Reactions of Germenes with Some para-Quinones: Formation of a

Organometallics 2010, 29, 4849–4857 DOI: 10.1021/om100113c

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Reactions of Germenes with Some para-Quinones: Formation of a Tricyclic Compound from 1,4-Benzoquinone Undergoing an Unexpected Rearrangement† Dumitru Ghereg,‡ Erwan Andre,§ Sakina Ech-Cherif El Kettani,^ Nathalie Saffon,4 Mohamed Lazraq,^ Henri Ranaivonjatovo,‡ Heinz Gornitzka,3 Karinne Miqueu,§ Jean-Marc Sotiropoulos,§ and Jean Escudie*,‡ ‡

Universit e de Toulouse, UPS, LHFA, 118 Route de Narbonne, F-31062 Toulouse, France, and CNRS, LHFA, UMR 5069, F-31062 Toulouse Cedex 09, France, §Institut Pluridisciplinaire de Recherche sur l’Environnement et les Mat eriaux, UMR-CNRS 5254, Universit e de Pau et des Pays de l’Adour, H elioparc, 2 Avenue du Pr esident Angot, F-64053 Pau Cedex 09, France, ^Laboratoire d’Ing enierie Mol eculaire et Organom etallique (LIMOM), BP 1796, Facult e des Sciences Dhar El Mehraz, Universit e Sidi Mohamed Abdellah, F es-Atlas, F es, Morocco, 4Structure F ed erative Toulousaine en Chimie Mol eculaire, FR 2599, Universit e Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex 09, France, and 3Laboratoire de Chimie de Coordination du CNRS, 205 Route de Narbonne, 31077 Toulouse Cedex 04, France Received February 11, 2010

Germenes Mes2GedCR2 1a (Mes = 2,4,6-trimethylphenyl; CR2 = fluorenylidene) and Mes2GedCR0 2 1b (CR0 2=2,7-di-tert-butylfluorenylidene) react with 1,4-benzoquinone and 2,3,5,6-tetramethyl-1,4-benzoquinone to give compounds 3a, 3b, and 5a, containing a 1,4-cyclohexadiene unit as central ring. Upon prolonged storage in diethyl ether or THF solution, compounds 3a and 3b undergo a double 1,3-hydrogen shift, leading to their structural isomers 4a and 4b. Theoretical calculations performed on the model compound H2GedCH2 1H postulate that the first steps of its reaction with 1,4-benzoquinone are a double [2þ2] cycloaddition between the GedC and CdO double bonds, leading to a transient dispiro compound containing two oxagermetane rings, followed by its isomerization to derivative 3H. Between 1a and 9,10anthraquinone, a double [2þ4] cycloaddition is observed involving the oxygens and the ortho-carbon atoms of one adjacent aromatic ring system, to generate the dioxadigermabenzopyrene derivative 2a. Introduction Alkenes are among the most important compounds in organic chemistry, due to the ability of the >CdC< double bond to furnish new organic functions by undergoing various addition or cycloaddition reactions. Some of their heavier analogues, metallaalkenes >M14dC< (M14 = Si (silenes),1 Ge (germenes),1f,2 and Sn (stannenes)1f,2b-d), have been synthesized and their chemical behavior has been † Part of the Dietmar Seyferth Festschrift. In honor of Professor Dietmar Seyferth, in recognition of his outstanding contribution to Organometallics as founding Editor-in-Chief. *Corresponding author. E-mail: [email protected]. (1) For reviews on silenes see: (a) Raabe, G.; Michl, J. Chem. Rev. 1985, 85, 419. (b) Brook, A. G.; Baines, K. M. Adv. Organomet. Chem. 1986, 25, 1. (c) Brook, A. G.; Brook, M. A. Adv. Organomet. Chem. 1996, 39, 71. (d) M€ uller, T.; Ziche, W.; Auner, N. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: New York, 1998; Vol. 2, Chapter 16. (e) Morkin, T. L.; Owens, T. R.; Leigh, W. J. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: New York, 2001; Vol. 3, Chapter 17. (f) Lee, V. Ya.; Sekiguchi, A. Organometallics 2004, 23, 2822. (g) Ottosson, H.; Ekl€of, A. M. Coord. Chem. Rev. 2008, 252, 1287. (2) For reviews on germenes, see: (a) Barrau, J.; Escudie, J.; Satge, J. Chem. Rev. 1990, 90, 283. (b) Baines, K. M.; Stibbs, W. G. Adv. Organomet. Chem. 1996, 39, 275. (c) Escudie, J.; Couret, C.; Ranaivonjatovo, H. Coord. Chem. Rev. 1998, 178, 565. (d) Escudie, J.; Ranaivonjatovo, H. Adv. Organomet. Chem. 1999, 44, 114. (e) Tokitoh, N.; Okazaki, R. In The Chemistry of Organic Germanium, Tin and Lead Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: New York, 2002; Vol. 2, Chapter 13.

r 2010 American Chemical Society

investigated in the last two decades. They display vast differences in their properties compared to the corresponding alkenes, and they constitute powerful building blocks in organometallic and heterocyclic chemistry owing to the great reactivity of the MdC double bond. These derivatives now constitute an established class of compounds and have been the topic of several review articles.1,2 Whereas the chemical behavior of metallaalkenes toward many different organic functions has been reported, their reactivity with quinones is poorly documented. The only results published very recently are the reactions of germene Mes2GedCR2 1a (Mes = 2,4,6-trimethylphenyl, CR2 = fluorenylidene)3 with various naphthoquinones4,5 (Scheme 1) and stannene Tip2SndCR0 26 (Tip = 2,4,6-triisopropylphenyl, CR0 2 = 2,7-di-tert-butylfluorenylidene) with para-benzoquinone, 1,4-naphthoquinone, and 9,10-anthraquinone7 (Scheme 2). A different behavior was observed (3) (a) Couret, C.; Escudie, J.; Satge, J.; Lazraq, M. J. Am. Chem. Soc. 1987, 109, 4411. (b) Lazraq, M.; Escudie, J.; Couret, C.; Satge, J.; Dr€ager, M.; Dammel, R. Angew. Chem., Int. Ed. Engl. 1988, 27, 828. (4) Ghereg, D.; Ech-Cherif El Kettani, S.; Lazraq, M.; Ranaivonjatovo, H.; Schoeller, W. W.; Escudie, J.; Gornitzka, H. Chem. Commun. 2009, 4821. (5) Ghereg, D.; Gornitzka, H.; Ranaivonjatovo, H.; Escudie, J. Dalton Trans. 2010, 39, 2016. (6) Abdoul Fatah; El Ayoubi, R.; Gornitzka, H.; Ranaivonjatovo, H.; Escudie, J. Eur. J. Inorg. Chem. 2008, 2007. (7) Ghereg, D.; Ranaivonjatovo, H.; Saffon, N.; Gornitzka, H.; Escudie, J. Organometallics 2009, 28, 2294. Published on Web 04/07/2010

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Organometallics, Vol. 29, No. 21, 2010

Ghereg et al.

