Palladium-catalysed synthesis of monodisperse, controlled-length, and functionalized oligoanilines

4960

J. Am. Chem. Soc. 1998, 120, 4960-4976

Palladium-Catalyzed Synthesis of Monodisperse, Controlled-Length,

and Functionalized Oligoanilines

Joseph P. Sadighi, Robert A. Singer, and Stephen L. Buchwald*

Contribution from the Department of Chemistry, Massachusetts Institute of Technology,

Cambridge, Massachusetts 02139

ReceiVed January 5, 1998

Abstract: The palladium-catalyzed amination of aryl halides, in conjunction with an orthogonal protective

group scheme, forms the basis of two routes to oligoaniline precursors. One method consists of a bidirectional

chain growth from a symmetric core piece, whereas the other involves a divergent-convergent synthesis of

nonsymmetric fragments, followed by coupling to a symmetric core fragment. The oligoaniline precursors

are soluble in a variety of common organic solvents and are easily converted to the deprotected oligoanilines.

The method allows the preparation of even or odd chain lengths and the incorporation of a variety of functional

groups. The synthesis of phenyl-capped heptaaniline through decaaniline, of four end-functionalized octaaniline

derivatives, and of phenyl-capped 16-mer and 24-mer is described. The effects of chain length and substitution

upon oligomer behavior have been investigated by electronic absorption spectroscopy and cyclic voltammetry.

Introduction

Soon after polyaniline was identified as an electrical conduc-

tor, Honzl et al. prepared and investigated the first phenyl-

capped oligoanilines of controlled chain length as models for

the poorly defined polymer.¹³The synthetic method involved

the condensation of small oligoanilines (dimer, trimer, and

tetramer)¹⁴with diethyl succinoylsuccinate, followed by hy-

drolysis, decarboxylation, and aromatization. This sequence

afforded phenyl-capped tetraaniline and hexaaniline; diazoti-

zation and reduction of tetraaniline gave rise to phenyl-capped

trianiline. The authors apparently did not isolate phenyl-capped

octaaniline from the condensation reaction when tetraaniline was

used.

In 1986, Wudl and co-workers modified the Honzl condensa-

tion approach (Scheme 1) and succeeded in obtaining phenyl-

capped octaaniline.¹⁵This compound proved identical to bulk

polyaniline by ESR, UV-vis, and IR spectroscopy and dis-

played conductivity on the same order of magnitude as that of

the bulk polymer, demonstrating that useful electrical properties

may be realized even in relatively short oligoaniline systems.

More recently, other methods for the synthesis of oligoanilines

have been reported. A titanium alkoxide-mediated coupling of

anilines with phenols has been used to prepare phenyl-capped

tetraaniline and pentaaniline.¹⁶An Ullmann coupling reaction

between acetanilides and 4-iodonitrobenzene was used in an

iterative coupling/reduction sequence, followed by deacetylation

to afford trianiline and tetraaniline, the starting anilines for the

Wudl-Honzl oligoaniline synthesis.¹⁷Finally, in a modern

variant of the Wilsta¨tter-Moore approach,¹⁴MacDiarmid and

Epstein et al. have oxidized N-phenyl-1,4-phenylenediamine to

tetraaniline; they report that oxidation of the latter compound

affords a 16-mer.¹⁸

Well over a century after the first oxidation of aniline,¹the

electrical conductivity of polyaniline was recognized.²Among

conductive polymers, polyaniline is remarkable for its excellent

environmental stability,³and unique in the ease with which its

properties may be tuned by changes in oxidation state⁴or in

degree of protonation.⁵Advances in the electropolymerization

of aniline,⁶and in solution-processing of the chemically

synthesized polymer,⁷have allowed the study of polyaniline in

numerous practical applications, including rechargeable organic

batteries,⁸electrochromic displays,⁹electromechanical actua-

tors,¹⁰anticorrosion coatings for steel,¹¹and electromagnetic

interference shielding.¹²

(1) Fritsche, J. J. Prakt. Chem. 1840, 20, 453-459.

(2) (a) Parini, V. P.; Kazakova, Z. S.; Berlin, A. A. Vysokomolekul

Soedin. 1961, 3, 1870-1873. (b) Pohl, H. A.; Engelhardt, E. H. J. Phys.

Chem. 1962, 66, 2085-2095.

(3) (a) Huang, W.-S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem.

Soc., Faraday Trans. 1 1986, 82, 2385-2400. (b) Chen, S.-A.; Fang, W.-

G. Macromolecules 1991, 24, 1242-1248.

(4) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89,

1441-1447.

(5) Chiang, J.-C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193-205.

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chem. 1980, 111, 111-114.

(7) (a) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1992, 48, 91-97.

(b) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 57, 3514-3519. (c)

Pron, A.; O¨ sterholm, J. E.; Smith, P.; Heeger, A. J.; Laska, J.; Zagorska,

M. Synth. Met. 1993, 57, 3520-3525.

(8) MacDiarmid, A. G.; Mu, S.-L.; Somasiri, N. L. D.; Wu, W. Mol.

Cryst. Liq. Cryst. 1985, 121, 187-190.

(9) Kobayashi, T.; Yoneyama, H.; Tamura, H. J. Electroanal. Chem.

