Fluorene-based stannylated polymers and their use as recyclable reagents in the Stille reaction

Journal of Organometallic Chemistry 696 (2011) 3316e3321

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Fluorene-based stannylated polymers and their use as recyclable reagents in the Stille reaction Nora Carrera a, Alfonso Salinas-Castillo b, Ana C. Albéniz a, *, Pablo Espinet a, *, Ricardo Mallavia b a b

IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, 47071-Valladolid, Spain Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, 03202-Elche, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 February 2011 Received in revised form 4 July 2011 Accepted 5 July 2011

Stannylated polymers based on the polyfluorene backbone have been synthesized and used in the Stille reaction as recyclable reagents, minimizing the generation of toxic tin residues. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Stille reaction Stannylated polymer Recyclable reagent Cross coupling Tin Polyfluorene

1. Introduction The Stille reaction belongs to the large family of palladiumcatalyzed cross coupling reactions (Scheme 1) [1e3]. Due to its versatility, functional group compatibility and the fact that additives such as bases or fluorides are not needed, the Stille coupling is very useful in organic synthesis [4], including natural product synthesis where it has mostly been used in the final steps [5]. Nonetheless, organometallic tin chemistry has a serious drawback in the formation of toxic tin residues difficult to separate from the target products. One solution to overcome this problem has been the use of a tin-containing polymeric matrix [6], easier to separate from the desired products at the end of the reaction. An additional advantage of this approach, when compared to other separation strategies used [7], is the potential for recycling of the tin byproduct. Although a few tin reagents not supported on polymers have been used in the Stille reaction and reused [7d,8], a polymer framework generally allows for an easier separation-recovery-regeneration sequence. The few polymers that have been used for this purpose are generally polystyrene resins, most of them synthesized by functionalization of a preformed polymer support [6]. We recently reported the synthesis of stannylated polynorbornenes, prepared

* Corresponding authors. Tel.: þ34 983184621; fax: þ34 983423013. E-mail address: [email protected] (A.C. Albéniz). 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.07.004

from stannylated norbornene monomers, and their application as recyclable reagents in the Stille reaction [9]. The utility of these polymers was illustrated by the low contents of tin in the crosscoupling products and the high activity of the tin polymers after successive recycling steps. Based on this previous work, we decided to extend this study to a different type of polymers. Here we use the fluorene-based poly [(9,9-bis(bromoalkyl)fluoren-2,7-diyl)-alt-co-(benzen-1,4-diyl)], which contains a rigidly planar biphenyl unit and halogenated hydrocarbon side chains at the C9 site and it is highly luminescent [10]. It was chosen because it undergoes facile substitution reactions of the halides for stannane. Moreover, the chains provide an aliphatic link for the tin group not prone to undergo transmetalation (equivalent to n-Bu in the traditional reagents Bu3SnR), and are flexible enough so the tin groups are easily accessible for reaction. Thus, we report in this paper the first stannylated fluorene-based polymer and its use as a recyclable reagent in the Stille reaction. 2. Results and discussion 2.1. Synthesis and characterization of stannylated polymers The polyfluorene poly[(9,9-bis(60 -bromohexyl)fluoren-2,7-diyl)alt-co-(benzen-1,4-diyl)] (2) depicted in Scheme 2, was used as the polymer precursor of the stannylated derivatives. It is prepared by Suzuki coupling of a dibromo derivative (2,7- dibromo-9,

N. Carrera et al. / Journal of Organometallic Chemistry 696 (2011) 3316e3321

Scheme 1. The Stille reaction.