Scheme 1

Scheme 2 Figure 1. Molecular structure of 2a (thermal ellipsoids at the 50% probability level). Hydrogen atoms and noncoordinated solvent molecules (CHCl3) are omitted; Mes and CR2 groups are simplified for clarity. Selected bond lengths (A˚) and angles (deg): Ge1-C15 2.010(4); Ge1-C41 1.978(4); Ge1-C50 1.968(4); Ge1-O1 1.816(3); C1-O1 1.362(5); C3-C15 1.587(6); C1-C2 1.371(6); C2-C3 1.516(6); C3-C4 1.493(6); C4-C5 1.306(7); C5-C6 1.497(6); C6-C7 1.511(6); C2-C7 1.441(6); C7-C8 1.376(6); C8-C9 1.407(6); C9-C14 1.422(6); C1-C14 1.425(6); C1-C2-C3 118.4(4); C2-C3-C15 111.8(3); C3-C15-Ge1 104.6(3); O1-Ge1-C15 99.49(15); C1-O1-Ge1 123.1(3); O1-C1-C2 122.2(4).

calculations are also presented to understand the mechanism of the reactions.

Results and Discussion

Scheme 3

between germene and stannene in their reaction with 1,4naphthoquinone, since ortho-quinodimethane-type compound I and derivative II (formed by a cycloaddition on the aromatic ring) have been obtained4 from the germene (Scheme 1), whereas it was only IV, the analogue of II, in the case of the stannene.7 We have now addressed the question of whether the germene would also afford cycloadducts similar to III and V with 1,4-benzoquinone and anthraquinone (Scheme 2). Herein we report our investigations in this field by studying the behavior of germenes toward 1,4-benzoquinone, 2,3,5, 6-tetramethyl-1,4-benzoquinone, and 9,10-anthraquinone; DFT

a. 9,10-Anthraquinone. The treatment of an ethereal solution of germene Mes2GedCR2 1a with 9,10-anthraquinone led to product 2a in 85% yield; whatever the ratio of the reactants, the analytical and spectral data were indicative of a 2:1 adduct (two germenes for one quinone) (Scheme 3). Thus, 1a behaves similarly to Tip2SndCR0 2, as previously reported.7 The formation of a pentacyclic ring system has also been observed in the addition of germylenes Ge[CH(SiMe3)2]2 and Ge[N(SiMe3)2]2 to 9,10-anthraquinone.8 The driving force for this reaction is presumed to be the well-known oxophilic character of germanium and the formation of two thermodynamically stable six-membered-ring heterocycles. The formation of 2a induces aromaticity on the central quinonic ring, thus compensating for the high thermodynamic cost required to break the aromaticity of one of the fused rings of the quinone. It is worthy to note that only one aromatic ring was involved in the reaction, as clearly corroborated by the 1H and 13C NMR spectra. The 1H NMR spectrum of 2a contains two singlets for the CR2-CH-CHd moiety at 4.41 (CR2-CH) and 4.33 (CR2CH-CHd) ppm. In the 13C NMR spectrum, the corresponding CH signals appear at 39.54 and 123.86 ppm, in the typical region for allylic and olefinic carbon atoms, respectively. The hydrogen atoms in the remaining aromatic ring of the former quinonic backbone resonate, like in the starting compound, as two doublets of doublets at 7.48 and 8.40 ppm. The X-ray determination of 2a (Figure 1) clearly shows the formation of two six-membered rings involving both oxygen (8) Sweeder, R. D.; Gdula, R. L.; Ludwig, B. J.; Banaszak Holl, M. M.; Kampf, J. W. Organometallics 2003, 22, 3222.

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Scheme 4

Figure 2. Molecular structure of 3a (thermal ellipsoids at the 50% probability level). Hydrogen atoms and noncoordinated solvent molecules (pentane) are omitted; Mes and CR2 groups are simplified for clarity. Selected bond lengths (A˚) and angles (deg): C2-C7 1.305(9); C7-C11A 1.527(8); C2-C11 1.484(8); O1-C7 1.369(7); Ge1-O1 1.824(4); Ge1-C4 2.015(6); C11A-C4 1.549(9); C7-C2-C11 123.2(6); C2-C11-C7A 112.4(5); C2-C7-C11A 124.3(6); O1-C7-C11A 111.9(5); C7-C11A-C4 107.5(5); C11A-C4-Ge1 96.4(4); O1-Ge1C4 90.0(2); C7-O1-Ge1 112.3(4).

and ortho-carbon atoms of one adjacent aromatic ring system. The C3-C15 (1.587(6) A˚) and C6-C28 (1.580(6) A˚) bonds are longer than standard C-C single bonds. The former quinonic ring has been converted into an aromatic ring, as confirmed by the shortening of the following C-C distances: C1-C2 = 1.371(6) A˚; C7-C8 = 1.376(6) A˚; C8-C9 = 1.407(6) A˚; and C1-C14 = 1.425(6) A˚. The ring C2 to C7 of the former 9,10-anthraquinone is no longer aromatic, retaining only one double bond (C4-C5=1.306(7) A˚). b. 1,4-Benzoquinone. Addition of 1,4-benzoquinone to germene 1a in an Et2O solution furnished a mixture of two products, 3a and 4a, which were separated by fractional crystallization immediately after the reaction completion and were isolated in 56% and 35% yields. Like in the case of 9,10-anthraquinone, two equivalents of 1a were necessary for one of quinone; when the reaction was performed with one equivalent of each reactant, a half equivalent of quinone was recovered. The structures of compounds 3a and 4a were determined by NMR and X-ray analysis (Figures 2 and 4). They are composed of a 1,4-cyclohexadiene backbone with two additional five-membered-ring skeletons. In contrast to the corresponding product obtained from the stannene Tip2SndCR0 2 (Tip = 2,4,6-triisopropylphenyl, CR0 2 = 2,7di-tert-butylfluorenylidene),7 the two cycloadditions occur exclusively on the opposite sides of the six-membered ring (Scheme 4). This surprising behavior motivated us to verify whether germene 1b (Mes = mesityl, CR0 2 = 2,7-di-tert-butylfluorenylidene), structurally similar to the stannene Tip2SndCR0 2 since it displays two tert-butyl groups in 2,7 positions on the fluorenylidene system, would furnish a different result. In fact, 1b does not yield a compound of type III like in the case of tin (Scheme 2) but behaves similarly to 1a, leading to the 1,4-cyclohexadiene derivatives 3b and 4b; the ratio after the reaction was 60/40. Thus, the high steric hindrance caused by the two tert-butyl groups has no influence on the course of the reaction. When the reaction mixture was stirred for 5 h before treatment, the ratio between compounds 3 and 4 changed to 20/80. This result is in favor of a rearrangement

