not possible in Et2O solution at room temperature but occurred instantly in THF at 3 °C and was performed with n-Bu6Li (1.1 equiv) at −30 °C for preparing the sample of [6Li]14 to be used in the kinetic experiments in THF. This precaution was necessary because 14 disappeared within 20 min at room temperature, forming some parent olefin 15 along with the benzyllithium derivative 17, which was stable for up to three days in this milieu. The constitution of 17 followed from the diminished (upfield) 13C NMR shifts (Tables S28 and S29)13 of C-3′and C-5′ (the o- and p-positions of the benzyllithium part of 17), in accord with its deuteriolysis to give 16 with the triplet (1:1:1) 1H and 13C NMR splittings of its CH2D group. Rotation by ca. 180° about the C-α/C-1′ single bond in 17 is slow on the 13C NMR time scale up to at least 25 °C, as recognized through the detection of f ive methyl signals instead of the three resonances (intensities 2:2:1) expected for a rapidly reversible half-rotation. Therefore, a 6′-methyl group, as the “smallest” of the 2′-/6′-substituents, suffices to retard the halfrotation in 17 and hence also in 12−16. B. Ground States of the 1-Aryl-1-alkenyllithiums of Type 3. The ground states of 3, 8a−d, 8f, and 14 in THF as the solvent are monomeric contact ion pairs, as revealed6 at low

Scheme 4

temperatures by the triplet (1:1:1) splitting of their 13C-α NMR resonances caused by scalar magnetic coupling with only one 6Li nucleus (spin quantum number I = 1). The magnitudes of the one-bond coupling constants 1J(13C,6Li) are all similar, as shown in the last two columns of Table 1; they indicate microsolvation by d = 3 THF donor ligands at Li according to the empirical relationship6 of eq 1, where n = a = 1 are the numbers of CLi contacts at 13C and 6Li, respectively, for monomers, and L = 41 (±3) Hz is a sensitivity parameter valid for the 1-aryl-1-alkenyllithiums in entries 2−8 of Table 1. Such trisolvation by THF is most important for the present purposes: Ground states with these 1JC,Li values and microsolvation numbers d = 3 cannot be solvent-separated ion pairs whose Li+ would be tetrasolvated (coordinatively saturated) and thus not in a direct contact with a carbanionic 13C nucleus (so that JC,Li ≈ 0). d = L × (n × 1J C,Li )−1 − a

(1)

The bent (angular) structures of the above 1-aryl-1alkenyllithiums (see 3) in solution followed from their NMR spectra, which established a molecular Cs symmetry (the C-α/ C-2 double-bond plane) of their ground states.6 The α-aryl groups of 3 and 8a−f (and more so of 14) are rotationally impeded by the two methyl groups at C-1, so that they adopt a close to orthogonal conformation with respect to the C-α/C-2 double bond, as confirmed6 by several single-crystal X-ray diffraction analyses. This conformation ensures an almost maximum possible overlap of the pz orbital at C-1′ with the charge-carrying, quasi-sp2 orbital of the Li−C(α) bond (ionic with a modest covalent contribution), as depicted in formulas 3 and 3′. This overlap leads to delocalization of negative electric charge into the α-aryl π-system, so that fractions of negative πcharge are expected to emerge at the 2′/6′ and 4′ positions (ortho and para, respectively), in analogy with π-conjugation in the benzyl anion (PhCH2−), whose predominant mesomeric form, 18alpha, is drawn in Scheme 5 (part a) with positional numbers corresponding to those of 3: If the pz orbital axes on the C-α and C-1′ atoms are parallel to each other (perpendicular to the molecular plane in part b), this provides 4072


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Table 1. Lithiation NMR Shifts Δδ = δ(α-Li Compound) − δ(α-H Parent) and Coupling Constants 1J(13C,6Li) of Benzyllithium (Li+&18) and of the Monomeric 1-Aryl-1-alkenyllithiums 3, 8a−d, 8f, and 14 in THF Solution Δδ (ppm)

1

J(13C,6Li)

entry

cpd no.

aryl substituent

temp, °C

2′-/6′-H

3′-/5′-H

4′-H

C-2′/-6′

C-3′/-5′

C-4′

C-1′

1 2 3 4 5 6 7 8

Li+&18 3 8a 8b 8c 8d 8f 14

(a) 4′-H 4′-SiMe3 4′-C6H5 4′-CH3 4′-Cl 4′-Li 2′-/6′-CH3

+25 −102 −72 −89 −90 −88 −90 −95

−1.03 −0.74 −0.75 −0.75 −0.69 −0.74 −0.67

−0.86 −0.57 −0.58 −0.38 −0.52 −0.55 −0.42 −0.46

−1.69 −0.98

−12.7 −8.2 −8.3 −9.0 −8.2 −6.4 −8.0 −11.5

−0.4 −1.5 −1.0 +1.4 −2.4 −1.6 +14.9 −1.3

−21.8 −13.2 −18.1 −14.1 −14.5 −14.5

+22.0 +23.2 +23.0 +22.9 +23.2 +22.8

−12.4

+21.0

−0.88

C-2

C-α

Hz

T, °C

−15.2 −16.1 −16.2 −14.6 −15.6 −15.8 −17.3

+15.4 +66.5 +66.1 +66.2 +66.4 +65.6 +65.9 +66.0

10.7 10.0 9.5 11.0 10.0 10.2 10.7

≤−37 −72 ≤−89 −90 ≤−75 −90 −61

temperatures (no thermal loss of THF, therefore), because the corresponding disolvated monomers are known6 to have significantly smaller Δδ values, which are practically equal for monomeric 3 in Et2O (d = 2), t-BuOMe (d = 2), and Et2O/ [D8]toluene as the solvents. The remaining Δδ data for the 1,1,3,3-tetramethyl-2-indanylidene skeleton (Table S8)13 verify the previously20 described conspicuous π-polarization at the positions C-8 ≠ C-9 (positive) and C-5 ≠ C-6 (negative), as also illustrated in Chart S2 of the Supporting Information of ref 6. It will now be established that the polar character of these CIP ground states (“tight ion pairs”) increases on the way to the transition states of trans/cis interconversion. C. Cis/trans Diastereotopomerization of the 1-Aryl-1alkenyllithiums of Type 3. The trans/cis sp2-stereoinversion (such as monomeric 3 → 3′, or 21 → 21′ in Scheme 6) leads to

Scheme 5. Benzyl Anion Seen (a) from above and (b) from Close to within the Molecular Plane (with pz Orbitals)

for the best possible charge delocalization into the phenyl ring, as visualized by the mesomeric forms 18ortho and 18para. Effects of such a charge distribution can be observed as “upfield” (less positive) NMR shifts δ for the ortho and para positions in the real ion pair benzyllithium (Li+&18) and will be utilized as follows. Looking at benzyllithium (Li+&18) as being derived from toluene through α-lithiation, we calculated its lithiation NMR shifts Δδ = δ(Li+&18) − δ(toluene15) in THF from its 1H (at 25 °C)16 and 13C NMR15 shifts, as depicted in entry 1 of Table 1. With due reservations, the Δδ values of at least the remote 4′-H and C-4′ positions might be considered to be π-charge indicators with the approximate conversion factors of −10.7 ppm (1H)17 and roughly −165 ppm (13C)18 per π-electron. Comparisons of these Δδ values with those for 4′-H and C-4′ of 3 in entry 2 of Table 1 suggest ca. 60% of the benzyllithium charge delocalization for trisolvated monomeric 3 in THF solution (“quasi-benzyllithium”). The total set of Δδ data for 3 (Table S8)13 was computed from the δ values (Tables S11 and S12)13 of 3 at −102 °C with respect to those of the parent olefin 9f, the temperature-dependent δ values19 of which were extrapolated to −102 °C in THF. In the same manner, lithiation shifts Δδ of 8a−d, 8f, and 14 (Table S8)13 were calculated from δ values in THF (Tables S13−S16, S19−S23, S26, S27)13 minus those of the corresponding parent olefins 9a−d, 9f, and 15 at the corresponding temperatures in THF. The results are listed in entries 3−8 of Table 1 and show that Δδ values are not always suitable indicators of π-charges: As the most obviously deviating example, Δδ(C-α) is strongly positive despite the negative charge expected for a carbanionic center. Nevertheless, the Δδ data of C-1′, C-2, and C-α are particularly suitable for assigning the alkenyllithiums in entries 2−8 to the family of trisolvated monomers. In addition, the weak temperature dependencies of δ (and Δδ) values for C-α, C-2, and C-1′ (Tables S11−S23, S26, S27)13 establish that 3, 8, and 14 remain the same trisolvated ground states at all investigated

Scheme 6. Proposed sp2-Inversion Mechanism of 21

the positional interchange (diastereotopomerization)7 within pairs of diastereotopic nuclei (1-CH3/3-CH3, 1-CH3/3-CH3, C1/C-3, C-8/C-9, etc.). This process may be monomolecular or bimolecular with respect to the ground-state species, as exemplified in the Introduction (Scheme 1) for sp3-inversions; if monomolecular, it might be dissociative (via free carbanion and free Li+) or occur by intramolecular migration of Li+ in an ion pair with a temporary abolition of the C−Li contact that defines the CIP ground state. Rate measurements can provide 4073