9-bis(60 -bromohexyl)fluorene) with 1,4-phenyldiboronic acid [11]. The polymer obtained in this way has an alternating structure and a Mw of 9400 g mol1 (Mw/Mn ¼ 1.9) based on polyfluorene calibration [11a]. The stannylated fluorene-based polymer 3 was synthesized by nucleophilic substitution of the bromine atoms in 2 with an organostannide (Scheme 2). The latter, LiSnBu2An (An ¼ panisyl), was prepared from anisyl dibutyltin choride, which in turn can be easily obtained by the efficient method we described recently, involving microwave irradiation and column chromatography in acidic alumina [12,13]. The 1H NMR spectrum of 3 exhibits welldefined signals for the anisyl moiety and lacks any broad signal at 3.28 ppm of the initial eCH2eBr protons of polymer 2 (Fig. 1), supporting complete stannylation of 2. The absence of Br in polymer 3 was further confirmed by elemental analysis. Polymer 3 can be used as an anisyl-transmetalating reagent in the Stille reaction and can be recycled as described below. Furthermore, 3 can be easily and selectively converted into PFP(SnBu2Cl)2 (4) by reaction with a stoichiometrical amount of HCl in Et2O. The complete conversion of 3 into 4 was confirmed by elemental analysis of the chloro content and by its 1H NMR spectrum, which clearly shows the total absence of the anisyl signals (Fig. 1). Polymer 4 can be used as a general starting material for the preparation of other PFP(SnBu2R)2 reagents [9]. Fig. 2 shows the fluorescence emission spectra for polymers 2e4 in THF solutions and room-temperature (at lex ¼ 370 nm). The emission peaks of the three blue polymers are very similar, presenting a maximum at 409e410 nm with a well-defined vibronic feature at 430 nm. Thus, the substituent at the end of the hexyl chain (Br, SnBu2An, or SnBu2Cl) has little influence in the fluorescent behaviour of the polymer. 2.2. Polymer use as reagent, recovery and recycling Polymer 3 was tested as reagent in the Stille reaction (Scheme 3) and a few examples using different organic electrophiles are given in Table 1. As can be observed, the polymeric stannanes are useful and high to moderate conversions are obtained. When compared to monomeric reagents, longer reaction times are generally needed. The coupling of allyl chloride and the anisyl group (entry 1, Table 1) was chosen as a model reaction to check if polymer 3 can be recycled (Scheme 4 and Table 2). In a first step (Step A) the Stille coupling of allyl chloride and polymer 3 was carried out following the conditions of Scheme 4 and Table 1 (entry 1). A molar ratio allyl chloride:SneAn ¼ 1:1.5 was used along with 0.5% mol of

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[Pd2(meCl)2(h3eC3H5)2] as catalyst and 1% of benzoquinone as coupling promoter, at 50  C in THF. The use of benzoquinone has been shown to accelerate the reductive elimination step, usually slower when allylic derivatives are used in cross-coupling reactions [14,15]. Allyl chloride converts into the coupling product 5 selectively, and the yield after 1 day was measured by 1H NMR spectroscopy and calculated by the integral ratio of the allylic signals of the allylchloride and 5. In almost every case the reaction was quantitative [16]. After running the Stille reaction, the polymer was precipitated in MeOH and separated from the coupling product by filtration (Step B). The recovery of 4 was almost quantitative and the coupling product 5 was obtained by evaporation of the solvents to dryness, treatment of the residue dissolved in Et2O with activated charcoal and filtration through silica gel. The recovered polymer was regenerated by treatment with the organolithium derivative LiAn (Step C). The complete substitution in this step was checked by elemental analysis of the chloro content in the polymer. In this way, polymer 3 can be recycled at least four times as shown in Table 2. This procedure gave 5 with different tin contents depending on the recycling experiment. As shown in Table 1, the first cycle leaves the largest amount of tin in the coupling product which is clearly reduced through the cycles. Even in the first cycle, the amount of residual tin is half the quantity expected when conventional methods are used, usually treatment with an aqueous solution of KF. The decrease of the tin content in 5 on going from cycle 1 to 4, could be explained by the decrease in polymer solubility which is observed as it is being reused. Even if we did not detect them by NMR, we cannot rule out that a small amount of tin oligomers formed by decomposition of LiSnBu2An in the preparation of 3 could contaminate the polymer and be washed off in the first cycle [13]. The effect of the decrease in the solubility of these polymers is also found when they are stored for months at room temperature, although the nature of the aging process responsible for this change in properties has not been determined. The decrease in solubility does not affect the efficiency of the polymer as reagent in the Stille reaction and thus it is a good feature for separation purposes (even the recovery and regeneration yields slightly increase in cycles 2e4 when compared to the first one). The high luminescence of the polymers at low concentrations is useful for qualitative detection of soluble polymer contamination, by visual inspection under UV light. However, we found that the fluorescence underestimated the quantitative results obtained by ICP-MS tin determination, so its use as quantitative assay was discarded in favor of the latter technique [17]. Finally, we checked the possibility of reuse of the same polymer after these recycling experiments in a different coupling reaction. Since we use excess of the stannylated polyfluorene in the Stille couplings we treated polymer 4 recovered from the last cycle in Table 2 with an ethereal solution of HCl. This eliminates the anisyl groups in the polymer affording pure 4. This polyfluorene was then reacted with Li(peFC6H4) to give a p-fluorophenyl functionalized polymer (R2 ¼ peFC6H5, 9). The Stille coupling of this reagent with allyl chloride in the same conditions used for the recycling experiments in Table 2, afforded 90% conversion to p-fluoro allyl benzene (10) in 1 day. 3. Experimental section 3.1. Materials and methods

