nmr spectroscopy of organolithium compounds. xxvi—the aggregation behaviour of methyllithium in...

MAGNETIC RESONANCE IN CHEMISTRYMagn. Reson. Chem. 2004; 42: 788–794Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/mrc.1443

NMR spectroscopy of organolithium compounds.XXVI† — The aggregation behaviour of methyllithiumin the presence of LiBr and LiI in diethyl etherand tetrahydrofuran‡

Thomas Fox, Heike Hausmann and Harald Gunther∗

University of Siegen, FB 8, OCII, D-67068 Siegen, Germany

Received 31 May 2004; Accepted 16 June 2004

1H, 6Li and 13C NMR spectroscopy were used to determine the structure of aggregates formed in mixturesof methyllithium, H3CLi, and lithium bromide and iodide in diethyl ether and tetrahydrofuran. From thechemical shifts, the signal intensity distribution and the isotope shifts observed for partially deuteratedsystems, it was shown that generally tetrameric structures with different halogen contents dominate. Formethyllithium–lithium bromide (1 : 1) in THF a considerable concentration of an H3CLi–LiBr dimer wasfound. For the first time, deuterium-induced 6Li isotope shifts over four bonds were observed. Copyright 2004 John Wiley & Sons, Ltd.

KEYWORDS: NMR; 6Li NMR; organolithium compounds; structure; aggregation; tetramer; dimer; dynamic behaviour;isotope shift

INTRODUCTION

The aggregation behaviour of organolithium compounds inorganic solvents is of continued interest for reactivity studieson these systems.2 In this context, the formation of mixedaggregates between organolithium compounds and lithiumsalts, which are often by-products of the synthetic routeused, is of importance (for a recent example of the effects oflithium salts on organometallic reactions, see Ref. 3). Sincethe pioneering work of Waack et al.4 and Novak and Brown,5

NMR spectroscopy of 1H, 13C, 6Li and 7Li has played a majorrole in these type of structural investigations (for a review,see Ref. 6).

With the isotope fingerprint method we introduceda technique which allows one to recognize the nextneighbour environment of a certain lithium site in anaggregate using the sensitivity of 6Li NMR in 6Li-enrichedsamples.7 Owing to deuterium-induced two-bond isotopeshifts of the 6Li resonance, 2�2H, 1H�6Li, in aggregateswith suitably labelled organic ligands, 1 : 1 mixtures ofdeuterated (d) and non-deuterated (h) compounds yield inthe region of slow inter- and intra-aggregate exchange, whichis usually accessible at low temperatures, typical multipletsfor the 6Li resonances which constitute fingerprints of the6Li environment. Thus, a dimer, to describe a typical result,

†Dedicated to Professor Wolfgang von Philipsbornon the occasion of his 75th birthday.ŁCorrespondence to: Harald Gunther, University of Siegen, FB8, OCII, D-67068 Siegen, Germany.E-mail: [email protected]†For Part XXV, see Ref. 1.

leads to a 1 : 2 : 1 6Li triplet due to the arrangements 6Lihh,6Lihd (D 6Lidh) and 6Lidd.

Earlier we used this approach to study aggregation in thesystem H3CLi–LiI (1 : 1) in diethyl ether (DEE).7 In this work,we investigated aggregate formation between H3CLi andLiBr in DEE and tetrahydrofuran (THF) (for a preliminaryreport, see Ref. 8) and, in addition, the system H3CLi–LiIin THF.

RESULTS AND DISCUSSION

The system H3CLi–LiBr (1 : 1) in DEEFor H3CLi–LiI (molar ratio 1 : 1) in DEE it was shown that inaddition to LiI the clusters A, B, C and D (Scheme 1, X D I)exist. In these aggregates, each lithium has three next andone remote neighbour.

The distribution of the different complexes in a 0.1 M

solution was not far from the statistical ratio exceptfor complex C which was removed from the solutionby precipitation. Similar results were recently reportedfor the system H3CLi–LiBr in toluene–DEE (9 : 1) usingvarious ratios for the mole fractions of both species.9

Here, the concentration of complex D (X D Br) was foundsystematically smaller than expected, which was explainedby the high energy of this complex as derived from theoreticalcalculations.