Interfacial Electrochem. 1984, 161, 419-423.

(10) (a) Kaneto, K.; Kaneko, M.; Min, Y.; MacDiarmid, A. G. Synth.

Met. 1995, 71, 2211-2213. (b) Takashima, W.; Kaneko, M.; Kaneto, K.;

MacDiarmid, A. G. Synth. Met. 1995, 71, 2265-2266.

(11) (a) DeBerry, D. W. J. Electrochem. Soc. 1985, 132, 1022-1026.

(b) Ahmad, N.; MacDiarmid, A. G. Synth. Met. 1996, 78, 103-110. (c)

Lu, W.-K.; Elsenbaumer, R. L.; Wessling, B. Synth. Met. 1995, 71, 2163-

2166.

(12) (a) Taka, T. Synth. Met. 1991, 41, 1177-1180. (b) Colaneri, N. F.;

Shacklette, L. W. IEEE Trans. 1nstrum. Meas. 1992, 41, 291-297. (c) Joo,

J.; Epstein, A. J. Appl. Phys. Lett. 1994, 65, 2278-2280.

(13) Honzl, J.; Tlusta´kova´, M. J. Polym. Sci. 1968, C22, 451-462.

(14) Willsta¨tter, R.; Moore, C. W. Chem. Ber. 1907, 40, 2665-2689.

(15) Lu, F.-L.; Wudl, F.; Nowak, M.; Heeger, A. J. J. Am. Chem. Soc.

1986, 108, 8311-8313.

(16) Ochi, M.; Furusho, H.; Tanaka, J. Bull. Chem. Soc. Jpn. 1994, 67,

1749-1752.

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65-66.

Published on Web 05/12/1998

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4961

Scheme 1. Oligoaniline Synthesis of Honzl¹³and Wudl^{15 a}

^aKey: (a) ∆, then (for R ) Et) NaOH, EtOH, dioxane, ∆; (b) ∆,

O₂.

The palladium-catalyzed amination of aryl halides and

triflates¹⁹has emerged as a powerful method for the synthesis

of a wide variety of arylamines. The high efficiency and broad

substrate scope of the reaction make it an ideal method for the

preparation of novel oligoaniline derivatives. We have devel-

oped a general route to oligoanilines, using palladium catalysis

to assemble the aryl-nitrogen framework and an orthogonal

protective group scheme to control the course of the reactions.

The protective groups confer excellent solubility upon the

products and are easily removed to form the electroactive

oligomers. This method offers great synthetic flexibility: even-

or odd-numbered oligomers may be prepared, and functional

groups may be introduced at the ends of the chains to modify

the properties of the materials without disrupting the coplanarity

between rings. We have prepared and investigated phenyl-

capped heptaaniline through decaaniline, a series of end-

functionalized octamers, and the phenyl-capped 16-mer and 24-

mer.

Figure 1. Possible strategies for the synthesis of oligoanilines by aryl

amination.

a protected 4-bromoaniline, followed by deprotection, would

result in an increase in chain length of one unit for each iteration.

The disadvantages of such a method are the relatively slow

increase in chain length for a given number of steps and the

increasing difficulty of separating the products from any

unreacted starting material or byproducts as the chain length

increases. An outward growth of the oligoaniline from a

symmetric core would permit the chain to grow by two units in

one iteration and result in a larger difference in size between

starting material and desired product. As in the monodirectional

strategy, the chain length increases by the same increment with

each iteration of the sequence.

A geometric growth in chain length is possible using a

divergent-convergent approach.²¹In this strategy, a suitably

protected oligomer is divided into two portions; one is converted

to an arylamine, and the other to an aryl bromide. The coupling

of the two produces a homologous oligomer, with a doubling

in chain length.

Results and Discussion

Oligomer Synthesis. The simplest palladium-catalyzed

synthesis of polyaniline would involve the polymerization of

4-bromoaniline or the copolymerization of 1,4-phenylenedi-

amine with 1,4-dibromobenzene. However, the coupling prod-

ucts would be easily oxidized, even short oligomers would

present problems in solubility and purification, and precise

control over chain length would be difficult.

Several strategies,²⁰illustrated in Figure 1, may be envisioned

for the synthesis of discrete oligoanilines by sequences of aryl

amination and deprotection. The reaction of an arylamine with

(18) Zhang, W. J.; Feng, J.; MacDiarmid, A. G.; Epstein, A. J. Synth.

Met. 1997, 84, 119-120.

For electrochemical studies and applications of oligoanilines,

symmetric products are desirable, to avoid the complications

of parallel and antiparallel orientations between chains. Our

synthetic methods combine the divergent-convergent approach

with a modified bidirectional approach, which links the chain

fragments to form a symmetric oligomer. This strategy requires

the use of suitable equivalents for the aryl bromide and

arylamine functional groups, so that each may be unmasked

without affecting the other.

(19) (a) Kosugi, M.; Kameyama, M.; Migita, T. Chem. Lett. 1983, 927-

928. (b) Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem.,

Int. Ed. Engl. 1995, 34, 1348-1350. (c) Wolfe, J. P.; Rennels, R. A.;

Buchwald, S. L. Tetrahedron 1996, 52, 7525-7546. (d) Wolfe, J. P.;

Wagaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118, 7215-7216.