Scheme 2. Synthesis of PFP(SnBu2An)2 (3).

1 H, 13C{1H}, and 119Sn NMR spectra were recorded using Bruker AC-300, ARX-300 and AV-400 instruments. Chemical shifts (d) are reported in ppm and referenced to Me4Si (1H and 13C) or SnMe4

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N. Carrera et al. / Journal of Organometallic Chemistry 696 (2011) 3316e3321

Fig. 1. 1H NMR spectra of polymers 2 (a), 3 (b) and 4 (c).

(119Sn). All of the NMR spectra were recorded at 293 K. The tin content of the products was determined by ICPeMS using Agilent 7500i equipment; the samples were dissolved in a mixture of HNO3:H2SO4 ¼ 7:3 using an ETHOS SEL Milestone microwave oven. The chloro or bromo content in the polymer was determined by oxygen-flask combustion of a sample and analysis of the residue by the mercurimetric titration of chloride [18]. Coupled size exclusion chromatography (SEC) was carried out on the starting polyfluorene 2 using an HP-1090 and light scattering detector; ELSD 3300 Alltec. The size exclusion chromatogram was carried out at room constant temperature using two column PLGel 5 mm MIXED- C (300  7.5 mm) in THF, and calibrated to polyfluorene standards. Emission spectra were acquired using a PTI Quantum Master-model QM62003SE, spectrofluorimeter at 90 detection angle, lex ¼ 370 nm and room-temperature. Excitation and emission wavelengths are selected by means of auto-calibrated, computercontrolled by FeliX32TM Software package. Corrected steady state fluorescence emission was performed using low absorption solutions. Solvents were dried over CaH2 or Na, distilled, and deoxygenated prior to use. SnBu2Cl2, LiAlH4 and NHi-Pr2 were purchased from Aldrich or Alfa Aesar. The compounds SnBu2H2 [19], SnBu2AnCl [12], Li(NiePr2) [20], [Pd2(meCl)2(h3eC3H5)2] [21], and copolymer 2 [11] were prepared according to the literature procedures. Compounds 5 [22], 6 [23], 7 [24], 8 [25] and 10 [26] have been reported before.

3.1.1. Anisyldi(n-butyl)tinhydride (1) SnBu2AnCl (18.3095 g, 48.6 mmol) dissolved in Et2O (80 mL) was added dropwise to a suspension of LiAlH4 (1.8455 g, 48.6 mmol) in Et2O (180 mL) and heated under reflux for 4 h. The mixture was cooled to room temperature and hydroquinone (27 mg), a 20% sodium potassium tartrate aqueous solution (100 ml) and water (20 mL) were added slowly stepwise. After separation of the ethereal layer and extraction of the aqueous layer with ether (3  100 mL), the combined organic extracts were washed with water (2  50 mL) and dried over MgSO4. The solution was filtrated and the filtrate was concentrated under vacuum to yield compound 1 (14.0345 g, 85%) as a colorless liquid. 1 H NMR (C6D6, 300.13 MHz): d 7.50 (m3JHH ¼ 8.3 Hz3JHSn ¼ 42.2 Hz, 2H; Hortho), 6.94 (m3JHH ¼ 8.3 Hz, 2H; Hmeta), 5.77 (s1JHe119Sn ¼ 1685.5 Hz1JH117Sn ¼ 1609.4 Hz, 1H; SneH) 3.38 (s, 3H; OCH3), 1.63 (m, 4H; CH2), 1.40 (m, 4H; CH2), 1.15 (m, 4H; CH2eSn), 0.93 (t, 6H; CH3). 13C {1H} NMR (C6D6, 75.4 MHz): d 160.7 (1C; CparaeOCH3), 138.9 (2JCSn ¼ 40.3 Hz, 2C; Cortho), 129.8 (1C; CipsoeSn), 115.2 (s3JCSn ¼ 52.1 Hz, 2C; Cmeta), 55.1 (1C; OCH3), 30.5 (2JCSn ¼ 22.6 Hz, 2C; CH2), 28.0 (3JCSn ¼ 56.5 Hz, 2C; CH2), 14.5 (s, 2C; CH3), 10.2 (1JC119Sn ¼ 367.4 Hz1JC117Sn ¼ 351.6 Hz, 2C; CH2eSn). 119Sn {1H} NMR (C6D6, 111.92 MHz): d110.5 (s). MS (EI, m/z (%)): 341 (5) [M þ H], 285 (50) [M þ eBu], 229 (96), 227 (100), 121 (60) [HSnþ].

