Pitteloud et al.
JOCArticle
to the Ge center that renders the Ge center more susceptible to
coordination by an external Lewis basic ligand (e.g., fluoride,
hydroxide, etc.), thereby also rendering the Ge center pentava-
lent.11-13,17,22 Although the effects of the fluoride or base activa-
tion and Pd-catalyst/ligand combination on the coupling of
organogermanes are still ambiguous, all but one8 of the success-
ful examples of organogermane couplings with aryl/alkenyl
halides involved activation with either base or fluoride. Proposed
reactive species were Nan[ArGe(OH)3þn] (n=1, 2),12 [ArGe-
(OH)3F]-,11 or RGe(OH)n(SiMe3)3-n/NaOH (n=1, 2, 3).17
FIGURE 1. Examples of organogermanes employed in Pd-catalyzed
cross-couplings.
We have recently communicated that (phenyl)n(chloro)4-n
-
germanes (n = 1, 2, or 3) undergo Pd-catalyzed cross-couplings
with aryl halides in the presence of TBAF in “moist” toluene,
with transfer of up to three phenyl groups from germane.22
Interestingly, organotin halides were initially considered to be
inactive in Pd-catalyzed couplings due to the deactivating
nature of the halogen.3,23 However, coupling between halos-
tannanes (e.g., ArBu2SnCl) and organic halides were later
found to be facilitated by TBAF.3,24 Hypervalent organotin
species3 were suggested as the active species in the Pd-catalyzed
coupling reactions of organotin trichlorides with aryl halides in
aqueous base.25,26 Moreover, alkenylsilyl chlorides and fluo-
rides were among the first alkenyl silanes to act as effective and
general substrates in the coupling reactions.4 Hatanaka and
Hiyama found that introduction of fluorine atom(s) into the
silicon substituent accelerates the cross-coupling reactions.27
Thus, reaction of the alkenyl/aryl-substituted fluoro(alkyl)-
silanes and difluoro(alkyl)silanes with organic halides or tri-
flates in the presence of fluoride promoters (TASF) provided
cross-coupling products in a stereo- and regioselective manner
and in good yields.27,28 Interestingly, trifluorosilanes were
ineffective in the cross-coupling with 1-iodonapthalene27 but
coupled with alkenyl triflates.28 The (aryl)halosilanes were also
employed successfully in the coupling with aryl halides pro-
moted by KF in DMF.29 Yet again, aryl(trifluoro)- and aryl-
(trichloro)silanes failed to generate cross-coupling products.29
Couplings of organochlorosilanes with aryl chlorides promoted
by KF30 or NaOH31 and aryl iodides in aqueous KOH media32
have been also reported.
with aryl bromides due to the activation of germanium by
internal coordination to nitrogen8 (Figure 1). The allyl, aryl,
alkenyl, and alkynyl oxagermatranes 2 were found to be more
efficient than carbagermatranes 1 and triethoxygermanes.9 The
couplings were promoted by fluoride, and in fact, arylalkynyl
oxagermatranes 2 underwent couplings even with less reactive
aryl chlorides and triflates under milder conditions than are
usually required for Sonogashira couplings with triorgano-
silicon reagents.10 The fluoride-promoted couplings with aryltri-
(2-furyl)germanes11 and the NaOH-activated couplings with ar-
ylgermanium trichlorides12 or their hydrolyzed and stable ses-
quioxide alternatives13 were also reported. Bis(2-naphthyl-
methyl)arylgermanes 3 were developed as the photochemically
activated “safety-catch” arylgermanes for the synthesis of
biaryls.14,15 The vinyl tris(trimethylsilyl)germanes 4 were emplo-
yed as transmetalation reagents in “ligand-free” and “fluoride-
free” cross-coupling reactions with aryl and alkenyl halides
under oxidative conditions (H2O2) in either aqueous or anhy-
drous basic conditions.16,17 Interestingly, the (R-fluoro)vinyl
germanes 5 provided a synthetic route to fluoroalkenes and
fluorodienes.18 In contrast, the use of (R-fluoro)vinyl stannanes
and silanes as transmetalation reagents in Pd-catalyzed cou-
plings proved to be of very limited value.19,20 Maleczka et al.
reported that the Pd-mediated coupling of vinyltributyl-
germanes with aryl halides occurred more efficiently under Heck
than Stille conditions to give preferentially Z-alkenes, especially
when oxygen was present at the allylic position.21
Couplings with organogermanes appear to be promoted
either by intramolecular chelation of a pendant Lewis basic
heteroatom which renders the Ge center “permanently” penta-
valent8,9,21 or by the presence of at least one heteroatom bound
Although systems which will allow direct comparison of
the cross-coupling reactions between the organometallic
reagents derived from group 14 metals and electrophiles
are lacking,33-35 organochlorogermanes can render a cou-
pling efficiency comparable to the usually more reactive
stannane and silane counterparts.12,22 Therefore, herein,
(8) Kosugi, M.; Tanji, T.; Tanaka, Y.; Yoshida, A.; Fugami, K.;
Kameyama, M.; Migita, T. J. Organomet. Chem. 1996, 508, 255–257.