(e) Wagaw, S.; Buchwald, S. L. J. Org. Chem. 1996, 61, 7240-7241. (f)

Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609-3612. (g) Driver,

M. S.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7217-7218. (h) Wolfe,

J. P.; Buchwald, S. L. J. Org. Chem. 1997, 62, 1264-1267. (j) Louie, J.;

Driver, M. S.; Hamann, B. C.; Hartwig, J. F. J. Org. Chem. 1997, 62, 1268-

1273. (k) Wolfe, J. P.; Buchwald, S. L. Tetrahedron Lett. 1997, 38, 6359-

6362. (l) A° hman, J.; Buchwald, S. L. Tetrahedron Lett. 1997, 38, 6363-

6366. (m) Wolfe, J. P.; A° hman, J.; Sadighi, J. P.; Singer, R. A.; Buchwald,

S. L. Tetrahedron Lett. 1997, 38, 6367-6370.

(21) For the first example of a divergent-convergent synthesis used to

prepare monodisperse polyethylenes, see: Igner, E.; Paynter, O. I.;

Simmonds, D. J.; Whiting, M. C. J. Chem. Soc., Perkin Trans. 1 1987,

2447-2454.

(20) For a review of synthetic approaches to conjugated macromolecules

with precise length, see: Tour, J. M. Chem. ReV. 1996, 96, 537-553.

4962 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Scheme 2^a

The synthesis of substituted octamers 18 was carried out by

the bidirectional approach illustrated in Scheme 6. The sym-

metric N₄-diamine 15 is obtained by the reaction of 1,4-

phenylenediamine with 2 equiv of monomer 1, followed by

BOC-protection and imine cleavage.²⁷Iteration of the sequence

using aryl bromide 5 allows more rapid growth, giving the N₈-

diamine 17. This diamine reacts with simple aryl bromides to

give a variety of R,ω-disubstituted phenyl-capped octamers

(18a-d) from a common precursor. Alternatively, the N₄-

diamine 15 may be converted directly to a capped octamer by

reaction with the appropriate N₂-aryl bromide, as in the synthesis

of the bis(methoxy)-substituted octamer (18e).

Symmetric oligomers also result from the reaction of ary-

lamines with symmetric dibromides, prepared as shown in

Scheme 7; odd- or even-numbered oligomers may be obtained,

depending on the core piece used. Regioselective para-

bromination of diphenylamine²⁸affords 4,4′-dibromodipheny-

lamine, which is activated toward aryl amination by conversion

to its BOC derivative (19). Even-numbered dibromides (20,

21) are prepared by the coupling of diamines with 2 equiv of

1,4-dibromobenzene (see the preparation of 14, above), followed

by BOC-protection. Scheme 8 illustrates the synthesis of

phenyl-capped heptamer 22 and a series of R,ω-bis-(trimeth-

ylsilyl) phenyl-capped oligomers: nonamer 23, decamer 24, 16-

mer 25, and 24-mer 26.

^aKey: (a) Ph₂CO, 5 Å molecular sieves, PhCH₃, 120 °C; (b)

n-C₄H₉Li, THF, -78 °C; (c) (CH₃)₃SiCl, THF, -78 °C; (d) H₂NOH‚HCl

(1.5 equiv), NaOAc (2 equiv), CH₃OH.

The facile electrophilic substitution of the trimethylsilyl

group²²allows it to function as a masked aryl bromide. The

nitrogen protecting group was therefore required to be stable

to the reaction conditions of bromodesilylation as well as to

those of aryl amination, and to be removable without the use

of strong acid, which would cleave the aryl-silicon bond.

After investigating a number of possibilities, we found the

diphenylmethylene group to be extremely useful for several

reasons. Condensation of 4-bromoaniline with benzophenone

is easily carried out on large scale and in high yield. The

resulting N-(diphenylmethylene)-4-bromoaniline (1) is a con-

venient substrate for palladium-catalyzed aryl amination; the

reactions proceed rapidly and cleanly, with no detectable

transamination, and the diphenylmethylene group imparts excel-

lent crystallinity to the products. This protective group is stable

to bromine under the conditions used in bromodesilylation. The

free primary amine may be liberated by hydrogenolysis²³or by

treatment with hydroxylamine under weakly acidic conditions.²⁴

Finally, the stability of the imine to alkyllithium reagents at

low-temperature allows halogen-metal exchange to be carried

out on 1, leading to a convenient preparation of 4-(trimethyl-

silyl)aniline (2).²⁵The preparation of 4-bromoaniline equiva-

lents 1 and 2 is outlined in Scheme 2.

Palladium-catalyzed coupling of 1 and 2 affords an aniline

dimer with a masked bromide at one end and a protected amine

at the other. Protection of the internal NH group as its tert-

butyl carbamate (BOC) derivative forms a dimer derivative (3)

which may be homologated by the divergent-convergent

approach. The tert-butyl carbamate confers excellent solubility

upon intermediates and products, prevents the oxidation of the

phenylenediamine moieties in higher oligomers to quinonedi-

imines, and allows bromodesilylation to occur without detectable

overbromination. The divergent-convergent process is easily

carried out on multigram scale; the yield for each step is high,

and the intermediates are easily purified by crystallization.