Fig. 2. Fluorescence spectra of polymers 2e4 in THF.

Scheme 3. Use of stannylated polyfluorene 3 in Stille couplings.

3.1.2. Poly[(9,9-bis(60 -dibutyl(4-methoxyphenyl)stannylhexyl)fluoren2,7-diyl)-alt-co-(benzen-1,4-diyl)] (3) A solution of butyllithium in n-hexane (10.3 mL, 1.6 M, 16.418 mmol) was added dropwise to a solution of NHi-Pr2 (1.611 g, 15.889 mmol) in dry THF (22 mL) at 78  C. After stirring for 1 h the temperature was allowed to rise to 60  C and SnBu2AnH (5.4185 g, 15.889 mmol) was added dropwise. The mixture was stirred for 1 h and the yellow solution of LiSnBu2An was added to a brown viscous solution of 2 (3.000 g, 5.296 mmol) in dry THF (110 mL) at 60  C.

N. Carrera et al. / Journal of Organometallic Chemistry 696 (2011) 3316e3321 Table 1 Stille couplings using stannylated polyfluorene 3 as reagent. Entry b,c

1 2d 3e 4b

Table 2 Recycling experiments using PFP(SnBu2An) (3) in the Stille reaction.

R1X

Solvent

T/ C

time

R1eR2 (%)a

CH2 ¼ CHeCH2Cl C6F5I peNO2eC6H4I PhCOCl

THF Dioxane THF THF

50 90 50 50

1d 5d 5d 2.5 d

5 6 7 8

(98) (85) (61) (78)

a Determined by relative integration of 1H or 19F NMR signals of reagent and product. b [Pd(m-Cl)(h3eC3H5)]2 (0.5% mol for entry 1 and 2.5% mol for entry 4) as catalyst. c Benzoquinone (1% mol) was added. d [Pd(meBr)(C6F5)(AsPh3)]2 (2.5% mol) was used as catalyst. e [Pd(PPh3)4] (2.5% mol) was used as catalyst.