Scheme 5

from 3 to 4, which was proved by stirring the pure compound 3a overnight in Et2O at room temperature: the subsequent analytical and spectral data revealed the presence of a mixture of 3a and 4a in a 10/90 ratio, indicating that under these conditions cycloadduct 3a undergoes a double 1,3hydrogen shift leading to 4a. When compound 3a was put in solution in pentane, it was recovered unchanged after one day, and thus no rearrangement occurred. c. 2,3,5,6-Tetramethyl-1,4-benzoquinone. To determine whether a similar cycloaddition process is also possible with a substituted 1,4-benzoquinone, germene 1a was allowed to react with 2,3,5,6-tetramethyl-1,4-benzoquinone (Scheme 5). The reaction occurred nearly quantitatively (96% yield), and no product other than 5a was observable in the crude reaction mixture. As in the cases of 1,4-benzoquinone and 9,10-anthraquinone, whatever the ratio of the reactants, the final product has two germene moieties for one quinone. Physicochemical measurements (1H, 13C NMR and mass spectrometry) of pure 5a obtained by crystallization from a pentane solution supported the formation of a cycloadduct analogous to 3a. Thus, the increased steric shielding in the quinone did not prevent the attack of CR2 groups on the carbon atoms substituted by the methyl groups. In contrast with 3a, 5a is stable since any subsequent isomerization by a Me shift does not occur. d. 1H and 13C NMR Data for 3-5. Due to complex spectra, the assignment of the 1H and 13C signals was based in all cases on the homonuclear (COSY) and heteronuclear (HSQC and HMBC) experiments. The presence of two stereogenic centers in 3a complicates the assignment of the 1H and 13C signals, due to the diastereotopic mesityl groups. For each Mes2Ge unit, one extremely

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broad singlet at 1.98 ppm (half-width = 210 Hz) is observed for the ortho-methyl groups of one mesityl unit and two broad singlets at 0.79 and 2.25 ppm for the ortho-methyl groups of the other one, suggesting that the rotation of one mesityl on the NMR time scale is more hindered than that of the other one: in one case the ortho methyls are broadening out, and in the other case, the ortho methyls have already been separated in two different signals. As expected, an NMR study performed at -60 C displays that the broad signal at 1.98 ppm splits into two signals at 1.80 and 2.26 ppm. para-Methyl groups appear as sharp singlets (at 2.07 and 2.27 ppm), since they are not influenced by the slow or hindered rotation around the ipso-C-Ge bond. The four hydrogen atoms of the former quinonic ring give two singlets at 4.01 ppm (CHdCO) and 4.15 ppm (CHCR2), and the corresponding carbons give signals at 93.99 ppm (CHdCO) and 50.30 ppm (CHCR2). In contrast to 3a, the rearrangement product 4a exhibits fully unrestricted free rotation on the NMR time scale; that is, all 1H and 13C resonances appear as sharp signals. The aliphatic zone in the 1H NMR spectrum displays, among ortho- and para-methyl resonances of mesityl units (at 1.71 and 2.16 ppm, respectively), one signal at 2.29 ppm corresponding to four methylene protons. In this case, mesityl groups are equivalent. Together with the 13C NMR spectrum showing one secondary carbon at 27.56 ppm (CH2) and a quaternary one at 107.03 ppm (CdCO), these observations are in agreement with the structure of the isomerization product 4a (Scheme 4). The 1H and 13C spectra of 3b are similar to those of 3a despite the larger spatial requirement of the 2,7-di-tertbutylfluorenylidene group than that of fluorenylidene. Almost identical mass spectra were obtained for 3a and its isomer 4a and 3b/4b, respectively. As expected, mesityl moieties in 5a are not equivalent due to the presence of two stereogenic carbon atoms. Moreover, due to the steric crowding, the germanium mesityl bonds rotate slowly, causing the methyl groups of mesityl fragments to exhibit two singlets at 2.05 and 2.22 ppm for paraMe protons and four sharp singlets at 0.56, 1.45, 2.42, and 2.47 ppm for ortho-Me protons. A variable-temperature NMR was performed between 20 and 60 C in toluene-d8. The signals, sharp at 20 C, become broad at 60 C, but the coalescence effect has not been observed in this temperature range. The patterns of 13C NMR spectra for 5a are similar to those of 3a and 3b. The CCR2, CdCO, and C-O of the former quinonic fragment display signals at 54.59, 108.05, and 152.49 ppm, respectively. e. X-ray Structures of 3a, 3b, 4a, and 5a. The constitution of 3a, 3b, 4a, and 5a was confirmed by X-ray structure analyses (3a Figure 2, 3b Figure 3, 4a Figure 4, and 5a Figure 5). Suitable crystals for structure determination were obtained at room temperature from pentane (3a), THF (3b), chloroform (4a), and dichloromethane (5a). The structures of 3a and 4a are easily differentiated by X-ray structure analyses since the bond lengths in the sixmembered ring of the former quinonic ring are completely different. The C-C distance in the two bonds joining the central six-membered ring to oxagermanolane units changes from 1.527(8) A˚ (C7-C11A in 3a, single bond) to 1.336(9) A˚ (C1-C2 in 4a, double bond). The other C-C distances in this six-membered ring change from two single bonds (C2C11 = 1.484(8) A˚) and two double bonds (C2-C7 = 1.305(9) A˚) in 3a to four remarkably short single bonds

Ghereg et al.

Figure 3. Molecular structure of 3b (thermal ellipsoids at the 50% probability level). Hydrogen atoms, rotation disorder of tert-butyl groups, and noncoordinated solvent molecules (THF) are omitted; Mes and CR0 2 groups are simplified for clarity. Selected bond lengths (A˚) and angles (deg): C40-C41 1.494(9); C41-C42 1.500(9); C42-C40A 1.319(9); C19-C41 1.570(9); O1-C40 1.358(8); Ge1-O1 1.826(4); Ge1-C19 2.001(7); Ge1-C1 1.954(7); Ge1-C10 1.962(7); C40-C41-C42 114.1(5); C42A-C40-C41 124.3(6); C41-C42-C40A 121.5(6); O1C40-C41 112.5(5); C40-C41-C19 108.0(5); C41-C19-Ge1 96.3(4); O1-Ge1-C19 90.3(2); C40-O1-Ge1 112.2(4).