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with R = Ph) and in Figures S1−S6 for the others;13 this establishes the monomolecular, kinetic first-order role that the trisolvated monomers 21 play in the stereoinversion rate. Such insensitivity to concentrations of 21 includes losses of 21 through decomposition, possibly with a concomitant generation of free Li+(THF)4 that could have reduced the kψ values through a mass-law suppression of ionic dissociation of 21; however, such a deceleration could not be inferred from Figures 1 and S1−S6. The conventional extraction of pseudoactivation enthalpies ΔHψ⧧ and entropies ΔSψ⧧ from such kψ diagrams furnished the results collected in Table 2, where the free pseudoactivation enthalpies ΔGψ⧧(273 K) = ΔHψ⧧ − (273 K) × ΔSψ⧧ are included for comparisons at the common temperature of 273 K. Evidence for an increasing charge transfer from C-α into the α-aryl groups on the way from 21 to a transition state was obtained through the Hammett correlation (Figure S7)13 of these ΔGψ⧧(273 K) values versus the p-substituent constants σp−, which quantify the interaction of negative π-charge with the 4′-substituents. (Our choice of the σp− values is documented in Section S3.)13 The slope of the regression line in Figure S713 equals −2.303RTρ and leads to the rather large Hammett reaction constant ρ(273 K) = +5.2 (±0.2); this positive value indicates a transition state that is energetically stabilized more than ground state 21 (or 21′) by electron-withdrawing 4′-substituents (SiMe3, Cl, phenyl) or destabilized by 4′-CH3, so that the diastereotopomerization7 rate constants increase or decrease, respectively. This establishes the temporary development of considerably more negative π-charge density in the 4′-position during stereoinversion. That property is thought to apply also to 8f (4′-Li) with its similar 1J(13C,6Li) and pseudoactivation parameters in entry 6 of Table 2, although the modest acceleration as compared with 3 remains unexplained in default of a reliable σp− value for 4′-Li. The addition of the tridentate amine PMDTA (pentamethyl diethylenetriamine) to 8f caused only a roughly 2-fold decrease of the rate constant. D. Mechanistic Deductions. With an increasing transfer of electron density from C-α to the α-aryl group on the way to the transition state experimentally settled (Hammett ρ = +5.2) for 3 and 8a−d, an increasing separation of positive from negative electric charges appears established as an essential feature of the

the keys for eliminating the incorrect ones of such possibilities as follows. The temperature-dependent pseudo-first-order (explained below) rate constants kψ of the 21/21′ interconversion were measured by 1H and 13C NMR line shape analyses through their simulation by a computer program21 for the interchange of noncoupled nuclear spins. These kψ did not depend on the concentrations of 21, as exemplified in Figure 1 for 8b (= 21

Figure 1. Arrhenius diagram of the natural logarithms of pseudo-first order sp2-stereoinversion rate constants kψ [s−1] versus 1000/T [K−1] for 8b in THF solution. Concentrations: open symbols, 0.3 M; hatched, 0.1 M; filled, 0.02 M.

Table 2. Pseudoactivation Parameters ΔGψ⧧ (kcal mol−1 at 0 °C), ΔHψ⧧ (kcal mol−1), and ΔSψ⧧ (cal mol−1 K−1) of Cis/Trans Diastereotopomerization Rates of the Monomeric 1-Aryl-1-alkenyllithiums 21 (= 3, 8a−d, 8f, or 14) in THF, Depending on Available Hammett Parameters σp− for Conjugating 4′-Substituentsa

a

entry

cpd no.

aryl substituent

1

3

4′-H

2

8a

4′-SiMe3

3

8b

4′-C6H5

4

8c

4′-CH3

5

8d

4′-Cl

6

8f

4′-Li

7

14

2′-/6′-CH3

ΔGψ⧧ (0 °C)

ΔHψ⧧

ΔSψ⧧

13.35 ±0.03 12.40 ±0.01 11.41 ±0.01 14.44 ±0.01 12.26 ±0.02 13.02 ±0.01 12.47 ±0.01

6.63 ±0.24 5.83 ±0.06 5.81 ±0.05 8.56 ±0.16 5.90 ±0.09 6.09 ±0.19 6.77 ±0.18

−24.6 ±1.0 −23.9 ±0.3 −20.5 ±0.2 −21.5 ±0.5 −23.3 ±0.4 −25.4 ±0.7 −20.8 ±0.7

1

J(13C,6Li), Hz

σp−

10.7

0.0

10.0

11.0

+0.18 ±0.01 +0.28 ±0.02 −0.17

10.0

+0.19

9.5

10.2 10.7

The averaged “true” activation entropy would be ΔS⧧(av) ≈ ΔSψ⧧(av) − R ln[free THF] ≈ − 22.9 − 5.1 = −28.0 cal mol−1 K−1. 4074


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sp2-carbanionic stereoinversion mechanism. A straightforward interpretation infers Li−C bond heterolysis as the causal process, which is usually induced and supported by the immobilization of THF molecules at Li+ in a solvent-separated ion pair such as 19 (SSIP in Scheme 6). As 19 maintains a short distance of cation and anion (without direct contact of Li to Cα), which precludes the stereoinversion, that distance must be modified within the ion pair 19 in a subsequent step toward a more charge-delocalized transition state, because the conversion of 19 into 19′ requires lithium migration. A sufficiently fast C-α/C-1′ half-rotation with Li riding on one of the two πfaces of α-aryl in 19 appears unlikely, recalling that such a halfrotation in the ground state 14 had been concluded in Section A (see 17) to occur much more slowly than would be visible on the NMR time scales. One might be tempted to assume more rotational freedom (less hindrance by the 1-CH3 groups) for αaryl on rehybridization of C-α to a roughly linear (sphybridized) arrangement, as shown in the ion pair 20, which is attained through a lateral α-aryl motion that appears reasonable on account of steric congestion and energetic resistance against torsion of the C-2/C-α double bond. However, this arrangement implies a more developed quasibenzyl carbanion π-system in 20 with parallel pz orbital axes (perpendicular to the α-aryl plane) and a correspondingly enhanced charge delocalization away from C-α toward C-4′ as compared with the less charge-delocalized (because bent) quasi-benzyl carbanion parts in 19 and 21 (hence the positive Hammett ρ value). Such an increased benzyl-type carbanion resonance and its energetic lowering of 20 would be sacrificed on C-2/C-α half-rotation, so that a substantial energetic barrier against that half-rotation should be expected. Of course, the latter consideration would also argue against 1-aryl rotation in less congested transition states of 1-aryl-1-alkenyllithiums without bulky substituents at C-2. What role may be expected for Li+(THF)4 during the stereoinversion? Ionic dissociation of 19 would leave the free carbanion 22 (Scheme 6) behind, whose mechanistic participation would be recognized through a faster sp2stereoinversion in a more diluted solution because of an increased dissociation; however, the dilution-independent kψ data in Section C (Figure 1) excluded this possibility. Entries 1 and 7 of Table 2 show that 3 and 14 have almost the same pseudoactivation enthalpies ΔHψ⧧, which suggests that the 2′-/ 6′-methyls in 14 do not significantly aggravate the circumnavigation of Li+(THF)4 about the anion and that the increasing negative π-charge in position 4′ is more important for an energetic lowering of the transition state than the πcharges in the 2′- and 6′-positions are. This may be so if Li+(THF)4 migrates along the charge gradient (C-3′/-5′ → C4′) toward closer to the 4′-position, as proposed in formula 20 (SSIP), where the indicated motion of Li+(THF)4 may occur via a Cs-symmetric structure that contains the α-aryl ring in the plane of symmetry. The term “conducted tour” mechanism had been coined by D. J. Cram22 for a corresponding cation migration about a chiral carbanion, because that process occurred with isotope-reported cation retention during the enantiomerization (that is, sp3-stereoinversion) of the carbanion. In our system, the conducted tourist Li+(THF)4 in 20 may be envisioned as being steered across the α-aryl edge by the charge gradient as a guide. k ψ = k 0[free Don]m

The inferred participation of THF shown in 19 (SSIP) and 20 (SSIP) means that we have been measuring pseudo-firstorder rate constants kψ that contain the concentration of Don = THF in eq 2 in an experimentally unconfirmed kinetic order (m) of reaction, but the obvious experimental tests could not be carried out, because significant (at least 2-fold) diminutions of [free THF] would be accompanied by serious changes of the solvent polarity and hence would furnish questionable evidence. Therefore, we leave the constant THF concentration latent in kψ, conceding that the “true” ΔS⧧ values (as derived from unconfirmed “true” rate constants) would be more negative than our ΔSψ⧧ (average ca. −22.9 cal mol−1 K−1 in Table 2 from kψ data) by a mathematical correction of roughly23 R ln [THF]m = ca. 5 × m cal mol−1 K−1 (where R = 1.986 cal mol−1 K−1), so that ΔS⧧(average) = ca. −28 cal mol−1 K−1 if m = 1. If desired, transformations of the pseudoactivation parameters of 3, 8a−f, and 14 in Table 2 into the “true” values ΔG⧧, ΔH⧧, and ΔS⧧ would be simple: In the case of m = 1 (eq 2) as the kinetic order of Don = THF as the solvent, the primary kψ values13 may be divided by the practically constant solvent concentration of [free THF] (up to 13 M); this affords the second-order k0 data to be used for the extraction of “true” ΔH⧧ and ΔS⧧. For the example of 8b, this would provide ΔS⧧ = −25.6 (in place of ΔSψ⧧ = −20.5) cal mol−1 K−1 and an unchanged ΔH⧧ = 5.81 kcal mol−1 = ΔHψ⧧, so that the expression R ln[free THF] = R ln(kψ/k0) = −(ΔHψ⧧ − ΔH⧧)/ T + ΔSψ⧧ − ΔS⧧ (as derived from eq 2) simplifies to ΔS⧧ = ΔSψ⧧ − R ln[free THF]. In fact, there can be little doubt that the pseudo-first-order stereoinversion rate constants kψ (eq 2), as measured in Section C, must contain the concentration of Don = THF in a first-order (m = 1) of reaction: As the ground states 21 are already trisolvated by THF,6 only one further THF ligand can be expected to become immobilized in 19 (SSIP), because a fifth THF molecule would bind only very weakly. Consequently, one independent particle (19) is formed from two (21 plus THF), which should contribute23 roughly −11 cal mol−1 K−1 in ΔSψ⧧, whereas the experimental ΔSψ⧧ values in Table 2 are clustered at about −22.9 cal mol−1 K−1; this difference confirms the proposal in Scheme 6 that the stereoinversion rate is not determined by the formation of 19 alone but is co-determined by a subsequent transition state (20). Thus, this process represents a pseudomonomolecular mechanism because it is monomolecular with respect to the ground-state species 21 but also “catalyzed” by THF as the solvent. A final look at the above entropy difference of ca. −22.9 − (−11) ≈ −12 cal mol−1 K−1 is instructive: That entropic penalty may have to do more with Li+(THF)4 migration than with the lateral motion (19 → 20) of α-aryl in Scheme 6. This expectation is based on observations24,25 of practically zero activation entropies13 for the sp2-stereoinversion of the lone electron pair at nitrogen in the Schiff base families 23 and 24 (Scheme 7). If the ground states and diastereotopomerizations7 of 23 and 23′ can be taken as isoelectronic models for the carbanion parts of 19 and 19′ in the absence of lithium, it may be concluded that the lateral motion of α-aryl alone toward sphybridization as in 20 would not necessarily create a strong contribution to the activation entropy. With regard to ΔH⧧, that lateral motion may be expected to release the steric strain13 inherent in 19 (where α-aryl is repelled by two 1-CH3) to a similar degree to that determined24 for 23 (namely, by −4.7 kcal/mol).