In the present study of a 0.1 M 1 : 1 mixture of H3CLi andLiBr in DEE, we observed, at 188 K, three 1H NMR signals at�1.84, �1.90 and �1.97 ppm (relative to external TMS) withan intensity ratio of 19 : 71 : 10 [Fig. 1(a)], which must resultfrom the methyl resonances of clusters A, B, and C already

Copyright 2004 John Wiley & Sons, Ltd.

Aggregation behaviour of MeLi in the presence of Li salts 789

Li-3

CH3

CH3

CH3

Li-2

Li-3

Li-3

Li-6

CH3

Li-6

Li-7

Li-6

Li-1

CH3

CH3

CH3

CH3

Li-1

Li-1

Li-1

Li-5

CH3

CH3Li-4

Li-5

Li-4

X

X

X

X

X

X

A CB

D

Li-8

CH3

Li-8

X

E

Scheme 1. Aggregates of H3CLi-LiX mixtures.

-2.0 -2.1 -2.2

-1.8 -2.0

-2.0 -2.1 -2.2 -2.3 ppm

ppm

(a) (b)

(c)

188 K 193 K

158 K

183 K

-1.9

A

B

CB

A

E

A

B

E

C

ppm

-2.20 -2.25 -2.30 -ppm

Figure 1. 400 MHz 1H NMR spectra of H3CLi–LiBr in DEE (a),H3CLi–LiBr in THF (b) and H3CLi–LiI in THF (c).

found by Novak and Brown from 7Li NMR studies.5 Theirassignment will be discussed below.

The 6Li NMR spectrum [Fig. 2(a)] showed six majorsignals 1 and 4–8 with the intensity distribution given inTable 1 and in addition two small singlets 2 and 3. Accordingto their chemical shift range, these signals belong to Li nextneighbour environments [CH3, CH3, CH3] (7, 8), [CH3, CH3,Br] (5, 6), and [CH3, Br, Br] (4), respectively, while the singletat lowest frequency (1) can be assigned to LiBr existing asa tetramer or dimer. From the NOE data measured in anNOE difference experiment [Fig. 2(b) and Table 1], the high-frequency signals 8 and 7 must arise from Li-1 of A and Li-2of B or vice versa, with CH3 and Br as remote neighbours,

Table 1. 1H NMR data υ�1H� for aggregates observed forH3CLi–LiX (X D Br, I) 1 : 1 mixtures in DEE and THF (relativeintensity in italics; for assignments see text ext. standard TMS)

SampleTemperature

(K)

H3CLi–LiBr in 188 �1.84 �1.90 �1.97DEE 19 71 10Aggregate C B A

H3CLi–LiBr in 193 �2.08 �2.15 �2.17THF 66 17 17Aggregate A B E

H3CLi–LiI in 183 �2.09 �2.16 �2.22 �2.28THF 90 9 1 0.34Aggregate A B E C

respectively. The next group at lower frequency and thusmore shielded, signals 6 and 5, show reduced NOE effectsand must belong to Li-3 of B and Li-4 of C with CH3 and Br asremote neighbours, respectively. Finally, signal 4 originatesfrom Li-5 in C or Li-6 in D.

In agreement with this assignment is the intensity ratio of1 : 3 for signals 8 and 6, which thus come from aggregate B,while equal intensity for signals 5 and 4 allows an assignmentof both signals to cluster C. Signal 4 then belongs to lithiumin environment [CH3, Br, Br] with a CH3 group as remoteneighbour as found for Li-5 in C. The remaining high-frequency signal 7 is thus due to cluster A.

These assignments are fully supported by the isotopicfingerprint method [Fig. 2(c)]. For an H3CLi–D3CLi–LiBr(1 : 1 : 2) mixture, signals 8 and 7 show quartets, signals6 and 5 triplets and signal 4 a doublet as expected forlithium with next-neighbour environments [CH3, CH3, CH3],[CH3, CH3, Br] and [CH3, Br, Br], respectively. Sincedeuterium in the CD3 groups leads to high-frequencyshifts,7 the intensity distribution in the multiplets reflectsthe decreasing NOE effects for 6Li with the decreasingnumber of CH3 groups as next neighbours. Noteworthyis the extra doublet splitting for the Li-3 signal [Fig. 2(d)]

Copyright 2004 John Wiley & Sons, Ltd. Magn. Reson. Chem. 2004; 42: 788–794

790 T. Fox, H. Hausmann and H. Gunther

( )