Figure 4. Molecular structure of 4a (thermal ellipsoids at the 50% probability level). Hydrogen atoms and noncoordinated solvent molecules (CHCl3) are omitted; Mes and CR2 groups are simplified for clarity. Selected bond lengths (A˚) and angles (deg): C1-C2 1.336(9); C2-C3 1.478(9); C1-C3A 1.489(9); C1-O1 1.381(8); Ge1-O1 1.843(4); Ge1-C4 2.034(6); C2-C4 1.510(9); Ge1-C26 1.974(7); Ge1-C17 1.970(7); C1-C2-C3 122.8(6); C2-C3-C1A 112.8(5); C2-C1-C3A 124.5(6); C1-C2C4 117.1(6); Ge1-C4-C2 97.5(4); O1-Ge1-C4 89.2(2); Ge1-O1-C1 106.8(4); O1-C1-C2 120.1(6).

(C2-C3 = 1.478(9) A˚ and C1-C3A = 1.489(9) A˚) in 4a. In derivatives 3b and 5a the bond lengths and angles in the central 1,4-cyclohexadiene ring are not appreciably different from those of compound 3a, proving that a 1,3-shift did not occur. In all cases, the six-membered ring is nearly planar. In derivative 4a, the five-membered rings adopt an envelope conformation with germanium atoms being out of the mean plane (in trans position in relation to this plane): Ge1-C4-C2-C1 = -23.39, Ge1-O1-C1-C2 = 16.80. In the nonrearranged compound 3a, the five-membered rings are more distorted: Ge1-O1-C7-C11 = -15.23,

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Figure 5. Molecular structure of 5a (thermal ellipsoids at the 50% probability level). Hydrogen atoms and noncoordinated solvent molecules (CH2Cl2) are omitted; Mes and CR2 groups are simplified for clarity. Selected bond lengths (A˚) and angles (deg): C32-C33 1.332(5); C33-C34 1.514(5); C33-C35 1.520(5); C35-C32A 1.512(5); C35-C36 1.557(5); C32-O1 1.377(4); Ge1-O1 1.817(3); Ge1-C1 2.019(4); Ge1-C14 1.978(4); Ge1-C23 1.973(4); C1-C35A 1.603(5); C32C33-C35 120.6(3); C33-C32-C35A 127.9(3); C32AC35-C33 110.8(3); C35A-C32-O1 113.2(3); C32-O1-Ge1 113.3(2); C1-Ge1-O1 89.74(14); Ge1-C1-C35A 99.8(2), C1-C35A-C32 105.0(3).

Ge1-C4-C11-C7 = 46.32. A similar trend is observed in 3b. In all compounds, Ge-O distances (1.817-1.843 A˚) are slightly beyond the upper limit of the standard range (1.73-1.79 A˚).9 The intracyclic Ge-C bonds are also elongated (2.001-2.034 A˚), whereas the acyclic Ge-C(Mes) bond lengths (1.954-1.978 A˚) lie in the normal range (1.93-1.98 A˚).9 f. Theoretical Calculations. In order to get further insight on the reaction of germene with 1,4-benzoquinone (Scheme 4), a DFT study was carried out at the B3LYP/631G** level of theory. Different hypotheses have been investigated, and a reaction pathway, calculated in the gas phase, has been proposed. In this study, a model of Mes2GedCR2 where bulky substituents are replaced by hydrogen atoms, H2GedCH2 1H, has been chosen to reach reasonable computation times. In a first step, we had to determine whether the reaction between two molecules of 1H and 1,4-benzoquinone is a onestep concerted process or a multistep mechanism. A meticulous search on the potential energy surface (PES) did not allow us to find a transition state corresponding to a direct [2þ3] cycloaddition pathway. Only a mechanism involving first [2þ2] cycloadditions followed by successive rearrangements leading to the formal [2þ3] adduct has been found on the PES (see Figures 6 and 7). This mechanism consists in two successive additions of one molecule of 1H on the 1,4benzoquinone, which is consistent with the really low probability of a concerted reaction involving three different molecules. The energy profile of the first steps of the reaction and the structures of the different minima and transition states (labeled “M” or “T”, respectively) are depicted in Figure 6. The [2þ2] cycloaddition of the GedC double bond on the carbonyl group of the quinone leads to M1 via the transition state T1, slightly higher in energy than the reactants (9) Baines, K. M.; Stibbs, W. G. Coord. Chem. Rev. 1995, 145, 157.


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Figure 6. Energy profile (ΔE in kcal 3 mol-1) of the reaction between 1H and 1,4-benzoquinone: formation of the [2þ2] intermediates M1 and M2.

Figure 7. Energy profile (ΔE in kcal 3 mol-1) of the reaction of 1H with 1,4-benzoquinone; comparison between pathways leading to the formation of 3H and its corresponding type IIIH compound. &Interconversion isomers. The arrows represent the motion of the cycle leading to the corresponding isomer.

(∼2 kcal 3 mol-1). The latter presents a Ge-O bond (1.946 A˚) longer than the one in M1 (1.836 A˚) and a C-C1 bond not yet formed (3.331 A˚), consistent with an asynchronous mechanism. It is noteworthy that Mosey and co-workers have theoretically investigated the [2þ2] cycloaddition between formaldehyde and germene.10 In this case a synchronous mechanism has been proposed with a free energy barrier around 16 kcal 3 mol-1 (B3LYP/6-311þþG**), more important than ours. The small energy barrier found for the mechanism involving the quinone is probably due to the asynchronous mechanism for which the geometry of the TS is close to the reactants. The next step of the reaction corresponds to a [2þ2] cycloaddition of a second molecule 1H to give M2. This second step also occurs easily since an activation barrier around 7 kcal 3 mol-1 is calculated and leads to the intermediate M2 (-69 kcal 3 mol-1). This result is not surprising since a [2þ2] cycloaddition between the GedC double bond of germene 1a and one CdO (10) Mosey, N. J.; Baines, K. M.; Woo, T. K. J. Am. Chem. Soc. 2002, 124, 13306.