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CLi−Ph was surprisingly soluble in hexane/pentane solution and rearranged in this milieu quantitatively to the insoluble E isomer within ca. four hours at room temperature. A mechanistic stereoinversion study with such less congested examples will meet the problem that their ground states are certainly aggregated rather than monomeric, judging from the observations6,23 of one trimeric and several dimeric forms of our more congested model compound 3. The spatial limitations at Li in monomeric 3 had been proposed6 to explain why trisolvation is unattained in Et2O, tBuOMe, or TMEDA as the solvents. Because this microsolvation privilege (d = 3) of THF turned up here for the rates of stereoinversion, we sought independent confirmations of that proposal6 through examination of the mobility about the C(O) carbon center within the ketone adducts 25−27 (Scheme 8) of 3. While t-Bu2CO did not form a covalently bound

Scheme 7

E. The Privilege of THF. In accord with the inferred formation (Scheme 6) of NMR-invisible amounts of 19 (SSIP) from 21 (CIP), the high concentration of [free Don] of the solvent THF is essential for the high configurational lability of our monomeric 1-aryl-1-alkenyllithiums of type 3: The same trisolvated ground states 21 did not show any indication of stereoinversion on the NMR time scales in hot toluene, where the concentration of [free THF] was diminished by ca. 2 orders of magnitude. (We cannot separate this effect from the supposed retardation caused by the nonpolar character of toluene.) Likewise, a mixture (0.12 M) of monomeric 3 and its dimer in [D8]toluene with THF (0.57 M) showed no hint of cis/trans diastereotopomerization7 up to 90 °C: from the smallest preserved NMR frequency difference (16 Hz for the diastereotopic pair C-4/C-7), we estimated ΔGψ⧧(90 °C) ≫ 19.0 kcal mol−1 in this solution, to be compared with ΔGψ⧧(90 °C) = 15.6 kcal mol−1 for 3 in THF as the solvent (calculated from entry 1 of Table 2). The kinetic privilege of THF is evident on comparison with other donor solvents: The cis/trans diastereotopomerization7 was also slower (albeit not quantified) for the disolvated6 monomers 3&2Don in the solvent Et2O, t-BuOMe, or TMEDA (experimental estimate ΔGψ⧧ > 17.1 kcal mol−1 at +52 °C in Et2O). The ionic mechanism in Scheme 6 would now require the immobilization of m = 2 additional Et2O or t-BuOMe donor ligands for arriving at a possibly tetrasolvated transition state [20 with (Don)4 in place of (THF)4]. This implies a more negative activation entropy (by another ca. −11 cal mol−1 K−1 relative to Don = THF with m = 1)23 whose decelerating effect appears to be incompletely balanced by a presumably (because of m = 2) lowered activation enthalpy. Such an entropic handicap (for m = 2) would get exacerbated in toluene as the solvent by the squared concentration factor [free Don]2 in eq 2, where a ca. 100-fold diminished concentration of [free Don] might entail a 104-fold rate reduction for the disolvated monomer 3&2Don. Under such strongly adverse conditions, the pseudomonomolecular ionic mechanism (Scheme 6) might perhaps be outrun on a route involving more highly aggregated intermediates, as supposed1 for 2. However, heating our disolvated dimers (d = 1)6 in toluene provided no evidence of such a stereolability. The microsolvation privilege of THF has not yet been cleared up for less congested 1-aryl-1-alkenyllithiums whose cis/trans sp2-stereoinversion was observed on the NMR time scale8 at 60 MHz in TMEDA solution and on the laboratory time scale with Et2 O in hydrocarbons,26,27 while the appertaining mechanisms have remained uncertain. Even the absence of any donor ligands (which precludes the ionic SSIP mechanism of Scheme 6) did not prevent the slow Z/E stereoinversion of Ph−CHCLi−Ph in neat benzene26 at 27 °C. Likewise, the unsolvated, pure Z isomer28 of Ph−CMe

Scheme 8

adduct,6 the more compact adamantan-2-one added readily to give 25. NMR spectra of 25 at 25 °C showed apparent Cs symmetry (the C-α/C-2 double-bond plane), but hindered rotation about the C(α)−C(O) single bond became evident at −63 °C in CDCl3 solution. The resultant asymmetric conformation was recognized through unequal chemical shifts for the two 3-CH3 groups and all diastereotopic 13C nuclei: C2′ ≠ C-6′, C-3′ ≠ C-5′, C-1″ ≠ C-3″, C-4″ ≠ C-9″, C-8″ ≠ C10″, two 1-CH3, and two 3-CH3. This asymmetry indicates that the volume of 2-adamantyl is sufficiently small to fit in one halfspace (below or above the C-α/C-2 double-bond plane) that would be available for donor coordination at Li+ in 3. No such rotational deceleration was observed for the less congested adducts of dicyclopropyl ketone (27b, the smaller cousin of 25), fluorenone (26 at −60 °C), and benzophenone (27a). (The attempted addition of 3 to diisopropyl ketone was slow enough to be suppressed by a faster proton transfer to 3 with enolate formation.) Thus, the hindered rotation in front of the plane C-1′/C-2′/C-3′ in 25 is thought to confirm the sufficient but not excessive availability of space for the corresponding fragments O−Li−O of two Et2O or two t-BuOMe ligands in the two half-spaces of monomeric 3, whereas trisolvation would be a privilege of the more compact donor THF.

■

CONCLUSION Our deduction of the ionic mechanism of stereoinversion is based on the following considerations: (i) The pseudo-firstorder rate constants kψ are independent of the concentrations and decomposition products of the monomeric substrates (hence no dimeric transition states, no free carbanions as intermediates); the stereoinversion is monomolecular with 4076