Scheme 3 shows the synthesis of several chain fragments (5, 7,

10) used in the preparation of symmetric oligomers.

The protected oligomers exhibit good solubility in numerous

common solvents; they are moderately soluble in tetrahydrofuran

and hot alcohols, highly soluble in toluene, and extremely

soluble in dichloromethane and chloroform. Removal of the

tert-butyl carbamate groups decreases the solubility of the

materials considerably; however, the deprotected oligoanilines

are sufficiently soluble in polar aprotic solvents such as N,N-

dimethylformamide and N-methylpyrrolidinone to permit their

characterization by UV-vis spectroscopy and the casting of

films for electrochemical studies. Deprotected oligoanilines as

1

long as the decamer could be characterized by H NMR. To

examine the solubilizing influence of alkyl groups at the termini

of oligoanilines, we prepared the bis(tert-butyl)- and bis(n-

dodecyl)-substituted octaanilines 27c and 27d, but these exhib-

ited the same solubility as the other oligoanilines. In any case,

the facile cleavage of the BOC groups allows them to function

as removable solubilizing groups.

Oligomer Deprotection. Thermolysis of the protected

oligomers under an inert atmosphere results in clean and

quantitative removal of the BOC group,²⁹affording the oligoa-

niline in its lowest oxidation state as shown in Scheme 9.

Infrared spectroscopy of a thin film of 18a on a NaCl plate,

heated under argon at 185 °C, showed that the complete

disappearance of the carbonyl absorption required a reaction

time of approximately 7 h. Likewise,¹H NMR spectroscopy

of 18a, heated at 185 °C in DMSO-d₆solution, indicated a

reaction time of nearly 7 h for the complete loss of the tert-

butyl resonance. The preparation of octaanilines 27a-e was

accomplished by heating the powders in Schlenk tubes under

argon for 9 h.

Other nonsymmetric chain fragments may be prepared by

modifications of this synthetic methodology; the synthesis of a

trimer derivative (12) is shown in Scheme 4. The synthesis of

aryl bromide 14 (Scheme 5) is noteworthy for the selective

monoamination of 1,4-dibromobenzene; the highly electron-rich

coupling product 13.2 reacts so slowly with the palladium

catalyst that, under these conditions, the amination stops cleanly

at this stage. Protection of the secondary amine as its BOC

derivative results in an aryl bromide substrate (14) which is

activated toward oxidative addition.

(26) For reasons of availability at the time that this work was carried

out, we employed S-BINAP. The significantly less expensive racemic form,

now available commercially from Strem Chemical Company, is an equally

effective ligand in these coupling reactions, with no observable differences

in yields.

(27) The corresponding N₃-diamine, a core piece for odd-numbered

oligomers, has been prepared; see ref 19m.

(28) Berthelot, J.; Guette, C.; Essayegh, M.; Desbene, P. L.; Basselier,

J. J. Synth. Commun. 1986, 16, 1641-1645.

(29) Thermal deprotection of BOC-protected pyrroles and indoles has

been reported to proceed more rapidly: Rawal, V. H.; Jones, R. J.; Cava,

M. P. J. Org. Chem. 1987, 52, 19-28.

(22) For a review of electrophilic substitutions of arylsilanes, see:

Bennetau, B.; Dunogues, J. Synlett 1993, 171-176.

(23) Wessjohann, L.; McGaffin, G.; de Meijere, A. Synthesis 1989, 359-

363.

(24) Fasth, K.-J.; Antoni, G.; Långstro¨m, B. J. Chem. Soc., Perkin Trans.

1 1988, 3081-3084.

(25) This compound had been obtained previously by an analogous

sequence using 4-bromo-N,N-bis(trimethylsilyl)aniline: Walton, D. R. M.

J. Chem. Soc. C 1966, 1706-1707. We found it more convenient to use

the crystalline and moisture-stable N-(diphenylmethylene)-4-bromoaniline.

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4963

Scheme 3^a

^aKey: (a) Pd₂(dba)₃(0.25 mol %), S-BINAP²⁶(0.75 mol %), NaOtBu (1.4 equiv), THF, reflux; (a′) Pd₂(dba)₃(1 mol %), S-BINAP (2.4 mol %),

NaOtBu (1.4 equiv), toluene, 80 °C; (a′′) Pd₂(dba)₃(2 mol %), S-BINAP (4.8 mol %), NaOtBu (1.4 equiv), toluene, 80 °C; (b) (BOC)₂O (1.3

-

equiv), 4-DMAP (0.2 equiv), THF, reflux; (c) NH₄⁺HCO₂(12 equiv), 10% Pd/C (0.1 equiv of Pd), THF/CH₃OH, 60 °C; (c′) NH₄⁺HCO₂(>30

equiv), 20% Pd(OH)₂/C (0.5 equiv of Pd), iPrOH, 80 °C; (d) NaOAc (1 equiv), Br₂(2 equiv), THF, -78 °C to 0 °C.

Scheme 4^a

Scheme 5^a

^aKey: (a) Pd₂(dba)₃(1 mol %), S-BINAP (2.5 mol %), NaOtBu

(1.4 equiv), toluene, 80 °C; (b) (BOC)₂O (1.5 equiv), 4-DMAP (0.2

-

equiv), THF, 60 °C; (c) 5% Pd/C (0.1 equiv of Pd), NH₄⁺HCO₂(15

equiv), THF/CH₃OH, 60 °C.