Figure 2. (a) 58.88 MHz 6Li NMR spectra of H3CLi–LiBr (1 : 1)in DEE at 183 K; (b) NOE difference spectrum; (c) 58.88 MHz6Li NMR spectra of H3CLi–D3CLi–LiBr (1 : 1 : 2) in DEE.

which indicates a deuterium-induced isotope shift overfour bonds. Thus, under the conditions used in thisstudy, we find that clusters A, B and C (Scheme 1,X D Br) are present in a molar ratio of 7 : 66 : 27. The 1Hsignal yield, considering the number of methyl groupsper aggregate, 7 : 66 : 27, in excellent agreement with the6Li NMR results and the earlier work of Novak andBrown.5

It remains to discuss the presence of cluster D (X D Br)and the origin of the two small singlets 2 and 3 at 0.28and 0.30 ppm, respectively, which show doublets in thefingerprint spectrum [Fig. 2(c)]. They are compatible with a6Li site with environment [CH3, Br, Br] and Br as remoteneighbour in a tetrameric structure as Li-6 in D, or a lithiumsite in a mixed dimer E (Li-8). The remaining signal for Li-7of D, only one-third or half of the intensity of signal 2 or3, respectively, is expected to coincide with the LiBr signal1 at lowest frequency as shown in Ref. 9 where 6Li wasmeasured at 73 MHz. The Li environment [CH3, Br, Br]Br inD is thus shielded by 0.12 or 0.14 ppm as compared withthe arrangement Li[CH3, Br, Br]CH3, which corresponds tosignal 4. Similarly, the shift difference of 0.04 ppm observedbetween signals 6 and 5 corresponds to the introductionof a remote Br ligand (Li-3[CH3, CH3, Br]CH3 vs Li-4[CH3,CH3, Br]Br). Although one of the signals 2 and 3 is thereforewithout doubt due to cluster D, especially since D (X D I)was also found for the system H3CLi–LiI,7 it is not clear if

it is signal 2 or 3. The other signal should then result fromdimer E. The concentration of both species D and E can beestimated from the signal intensity relative to signal 4 asabout 1.5%.

It is interesting to compare our results with thosereported by Desjardins et al.9 for the H3CLi–LiBr system intoluene–DEE (9 : 1). Different molar ratios of H3CLi to LiBrwere used in both studies, but our results for the 1 : 1 mixturefit fairly well into the data sets reported by Desjardins et al.for 1 : 0.5 and 1 : 1.4 mixtures (A : B : C : D D 18 : 54 : 28 : 0 and0 : 12 : 39 : 24, respectively). Both studies were performed at183 K and agree that in the range of these molar ratios clusterB is dominant, followed by cluster C. Only small amounts ofclusters A and D are present in our 1 : 1 mixture, in line withthe decrease of c(A) from 18 to 0 and the increase of c(D) from0 to 24 on going from the 1 : 0.5 to the 1 : 1.4 mixture. Our find-ing υ�6Li[CH3, CH3, CH3]CH3� < υ�6Li[CH3, CH3, CH3]Br�indicates, however, that the assignment for the signals ofLi-1 and Li-2 given in Ref. 9, Fig. 3(B), for the 2 : 1 mixturehas to be reversed. In fact, from the spectra shown a reverseassignment is most likely by comparing the chemical shift ofthe signals.

The system H3CLi–LiBr (1 : 1) in THFTurning now to the solvent THF, where in the earlier workno tendency to mixed aggregate formation was found,4 weobserved for H3CLi–LiBr (1 : 1) at 193 K three 1H signalsat �2.08, �2.15 and �2.17 ppm (relative to external TMS)[Fig. 1(b)] with an intensity distribution of 66 : 17 : 17. At

d(6Li)

( )

Figure 3. (a) 58.88 MHz 6Li NMR spectra of H3CLi–LiBr (1 : 1)in THF; (b) 58.88 MHz 6Li NMR spectra of H3CLi–D3CLi–LiBr(1 : 1 : 2) in THF.