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constant. Natural logarithms (ln) are understood to be calculated with the dimensionless magnitudes of the employed quantities. 2-(α-[6Li]Lithiobenzylidene)-1,1,3,3-tetramethylindane (3). The donor-free, crystalline cyclotrimer of 3 (compound 8 in ref 6) was prepared as described on pp 14187 and S21 of ref 6. Dissolved in anhydrous THF, 3 is purely monomeric (sept, 3J = ca. 3.4 Hz, C-2); IR (KBr) ν 2988, 2961, 2924, 2864, 1650 (w), 1591 (w), 1509, 1484, 1456, 1362, 807, and 758 cm−1. Anal. Calcd for C21H23Cl (310.9): C, 81.14; H, 7.46; Cl, 11.40. Found: C, 81.54; H, 7.53; Cl 11.21. 2-(α,4′-Dichlorobenzylidene)-1,1,3,3-tetramethylindane (6d). p-Chlorophenyllithium (pseudo-AB proton system with 3J = 7 Hz at δH = 7.07 and 7.96 ppm) is fairly stable in t-BuOMe at 40 °C but does not work well as the initiating base and nucleophile in the carbenoid chain reaction. Therefore, the α-chloro-4′-silyl compound 6a (500 mg, 1.35 mmol) and N-chlorosuccinimide (434 mg, 3.25 mmol) in glacial acetic acid (8 mL) were stirred at 80 °C overnight. The mixture was diluted with distilled water (20 mL) and extracted with Et2O (4 × 20 mL). The combined extracts were washed with 2 M NaOH (2 × 20 mL) and then with water (20 mL), dried over MgSO4, and concentrated to yield a crude material containing almost only 6d. Crystallizations from EtOH yielded pure 6d (270 mg, 60%), mp 139.5−141 °C; 1H NMR (400 MHz, CDCl3) δ 1.18 (s, 6 H, 2 × 1CH3), 1.74 (s, 6 H, 2 × 3-CH3), 7.04 (dm, 3J = 7.5 Hz, 1 H, 7-H), 7.20 (dm, 1 H, 4-H), ca. 7.24 (m, 2 H, 5-/6-H), 7.29 (dm, 3J = 8.5 Hz, 2 H, 3′-/5′-H), 7.36 (dm, 3J = 8.5 Hz, 2 H, 2′-/6′-H), assigned through comparison with chlorobenzene; 13C NMR (100.6 MHz, CDCl3) δ 27.9 (qq, 1J = 127.5 Hz, 3J = 4 Hz, 2 × 3-CH3), 31.2 (qq, 1J = 127.5 Hz, 3J = 4 Hz, 2 × 1-CH3), 49.1 (m, C-1), 50.0 (m, C-3), 122.2 (dd, 1J = 158 Hz, 3J = 7.5 Hz, C-7), 122.5 (dd, 1J = ca. 156 Hz, 3J = 7.5 Hz, C4), 126.3 (t, 3J = 4 Hz, C-α), 127.3 (dm, 1J = 160 Hz, C-6), 127.4 (dm, 1 J = 160 Hz, C-5), 128.3 (dd, 1J = 166 Hz, 3J = ca. 5 Hz, C-3′/5′), 131.5 (dd, 1J = 163 Hz, 3J = 7.2 Hz, C-2′/6′), 134.1 (tt, 3J = 10.1 Hz, 2 J = ca. 3.0 Hz, C-4′), 139.0 (sharp t, 3J = 7.6 Hz, C-1′), 148.8 (m, C8), 149.5 (m, C-9), 156.2 (m, C-2), assigned through comparison of δ and 1JCH values with those of chlorobenzene; IR (KBr) ν 2987, 2962, 2928, 1652 (w), 1594 (w), 1485, 1362, 1090, 814, and 760 cm−1. Anal. Calcd for C20H20Cl2 (331.3): C, 72.51; H, 6.09; Cl, 21.40. Found: C, 72.08; H, 5.82; Cl 21.64. 2-(4′-Bromo-α-chlorobenzylidene)-1,1,3,3-tetramethylindane (6e). See ref 13. 2-(4′-Trimethylsilyl-α-trimethylstannylbenzylidene)-1,1,3,3tetramethylindane (7a). See refs 12 and 13 2-(4′-Phenyl-α-trimethylstannylbenzylidene)-1,1,3,3-tetramethylindane (7b). See ref 32. This was prepared in analogy with 12 7a from 6b (672 mg, 1.80 mmol) with Me3SnLi (ca. 3.6 mmol) in anhydrous THF (6 mL). Aqueous workup6 after 10 min at room temperature afforded an oil (976 mg), which crystallized from EtOH to give pure 7b (605 mg, 69%) with mp 127.5−128.5 °C; 1H NMR (400 MHz, CDCl3) δ 0.08 (s, 9 H; 119Sn satellites, 2J = 51.8 Hz; SnMe3), 1.24 (s, 6 H, 2 × 1-CH3), 1.53 (s, 6 H; 119Sn satellites, 5J = 2.8 Hz; 2 × 3-CH3), 7.04 (d, 3J = 8.2 Hz, 2 H, 2′-/6′-H), 7.05 (m, 1 H, 7-H), 7.16−7.23 (m, 2 H, 5-/6-H), 7.18 (dm, 1 H, 4-H), 7.32 (tt, 3J = 7.2 Hz, 4J = 1.2 Hz, 1 H, p′-H), 7.43 (tt, 3J = 7.5 Hz, 2 H, 2 × m′-H), 7.53 (dm, 3J = 8.4 Hz, 2 H, 3′-/5′-H), 7.66 (dm, 3J = ca. 8 Hz, 2 H, 2 × o′-H), assigned through comparison with 6b and 7c and the following selective {1H} decoupling experiments: {o′-H} → m′-H as a d, {m′-H} → o′-H as a s, {3′-/5′-H} → 2′-/6′-H as a s, {p′-H} → m′H as a dm; 13C NMR (100.6 MHz, CDCl3) δ −5.0 (broadend q, 1J = 128.7 Hz; 119Sn satellites, 1J = 339 Hz; SnMe3), 31.9 (qq, 1J = 127 Hz, 3 J = 4.4 Hz, 2 × 1-CH3 + 2 × 3-CH3), 48.4 (unresolved; 119Sn satellites, 3J(cis) = 21.5 Hz; C-3), 50.6 (unresolved; 119Sn satellites, 3 J(trans) = 61.2 Hz; C-1), 122.25 (dd, 1J = 156 Hz, C-7), 122.32 (dd, 1 J = 156 Hz, C-4), 125.9 (dd, 1J = 157 Hz; 119Sn satellites, 4J = 9.9 Hz, C-3′/-5′), 126.74 (dm, 2 × C-o′), 126.79 (dm, C-5), 126.94 (2 dm, C6 and C-p′), 128.70 (dd, 1J = 158 Hz, 2 × C-m′), 128.85 (dd, 1J = 158 Hz; 119Sn satellites, 3J = 19.0 Hz; C-2′/6′), 137.1 (tt, 3J = 7.2 and 3.6 Hz; 119Sn satellites, 5J = 12.7 Hz; C-4′), 138.6 (unresolved; 119Sn

satellites, 1J = 441 Hz; C-α), 140.9 (m, apparent 3J = ca. 3 Hz, C-i′), 144.7 (sharp t, 3J = 7.5 Hz; 119Sn satellites, 2J = 20.5 Hz; C-1′), 149.9 (unresolved, C-9), 150.3 (unresolved, C-8), 167.7 (m; 119Sn satellites, 2 J = 16.0 Hz; C-2), assigned through the 119Sn satellites, comparisons with 7c and the olefin 9b, and selective {1H} decoupling as follows: {m′-H} → C-m′ simplified and C-i′ as a broadened s, {3′-/5′-H} → C1′ as a s, C-i′ and C-3′/5′ simplified, {o′-H} → C-4′ as a t 7.2 Hz; IR (KBr) ν 2986, 2960, 2922, 1608 (w), 1484, 762 (s), 735, 698, and 528 cm−1. Anal. Calcd for C29H34Sn (501.3): C, 69.48; H, 6.84. Found: C, 69.64; H, 6.77. 2-(4′-Methyl-α-trimethylstannylbenzylidene)-1,1,3,3-tetramethylindane (7c). This was prepared in analogy with 7a12 from 6c (250 mg, 0.80 mmol) with Me3SnLi (ca. 2.0 mmol) in anhydrous THF (5 mL) first at room temperature for five minutes and thereupon in an ice-bath for 60 minutes. Aqueous workup6 furnished a 3:2 mixture (358 mg) of 7c and its parent olefin 9c. Crystallization from ethanol provided thin, cottony needles of 7c (183 mg, 52%) with mp 65−68 °C; recrystallization gave analytically pure 7c, mp 73−74.5 °C; 1H NMR (400 MHz, CDCl3) δ 0.04 (s, 9 H; 119Sn satellites, 2J = 51.5 Hz; SnMe3), 1.20 (s, 6 H, 2 × 1-CH3), 1.51 (s, 6 H; 119Sn satellites, 5J = 3 Hz; 2 × 3-CH3), 2.34 (s, 3 H, 4′-CH3), 6.85 (dm, 3J = 8 Hz, 2 H, 2′-/ 6′-H), 7.04 (dm, 1 H, 7-H), 7.06 (broadened d, 3J = 8 Hz, 2 H, 3′-/5′H), 7.16 (m, 1 H, 4-H), 7.20 (m, 2 H, 5-/6-H), assigned through NOESY interactions (see below), 119Sn satellites, and comparison with 7b; 13C NMR (100.6 MHz, CDCl3) δ −5.1 (qm, 1J = 128.6 Hz; 119Sn satellites, 1J = 337.5 Hz; SnMe3), 21.1 (qt, 1J = 126 Hz, 3J = 4.7 Hz, 4′CH3), 31.80 and 31.85 (2 qq, 1J = 127 Hz, 3J = 4.5 Hz, 2 × 1-CH3 + 2 × 3-CH3), 48.3 (m; 119Sn satellites, 3J(cis) = 22 Hz; C-3), 50.5 (m; 119 Sn satellites, 3J(trans) = 63 Hz; C-1), 122.23 (dm, 1J = 156 Hz, C7), 122.32 (dm, 1J = 156 Hz, C-4), 126.73 (dd, 1J = 159 Hz, C-5), 126.88 (dd, 1J = 159 Hz, C-6), 128.0 (dm, 1J = 156 Hz; 119Sn satellites, 4 J = 9.6 Hz; C-3′/-5′), 128.3 (dm, 1J = 156 Hz; 119Sn satellites, 3J = 18.8 Hz; C-2′/6′), 133.9 (pseudosextet, apparent J = 6.6 Hz; 119Sn satellites, 5J = 12 Hz; C-4′), 138.9 (unresolved; 119Sn satellites, 1J = ca. 460 Hz; C-α), 142.3 (sharp t, 3J = 7.8 Hz; 119Sn satellites, 2J = 20.4 Hz; C-1′), 150.0 (unresolved, C-9), 150.4 (unresolved, C-8), 167.5 (unresolved; 119Sn satellites, 2J = 18 Hz; C-2), assigned as above; IR (KBr) ν 3018, 2983, 2957, 2922, 2862, 1615 (w), 1502, 1485, 1454, 1360, 1110, 1026, 798, 771, and 755 cm−1. Anal. Calcd for C24H32Sn (439.2): C, 65.63; H, 7.34. Found: C, 65.79; H, 7.67. The above 119Sn/1H scalar coupling (3 Hz) was observed only for the 3-CH3 protons in the cis position relative to SnMe3, whereas the 1CH3 (trans) protons exhibited no 119Sn satellites. Therefore, an alternative to the tentative proposal of coupling through five bonds (5J) may pose this 3 Hz splitting as a case of through-space coupling, in accord with the close spatial proximity of the SnMe3 and 3-CH3 protons as established in the following NOESY interactions: Sn(CH3)3 ↔ 3-CH3 ↔ 4-H, and 7-H ↔ 1-CH3 ↔ 2′-/6′-H ↔ 3′-/5′-H ↔ 4′CH3. 2-(4′-Chloro-α-trimethylstannylbenzylidene)-1,1,3,3-tetramethylindane (7d). This was prepared in analogy with 7a12 from the α,4′-dichloride 6d (66 mg, 0.20 mmol) with Me3SnLi (ca. 0.4 mmol) in anhydrous THF (0.7 mL). Aqueous workup6 after five minutes at room temperature afforded an oily 4:1 mixture (81 mg) of 7d and its parent olefin 9d. Crystallization from a little ethanol furnished spectroscopically pure 7d (61 mg, 66%). The analytic sample had mp 120.5−122 °C; 1H NMR (400 MHz, CDCl3) δ 0.07 (s, 9 H; 119Sn satellites, 2J = 51.8 Hz; SnMe3), 1.19 (s, 6 H, 2 × 1-CH3), 1.50 (s, 6 H; 119 Sn satellites, 5J = 3.0 Hz; 2 × 3-CH3), 6.91 (dm, 3J = 8.5 Hz, 2 H, 2′-/6′-H), 7.05 (dm, 1 H, 7-H), 7.16 (dm, 1 H, 4-H), 7.23 (m, 2 H, 5-/6-H), 7.24 (dm, 3J = 8.5 Hz, 2 H, 3′-/5′-H), assigned through 119Sn satellites and comparison with 7c; 13C NMR (100.6 MHz, CDCl3) δ −5.1 (119Sn satellites, 1J = 341 Hz; SnMe3), 31.8 (2 × 1-CH3 + 2 × 3CH3), 48.45 (119Sn satellites, 3J(cis) = 20.9 Hz; C-3), 50.59 (119Sn satellites, 3J(trans) = 59.6 Hz; C-1), 122.23 (C-7), 122.31 (C-4), 126.88 (C-5), 127.01 (C-6), 127.6 (119Sn satellites, 4J = 9.4 Hz; C-3′/5′), 129.7 (119Sn satellites, 3J = 18.4 Hz; C-2′/6′), 130.3 (119Sn satellites, 5J = 14.4 Hz; C-4′), 137.8 (119Sn satellites, 1J = 438 Hz; Cα), 144.09 (119Sn satellites, 2J = 21.8 Hz; C-1′), 149.7 (C-9), 150.0 (C8), 168.4 (119Sn satellites, 2J = 15.0 Hz; C-2), assigned as above; IR 4078