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double bond of maleic anhydride has previously been observed;11 in the case of 4-cyclopentene-1,3-dione, a double [2þ2] cycloaddition onto both carbonyl groups occurred;11 these five-membered-ring carbonyl reagents also contain a conjugated system OdC-CdC. Let us consider the pathway from M2 to 3H (Figure 7). This reaction occurs in two steps. The first one, which can be described as a synchronous process via the transition state T3, leads to the intermediate M3. In T3, the CR-C1 bond is elongated (2.27 A˚) compared to that found in M2 and the CR-C6 distance decreases and becomes intermediate between those of M2 and M3 (2.06 A˚ versus 2.55 and 1.55 A˚, respectively). Moreover the carbon atom CR, located between C1 and C6, remains in a tetrahedral environment P ( CR ∼349), indicating the presence of a weak interaction between C6 and/or C1. The intermediate M3 exists in four isoenergetic isomers (M3a0 , M3b0 , M3a00 , M3b00 ), which correspond in fact to interconversion isomers. In the second step, which is the key step of the reaction, two different pathways can be followed. When Cβ moves toward the carbon atom C3, the reaction leads to the formation of 3H via the transition state T4, while the product IIIH (observed in the case of the tin analogue, Scheme 2) is obtained via the transition state T40 when Cβ moves toward carbon C5. The second step is predicted, in both cases, to be exothermic (ΔE ≈ -35 kcal 3 mol-1). The energy gap between 3H and IIIH (2 kcal 3 mol-1) is too small to favor one product over the other one. The comparison of the activation barriers reveals that the formation of 3H is kinetically favored. However, the difference in energy (8 kcal 3 mol-1 between T4 and T40 ) is fairly small and is not sufficient to explain that III is not observed experimentally. This hints that other factors should be in favor of 3a formation, such as the steric hindrance of the fluorenylidene groups on CR and Cβ, which has not been taken into account in this study. From the experimental point of view, the last part of this reaction (3 f 4) can be described as a 1,3 prototropy and clearly shows that this mechanism is assisted by the solvent since it occurs in Et2O and not in pentane. When calculations take into account the solvent both explicitely12 and implicitely (PCM formalism), the pathway from 3H to 4H shows that the intermediate MH2 and the product of this reaction, 4H, are slightly thermodynamically favored compared to 3H by 6 and 9 kcal 3 mol-1, respectively (Figure 8). Furthermore two transitions states, corresponding to the two 1,3-H shifts, have been found (TH1 and TH2, respectively). It is clear that the short H-O distances (respectively 1.057 and 1.054 A˚ for TH1 and TH2) combined with the long C3,5-H and C2,6-H distances respectively for TH1 and TH2 (around 1.1 A˚) attest that the reaction is explicitly assisted by the solvent, which plays a major role in the reaction. It is interesting to note that, without the presence of one molecule of ether, no 1,3-H migration mechanism can be found in the gas phase. This result can be compared to the reaction performed in pentane, where no hydrogen shift is observed, confirming definitely that with a lack of interaction with the solvent, a rearrangement is not possible. Even considering the stabilization due to solvent effects, the energy barriers corresponding to the two H shifts are (11) Ech-Cherif El Kettani, S.; Lazraq, M.; Ranaivonjatovo, H.; Escudie, J.; Gornitzka, H.; Ouhsaine, F. Organometallics 2007, 26, 3729. (12) The explicit assistance of the solvent was modelized by taking into account one molecule of dimethyl ether in the reaction profile.

Ghereg et al.

Figure 8. Energy profile (ΔE in kcal 3 mol-1, PCM formalism) of the prototropy reaction from 3H to 4H taking into account one molecule of dimethyl ether.

noticeably higher than the other steps of the reaction pathway. Nevertheless, this is consistent with the slow kinetics of the hydrogen shift observed experimentally. We are aware that the proposed mechanisms possess rather high activation barriers, but some factors that have not been taken into account in this study should affect the energy profile of the reaction. In particular, the steric hindrance of the germene substituents could contribute to lower all the previously discussed barriers. Nevertheless, the proposed reaction pathway gives trends of what happens experimentally. In conclusion germenes react with 1,4-benzoquinones and 9,10-anthraquinone in the molar ratio of 2/1 to give polycyclic derivatives. With 1,4-benzoquinone and 2,3,5,6-tetramethyl-1,4-benzoquinone, formal [2þ3] cycloadducts are obtained with the two oxagermanolane rings on opposite sides of the central 1,4-cyclohexadiene ring; according to DFT calculations, the first steps of these reactions are [2þ2] cycloadditions between the GedC and CdO double bonds followed by a rearrangement. A double 1,3-H shift is then observed in diethyl ether in the case of 1,4-benzoquinone. Germenes and stannenes showed a different behavior in their reactions with 1,4-benzoquinones; by contrast, in the case of 9,10-anthraquinone, the same type of cycloaddition (involving one adjacent aromatic ring) occurs.

Experimental Section General Experimental Details. All experiments were performed in flame-dried glassware under an argon atmosphere using standard vacuum-line, Schlenk, and cannula techniques with solvents being distilled over standard drying agents and degassed before use. All reagents were purchased from Aldrich and were used without further purification. Deuterated solvents were dried and stored over 4 A˚ molecular sieves. NMR spectra were recorded in CDCl3 on Bruker Avance 300 and 400 instruments at the following frequencies: 300.13, 400.13 MHz (1H) and 75.47, 100.62 MHz (13C{1H}). 1H and 13C{1H} NMR assignments were confirmed by 1H COSY, HSQC (1H-13C), and HMBC (1H-13C) experiments. Mass spectra were measured on a Nermag R10-10 spectrometer by CI (NH3 or CH4). Melting points were determined on a Leitz microscope heating stage 250 or Electrothermal apparatus (capillary). Elemental analyses were performed by the “Service de Microanalyse de

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Table 1. Crystal Data for 2a, 3a, 3b, 4a, and 5a

empirical formula fw temperature (K) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) Z abs coeff (mm-1) reflns collect. indep reflns abs corr data/restraints/params goodness-of-fit (F2) final R indices (I > 2σ(I)) R indices (all data) largest diff peak, hole (e A˚-3)

2a 3 2CHCl3

3a 3 2C5H12

3b 3 THF

4a 3 2CHCl3

5a 3 CH2Cl2

C78H70Cl6Ge2O2 1397.22 173(2) triclinic P1 11.014(2) 16.376(3) 20.416(3) 67.051(3) 89.171(3) 85.840(3) 3381.6(9) 2 1.173 15 047 9549 [R(int) = 0.0404] semiempirical 9549/186/878 1.016 0.0481 0.1020 0.0854 0.1162 0.569; -0.450