158 K the intensities of both low-frequency signals aredrastically reduced [Fig. 1(b)].

More informative is the 6Li NMR spectrum (Fig. 3), whichshows at 193 K four sharp singlets and one broad signal atchemical shifts typical for the 6Li resonances in the differentenvironments of tetrameric aggregates (Table 1). The mostintense signal 5 at 1.68 ppm must belong to cluster A (X D Br).It shows 13C satellite signals (*) separated by 5.7 Hz, acoupling typical for a tetramer.6 This result is confirmed inthe 13C NMR spectrum, where a 1 : 3 : 6 : 7 : 6 : 3 : 1 septet withthe same splitting is observed at �15.2 ppm (Fig. 4). Signals4 and 3 of the 6Li NMR spectrum have an intensity ratioof 1 : 3 and should result from aggregate B (X D Br) wheresignal 4 corresponds to the [CH3, CH3, CH3]Br environmentof Li-2 and signal 3 to the [CH3, CH3, Br]CH3 environmentof Li-3. Finally, signal 1 is due to LiBr, but signal 2, that liesin the region typical for a next neighbour environment of thetype [CH3, CH3, Br] has no counterpart left. In addition, itsbroadened shape does not correspond to the relatively sharpresonances of the other 6Li species.

The isotopic fingerprint method [Fig. 3(b)] confirms theseassignments with quartets observed for signal 5 and 4 andthe triplet for signal 3 again with an extra splitting. Thisspectrum was recorded at 183 K and signal 2 sharpened. Itshows, however a doublet and could therefore belong to anenvironment 6Li[CH3, Br, Br]Br as found for Li-6 in clusterD (X D Br). Li-7 of this cluster should then coincide with theLiBr signal 1. However, the chemical shift of signal 2 is largerthan expected for Li-6 in D, where we found 0.30 ppm inDEE (see above). As an alternative explanation remains theassignment of signal 2 to a mixed dimer E (X D Br). Indeed,variable-temperature and 2D EXSY spectra10 show chemicalexchange between lithium signals 2 and 1 whereas the othersignals are not effected (Figs 5 and 6). The final proof for theexistence of E comes from the 13C NMR spectrum, whichshows a quintet at �13.5 ppm with a splitting of 9.75 Hz,typical for the 13C, 6Li coupling of a dimer6 (Fig. 4).

After the 6Li assignment was complete, the 1H assignmentwas derived by 1H, 6Li HOESY spectroscopy11 (Fig. 7).HOESY spectra at 193 and 158 K show strong cross peaksbetween the 6Li signal 5 (Li-1 in A) and the larger high-frequency 1H signal which therefore comes from cluster A.The correlation of Li-2 of cluster B (6Li signal 3) with the1H signal at �2.15 ppm was observed at 193 K, whereas the1H, 6Li correlation for the mixed dimer E (�2.17/0.73 ppm)was observed only at 158 K where the chemical exchangebetween E and LiBr is slow on the NMR time-scale. This

Figure 4. Partial 100.16 MHz 13C NMR spectrum ofH3CLi–LiBr (1 : 1) in THF internal standard TMS.

Figure 5. Temperature dependence of the 6Li NMR spectrumof H3CLi–LiBr (1 : 1) in THF showing the Li exchange E � LiBr.

Figure 6. 6Li,6Li EXSY spectrum of H3CLi–LiBr (1 : 1, 0.15 M) inTHF at 162 K.

shows that the low-frequency 1H signal at �2.17 ppm comesfrom the dimer E. It is interesting that the order of the 1Hsignals of cluster A and B in THF is opposite to that in DEE,where the increasing number of Br ligands caused a low-fieldshift.

From the signal intensities of the 1H and 6Li spectra,we derive at 193 K molar ratios of 42 : 14 : 45 and 51 : 25 : 24,respectively, for the aggregates A, B and E (X D Br). Theconcentration of A increases at lower temperatures on theextent of the other two aggregates, perhaps as a result of LiBrprecipitation.