Organometallics

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(KBr) ν 2988, 2958, 2923, 1606 (w), 1482 (s), 1088, 801, 761 (s), and 523 cm−1. Anal. Calcd for C23H29ClSn (459.64): C, 60.10; H, 6.36; Cl, 7.71. Found: C, 59.95; H, 6.15; Cl, 7.94. 2-[α,4′-Bis(trimethylstannyl)benzylidene]-1,1,3,3-tetramethylindane (7e). This was prepared in analogy with 7a12 from the 4′bromo-α-chloro derivative 6e (800 mg, 2.13 mmol) with Me3SnLi (ca. 6 mmol) in anhydrous THF (12 mL). After 30 min at ice temperature, aqueous workup6 gave an oily 4:1 mixture (1220 mg) of 7e and the source 6e along with only a trace of the olefin (α-H in place of the Cl in 6e). Treatment with ethanol (5 mL) furnished spectroscopically pure platelets of 7e (1059 mg, 85%) with mp 90−92.5 °C; analytically pure 7e had mp 98.5−100 °C; 1H NMR (400 MHz, CDCl3) δ 0.04 (s, 9 H; 119Sn satellites, 2J = 51.7 Hz; α-SnMe3), 0.29 (s, 9 H; 119Sn satellites, 2J = 55 Hz; 4′-SnMe3), 1.20 (s, 6 H, 2 × 1-CH3), 1.51 (s, 6 H; 119Sn satellites, 5J = 2.9 Hz; 2 × 3-CH3), 6.93 (dt, 3J = 7.9 Hz, 2 H, 2′-/6′-H), 7.04 (dm, 1 H, 7-H), 7.17 (dm, 1 H, 4-H), 7.21 (m, 2 H, 5-/6-H), 7.34 (dt, 3J = 7.9 Hz, 2 H, 3′-/5′-H), assigned through comparison with 7c and 7d; 13C NMR (100.6 MHz, CDCl3) δ −9.4 (119Sn satellites, 1J = 348 Hz; 4′-SnMe3), −5.1 (119Sn satellites, 1J = 338 Hz; α-SnMe3), 31.82 and 31.84 (2 × 1-CH3 + 2 × 3-CH3), 48.3 (119Sn satellites, 3J(cis) = 22.0 Hz; C-3), 50.5 (119Sn satellites, 3J(trans) = 62.3 Hz; C-1), 122.22 (C-7), 122.31 (C-4), 126.74 (C-5), 126.89 (C-6), 128.1 (119Sn satellites, 3J = 18.9 Hz with α-Sn, 3J = 47.5 Hz with 4′-Sn; C-2′/6′), 134.7 (119Sn satellites, 4J = 9.4 Hz with α-Sn, 2J = 37.4 Hz with 4′-Sn; C-3′/-5′), 137.3 (119Sn satellites, 1J = 480 Hz, 5J = 13.2 Hz; C-4′), 138.9 (119Sn satellites, 1J = 444 Hz; C-α), 145.3 (119Sn satellites, 2J = 20.6 Hz with α-Sn, 4J = 10.6 Hz with 4′-Sn; C-1′), 149.9 (C-9), 150.4 (C-8), 167.2 (119Sn satellites, 2J = 16.8 Hz; C-2), assigned as above; IR (KBr) ν 2985, 2958, 2922, 1609 (w), 1485, 1360, 770, 754 (s), and 526 (s) cm−1. Anal. Calcd for C26H38Sn2 (588.0): C, 53.11; H, 6.51. Found: C, 53.37; H, 6.42. 2-(α-[6Li]Lithio-4′-trimethylsilylbenzylidene)-1,1,3,3-tetramethylindane (8a). The crystalline t-BuOMe solvate of 8a was prepared as reported on pp 14188 and S36 of ref 6 (12b therein), but finally dissolved in anhydrous THF (1H and 13C NMR at −72 °C described on p S39 of ref 6), where it was monomeric up to at least 46 °C (Tables S13 and S14).13 2-(α-[6Li]Lithio-4′-phenylbenzylidene)-1,1,3,3-tetramethylindane (8b). The procedure described for 8a was carried out under argon gas cover with 7b (120 mg, 0.24 mmol) in cyclopentane (0.8 mL) plus Et2O (0.074 mL, 0.71 mmol) and n-Bu6Li (0.36 mmol) in cyclopentane (0.21 mL), which precipitated slightly yellow rodlets overnight at room temperature. After washing with dry cyclopentane and blowing with dry argon gas for five seconds, the rodlets were dissolved in anhydrous THF or in dry Et2O and measured at variable temperatures (Tables S15−S18);13 they are almost insoluble in tBuOMe or in [D8]toluene. Finally, the addition of D2O (ca. 0.06 mL) and aqueous workup afforded the parent olefins 9b and [α-D]9b (42:58, 39 mg, 48%) with mp 122−124.5 °C (see further below). 8b is monomeric in THF between at least −94 and +25 °C (Tables S15 and S16)13 and in Et2O/[D6]benzene (6:1) between at least −72 and +52 °C (Tables S17 and S18).13 The two 1-/3-CH3 signals in Et2O showed slight broadening but not yet 1H NMR coalescence at +52 °C and 400 MHz (where Δν = 18 Hz), so that kψ < 40 s−1 and ΔG⧧ > 17.1 kcal mol−1 for the cis/trans diastereotopomerization in Et2O (that is, >5 kcal mol−1 higher than in THF). 2-(α-[6Li]Lithio-4′-methylbenzylidene)-1,1,3,3-tetramethylindane (8c). As described for 8a, the α-SnMe3 compound 7c (60 mg, 0.14 mmol) was partially dissolved in pentane (0.5 mL) with Et2O (0.043 mL, 0.41 mmol) and then treated with n-Bu6Li (0.34 mmol) in cyclopentane (0.20 mL). The slow growth of a few rather big cubes (= dimeric Et2O solvate of 8c) was observed during four days at room temperature, but the accompanying very small cubes could not be recrystallized by gentle warming. In a THF solution, 8c was totally monomeric between −90 and +53 °C (Tables S19 and S20),13 but it rearranged slowly (one hour at 53 °C) to produce the less basic benzyllithium isomer 11. 2-(4′-Chloro-α-[6Li]lithiobenzylidene)-1,1,3,3-tetramethylindane (8d). A dry NMR tube was charged with the 4′-chlorostannane 7d (70 mg, 0.15 mmol) in anhydrous THF (0.50 mL), cooled to −70