Alternatively, the BOC group may be cleaved using iodot-

rimethylsilane. The protected oligomers react rapidly with

iodotrimethylsilane to form the corresponding trimethylsilyl

carbamates.³⁰The trimethylsilyl carbamate group confers the

same solubility as the tert-butyl carbamate but is extremely labile

in the presence of moisture or protic solvents. For preparative

purposes, a solution of the trimethylsilyl carbamate is prepared

in dichloromethane; subsequent addition of excess methanol

causes the deprotected oligoaniline to precipitate immediately.

Phenyl-capped heptaaniline (28), nonaaniline (29), decaaniline

(30), 16-mer (31), and 24-mer (32) were prepared by this

method, as shown in Scheme 10. Note that the acid generated

upon reaction of the remaining iodotrimethylsilane with metha-

nol effects the protodesilylation of arylsilanes 23-26 in the same

operation. Octaanilines prepared by this method were analyti-

cally and spectroscopically identical to those prepared by

thermolysis. Solutions of the trimethylsilyl carbamates in

dichloromethane may be cast into films, which are converted

^aKey: (a) Pd₂(dba)₃(0.5 mol %), S-BINAP (1.5 mol %), NaOtBu

(1.4 equiv), THF, reflux; (a′) Pd₂(dba)₃(1 mol %), S-BINAP (3 mol

%), NaOtBu (1.4 equiv), THF, reflux; (b) (BOC)₂O (1.5 equiv),

4-DMAP (0.1 equiv), THF, reflux; (c) 20% Pd(OH)₂/C (0.1 equiv of

-

Pd), NH₄⁺HCO₂(20 equiv), EtOH, 60 °C.

to their redox-active, deprotected forms by immersion in

alcohols or in aqueous solutions. All samples used in electro-

chemical studies were prepared in this manner.

Oxidation States of Aniline Oligomers. The lowest oxida-

tion state of polyaniline is the insulating leucoemeraldine form,

in which all nitrogen atoms are neutral and sp³-hybridized, and

all aromatic rings are in the benzenoid form. Oxidation of half

(30) Removal of benzyl and tert-butyl carbamate groups from peptides

using TMSI is well-known: Lott, R. S.; Chauhan, V. S.; Stammer, C. H.

J. Chem. Soc., Chem. Commun. 1979, 495-496.

4964 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Scheme 6^a

^aKey: (a) Pd(OAc)₂(1 mol %), S-BINAP (1.5 mol %), NaOtBu (4.5 equiv), toluene, 80 °C; (a′) Pd₂(dba)₃(6 mol %), S-BINAP (7 mol %),

NaOtBu (2.8 equiv), toluene/Et₃N, 90 °C; (a′′) Pd₂(dba)₃(2 mol %), S-BINAP (6 mol %), NaOtBu (2.5 equiv), THF, reflux; (a′′′) Pd₂(dba)₃(2 mol

%), S-BINAP (5 mol %), NaOtBu (2.5 equiv), toluene, 80 °C; (b) (BOC)₂O (3 equiv), 4-DMAP (0.1 equiv), THF/toluene, reflux; (b′) (BOC)₂O (3

equiv), 4-DMAP (0.1 equiv), THF, reflux; (c) H₂NOH‚HCl (2.5 equiv), pyridine (4 equiv), CHCl₃/THF/EtOH, room temperature; (d) 20% Pd(OH)₂/C

-

(0.4 equiv of Pd), NH₄⁺HCO₂(20 equiv), THF/EtOH, 70 °C.

Scheme 7^a

when the imine nitrogen atoms are protonated. This form has

been described as a repeating semiquinoid cation to explain its

paramagnetism and electrical conductivity. Oxidation of all

phenylenediamine moieties to their quinoid forms gives rise to

the pernigraniline form, with significant (though not necessarily

complete) deprotonation under most conditions. Even when

generated by oxidation under extremely nonbasic conditions,

and thus probably in its fully protonated form, pernigraniline

is an insulator.³¹These oxidation states are illustrated in Figure

2.

Electrochemical studies of polyaniline show two oxidation

waves of equal intensity, consistent with the transitions shown

in Figure 2. In contrast, the cyclic voltammogram of phenyl-

capped octaaniline, published by Wudl,³²displays a distinct split

in the second oxidation wave, suggesting an intermediate

“nigraniline” form³³in the oxidation from the emeraldine to

the pernigraniline form.

We wished to investigate effects of susbtitution in octaanilines

and the effects of chain length on oligoaniline redox behavior.

We have examined the oligoanilines in varying degrees of

oxidation and protonation by UV-vis spectroscopy and have

studied their electrochemical behavior by cyclic voltammetry.

(31) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1990,

112, 7869-7879. For other discussions of polyaniline oxidation states,

see: refs 3-5.