The system H3CLi–LiI (1 : 1) in THFAfter aggregate formation between H3CLi and LiI had beenstudied in DEE as solvent,7 it was of interest to investigatethis system also in THF. Here we found at 183 K four 1H



Figure 7. 1H, 6Li HOESY spectra of H3CLi–LiBr (1 : 1) (above)and H3CLi–D3CLi–LiBr (1 : 1 : 2) (below) in THF.

signals at �2.09, �2.16, �2.22 and �2.28 ppm in an intensityratio of 100 : 10 : 1:0.34 [Table 1, Fig. 1(c)], which indicates thepresence of two major and two minor aggregates. The 6Lispectrum shows seven signals [Fig. 8(a) and Table 2], whichcan be identified on the basis of the fingerprints observed forthe H3CLi–D3CLi–LiI (1 : 1 : 2) mixture and their intensityratio. The dominant signal 7 at 1.93 ppm yields the quartetexpected for Li-1 of cluster A [Fig. 8(b)], while cluster Bgives rise to a quartet for signal 6 at 1.81 ppm [Fig. 8(c)]and a triplet for signal 5 at 1.07 ppm [Fig. 8(d)]. There is noindication of an isotope shift over four bonds in B as foundfor the LiBr case. The presence of cluster C is shown by thesmall singlets 4 and 2 at 1.00 and 0.35 ppm, respectively,with an intensity ration of 1 : 1, which yield a triplet for Li-4and a doublet for Li-5 in the isotopic fingerprint mixture[Fig. 8(g) and (h)]. A further singlet (signal 3) at 0.97 ppmis slightly broadened and yields a broadened doublet as afingerprint which overlaps with the triplet of Li-5 [Fig. 8(g)].Since this signal has no visible counterpart, we can assign itto a mixed dimer E (Scheme 1, X D I), where the broadeningindicates exchange with the LiI signal at 0.0 ppm. Finally,the 13C satellites at signal 5 yield a scalar 6Li,13C couplingof 5.8 Hz, typical for a tetramer.6 This is confirmed in the13C NMR spectrum which shows at 189 K a 1 : 3 : 6 : 7 : 6 : 3 : 1septet at υ � 15.3 ppm with the same splitting. Thus onlyaggregate D is not observed. From the intensity of the 6Lisignals the assignment given for the 1H signals in Table 1 andFig. 1(c) can be derived. These compare well with the shiftsfound for H3CLi–LiBr (1 : 1) in THF [Fig. 1(b) and Table 1].From the 1H intensity and considering the number of methyl

1234

6

7

4

3

2

2.0 1.6 1.2 0.8 0.4 0.0

d (6Li)

+ +(a)

(f)

(c)

(h)(g)

(b)

(e)

(d)

5

Figure 8. (a) 58.88 MHz 6Li NMR spectra of H3CLi–LiI (1 : 1) inTHF; at C13C satellites due to 13C, 6Li coupling (5.78 Hz).Insets: (b) fingerprint quartet of signal 7; (c) fingerprint quartetof signal 6; (d) fingerprint triplet of signal 5; (e), (f) enlargedsignals 4, 3 and 2; (g) overlapping fingerprint triplet for signal 4and doublet for signal 3; (h) fingerprint doublet of signal 2.

groups in each aggregate the mole faction of the aggregatesA : B : E : C ³ 84 : 11 : 4 : 1 can be derived.

Dynamic exchange between E (X = Br) and LiBrIt was also of interest to derive an estimate for the barrier ofthe chemical exchange between E (X D Br) and LiBr foundfor the H3CLi–LiBr mixture in THF (see above) using 2DEXSY spectroscopy.10 In the region of slow exchange the rateconstant k for a system with equally populated sites is relatedto the mixing time tM of the EXSY sequence and the intensityratio of diagonal and cross signal, ID and IC, respectively, bythe well-known equation10

k D 1/[tM�ID/IC C 1�] �1�

From measurements at four different temperatures wederived on this basis the data given in the Experimentalsection. The uncertainty in these results comes from thedifferent populations of both sites in the present case andthe difficulties associated with correct integration of the 2Dsignals. However, owing to competing exchange processesbetween LiBr and the other aggregates present in the mixture,which were slow but certainly not completely negligible, thepopulation of the LiBr site in exchange with E cannot bedetermined and a more complete treatment was thereforenot possible. The present estimate, based on a rate constantk of 0.2 at 173 K, yields with the Eyring equation a G‡ (173)of 44 kJ mol�1. In an independent approximation a value



Table 2. Chemical shifts υ�6Li� (ppm) of lithium sites in H3CLi–LiX (X D Br, I) aggregates A—E (see Scheme 1)