°C under argon gas cover, treated with n-Bu6Li (0.16 mmol) in cyclopentane (0.10 mL), and stored at −70 °C. The solution contained only monomeric 8d between at least −88 and −2 °C (Tables S21 and S22)13 and decomposed at room temperature within five minutes to give the parent olefin 9d. 2-(α-[6Li]Lithio-4′-trimethylstannylbenzylidene)-1,1,3,3-tetramethylindane (8e). The bis(stannyl) derivative 7e (120 mg, 0.204 mmol) and anhydrous THF (0.50 mL) with [D6]benzene (0.08 mL) and a trace of TMS in a dry NMR tube under argon gas at −70 °C were treated with n-Bu6Li (0.207 mmol) in cyclopentane (0.120 mL) and measured immediately at 25 °C only. 1H NMR (400 MHz, THF) δ 0.064 (s, 9 H, 4′-SnMe3), 1.34 (s, 12 H, 2 × 1-CH3 and 2 × 3-CH3), 6.57 (d, 3J = 7.8 Hz, 2 H, 2′-/6′-H), 6.92 (d, 3J = 7.8 Hz, 2 H, 3′-/5′H); 13C NMR (100.6 MHz, THF) δ −9.54 (SnMe3), 33.7 (broadened, 4 C, 2 × 1-CH3 + 2 × 3-CH3), ca. 49.0 (very broad, C-3/-1), ca. 122.1 (C-2′/6′), 122.8 (C-7/-4), 125.8 (C-5/-6), ca. 127.8 (C-4′), 135.8 (C3′/-5′), 147.8 (broad, C-2), 153.9 (broadened, C-8/-9), 162.4 (broad, C-1′), 188.3 (broad, C-α). All signals were obviously above coalescence (1-CH3 = 3-CH3, C-3 = C-1, C-4 = C-7, C-5 = C-6, C8 = C-9), but C-2′/-6′ and C-3′/5′ showed multiple signals presumably due to partial conversion of 4′-SnMe3 to 4′-SnMe2Bu and 4′-SnMeBu2. 2-(α,4′-[6Li]Dilithiobenzylidene)-1,1,3,3-tetramethylindane (8f). See ref 13. 2-(4′-Trimethylsilylbenzylidene)-1,1,3,3-tetramethylindane (9a). This was published as compound 4l on p S9 in the Supporting Information of ref 10. 2-(4′-Phenylbenzylidene)-1,1,3,3-tetramethylindane (9b). See ref 32.This was isolated from the mother liquors of 8b as colorless crystals with mp 127.5−129 °C (from very little ethanol); 1H NMR (400 MHz, CDCl3) δ 1.35 (s, 6 H, 2 × 1-CH3), 1.50 (s, 6 H, 2 × 3-CH3), 6.68 (s, 1 H, α-H), 7.12 (dm, 1 H, 7-H), 7.20−7.24 (m, 3 H, 4-/5-/6-H), 7.337 (overlaid tt, p′-H), 7.343 (d, 3J = ca. 8 Hz, 2 H, 2′-/6′-H), 7.44 (tm, 3J = 7.5 Hz, 2 H, 2 × m′-H), 7.57 (d, 3J = 8 Hz, 2 H, 3′-/5′-H), 7.64 (dm, 3J = 8 Hz, 2 H, 2 × o′-H), assigned through an NOE difference spectrum ({1-CH3} → only 7-H and 2′-/6′-H intensified) and selective {1H} decoupling as follows: {2′-/6′-H} → 3′-/5′-H as s, {o′-H} → m′-H as d and p′-H as a sharp t, {m′-H} → o′H as s and p′-H as s; 13C NMR (100.6 MHz, CDCl3) δ 31.02 (qq, 1J = 127 Hz, 3J = 4.5 Hz, 2 × 1-CH3), 32.43 (qq, 1J = 127 Hz, 3J = 4.5 Hz, 2 × 3-CH3), 47.23 (broader m, C-1), 47.81 (narrower m, hence C-3), 122.29 (dd, 1J = 156 Hz, 3J = 6.8 Hz, C-7), 122.39 (dt, 1J = 150 Hz, 3J = 4 Hz, C-α), 122.52 (dm, 1J = 156 Hz, C-4), 126.36 (dd, 1J = 157 Hz, C-3′/-5′), 126.91 (dd, 1J = 159 Hz, C-5), 126.95 (overlaid dm, 1J = ca. 160 Hz, 2 × C-o′), 127.11 (overlaid dm, 1J = ca. 160 Hz, C-6), 127.15 (overlaid dm, 1J = ca. 160 Hz, C-p′), 128.75 (dm, 1J = 156 Hz, 2 × Cm′), 129.81 (dm, 1J = 159 Hz, C-2′/6′), 138.0 (sharp t, 3J = 7.5 Hz, C1′), 139.0 (tt, 3J = ca. 7 and 3.6 Hz, C-4′), 140.9 (tt, 3J = 7 and 3.2 Hz, C-i′), 148.6 (unresolved, C-9), 150.6 (unresolved, C-8), 162.3 (m, 3J = 3.5 Hz, C-2), assigned through selective {1H} decoupling as follows: {1-CH3} → 1-CH3 as s, C-1 narrowed, C-8 as a t; {3-CH3} → 3-CH3 as s, C-3 narrowed, C-9 as t 6 Hz; {7-H} → C-7 as s, C-9 narrowed; IR (KBr) ν 2959, 2921, 2860, 1484, and 755 cm−1. Anal. Calcd for C26H26 (338.49): C, 92.26; H, 7.74. Found: C, 92.44; H, 7.71. 2-(4′-Methylbenzylidene)-1,1,3,3-tetramethylindane (9c). The mother liquors from 7c were stirred in Et2O (5 mL) with concentrated HCl (5 mL) for 24 h at room temperature. The aqueous layer was extracted with Et2O (3 × 3 mL), and the combined ethereal phases were washed with distilled water, dried over MgSO4, and concentrated. Pure 9c distilled at 118−123 °C (bath temp)/0.001 mbar with mp 50−51.5 °C; 1H NMR (400 MHz, CDCl3) δ 1.30 (s, 6 H, 2 × 1-CH3), 1.47 (s, 6 H, 2 × 3-CH3), 2.36 (s, 3 H, 4′-CH3), 6.62 (s, 1 H, α-H), 7.11 (m, 1 H, 7-H), 7.13 (d, 3J = 8 Hz, 2 H, 3′-/5′-H), 7.15 (d, 3J = 8 Hz, 2 H, 2′-/6′-H), 7.21 (m, 3 H, 4-/5-/6-H), assigned through comparison with 9b; 13C NMR (100.6 MHz, CDCl3) δ 21.1 (qt, 1J = 126 Hz, 3J = 4 Hz, 4′-CH3), 31.0 (qq, 1J = 127 Hz, 2 × 1CH3), 32.4 (qq, 1J = 127 Hz, 2 × 3-CH3), 47.2 (broader, C-1), 47.7 (narrower, hence C-3), 122.3 (dm, 1J = 156 Hz, C-7), 122.5 (dm, 1J = 156 Hz, C-4), 122.7 (dt, 1J = 150 Hz, 3J = 3.8 Hz, C-α), 126.8 (dm, 1J = 159 Hz, C-5), 127.0 (ddm, 1J = 159 Hz, C-6), 128.4 (dqi, 1J = 156 4079


Organometallics

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J = 5 Hz, 2′-/6′-CH3), 30.5 (qm, 1J = 126 Hz, 2 × 1-CH3), 33.8 (qm, J = 126 Hz, 2 × 3-CH3), 47.3 (unresolved, C-3), 48.6 (unresolved, C1), 114.9 (sharp d, 1J = 156 Hz, C-4′), 122.8 (dm, 1J = ca. 154 Hz, C7), 122.9 (dm, 1J = ca. 154 Hz, C-4), 125.1 (m, apparent J = 6 Hz, C2′/6′), 125.7 (dd, 1J = 156 Hz, C-5), 126.0 (dd, 1J = 156 Hz, C-6), 126.3 (dm, 1J = 150 Hz, C-3′/5′), 142.5 (unresolved, C-2), 153.1 (unresolved, C-9), 154.2 (unresolved, C-8), 158.5 (unresolved, C-1′), 186.9 (t, 1J(13C,6Li) = 10.7 Hz, C-α). 2-(2′-/6′-Dimethylbenzylidene)-1,1,3,3-tetramethylindane (15). See ref 13. 2-[2′-(Monodeuteriomethyl)-6′-methylbenzylidene]-1,1,3,3tetramethylindane (16). See ref 13. 2-[2′-(Lithiomethyl)-6′-methylbenzylidene)-1,1,3,3-tetramethylindane (17). 17 was formed from 14 through trans-lithiation and was stable in THF for up to three days at 25 °C. The temperatureindependent 13C and 1H NMR nonequivalences of the two 3-CH3 groups (Tables S28 and S29)13 show that (half-)rotation about the Cα/C-1′ bond is slow on our NMR time scales. 1H NMR (400 MHz, THF, 25 °C) δ 1.32 (s, 3 H, 1 × 1-CH3; second 1-CH3 not detected), 1.42 and 1.46 (2 s, 2 × 3 H, 2 × 3-CH3), 1.93 (s, 3 H, 6′-CH3), (2′CH2Li not detected), 5.44 (d, 3J = 7.0 Hz, 1 H, 5′-H), 6.00 (s, 1 H, αH), 6.11 (d, 3J = 8.0 Hz, 1 H, 3′-H), 6.26 (t, 3J = 7.6 Hz, 1 H, 4′-H), 7.07 (m, 4 H, 4-/5-/6-/7-H), assigned through comparison of the lithiation shifts Δδ (Table 1) with those of benzyllithium;16 13C NMR (100.6 MHz, THF, 25 °C) δ 21.9 (6′-CH3), 29.7 and 30.1 (2 × 1CH3), 32.3 and 33.4 (2 × 3-CH3), 38.4 (2′-CH2Li), 47.6 (C-1), 47.9 (C-3), 106.5 (C-5′), 115.1 (C-3′), 121.3 (C-1′), 122.7 (C-7), 122.9 (C-4), 126.0 (C-α), 126.6 (C-5), 126.7 (C-6), 127.1 (C-4′), 133.8 (C6′), 150.1 (C-9), 153.1 (C-8), 156.0 (C-2′), 157.9 (C-2), assigned through comparison with 11 and as above with Δδ of benzyllithium.15 2-[α-(1,1,3,3-Tetramethyl-2-indanylidene)benzyl]adamantan-2-ol (25). Prepared from 3 with adamantan-2-one in Et2O; mp 160.5−162 °C (pentane); 1H NMR (400 MHz, CDCl3, 25 °C) δ 0.98 (broadened s, 6 H, 2 × 1-CH3), 1.48 (d, 2J ≈ 12 Hz, 4 H, 1 × 4″-H, 1 × 8″-H, 1 × 9″-H, and 1 × 10″-H), 1.64 (broad t, 3J ≈ 3 Hz, 2 H, 2 enantiotopic 6″-H), 1.74 (m, 3J ≈ 3 Hz, 1 H, 5″-H), 1.76 (s, 6 H, 2 × 3-CH3), 1.78 (m, 3J ≈ 3 Hz, 1 H, 7″-H), 1.83 (broad d, 2J ≈ 12 Hz, 2 H, 1 × 8″-H and 1 × 10″-H), 2.30 (broad d upon broad m, 2J ≈ 12 Hz, 4 H, 1 × 4″-H and 1 × 9″-H upon 1″-/3″-H), 6.98 (dm, 3J = 7.5 Hz, 1 H, 7-H), 7.13 (dm, 3J = 7.5 Hz, 1 H, 4-H), 7.16 (td, 3J = 7.2 Hz, 1 H, 6-H), 7.21 (td, 3J = 7.2 Hz, 1 H, 5-H), 7.24 (partially hidden t, 3′-/5′-H), 7.25 (hidden, 4′-H), 7.35 (dm, 2 H, 2′-/6′-H), assigned through SCS,20 comparison with 2-tert-butyladamantan-2-ol,33 and the NOESY correlations 2′-/6′-H ↔ 3′-/5′-H ↔ 1-CH3 (w, no crosspeaks with adamantyl signals), 6-H ↔ 7-H ↔ 1-CH3 ↔ 2′-/6′-H ↔ 8″-/10″-H (at δ = 1.83) ↔ 8″-/10″-H (at δ = 1.48), and 5-H ↔ 4-H ↔ 3-CH3 ↔ 1″-/3″-H ↔ 4″-/9″-/8″-/10″-H (at δ = 1.48) ↔ (5″-/ 7″-H) ↔ 6″-H; 1H NMR (400 MHz, CDCl3, −63 °C) δ = 0.50 (broadened s, 3 H, 1 × 1-CH3), 1.42 (s, 3 H, 1 × 3-CH3), 1.60 (s, 3 H, 1 × 1-CH3), 1.62 (not resolved, 2 H, 2 × 6″-H), 1.76 (1 H, 5″-H), 1.80 (1 H, 7″-H), 1.96 (s, 3 H, 1 × 3-CH3), 3.33 (s, 1 H, OH), 7.05 (1 H, 7-H), 7.44 (very broad, 1 × 2′-/6′-H); 13C NMR (100.6 MHz, CDCl3, 25 °C) δ 26.1 (dm, 1J = 132 Hz, C-5″), 26.7 (dm, 1J = 132 Hz, C-7″), 31.3 (sharp qq, 1J = 127 Hz, 3J = 4.5 Hz, 2 × 3-CH3), 32.7 (broad q, 1J = 127 Hz, 2 × 1-CH3), 33.4 (t, 1J = 128 Hz, C-4″/-9″), 35.3 (t, 1J = 128 Hz, C-8″/-10″), 36.2 (d, 1J = 132 Hz, C-1″/-3″), 37.4 (t, 1J ≈ 130 Hz, C-6″), 48.1 (unresolved, C-3), 51.0 (m, C-1), 79.9 (broad, C-2″), 122.0 (dd, 1J = 156 Hz, 3J = 7.5 Hz, C-7), 122.2 (dd, 1J = 157 Hz, 3J = 7.5 Hz, C-4), 125.8 (dd, 1J = 159 Hz, 3J ≈ 7.5 Hz, C3′/-5′), 126.4 (dt, 1J = 159.5 Hz, 3J = 7.5 Hz, C-4′), 126.6 (dd, 1J = 159.5 Hz, 3J = 7.5 Hz, C-5), 126.9 (dd, 1J = 159 Hz, 3J = 7.5 Hz, C-6), 132.3 (dt, 1J = 159 Hz, 3J = 6.5 Hz, C-2′/-6′), 140.7 (t, 3J ≈ 7 Hz, C1′), 145.1 (unresolved, C-α), 148.8 (broad, C-8), 151.6 (broad, C-9), 157.7 (unresolved, C-2), assigned through SCS,20 comparison with 2tert-butyladamantan-2-ol,33 and 13C/1H heterocorrelation (HSQC); 13 C NMR (100.6 MHz, CDCl3, −63 °C) δ 25.5 (C-5″), 26.0 (C-7″), 30.0 (1 × 3-CH3), 30.2 (1 × 1-CH3), 31.3 (1 × 1-CH3), 32.2 (C-4″ or C-9″), 33.5 (C-9″ or C-4″), 34.1 (C-8″ or C-10″), 34.4 (1 × 3-CH3), 34.8 (C-1″ or C-3″), 35.4 (C-10″ or C-8″), 36.5 (C-3″ or C-1″), 36.8 (C-6″), 47.8 (C-3), 50.8 (C-1), 79.7 (C-2″), 122.0 (C-7), 122.3 (C-4),