^aKey: (n-C₄H₉)₄N⁺Br₃^-(2 equiv), CH₂Cl₂, room temperature, 5 min;

(b) Pd₂(dba)₃(1.2 mol %), S-BINAP (3.7 mol %), NaOtBu (2.6 equiv),

THF, reflux; (b′) Pd(OAc)₂(4 mol %), S-BINAP (4.8 mol %), NaOtBu

(2.6 equiv), toluene/Et₃N, 90 °C; (c) (BOC)₂O (1.1 equiv), 4-DMAP

(0.2 equiv), THF, reflux; (c′) (BOC)₂O (3.5 equiv), 4-DMAP (0.2

equiv), THF, reflux; (c′′) (BOC)₂O (3.5 equiv), 4-DMAP (0.1 equiv),

THF/toluene/Et₃N, 67 °C.

(32) Wudl, F.; Angus, R. O.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.;

Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 3677-

3684.

(33) Early studies described a “nigraniline” oxidation state in polyaniline.

Described as an intermediate between emeraldine and pernigraniline, it may

well have been a mixture of the two. This possibility was not discussed,

and the evidence was inconclusive: (a) Willsta¨tter, R.; Dorogi, S. Chem.

Ber. 1909, 42, 2147-2168. (b) Willsta¨tter, R.; Dorogi, S. Chem. Ber. 1909,

42, 4118-4135. (c) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1910,

97, 2388-2403. (d) Green, A. G.; Woodhead, A. E. J. Chem. Soc. 1912,

101, 1117-1123.

of the phenylenediamine moieties to their quinoid forms results

in the insulating emeraldine form, which becomes conductive

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4965

Scheme 8^a

^aKey: (a) Pd₂(dba)₃(2 mol %), S-BINAP (4.8 mol %), NaOtBu (2.9 equiv), toluene, 80 °C; (a′) Pd(OAc)₂(4 mol %), S-BINAP (4.8 mol %),

NaOtBu (2.8 equiv), toluene/Et₃N, 90 °C; (a′′) Pd(OAc)₂(6 mol %), S-BINAP (7.2 mol %), NaOtBu (3 equiv), toluene/Et₃N, 90 °C; (b) (BOC)₂O

(2.5 equiv), 4-DMAP (0.5 equiv), THF, 60 °C; (b′) (BOC)₂O (3.5 equiv), 4-DMAP (0.1 equiv), THF/toluene/Et₃N, 67 °C; (b′′) (BOC)₂O (4 equiv),

4-DMAP (0.2 equiv), THF/toluene/Et₃N, 67 °C.

Scheme 9

Scheme 10^a

Electronic Absorption Spectroscopy. Under neutral condi-

tions, the UV-vis spectra of the oligoanilines (27-32) in a

given oxidation state are essentially identical; no significant

changes result from substitution or from variations in chain

length. The leucoemeraldine forms exhibit a single strong

absorption at 334-338 nm; lower-energy transitions are ob-

served for the partially and fully oxidized states. Oxidation of

a colorless leucoemeraldine solution in dilute DMF by silver-

(I) oxide results in an intense blue-purple solution of the

emeraldine base, with a sharp peak at 320 nm and a broad band

at 620 nm. Silver(II) oxide in DMF converts the leucoemer-

aldine to a red-pink pernigraniline solution, with a sharp peak

at 320 nm and a broad band at 520 nm. The strong blue shift

of this band, compared to that of emeraldine, reflects the

decreased charge-transfer absorption in the pernigraniline state.

The addition of a drop of sulfuric acid (a large excess) to the

UV-vis samples of the emeraldine and pernigraniline forms

produces a green color. Protonation of the emeraldine causes

the higher-energy absorption to broaden and split; the lower-

^aKey: (a) (CH₃)₃SiI (1.2n equiv), CH₂Cl₂, room temperature; (b)

CH₃OH (excess), Et₃N, CH₂Cl₂, room temperature.

energy absorption begins at ca. 540 nm and increases in intensity

up to the spectrometer’s limit at 1050 nm. In the case of the

pernigraniline, the lower-energy absorption is broadened, and

4966 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Figure 2. Principal oxidation states of polyaniline.

its maximum is red-shifted from 520 to 830 nm. The UV-vis

spectra of phenyl-capped octaaniline (27a) are shown in Figure

3.

The spectra of the protonated emeraldine forms vary con-

siderably with changes in electron density or in chain length,

as shown in Figure 4. The absorbance in the near-IR region of

27 becomes more intense with the increase in electron density

from the cyano-substituted to the methoxy-substituted octamer.

The twin peaks at higher energy, of nearly equal intensity for

phenyl-capped octaaniline, show complementary patterns for

the cyano- and methoxy-substituted analogues. A comparison

of the spectra for different chain lengths shows subtle differences

between heptamer, nonamer, and decamer; the distinct curvature

in the shape of the near-IR absorption contrasts with the near-

linear slope observed for the octamer. In the longer oligomers

(16-mer and 24-mer), this absorption shows a much more

definite maximum, occurring at somewhat shorter wavelengths.

Figure 3. UV-vis spectra of phenyl-capped octaaniline (27a) in DMF.

(top) Leucoemeraldine, ;;; emeraldine, s -; pernigraniline, - -.

Emeraldine (middle) and pernigraniline (bottom): in neutral solution,

;;; acidified, s -.