H3CLi–LiBr in DEE; T D 183 K Signal 8 7 6 5 4 3 2 1υ�6Li�a 1.80 1.72 1.08 1.04 0.44 0.30 0.28 0.0Rel. int. (%) 12.0 5.3 37.6 10.5 10.3 1.7 1.5 21.1NOE 0.75 0.76 0.40 0.36 0.19 —b —b 0.0Assignment Li-2 Li-1 Li-3 Li-4 Li-5 Li-6c Li-8c —Aggregate B A B C C Dc Ec LiBr

H3CLi–LiBr in THF; T D 193 K Signal 5 4 3 2 1υ�6Li�a 1.68 1.55 0.81 0.73 0.0Rel. int. (%) 43.4 5.4 16.1 10.3 24.8Assignment Li-1 Li-2 Li-3 Li-8 —Aggregate A B B E LiBr

H3CLi–LiBr in toluene–DEE (9 : 1); υ�6Li�d 1.89 1.85 1.20 1.13 0.57 0.49T D 183 K Assignment Li-2 Li-1 Li-3 Li-4 Li-5 Li-6

Aggregate B A B C C D

H3CLi–LiI in DEE; T D 183 K υ�6Li�e 1.96 1.84 1.11 1.11 0.40 0.40Assignment Li-2 Li-1 Li-3 Li-4 Li-5 Li-6Aggregate B A B C C D

H3CLi–LiI in THF; T D 195 K Signal 7 6 5 4 3 2 1υ�6Li�f 1.93 1.81 1.07 1.00 0.97 0.35 0.0Rel. int. (%) 59.9 2.8 8.5 0.6 2.3 0.6 3.0Assignment Li-1 Li-2 Li-3 Li-4 Li-8 Li-5 —Aggregate A B B C E C LiI

a This work, relative to internal LiBr.b Not measured.c Assignment may be reversed.d From Ref. 9 converted to internal LiBr by subtracting 1.14 ppm from the reported data; assignment of Li-1 and Li-2 reversed (seetext).e From Ref. 7 relative to internal LiI.f This work, relative to internal LiI.

Table 3. Deuterium-induced two-bond isotope shifts, 2�2H, 1H�6Li (ppb) for 6Li sites in aggregates A—E (X D Br, I)a

Aggregate Li-1 Li-2 Li-3 Li-4 Li-5 Li-6 Li-8

H3CLi–LiBr in DEE 15.5 18.8 20.2, 4.1b 19.4 24.1 16.6 14.6H3CLi–LiBr in THF 17.9 16.8, 3.3b 17.6H3CLi–LiI in DEEc 16.0 19.6 19.7 15.0H3CLi–LiI in THF 16.0 14.0 14.5 13.0 13.0 13.0

a Upper limit of experimental error š0.2 ppb.b Isotope shift over four bonds.c Data from Ref. 7.

of 40 kJ mol�1 for G‡ (195) is obtained on the basis of thechemical shift between both sites (43 Hz; see Table 2) and thecoalescence temperature of 195 K. A linear regression of ourdata points indicates a high negative activation entropy forthe exchange as expected for a highly ordered transition state.

Isotope effectsThe deuterium-induced isotope shifts observed for 6Li inthe samples investigated are summarized in Table 3. Theyhave the magnitude typical for the two-bond shift observedearlier for D3CLi–LiI aggregates.7 There is no systematicsolvent effect, however for the first time an 2H, 6Li, isotopeshift over four bonds was observed. As shown in the insetsin Figs. 2(d) and 3(c), the triplet due to Li-3 in cluster B of

the H3CLi–D3CLi–LiBr mixtures in DEE and THF shows anextra splitting of 4.1 and 3.3 ppb, respectively, which mustbe caused by the remote CD3 group. Such effects are notobserved for related 6Li environments in the other clustersas, for example, Li-1 in A or Li-5 in C, or in the mixtureswith LiI. The distortion of the tetramers from a regular cubedue to the different size of the metal and the halogen ionsapparently leads in some cases to a bond orientation thatfavours long-range isotope effects.