C78H88Ge2O2 1202.66 193(2) triclinic P1 9.126(2) 13.234(2) 15.561(3) 113.855(6) 90.973(6) 107.554(7) 1617.8(5) 1 0.975 10 216 4026 [R(int) = 0.0687] none 4026/154/422 1.022 0.0596 0.1432 0.1118 0.1713 0.692; -0.732

C88H104Ge2O3 1354.89 193(2) triclinic P1 11.142(2) 13.497(2) 14.854(2) 100.188(3) 110.955(3) 107.521(3) 1884.9(5) 1 0.845 8081 4995 [R(int) = 0.0451] none 4995/82/479 1.044 0.0639 0.1866 0.0954 0.2415 0.555; -0.569

C70H66Cl6Ge2O2 1297.11 133(2) triclinic P1 8.861(2) 10.974(2) 16.164(3) 91.254(4) 101.916(4) 97.471(4) 1522.9(5) 1 1.296 6371 4015 [R(int) = 0.0901] none 4015/108/404 1.024 0.0699 0.1711 0.0922 0.1886 1.098; -1.659

C73H74Cl2Ge2O2 1199.40 193(2) triclinic P1 11.966(1) 14.425(1) 17.809(1) 86.197(2) 85.504(1) 87.695(1) 3055.9(2) 2 1.117 30 735 12 326 [R(int) = 0.0720] semiempirical 12326/51/756 1.025 0.0576 0.1172 0.1099 0.1391 0.742; -0.578

l0 Ecole de Chimie de Toulouse”. The yields were calculated from the starting Mes2Ge(F)-CHR2 and Mes2Ge(F)-CHR0 2. All data for structures were collected at low temperature using an oil-coated shock-cooled crystal on a Bruker-AXS Apex II diffractometer with Mo KR radiation (λ = 0.71073 A˚) and are summarized in Table 1. The structures were solved by direct methods,13 and all non-hydrogen atoms were refined anisotropically using the least-squares method on F2.14 For the 1H and 13C NMR study the carbon atoms of the fluorenyl and 2,7-di-tert-butylfluorenyl groups are numbered as shown in Figure 9. Computational Details. Calculations were performed with the Gaussian 03 suite of programs,15 using the density functional method.16 The hybrid exchange functional B3LYP17 set was used. B3LYP is a three-parameter functional developed by Becke that combines the Becke gradient-corrected exchange functional and the Lee-Yang-Parr and Vosko-Wilk-Nusair correlation functional with part of the exact HF exchange energy. All Gaussian calculations were done in combination with the (13) SHELXS-97: Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467. (14) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement; University of G€ottingen, 1997. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, R.; Fukuda, J.; Hasegawa, M.; Ishida, T.; Nakajima, Y.; Honda, O.; Kitao, H.; Nakai, M. X.; Klene, X.; Li, K.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, D. K.; Malick, A. D.; Rabuck, K.; Raghavachari, J. B.; Foresman, J. V.; Ortiz, Q.; Cui, A. G.; Baboul, S.; Clifford, O.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D-02; Gaussian, Inc.: Pittsburgh, PA, 2003. (16) Parr, R. G.; Yang, W. Functional Theory of Atoms and Molecules; Breslow, R.; Goodenough, J. B., Eds.; Oxford University Press: NewYork, 1989. (17) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

Figure 9 6-31G(d,p) basis set for all atoms. Geometry optimizations were carried out without any symmetry restrictions; the nature of the extrema (minimum and transition states) was verified with analytical frequency calculations. To ensure that the transition states are connected to the right minima, intrinsic reaction coordinate (IRC) calculations were conducted using the Schlegel-Gonzalez algorithm.18 All the calculated energies have been zero-point energy (ZPE) corrected using unscaled density functional frequencies. Solvent effects were taken into account through the polarized continuum model (IEF-PCM),19 as implemented in Gaussian03, with the UAKS standard radii set (optimized for the PBE0/6-31G(d) level of theory). Synthesis of Germenes 1a and 1b. Germenes 1a3 and 1b5 were prepared as previously described, by dropwise addition of 1.05 molar equiv of tert-butyllithium (1.7 M in pentane) to a solution of 2.00 mmol of the starting fluorogermanes Mes2Ge(F)CHR2 1a (CR2 = fluorenylidene) or 1b (CR0 2 = 2,7-di-tert-butylfluorenylidene) in 20 mL of diethyl ether cooled to -78 C. Warming to room temperature afforded an orange solution of 1a or 1b with a precipitate of LiF. According to an 1H NMR experiment, germenes 1 were produced in a nearly quantitative yield. Thus, all reactions were performed on the crude reaction mixtures. Synthesis of 2a. 9,10-Anthraquinone (1.0 mmol, 0.208 g) dissolved in 20 mL of THF was added dropwise to a crude solution of 1a (2.0 mmol) in 20 mL of Et2O at -78 C. The (18) (a) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154. (b) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (19) (a) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43. (b) Tomasi, J.; Cammi, R.; Mennucci, B.; Cappelli, C.; Corni, S. Phys. Chem. Chem. Phys. 2002, 4, 5697.