The reaction mixture was allowed to slowly warm to room temperature and stirred for 24 h. The polymer was precipitated by pouring the mixture onto MeOH (600 mL). After stirring thoroughly, the polymer was filtered, washed with MeOH and vacuum dried at 50  C. The polymer was recrystalized by dissolving in CH2Cl2, treating with activated charcoal and filtering through kieselgur. The filtrate was added over MeOH and the grey solid was stirred for 2 h, filtered, washed with MeOH and dried under vacuum to yield 3 (4.6982 g, 82%) as a grey-greenish solid. 1 H NMR (THF-d8, 400.13 MHz): d 7.9e7.4 (br, 10Harylfluorene), 7.28 (m3JHH ¼ 7.6 Hz, 4Hanisole; Hortho), 6.81 (m3JHH ¼ 7.6 Hz, 4Hanisole; Hmeta), 3.69 (br, 6H; OCH3), 2.16 (a, 4Hlinker; CH2efluorene), 1.49 (br, 8HBu; CH2), 1.41 (br, 4Hlinker; CH2), 1.30 (br, 8HBu; CH2eCH3), 1.16 (br, 4Hlinker; CH2), 0.98 (br, 8HBu; CH2eSn), 0.92 (br, 4Hlinker; CH2eSn), 0.83 (br, 12HBu; CH3), 0.78 (br, 4Hlinker; CH2eCH2efluorene). 13C{1H} NMR (THFed8, 100.613 MHz): d 160.0 (2C; CparaeOCH3), 151.5 0 0 (2CarylfluoreneC8a C9a), 140.2 (4Carylfluorene C1 C4 , C4a, C4b), 139.6 2 7 2 (2Carylfluorene C C ), 137.0 ( JCSn ¼ 35.9 Hz, 4Canisole; Cortho), 130.7 0 0 0 0 (2Canisole; CipsoeSn), 127.2 (4Carylfluorene C2 C3 C5 C6 ), 125.8 (2Car1 8 6 3 4 5 ylfluoreneC C ), 121.0, (2Carylfluorene C C ), 119.9 (2Carylfluorene C C ), 113.7 (3JCSn ¼ 45.5 Hz, 4Canisole; Cmeta), 55.1 (1Cfluorene C9), 54.0 (2C, OCH3), 40.2 (2Clinker; CH2efluorene), 33.9 (2Clinker, CH2), 29.0 (2JCSn ¼ 10.1 Hz, 4CBu; CH2), 27.2 (3JCSn ¼ 55.0 Hz, 4CBu; CH2eCH3), 26.7 (2Clinker, CH2), 23.7, (2Clinker; CH2eCH2efluorene), 13.0 (4CBu; CH3), 9.3 (2Clinker; CH2eSn), 9.1 (1J119 CSn ¼ 338.2 Hz, 4CBu; CH2eSn). 119 Sn {1H} NMR (CDCl3, 149.211 MHz): d41.9 (br). 3.1.3. Poly[(9,9-bis(60 -chlorodibutylstannylhexyl)fluoren-2,7-diyl)alt-co-(benzen-1,4-diyl)](4) A solution of HCl in Et2O [27] (0.208 mL, 1.18 M, 0.246 mmol) was added dropwise to a viscous solution of 3 (0.1215 g,

1% bzq, 0.5% [Pd] THF, 50 ºC, 1 day

Cl

Step A Stille Reaction

PFP(SnBu2An)2

PFP(SnBu2Cl)2 +

3

LiCl

Step C Regeneration

An

Step B MeOH Separation and recovery

An LiAn

5 PFP(SnBu2Cl)2 4

[Pd] =

3319

Cl

Pd 2

An =

Scheme 4. Use and recycling of polymer 3 in the Stille reaction.

OMe

Cycle no.

Step A Yield [%]a

Step B 4 yield [%]

Step C 3 yield [%]

Sn content in 5 [wt %]b

1 2 3 4

100 98 100 95

93 99 97 96

88 92 92 95

2.51 0.67 0.3 0.1

a b

Determined by relative integration of 1H NMR allylic signals. Determined by ICP-MS.