CONCLUSION

The results show that the aggregate composition forH3CLi–LiBr and H3CLi–LiI mixtures in DEE is similar



and in the case of 1 : 1 compositions close to the statisticalratio. In THF, H3CLi–LiBr shows a different result with thedominance of cluster A, a large concentration of dimer E andfast exchange between E and LiBr in a temperature regionwhere the exchange between LiBr and tetramers is slow. Inthe H3CLi–LiI mixture in THF again cluster A is dominant,followed by cluster B. In addition, small amounts of thetetramer C and the dimer E are found. Hence DEE as solventfavours the formation of mixed tetramers in a distributionwhich is close to the statistical ratio. THF, on the otherhand, favours the halogen-free tetramers and supports theformation of mixed dimers. Noteworthy also is the different1H chemical shift order with increasing halogen content inthe samples with DEE and THF as solvent. In addition, theorder of the chemical shifts for Li-1 and Li-2 is different inDEE and THF with υ�Li-2� > υ�Li-1� in DEE and the oppositerelation in THF. For cluster B (X D Br), deuterium-induced13C isotope shifts over four bonds are observed in bothsolvents DEE and THF.

EXPERIMENTAL

CompoundsMixed samples H3CLi–LiX (X D Br or I) were preparedunder argon from the methyl halides CH3X (X D Br orI) by reaction with lithium-6 powder according to knownprocedures12,13 in diethyl ether. After filtration the solventwas removed under reduced pressure and replaced byEt2O–�D10�Et2O (7 : 3) or (D8)THF. The sample concentrationwas determined by Gilman titration14 and adjusted to0.1–0.2 M by adding the appropriate solvent. Solvents werecarefully degassed and dried over Na–K alloy. Methylbromide was dried by contact with KOH pellets andphosphorus pentoxide.

SpectraNMR spectra were recorded with a Bruker AMX-400spectrometer operating at 400.16 MHz for 1H, 100.15 MHzfor 13C and 58.885 MHz for 6Li and equipped with a low

temperature unit. Temperatures were measured with amethanol thermometer.15 Inverse-gated, NOE difference and2D spectra were measured using Bruker software. 1H, 6LiHOESY spectra11 were recorded with a sweep width 2890 Hzin F1 (1H) and 283 Hz in F2 (6Li) and a mixing time of 1.8 s.The total experimental time was 2.5 h. For the 6Li, 6Li EXSYspectra the sweep width was 128 Hz in both dimensions, themixing time, tM, 1.6 s and the total experimental time 1.6 h. Atthe temperatures given, the following intensity ratios ID/IC

and rate constants k were obtained: 162 K, 2.65, 0.171; 168 K,1.95, 0.212; 173 K, 1.63, 0.238; 175 K, 1.47, 0.253.

AcknowledgementWe are indebted to the Deutsche Forschungsgemeinschaft and theFonds der Chemischen Industrie for support.

REFERENCES1. Kuhnen M, Gunther H, Amoureux J-P, Fernandez C. Magn.

Reson. Chem. 2002; 40: 24.2. Rapport Z, Marek I (eds). The Chemistry of Organolithium

Compounds. Chichester: Wiley: 2004.3. Sosda-Rivadeneyra M, Munoz-Muniz O, Anaya de Parodi C,

Quintero L, Juaristi E. J. Org. Chem. 2003; 68: 2369.4. Waack R, Doran MA, Baker EB. Chem. Commun. 1967; 1291.5. Novak DP, Brown TL. J. Am. Chem. Soc. 1972; 94: 3793.6. Gunther H. In Advanced Applications of NMR to Organometallic

Chemistry, Gielen M, Willem R, Wrackmeyer B (eds). Wiley:Chichester, 1996; 247–290.

7. Eppers O, Gunther H. Helv. Chim. Acta 1990; 73: 2071.8. Gunther H. J. Braz. Chem. Soc. 1999; 10: 241.9. Desjardins S, Flinois K, Oulyadi H, Davoust D, Giessner-

Prettre C, Parisel O, Maddaluno J. Organometallics 2003; 22:4090.

10. Jeener J, Meier BH, Bachmann P, Ernst RR. J. Chem. Phys. 1979;71: 4546.

11. Bauer W. In Lithium Chemistry–a Theoretical and ExperimentalOverview, Saspe A-M, Schleyer PvR. Wiley: Chichester, 1995;125.

12. Wittig G. Angew. Chem. 1940; 53: 241.13. Pierce OR, McBee ET, Judd GF. J. Am. Chem. Soc. 1954; 76:

476.14. Gilman H, Cartledge FK. J. Organomet. Chem. 1964; 2: 447.15. Van Geet AL. Anal. Chem. 1970; 42: 679.


nmr spectroscopy of organolithium compounds. xxvi—the aggregation behaviour of methyllithium in...

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