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mixture was slowly warmed to ambient temperature and stirred overnight. Volatiles were removed in vacuo, and the resulting brownish solid was dissolved in 30 mL of pentane; LiF was eliminated by filtration. Recrystallization from pentane at -30 C gave pure crystals of 2a (0.98 g, 85%, mp 378 C). 1H NMR (300.13 MHz): 0.67 and 2.73 (2s, 2 6H, o-Me of Mes), 1.27 and 2.61 (2 br s, 2 6H, o-Me of Mes), 2.10 and 2.26 (2s, 2 6H, p-Me of Mes), 4.33 (s, 2H, CH-CHCR2), 4.41 (s, 2H, CHCR2), 6.34 and 6.54 (2d, 3JHH = 7.5 Hz, 2 2H, H1 and H8), 6.36 and 6.80 (2s, 2 2H, m-CH of Mes), 6.58 and 6.91 (2 br s, 2 2H, m-CH of Mes), 6.82 and 6.85 (2t, 3JHH = 7.5 Hz, 2 2H, H2 and H7), 7.18 and 7.21 (2t, 3JHH = 7.5 Hz, 2 2H, H3 and H6), 7.48 and 8.40 (2dd, 3JHH = 7.5 Hz, 4JHH = 1.5 Hz, CHCHCCO), 7.64 and 7.67 (2d, 3JHH = 7.5 Hz, 2 2H, H4 and H5). 13C NMR (75.47 MHz): 20.78 and 21.02 (p-Me of Mes), 22.63, 22.76, and 23.30 (2 overlapping signals) (o-Me of Mes), 39.54 (CHCR2), 54.93 (CR2), 117.42 (Carom-CHCR2), 119.04 and 119.38 (C4 and C5), 123.23 and 124.97 (CHCHCCO), 123.86 (CH-CHR2), 124.87 and 125.76 (C1 and C8), 126.24, 126.33, 126.39, and 126.83 (C2, C3, C6, and C7), 127.35 (CaromCO), 127.84 and 129.50 (m-CH of Mes), 128.80 and 130.80 (br signals, m-CH of Mes), 133.77 and 136.29 (ipso-C of Mes), 138.83 and 139.38 (p-C of Mes), 141.07 and 141.93 (C12 and C13), 141.28 and 143.75 (o-C of Mes), 144.72 and 145.96 (C10 and C11), 145.59 (C-O). MS m/z (% relative intensity): 1158 (M, 20), 1039 (M - Mes, 4), 477 (Mes2GedCR2 þ 1, 40), 311 (Mes2Ge - 1, 100), 209 (M - 2Mes2GedCR2 þ 2). Anal. Calcd for C76H68Ge2O2 (1158.632): C, 78.78; H, 5.92. Found: C, 78.90; H, 5.95. Synthesis of 3 and 4. To a crude solution of 1 (2 mmol) in Et2O (20 mL) cooled to -78 C was added 1 mmol of 1,4-benzoquinone dissolved in 5 mL of diethyl ether. The reaction mixture was allowed to warm to room temperature, during which time its color slowly turned from orange to brown. After 2 h of stirring at room temperature, solvents were removed in vacuo, and the resultant residue was dissolved in 100 mL of toluene and filtered to eliminate lithium salts. The solvent (toluene) was removed in vacuo and replaced by 20 mL of diethyl ether. The mixture was filtered again; the precipitate was washed with 10 mL of diethyl ether and then recrystallized from 10 mL of pentane to give 3. The filtrate was concentrated to 5 mL and cooled to -20 C to afford pure crystalline 4. 3a (0.59 g, 56%, mp 338 C). 1H NMR (300.13 MHz): 0.79 and 2.25 (2 br s, 2 6H, o-Me of Mes), 1.98 (vbr s, Δν1/2 = 210 Hz, 12H, o-Me of Mes), 2.07 and 2.27 (2s, 2 6H, p-Me of Mes), 4.01 (s, 2H, CHdCO), 4.15 (s, 2H, CHCR2), 6.42 and 6.63 (2s, 2 2H, m-CH of Mes), 6.58 and 7.38 (2d, 2JHH = 7.5 Hz, 2 2H, H1 and H8 of CR2), 6.73 (s, 2H, 4H, m-CH of Mes), 7.03, 7.15, 7.34, and 7.37 (4t, 2JHH = 7.5 Hz, 4 2H, H2, H3, H6, and H7 of CR2), 7.83 and 7.86 (2d, 2JHH = 7.5 Hz, 2 2H, H4 and H5 of CR2). 13C NMR (75.47 MHz): 20.82 and 21.08 (p-Me of Mes), 22.15, 23.06, and 23.91 (o-Me of Mes), 50.30 (CHCR2), 58.32 (CR2), 93.99 (CHdCO), 119.38 and 119.74 (C4 and C5 of CR2), 122.64 and 124.35 (C1 and C8 of CR2), 126.11, 126.24, 126.37, and 126.77 (C2, C3, C6, and C7 of CR2), 129.45, 129.47, and 129.50 (m-CH of Mes), 132.52 and 132.56 (ipso-C of Mes), 139.29 and 139.47 (p-C of Mes), 140.91 and 141.14 (C12 and C13 of CR2), 142.74, 143.23, and 143.74 (o-C of Mes), 144.55 and 146.33 (C10 and C11 of CR2), 155.77 (C-O). MS m/z (% relative intensity): 1058 (M, 25), 939 (M - Mes, 10), 893 (M CR2 - 1, 5), 584 (M - Mes2GedCR2, 5), 477 (Mes2GedCR2þ1, 15), 311 (Mes2Ge - 1, 100), 167. Anal. Calcd for C68H64Ge2O2 (1058.515): C, 77.16; H, 6.09. Found: C, 77.74; H, 6.17. 3b (0.87 g, 68%, mp >400 C). 1H NMR (300.13 MHz): 0.78 and 2.29 (2s, 2 6H, o-Me of Mes), 1.06 and 1.13 (2s, 2 18H, CMe3), 1.82 (br s, 12H, o-Me of Mes), 2.06 and 2.24 (2s, 2 6H, p-Me of Mes), 4.01 (s, 2H, CHdCO), 4.03 (s, 2H, CHCR0 2), 6.38 and 6.62 (2s, 2 2H, m-CH of Mes), 6.56 (d, 4JHH = 1.5 Hz, 2H, H1 of CR0 2), 6.71 (s, 2H, 4H, m-CH of Mes), 7.34-7.38 (m, 6H, H3, H6, and H8 of CR0 2), 7.69 and 7.73 (2d, 3JHH = 7.8 Hz,