Electrochemistry. Cyclic voltammetry of the oligoanilines

affords valuable insight into the electronic structures of the

oxidized forms. We wished to examine, for instance, whether

the presence of electron-donating or electron-withdrawing

groups at the chain ends would affect the redox behavior of

phenyl-capped octaaniline, or whether the electronic effects

would be insignificant for the chain as a whole. The salient

question with regard to chain length is the behavior of those

oligoanilines that do not correspond to the tetraaniline-based

model depicted in Figure 2. Phenyl-capped heptaaniline,

nonaaniline, and decaaniline behave quite similarly to the

octaaniline upon chemical oxidation, but the nature of the

“emeraldine” and “pernigraniline” forms obtained for these chain

lengths is not obvious a priori. If the oligoaniline framework

were able to stabilize radical cations effectively, either by

resonance or by π-stacking between chains,³⁴several odd-

electron states would be accessible for the heptamer and

nonamer, and the decamer emeraldine might be the five-electron

oxidation product, containing five equivalent semiquinoid

moieties.

The electrochemical studies discussed below employed thin

films of the oligoanilines on ITO (indium-tin oxide) coated

glass electrodes. The films were prepared by evaporation of a

dilute solution of the trimethylsilyl carbamate in dichlo-

romethane, followed by immersion in the electrolyte, dilute

aqueous sulfuric acid.³⁵The first cycle of each film indicated

significant loss of material (approximately 20-30%) during the

reduction,³⁶but the films exhibited good stability after this

break-in scan.

(35) The indium-tin oxide coating is known to be unstable to strong

acids; however, immersion of the slides in 1.0 M sulfuric acid, for the short

time periods involved in these CV studies, caused no discernible degradation.

(36) The cause of this material loss is unclear. It is possible that the

evolution of carbon dioxide during hydrolysis causes blistering of the films,

with some initial physical instability.

(34) For a review of π-dimers and π-stacks in conducting polymers,

see: Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417-423.

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4967

electron oxidation of emeraldine. The greater partial positive

charge adjacent to the chain ends, compared to the emeraldine

state, would be stabilized by resonance with the methoxy group,

as shown in Figure 5, and destabilized by conjugation with the

cyano group.

Figure 6 shows the cyclic voltammograms obtained for 27-

30, with proposed oxidation mechanisms for some of the

oxidation steps. The intermediates are depicted in their expected

major resonance forms. For many of the intermediates the

degree of protonation may vary, and several tautomers may exist

in addition to those shown.

In the cyclic voltammogram of phenyl-capped heptaaniline,

two reversible oxidations occur as the potential is increased from

-0.3 to 0.95 V. Integration of the first scan for a known film

quantity showed that only six electrons were removed per

molecule of heptaaniline, reproducibly within 3%. Comparison

of the areas of the two oxidation peaks showed the first to be

approximately twice as large as the second,³⁷suggesting that

the heptaaniline undergoes a four-electron oxidation followed

by a two-electron oxidation.

Similarly, oxidation of phenyl-capped nonaaniline within the

same potential range results in the removal of only eight

electrons per molecule. The cyclic voltammogram displays two

oxidation waves, corresponding in area to two four-electron

oxidations. The second of these displays a prominent shoulder

at the left side. We believe that the extra nitrogen lone pair,

relative to phenyl-capped octaaniline, allows oxidation to a

mixture of several nonequivalent but energetically similar

nigraniline-like states, beginning at relatively low potentials,

en route to the formation of the eight-electron oxidation product.

Oxidation of phenyl-capped decaaniline from -0.3 to 1.0 V

results in the removal of 10 electrons, consistent with the

conversion of all five phenylenediamine moieties to their quinoid

forms. The cyclic voltammogram displays two oxidation waves,

the first of which encompasses approximately 50% more area

than the second.³⁷The oxidation of phenyl-capped decaaniline

thus appears to proceed via a six-electron oxidation, followed

by a four-electron oxidation.

The simplest electrochemical behavior is that of the 16-mer

(31) and the 24-mer (32). Cyclic voltammograms of these

oligomers are shown in Figure 7; due to the high molecular

weight of 32, a smaller molar quantity (2.5 nmol) was used to

obtain a thin film. In contrast to phenyl-capped octaaniline,

these longer tetraaniline multiples display no distinct intermedi-

ate in the oxidation of their emeraldine forms. The oxidation

from the leucoemeraldine to the pernigraniline state, like that

of the bulk polymer, results in two peaks of equal area.

The even-numbered oligomers investigated here are stable

at potentials up to and beyond +1.0 V vs SCE, and polyaniline

in nonnucleophilic solvents has been found to be stable at very

high potentials.³¹In marked contrast, the odd-numbered oli-

gomers are unstable at potentials above 0.95 V (Figure 8). At

higher potentials a third oxidation peak is observed, at 1.07 V

for the heptaaniline and 1.08 V for the nonaaniline, with no

corresponding reduction peak. This two-electron oxidation

occurs only once for each film: a second scan to 1.25 V fails

to reproduce this peak, and the voltammogram resembles that

of an even-numbered oligomer.

Figure 4. (top) R,ω-Substituent effects upon the protonated emeraldine

form of phenyl-capped octaanilines in DMF: H (27a), ;;; CN (27b),

- - -; OCH₃(27e), s -. (bottom) Protonated emeraldines of phenyl-

capped oligoanilines in DMF: 7-mer (28), ;;; 8-mer (27a), s -;

9-mer (29), ‚‚‚; 10-mer (30), s s s; 16-mer (31), - - -; 24-mer (32),

s - - -.