(9) Faller, J. W.; Kultyshev, R. G. Organometallics 2002, 21, 5911–5918.
(10) Faller, J. W.; Kultyshev, R. G.; Parr, J. Tetrahedron Lett. 2003, 44,
451–453.
(11) Nakamura, T.; Kinoshita, H.; Shinokubo, H.; Oshima, K. Org. Lett.
2002, 4, 3165–3167.
(12) Enokido, T.; Fugami, K.; Endo, M.; Kameyama, M.; Kosugi, M.
Adv. Synth. Catal. 2004, 346, 1685–1688.
(22) Zhang, Z.-T.; Pitteloud, J.-P.; Cabrera, L.; Liang, Y.; Toribio, M.;
Wnuk, S. F. Org. Lett. 2010, 12, 816–819.
(23) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508–524.
(24) Fugami, K.; Ohnuma, S.-y.; Kameyama, M.; Saotome, T.; Kosugi,
M. Synlett 1999, 63–64.
(13) Endo, M.; Fugami, K.; Enokido, T.; Sano, H.; Kosugi, M. Adv.
Synth. Catal. 2007, 349, 1025–1027.
(25) Roshchin, A. I.; Bumagin, N. A.; Beletskaya, I. P. Tetrahedron Lett.
1995, 36, 125–128.
(14) Spivey, A. C.; Tseng, C.-C.; Hannah, J. P.; Gripton, C. J. G.; de
Fraine, P.; Parr, N. J.; Scicinski, J. J. Chem. Commun. 2007, 2926–2928.
(15) Spivey, A. C.; Gripton, C. J. G.; Hannah, J. P.; Tseng, C.-C.; Fraine,
P. d.; Parr, N. J.; Scicinski, J. J. Appl. Organomet. Chem. 2007, 21, 572–589.
(16) Wnuk, S. F.; Garcia, P. I., Jr.; Wang, Z. Org. Lett. 2004, 6, 2047–
2049.
(26) Rai, R.; Aubrecht, K. B.; Collum, D. B. Tetrahedron Lett. 1995, 36,
3111–3114.
(27) Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1989, 54, 268–270.
(28) Hatanaka, Y.; Hiyama, T. Tetrahedron Lett. 1990, 31, 2719–2722.
(29) Hatanaka, Y.; Goda, K.-i.; Okahara, Y.; Hiyama, T. Tetrahedron
1994, 50, 8301–8316.
(17) Wang, Z.; Wnuk, S. F. J. Org. Chem. 2005, 70, 3281–3284.
(18) Wang, Z.; Gonzalez, A.; Wnuk, S. F. Tetrahedron Lett. 2005, 46,
5313–5316.
(19) Chen, C.; Wilcoxen, K.; Zhu, Y. F.; Kim, K. i.; McCarthy, J. R.
J. Org. Chem. 1999, 64, 3476–3482.
(20) Hanamoto, T.; Kobayashi, T. J. Org. Chem. 2003, 68, 6354–6359.
(21) Torres, N. M.; Lavis, J. M.; Maleczka, R. E., Jr. Tetrahedron Lett.
2009, 50, 4407–4410.
(30) Gouda, K.-i.; Hagiwara, E.; Hatanaka, Y.; Hiyama, T. J. Org. Chem.
1996, 61, 7232–7233.
(31) Hagiwara, E.; Gouda, K.-i.; Hatanaka, Y.; Hiyama, T. Tetrahedron
Lett. 1997, 38, 439–442.
(32) Huang, T.; Li, C.-J. Tetrahedron Lett. 2002, 43, 403–405.
(33) For example, PhSnBu3 efficiently transfers a phenyl group,3 and so
does PhSi(OMe)3 when activated with fluoride.5 On the other hand PhGe-
(OEt)3 was basically ineffective under various conditions.9
8200 J. Org. Chem. Vol. 75, No. 23, 2010