0.112 mmol) in CH2Cl2 (5 mL) and heated under reflux for 10 h. After this time, the solvent was evaporated to dryness. The residue was washed several times with MeOH (3  10 mL), filtered and dried under vacuum to afford 4 as a grey-greenish solid (0.0755 g, 72%). 1 H NMR (CDCl3, 400.13 MHz): d 8.1e7.7 (br, 6Harylfluorene), 7.7e7.4 (br, 4Harylfluorene), 2.1 (br, 4Hlinker; CH2efluorene), 1.8e0.5 (br, 56H; 36HBu, 20Hlinker). 119Sn {1H} NMR (CDCl3, 149.211 MHz): d 156.2 (br). 3.1.4. Procedure for the Stille cross-coupling reactions using 3 (Table 1) 3 (0.1 g, 0.15 mmol), benzoylchloride (0.0169 g, 0.12 mmol), and [Pd(meCl)(h3eC3H5)]2 (0.0011 g, 0.003 mmol) in THF (4.5 mL) were mixed in a Schlenk flask under N2. The reaction mixture was heated at 50  C and monitored by 1H NMR every 6 h. The amount of 8 in the reaction mixture was determined by relative integration of the signals corresponding to 8 [9,25] and to the starting benzoylchloride. The reactions collected in Table 1 were carried out in the same way. 3.1.5. Procedure for the Stille cross-coupling reaction, separation and recovery of the stannylated polymer (steps A and B) 3 (3.9433 g, 5.808 mmol), allylchloride (0.2958 g, 3.872 mmol), benzoquinone (0.0042 g, 0.039 mmol), and a solution of [Pd(meCl)(h3eC3H5)]2 (0.0071 g, 0.019 mmol) in THF (1 mL) were added in a Schlenk flask under N2. The reaction mixture was heated at 50  C for 24 h. After that time, a small portion was taken and the formation of CH2 ¼ CHeCH2e(C6H4eOMeep) was observed by 1H NMR spectroscopy (100% conversion calculated by the integral ratio of the allylic signals of the allylchloride and the cross-coupling product). The mixture was poured onto MeOH (500 mL) and stirred for 30 min. The solvents were then evaporated by distillation to around 20 mL. The solid 4 was filtered, washed several times with MeOH (4  50 mL) and dried under vacuum for 1 day to afford a blue-grey solid (3.4129 g, 93%). The solvents were distilled and the residue was treated with Et2O (5 mL), activated charcoal and filtered through silica gel. After distillation to remove the solvents 5 was obtained as an oil (0.2870 g, 50%). Characterization data for 5 are identical to those reported in the literature [9,22]. 3.1.6. Procedure for the regeneration of polymer 3 (Step C) A solution of n-butyllithium in n-hexane (4.20 mL, 1.6 M, 6.703 mmol) was added to THF (50 mL) at 0  C. The mixture was cooled at 90  C and 4-bromoanisole (0.8035 g, 6.384 mmol) in THF (5 mL) was added over a 5 min period at that temperature. The temperature was allowed to rise to 50  C and the mixture was stirred for 15 min at this temperature [28]. The organolithium formed was then added to a solution of 4 (3.3129 g, 3.192 mmol) in THF (120 mL) at 50  C and the reaction mixture was allowed to slowly warm to room temperature and stirred for 24 h. After that time, the mixture was poured onto acidic MeOH (500 mL) and stirred for 1 h. The MeOH was decanted off and the resulting

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N. Carrera et al. / Journal of Organometallic Chemistry 696 (2011) 3316e3321

polymer was filtered, washed with MeOH and dried under vacuum to afford a blue-grey solid (3.1427 g, 88%). Polymer 9 (R2 ¼ peFC6H4) was prepared in the same way using p-bromofluorobenzene as reagent instead of 4-bromoanisole. 9: 1H NMR (CDCl3, 400.13 MHz): d 7.9e7.4 (br, 10Harylfluorene), 7.32 (m, 4Hfluoroaryl; Hortho), 7.0 (m, 4Hfluoroaryl; Hmeta), 2.10 (a, 4Hlinker; CH2efluorene), 1.49 (br, 8HBu; CH2), 1.41 (br, 4Hlinker; CH2), 1.30 (br, 8HBu; CH2eCH3), 1.15 (br, 4Hlinker; CH2), 1.0 (br, 8HBu; CH2eSn), 0.92 (br, 4Hlinker; CH2eSn), 0.85 (br, 12HBu; CH3, 4Hlinker). 13 1 C{ H} NMR (CDCl3, 100.613 MHz): 163.1 (1JCF ¼ 245 Hz, 2Cfluoroaryl; CparaeF), 151.6 (2Carylfluorene; C8a, C9a), 140.2, 140.4 (4Carylfluorene; 0 0 C1 ,C4 , C4a, C4b), 139.6 (2Carylfluorene; C2, C7), 137.0 (3JCF ¼ 6 Hz, 4Cfluoroaryl; Cortho), 136.6 (2Cfluoroaryl; CipsoeSn), 127.5 (4Carylfluorene; 0 0 0 0 C2 , C3 , C5 , C6 ), 126.0 (2Carylfluorene; C1, C8), 121.3, (2Carylfluorene; C6, 3 C ), 120.1 (2Carylfluorene; C4, C5), 115.2 (2JCF ¼ 20 Hz, 4Cfluoroaryl; Cmeta), 55.3 (1Cfluorene; C9), 40.5 (2Clinker; CH2efluorene), 33.9 (2Clinker, CH2), 29.0 (2JCSn ¼ 10.1 Hz, 4CBu; CH2), 27.5 (3JCSn ¼ 55.0 Hz, CH2eCH3), 26.2 (2Clinker, CH2), 23.8, (2Clinker; 4CBu; CH2eCH2efluorene), 13.6 (4CBu; CH3), 9.7 (2Clinker; CH2eSn), 9.6 (4CBu; CH2eSn). 119Sn {1H} NMR (CDCl3, 149.211 MHz): d 40.6 (br). 19 F NMR (CDCl3, 376.38): 115.5 (br).