Hz, C-3′/5′), 129.2 (dm, 1J = 157.5 Hz, C-2′/6′), 135.7 (tm, 3J = ca. 6.2 Hz, C-4′), 135.9 (sharp t, 3J = 7 Hz, C-1′), 148.7 (unresolved, C9), 150.7 (unresolved, C-8), 161.6 (unresolved, C-2), assigned through the 13C/1H coupling patterns and comparison with 9b; IR (KBr) ν 3020, 2962, 2923, 2861, 1512, 1483, 1457, 1360, 1109, 1025, 861, 793, 760, and 661 cm−1. Anal. Calcd for C21H24 (276.42): C, 91.25; H, 8.75. Found: C, 91.53; H, 8.66. 2-(4′-Chlorobenzylidene)-1,1,3,3-tetramethylindane (9d). See ref 13. 2-(Benzylidene)-1,1,3,3-tetramethylindane (9f). For the solvent and temperature dependencies of 1H and 13C NMR δ values, see Tables S6 and S7 on pp S19−S20 in the Supporting Information of ref 6. 2-(4′-Methylphenyl)-2-(1,1,3,3-tetramethyl-2-indanylidene)acetic Acid (10). See ref 13. 2-(4′-Lithiomethylbenzylidene)-1,1,3,3-tetramethylindane (11). The α-lithio-4′-methyl compound 8c vanished within one hour at 53 °C in THF solution, forming the olefin 9c together with 11, whose later deuteriolysis afforded the deuteriated olefin 9g. The temperature-independent (Tables S24 and S25)13 NMR data of 11 are characteristic of a p-substituted benzyllithium derivative in THF inclusive of an enhanced 1J(13CH2) value (expected15 134 Hz): 1H NMR (400 MHz, THF, 25 °C) δ 1.34 (s, 6 H, 2 × 1-CH3), 1.51 (s, 6 H, 2 × 3-CH3), 6.06 (d, 3J = 8.5 Hz, 2 H, 3′-/5′-H), 6.13 (s, 1 H, αH), 6.53 (d, 3J = 8.5 Hz, 2 H, 2′-/6′-H); 13C NMR (100.6 MHz, THF, 25 °C) δ 29.8 (qq, 1J = 126 Hz, 2 × 1-CH3), 33.0 (qq, 1J = 126 Hz, 2 × 3-CH3), 41.2 (t, 1J = 136 Hz, 4′-CH2Li), 46.8 (m, C-1), 48.5 (m, C3), 113.8 (t, 3J = 7.5 Hz, C-1′), 115.7 (dq, 1J = 150 Hz, 3J = 7 Hz, C3′/5′), 122.8 (C-7), 122.9 (C-4), 126.4 (dt, 1J = 141 Hz, C-α), 126.8 (dm, 1J = 159 Hz, C-5), 127.2 (dm, 1J = 158 Hz, C-6), 131.2 (dt, 1J = 149 Hz, 3J = 6 Hz, C-2′/6′), 146.6 (m, C-2), 150.2 (m, C-9), 153.6 (m, C-8), 158.4 (t, 3J = 7.2 Hz, C-4′). 2-(α-Chloro-2′/6′-dimethylbenzylidene)-1,1,3,3-tetramethylindane (12). See ref 13. 2-(2′-/6′-Dimethyl-α-trimethylstannylbenzylidene)-1,1,3,3tetramethylindane (13). This was prepared in analogy with 7a12 from 12 (100 mg, 0.308 mmol) with Me3SnLi (ca. 0.62 mmol) in anhydrous THF (3.4 mL). Aqueous workup6 after 90 min at room temperature gave a 68:32 mixture (129 mg) of 13 and its parent olefin 15. Analytically pure 13 (72 mg, 52%) had mp 104−105 °C (ethanol); 1 H NMR (400 MHz, CDCl3) δ 0.03 (s, 9 H; 119Sn satellites, 2J = 50.9 Hz; SnMe3), 1.17 (s, 6 H, 2 × 1-CH3), 1.57 (s, 6 H, 2 × 3-CH3), 2.17 (s, 6 H, 2′-/6′-CH3), 6.97 (quasi-s, 3 H, 3′-/5′-H and 4′-H), 7.05 (dm, 3 J = ca. 7 Hz, 1 H, 7-H), 7.17−7.23 (m, 3 H, 4-/5-/6-H), assigned through comparisons with 7b and with compound 21 of ref 20; 13C NMR (100.6 MHz, CDCl3) δ −3.78 (sharp q, 1J = 129 Hz, SnMe3), 21.3 (q, 1J = 126 Hz, 2′-/6′-CH3), 29.3 (qq, 1J = 127 Hz, 3J = 4.6 Hz, 2 × 1-CH3), 31.8 (q, 1J = 127 Hz, 3J = 4.6 Hz, 2 × 3-CH3), 49.2 (unresolved, C-3), 50.3 (unresolved, C-1), 122.1 (dm, 1J = 156 Hz, C7), 122.3 (dm, 1J = 156 Hz, C-4), 125.0 (sharp d, 1J = 158.7 Hz, C-4′), 126.8 (dd, 1J = 159 Hz, 3J = 7.5 Hz, C-5), 127.0 (dm, 1J = 159 Hz, C6), 127.1 (dm, 1J = 158 Hz, C-3′/-5′), 134.1 (m, C-2′/6′), 137.7 (s, Cα), 144.3 (m, C-1′), 149.6 (m, C-9), 150.3 (m, C-8), 165.0 (m, C-2), assigned as above; IR (KBr) ν 2987, 2958, 2921, 1602, 1486, 1460, 1361, 1027, 761 (s), and 521 cm−1. Anal. Calcd for C25H34Sn (453.25): C, 66.25; H, 7.56. Found: C, 66.44; H, 7.44. 2-(α-[6Li]Lithio-2′-/6′-dimethylbenzylidene)-1,1,3,3-tetramethylindane (14). A dry NMR tube containing the α-SnMe 3 compound 13 (50 mg, 0.11 mmol) in anhydrous THF (0.5 mL) was cooled to −30 °C, treated with n-Bu6Li (0.12 mmol) in cyclopentane (0.081 mL), and stored at −70 °C. 14 was totally monomeric between at least −95 and +25 °C (Tables S26 and S27)13 but suffered rapid trans-lithiation (within 20 min at 25 °C) to give the benzyllithium derivative 17 along with the parent olefin 15. The final addition of D2O and aqueous workup yielded 15 containing some 16 (total 31 mg, 97%). 1H NMR of 14 (400 MHz, THF, −95 °C) δ 1.11 (s, 6 H, 2 × 1-CH3), 1.37 (s, 6 H, 2 × 3-CH3), 2.04 (s, 6 H, 2′-/6′CH3), 6.19 (t, 3J = 7.3 Hz, 1 H, 4′-H), 6.56 (d, 3J = 7.3 Hz, 2 H, 3′-/ 5′-H), 6.95 (m, 1 H, 7-H), 7.00 (m, 2 H, 5-/6-H), 7.09 (m, 1 H, 4-H); 13 C NMR of 14 (100.6 MHz, THF, −95 °C) δ 22.7 (qd, 1J = 124 Hz,