Ghereg et al. 2 2H, H4 and H5 of CR0 2). 13C NMR (75.47 MHz): 20.83 and 20.95 (p-Me of Mes), 22.53 (2 overlapping signals) and 22.99 (o-Me of Mes), 31.05 and 31.42 (CMe3), 34.57 and 34.82 (CMe3), 50.96 (CHCR0 2), 57.95 (CR0 2), 93.85 (CHdCO), 118.47 and 119.01 (C4 and C5 of CR0 2), 119.11, 122.00, 122.55, and 123.51 (C1, C3, C6, and C8 of CR0 2), 127.65, 128.62, and 129.70 (m-CH of Mes), 132.55 and 132.84 (ipso-C of Mes), 138.38, 138.66, 138.94, and 139.27 (p-C of Mes, C12 and C13 of CR0 2), 142.17, 143.60, and 143.79 (o-C of Mes), 144.69 and 146.27 (C10 and C11 of CR0 2), 149.25 and 149.26 (C2 and C7 of CR0 2), 156.00 (C-O). MS m/z (% relative intensity): 1283 (M þ 1, 15), 1163 (M - Mes, 10), 1005 (M - CR0 2 - 1, 5), 587 (Mes2GedCR0 2 - 1, 22), 311 (Mes2Ge - 1, 25), 278 (R0 2CH2, 100), 263 (R0 2CH2 - Me, 53). Anal. Calcd for C84H96Ge2O2 (1282.939): C, 78.64; H, 7.54. Found: C, 78.37; H, 7.48. 4a (0.37 g, 35%, mp 373 C). 1H NMR (300.13 MHz): 1.71 (s, 24H, o-Me of Mes), 2.16 (s, 12H, p-Me of Mes), 2.29 (s, 4H, CH2), 6.60 (s, 4H, m-CH of Mes), 6.92 (d, 3JHH = 7.6 Hz, 4H, H1 and H8 of CR2), 7.09 and 7.35 (2t, 3JHH = 7.6 Hz, 2 4H, H2, H3, H6, and H7 of CR2), 7.83 (d, 3JHH = 7.6 Hz, 4H, H4 and H5 of CR2). 13C NMR (75.47 MHz): 20.87 (p-Me of Mes), 22.60 (o-Me of Mes), 27.56 (CH2), 60.77 (CR2), 107.03 (CdCO), 119.65 (C4 and C5 of CR2), 124.24, 126.28, 126.82 (C1, C2, C3, C6, C7, and C8 of CR2), 128.85 (m-CH of Mes), 132.90 (ipso-C of Mes), 139.19 (C12 and C13 of CR2), 140.56 (p-C of Mes), 143.33 (o-C of Mes), 147.38 (C10 and C11 of CR2), 153.85 (C-O). MS m/z (% relative intensity): 1058 (M, 100), 939 (M - Mes, 45), 893 (M - CR2 - 1, 30), 585 (M - Mes2GedCR2 þ 1, 15), 476 (Mes2Ged CR2, 15), 311 (Mes2Ge - 1, 45). Anal. Calcd for C68H64Ge2O2 (1058.515): C, 77.16; H, 6.09. Found: C, 77.52; H, 6.38. 4b (0.23 g, 18%, mp 335 C). 1H NMR (300.13 MHz): 1.11 (s, 36H, CMe3), 1.73 (br s, 24H, o-Me of Mes), 2.13 (s, 12H, p-Me of Mes), 2.34 (s, 4H, CH2), 6.58 (s, 8H, m-CH of Mes), 6.98 (d, 4 JHH = 1.5 Hz, 4H, H1 and H8 of CR0 2), 7.34 (dd, 3JHH = 8.1 Hz, 4JHH = 1.5 Hz, H3 and H6 of CR0 2), 7.69 (d, 3JHH = 8.1 Hz, 4H, H4 and H5 of CR0 2). 13C NMR (75.47 MHz): 20.76 (p-Me of Mes), 22.45 (o-Me of Mes), 27.35 (CH2), 31.27 (CMe3), 34.56 (CMe3), 60.64 (CR0 2), 107.04 (CdCO), 118.83 (C4 and C5 of CR0 2), 121.01 (C1 and C8 of CR0 2), 123.30 (C3 and C6 of CR0 2), 128.18 (m-CH of Mes), 133.49 (ipso-C of Mes), 137.95 (C12 and C13 of CR0 2), 138.92 (p-C of Mes), 143.22 (o-C of Mes), 147.41 (C10 and C11 of CR0 2), 149.59 (C2 and C7 of CR0 2), 153.47 (C-O). MS m/z (% relative intensity): 1283 (M þ 1, 15), 1163 (M - Mes, 6), 1005 (M - CR0 2 - 1, 10), 587 (Mes2Ged CR0 2 - 1, 4), 311 (Mes2Ge - 1, 20), 278 (R0 2CH2, 100), 263 (R0 2CH2 - Me, 65). Anal. Calcd for C84H96Ge2O2 (1282.939): C, 78.64; H, 7.54. Found: C, 78.85; H, 7.66. Synthesis of 5a. To a solution of 1a (2.0 mmol) in 20 mL of Et2O was added a solution of 2,3,5,6-tetramethyl-1,4-benzoquinone (0.164 g, 1.0 mmol) in Et2O at -78 C. The reaction mixture was allowed to warm to room temperature and stirred for 2 h, giving a yellow solution and a white precipitate. After filtration to remove LiF and elimination of the solvent, the remaining solid was washed with pentane, giving a white powder of 5a (1.07 g, 96%, mp 323 C). 1H NMR (400.13 MHz): 0.56, 1.45, 2.42, and 2.47 (4s, 4 6H, o-Me of Mes), 0.71 (s, 6H, MeCCR2), 1.62 (s, 6H, MeCdC), 2.05 and 2.22 (2s, 2 6H, p-Me of Mes), 6.32, 6.55, 6.60, and 6.86 (4s, 4 2H, m-CH of Mes), 6.97, 7.10, 7.37 (3t, 3 JHH = 7.2 Hz, 3 2H) and 7.30 (ddd, 3JHH = 8.0 Hz, 3JHH = 6.0 Hz, 4JHH = 2.8 Hz, 2H) (H2, H3, H6, and H7 of CR2), 6.98 and 7.19 (2d, 3JHH = 7.2 Hz, 2 2H, H1 and H8 of CR2), 7.76 and 7.83 (2d, 3JHH = 7.2 Hz, 2 2H, H4 and H5 of CR2). 13C NMR (100.62 MHz): 10.51 (MeCCR2), 20.73 and 20.91 (p-Me of Mes), 21.46, 23.08 (2 overlapping signals) and 23.86 (o-Me of Mes), 26.97 (MeCdC), 54.59 (CCR2), 62.92 (CR2), 108.05 (CdCO), 118.84 and 119.70 (C4 and C5), 125.77, 126.08, and 126.41 (C2, C3, C6, and C7 of CR2), 126.27 and 126.60 (C1 and C8), 127.80, 128.37, 128.99, and 131.10 (m-CH of Mes), 133.40 and 135.19 (ipso-C of Mes), 138.53 and 138.99 (p-C of Mes), 140.43, 142.18, 142.62, 143.01, 144.16, 144.60 (o-C of Mes, C12

Article and C13), 145.65 and 147.66 (C10 and C11 of CR2), 152.49 (C-O). MS m/z (% relative intensity): 1114 (M, 100), 995 (M - Mes, 40), 949 (M - CR2 - 1, 40), 476 (Mes2GedCR2, 50), 311 (Mes2Ge 1, 50). Anal. Calcd for C72H72Ge2O2 (1114.620): C, 77.58; H, 6.51. Found: C, 77.85; H, 6.77.

Acknowledgment. We are grateful to the CNRS (contract CNRS/JSPS no. PRC 450), the Agence Nationale


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pour la Recherche (contract ANR-08-BLAN-0105-01), and the MENESR for financial support of this work and the M3PEC mesocenter for computational facilities. Supporting Information Available: CIF files for 2a, 3a, 3b, 4a, and 5a. Theoretical calculation details. This material is available free of charge via the Internet at http://pubs.acs.org.

Reactions of Germenes with Some para-Quinones: Formation of a

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