[7]

4. Conclusion Polyfluorenes can be functionalized with stannylalkyl chains at C9. This leads to new polymers which can be used as reagents in the Stille reaction. They can be recovered and reused without activity loss. The reagent polymer becomes less soluble on recycling and this is an advantage since the tin contents of the coupling products decreases accordingly. The polymer can be reused for multiple Stille couplings, just introducing a new eSnBu2R group by reacting the recovered byproduct with eSnBu2Cl groups with a new LiR derivative.

[8]

[9] [10]

[11]

Acknowledgements Financial support from the Spanish MEC (DGI, grants CTQ201018901/BQU and MAT2008-05670; Consolider Ingenio 2010, Grant INTECAT, CSD2006-0003; FPU fellowship to NC), the Junta de Castilla y León (Grupos de Excelencia GR169; grant VA373A11-2) and the Fundación Caja Murcia is gratefully acknowledged.

[12] [13]

References [1] T.N. Mitchell, in: A. Meijere, F. Diederich (Eds.), second ed., Metal Catalyzed CrossCoupling Reactions, vol. 1 Wiley-VCH, Weinheim, 2004, pp. 125e161 ch. 3. [2] P. Espinet, A. Echavarren, Angew. Chem. Int. Ed. 43 (2004) 4704e4734. [3] J.K. Stille, Angew. Chem. Int. Ed. 25 (1986) 508e524. [4] (a) V. Farina, V. Krishanamurthy, W.J. Scott, The Stille Reaction. Wiley, New York, 1998; (b) P. Espinet, M. Genov, in: A.G. Davies, M. Gielen, K.H. Pannell, R.T. Tiekink (Eds.), Tin ChemistryeFundamentals, Frontiers and Applications, Wiley, Chichester, 2008, pp. 561e578. [5] (a) S. Pascual, A.M. Echavarren, in: A.G. Davies, M. Gielen, K.H. Pannell, R.T. Tiekink (Eds.), Tin ChemistryeFundamentals, Frontiers and Applications, Wiley, Chichester, 2008, pp. 579e607; (b) K.C. Nicolaou, P.G. Bulger, D.E. Sarlah, Angew. Chem. Int. Ed. 44 (2005) 4442e4489; (c) H.W. Lam, G. Pattenden, Angew. Chem. Int. Ed. 41 (2002) 508e511; (d) K.C. Nicolaou, J. Xu, F. Murphy, S. Barluenga, O. Baudoin, H. Wei, D.L.F. Gray, T. Ohshima, Angew. Chem. Int. Ed. 38 (1999) 2447e2451; (e) K.C. Nicolaou, T.K. Chakraborty, A.D. Piscopio, N. Minowa, P. Bertinato, J. Am. Chem. Soc. 115 (1993) 4419e4420. [6] Examples of the use of polymeric tin reagents in Stille couplings: (a) A.C. Albéniz, N. Carrera, Eur. J. Inorg. Chem. (2011) 2347e2360; (b) J.-M. Chrétien, J.D. Kilburn, F. Zammattio, E. Le Grognec, J.-P. Quintard, in: A.G. Davies, M. Gielen, K.H. Pannell, R.T. Tiekink (Eds.), Tin ChemistryFundamentals, Frontiers and Applications, Wiley, Chichester, 2008, pp. 653e665 Ch. 5; (c) G. Kerric, E. Le Grognec, F. Zammattio, M. Paris, J.-P. Quintard, J. Organomet. Chem. 695 (2010) 103e110; (d) J.-M. Chrétien, A. Mallinger, F. Zammattio, E. Le Grognec, M. Paris,

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