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Organometallics

Article

(7) Binsch, G.; Eliel, E.; Kessler, H. Angew. Chem. 1971, 83, 618− 619; Angew. Chem., Int. Ed. 1971, 10, 570−572. (8) Knorr, R.; Lattke, E. Tetrahedron Lett. 1977, 18, 3969−3972. (9) See compound 4m in ref 10. (10) Knorr, R.; Pires, C.; Behringer, C.; Menke, T.; Freudenreich, J.; Rossmann, E. C.; Böhrer, P. J. Am. Chem. Soc. 2006, 128, 14845− 14853. (11) See compound 4n in ref 10. (12) See the preparation of 7a in ref 6 (described therein as compound 11 with reversed positional numbering). (13) See the Supporting Information. (14) Reich, H. J.; Green, D. P.; Medina, M. A.; Goldenberg, W. S.; Gudmundsson, B. Ö .; Dykstra, R. R.; Phillips, N. C. J. Am. Chem. Soc. 1998, 120, 7201−7210 and Figure 13 therein. (15) (a) Takahashi, K.; Kondo, Y.; Asami, R.; Inoue, Y. Org. Magn. Reson. 1974, 6, 580−582. Notice that the assignments of C-o and Cm for toluene in THF (Table 1 therein) have to be reversed. (b) Compare also: van Dongen, J. P. C. M.; van Dijkman, H. W. D.; de Bie, M. J. A. Recl. Trav. Chim. Pays-Bas 1974, 93, 29−32. (16) Maercker, A.; Stötzel, R. J. Organomet. Chem. 1983, 254, 1−12 and Table 5 therein. (17) Schaefer, T.; Schneider, W. G. Can. J. Chem. 1963, 41, 966−982. (18) (a) O’Brien, D. H.; Hart, A. J.; Russel, C. R. J. Am. Chem. Soc. 1975, 97, 4410−4412 and references therein. (b) Hallden-Abberton, M.; Fraenkel, G. Tetrahedron 1982, 38, 71−74. (c) Bradamante, S.; Pagani, G. A. J. Org. Chem. 1984, 49, 2863−2870. (19) Tables S6 and S7 in the Supporting Information of ref 6. (20) Knorr, R.; von Roman, T.; Freudenreich, J.; Hoang, T. P.; Mehlstäubl, J.; Böhrer, P.; Stephenson, D. S.; Huber, H.; Schubert, B. Magn. Reson. Chem. 1993, 31, 557−565 on p 563. (21) Binsch, G. Top. Stereochem. 1968, 3, 97−192 on p 178. (22) (a) Cram, D. J.; Gosser, L. J. Am. Chem. Soc. 1964, 86, 2950− 2951. (b) Cram, D. J.; Gosser, L. J. Am. Chem. Soc. 1964, 86, 5457− 5465 Chart VI therein. (23) As explained by Knorr, R.; Menke, T.; Ferchland, K. Organometallics 2013, 32, 468−472. (24) Knorr, R.; Ferchland, K.; Mehlstäubl, J.; Hoang, T. P.; Böhrer, P.; Lüdemann, H.-D.; Lang, E. Chem. Ber. 1992, 125, 2041−2049 where ΔG⧧ of compound 3b was extrapolated to 126 °C for comparison with 7b in Table 2 therein. (25) Knorr, R.; Hoang, T. P.; Mehlstäubl, J.; Hintermeyer-Hilpert, M.; Lüdemann, H.-D.; Lang, E.; Sextl, G.; Rattay, W.; Böhrer, P. Chem. Ber. 1993, 126, 217−224 on p 220. (26) Curtin, D. Y.; Koehl, W. J. J. Am. Chem. Soc. 1962, 84, 1967− 1973. (27) Panek, E. J.; Neff, B. L.; Chu, H.; Panek, M. L. J. Am. Chem. Soc. 1975, 97, 3996−4000. (28) ten Hoedt, R. W. M.; van Koten, G.; Noltes, J. G. J. Organomet. Chem. 1979, 170, 131−149 eq 5 therein. (29) Curtin, D. Y.; Crump, J. W. J. Am. Chem. Soc. 1958, 80, 1922− 1926 formula X on p 1923. (30) This estimate rests on the premise that the monomer/dimer equilibrium parameters measured23 in [D8]toluene with limited amounts of THF apply also to solutions in preponderant THF. (31) Notice that we have chosen here an atom-numbering system that equals that in Scheme 1 of ref 10 but is the reverse of that used previously in the Supporting Information of ref 10 for compounds 4k and 4m therein. (32) The locants i′, o′, m′, and p′ refer to the 4′-phenyl group. (33) Duddeck, H.; Rosenbaum, D. J. Org. Chem. 1991, 56, 1700− 1707 and Supplementary Material therein.

125.4 (broad, C-3′ or C-5′), 126.1 (broad, C-5′ or C-3′), 126.2 (sharp, C-4′), 126.6 (C-5), 126.9 (C-6), 131.1 (broad, C-2′ or C-6′), 132.4 (broad, C-6′ or C-2′), 140.2 (C-1′), 144.9 (C-α), 148.3 (C-8), 150.9 (C-9), 156.6 (C-2); IR (KBr) ν 3636 (sharp O−H), 2910, 1601 (w), 1488, 762, and 715 cm−1. Anal. Calcd for C30H36O (412.6): C, 87.33; H, 8.79. Found: C, 88.00; H, 9.02. 9-[α-(1,1,3,3-Tetramethyl-2-indanylidene)benzyl]fluoren-9ol (26). See ref 13. 1,1,2-Triphenyl-2-(1,1,3,3-tetramethyl-2-indanylidene)ethanol (27a). See ref 13. 1,1-Dicyclopropyl-2-phenyl-2-(1,1,3,3-tetramethyl-2indanylidene)ethanol (27b). See ref 13.

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ASSOCIATED CONTENT

S Supporting Information *

Preparations of compounds 6e, 7a, 8f, 9d, 10, 12, 15, 16, 26, 27a, 27b, and the α,4′-dicarboxylic acid; cis/trans diastereotopomerization rate constants; Hammett p-substituent parameters σp− with Figure S7; lithiation NMR shifts; tabulated primary NMR data; Schiff base 23 as a carbanion model. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. † E/Z Equilibria, Part 20. Part 19: Knorr, R.; Behringer, C.; Nöth, H.; Schmidt, M.; Lattke, E.; Räpple, E. Chem. Ber./Recl. 1997, 130, 585−592.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft. This work is dedicated to Professor Herbert Mayr in recognition of his interest in and support of the “negative” part of ion kinetics.

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REFERENCES

(1) Reich, H. J.; Medina, M. A.; Bowe, M. D. J. Am. Chem. Soc. 1992, 114, 11003−11004. (2) The following references contain more recent instructive collections of contributions to the field of configurational stability of mainly quasi-sp3 carbanionic compounds in general; they do not cover the present topic of sp2-carbanion stereoinversion. (a) Basu, A.; Thuyamanavan, S. Angew. Chem. 2002, 114, 740−763; Angew. Chem., Int. Ed. 2002, 41, 716−738. (b) Gawley, R. E. Top. Stereochem. 2010, 26, 93−133. (c) Ott, H.; Däschlein, C.; Leusser, D.; Schildbach, D.; Seibel, T.; Stalke, D.; Strohmann, C. J. Am. Chem. Soc. 2008, 130, 11901−11911. (d) Capriati, V.; Florio, S.; Perna, F. M.; Salomone, A.; Abbotto, A.; Ahmedjkouh, M.; Nilsson Lill, S. O. Chem.−Eur. J. 2009, 15, 7958−7979. (e) Capriati, V.; Florio, S.; Perna, F. M.; Salomone, A. Chem.−Eur. J. 2010, 16, 9778−9788. (f) Perna, F. M.; Salomone, A.; Dammaco, M.; Florio, S.; Capriati, V. Chem.−Eur. J. 2011, 17, 8216− 8225. (g) Seel, S.; Dagousset, G.; Thaler, T.; Frischmuth, A.; Karaghiosoff, K.; Zipse, H.; Knochel, P. Chem.−Eur. J. 2013, 19, 4614−4622. (3) (a) Ahlbrecht, H.; Harbach, J.; Hoffmann, R. W.; Ruhland, T. Liebigs Ann. Chem. 1995, 211−216. (b) Ruhland, T.; Hoffmann, R. W.; Schade, S.; Boche, G. Chem. Ber. 1995, 128, 551−556. (4) Reich, H. J.; Dykstra, R. R. Angew. Chem. 1993, 105, 1489−1491; Angew. Chem., Int. Ed. 1993, 32, 1469−1470. (5) See Table 1 of ref 6. (6) Knorr, R.; Menke, T.; Ferchland, K.; Mehlstäubl, J.; Stephenson, D. S. J. Am. Chem. Soc. 2008, 130, 14179−14188. 4081


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