In dilute hydrochloric acid, the major peaks diminish in

intensity with each scan, while a broad peak grows in at ca.

0.55 V in the oxidation wave and 0.40 V in the reduction wave.

This degradation had been observed previously for both phenyl-

capped octaaniline and bulk polyaniline.³²In dilute sulfuric

acid, however, this degradation occurs more slowly.

Integration of the oxidation peaks of phenyl-capped octaa-

niline (5.0 nanomoles) in the first scan corresponded reproduc-

ibly, within 2%, to the removal of eight electrons per molecule,

but the oxidations occurred at markedly higher potentials than

in subsequent scans. The reduction peaks in the first scan

represent a significantly smaller area than the oxidation peaks,

but subsequent scans showed good reversibility. In the discus-

sion of oxidation states below, the total number of electrons

removed from each molecule is determined by integration of

the oxidation peaks in the first scan; the oxidation states of each

compound are determined by comparison of the relative peak

areas in the second (i.e., first stable) scan.

Phenyl-capped octaaniline (27a) oxidizes from the leucoe-

meraldine to the emeraldine form in one four-electron step. In

contrast, the oxidation from emeraldine to pernigraniline shows

a split, with peaks at 0.79 and at 0.90 V. This split is highly

sensitive to changes in electron density at the chain termini.

The methoxy groups of 27e cause a larger split in the

emeraldine-pernigraniline oxidation wave, with peaks at 0.66

and at 0.87 V, whereas the cyano groups of 27b cause the split

to disappear entirely, with a smooth four-electron oxidation

centered at 0.84 V. This disparity is consistent with formation

of the nigraniline form, with three quinoid moieties, by a two-

The irreversibility of the oxidation, and the fact that no

corresponding peak is observed for even-numbered oligomers,

is consistent with the formation of a highly unstable odd-electron

(37) Some ambiguity is involved in assigning the peak areas, but the

first peak is clearly larger, in the CVs of both the heptaaniline and the

decaaniline, for any reasonable choice of demarcation. The narrow peak at

high potential is smaller than it appears.

4968 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Figure 5. Stabilization of the nigraniline oxidation state by π-donating substituents.

an apparent multiplicity. Infrared spectroscopy was carried out on a

Perkin-Elmer 1600 Series FT-IR spectrometer. UV-vis spectra were

obtained using a Hewlett-Packard 8451A or 8453A spectrophotometer.

FAB mass spectra were recorded on a Finnigan MAT System 8200

using a 3-nitrobenzyl alcohol matrix. Elemental analyses were carried

out by E & R Microanalytical Laboratory Inc., Corona, NY. Gas

chromatographic analyses were carried out on a Hewlett-Packard HP-

5890 Series II gas chromatograph, fitted with an HP-1 capillary column

(25 m, 0.20 mm, 0.11 µm). Thin-layer chromatography was carried

out on E. Merck SIlica Gel 60 F-254 TLC plates. Melting points were

obtained using a Haake Buchler melting point apparatus and are

uncorrected.

Reactions under an argon atmosphere were carried out in oven-dried

glassware using standard Schlenk techniques. Tetrahydrofuran was

distilled under argon from sodium benzophenone ketyl. Toluene was

distilled under nitrogen from molten sodium. Dichloromethane used

in oligomer deprotections was purchased in anhydrous form from

Aldrich Chemical Co. and stored under nitrogen over activated 3 Å

molecular sieves. Absolute ethanol was purchased from Pharmco and

used as supplied. Diethyl ether, analytical reagent grade, was purchased

from Mallinckrodt and used as supplied. N-Methylpyrrolidinone,

anhydrous, and N,N-dimethylformamide, reagent grade, were purchased

from Aldrich Chemical Co. and used as supplied. Deuterated solvents

were purchased from Cambridge Isotope Laboratories and used as

supplied. All other solvents were of liquid chromatography grade

quality, purchased from EM Science and used as supplied.

Molecular sieves were purchased from Aldrich Chemical Co. and

activated at 180 °C and 10^-3mmHg for 12 h prior to use. Sodium

tert-butoxide was purchased from Aldrich Chemical Co. and stored in

a Vacuum Atmospheres glovebox under nitrogen. Small amounts were

removed from the glovebox as needed, stored in a desiccator for up to

1 week, and weighed in the air. 4-Bromoaniline, benzophenone,

chlorotrimethylsilane, p-anisidine, di-tert-butyl dicarbonate solution (1.0

M in tetrahydrofuran), tetra-n-butylammonium tribromide, palladium

hydroxide (moist, 20% on carbon), 1,4-phenylenediamine dihydrochlo-

ride, aniline, diphenylamine, 4-bromo-tert-butylbenzene, 4-bromoben-

zonitrile, ammonium formate, hydroxylamine hydrochloride, and

hexamethyldisilane were purchased from Aldrich Chemical Co. and

used as supplied. Di-tert-butyl dicarbonate and 4-(dimethylamino)-

pyridine were purchased from Lancaster Synthesis Inc. and used as

supplied. 4-Bromo-n-dodecylbenzene was purchased from TCI America

and used as supplied. S-BINAP, a gift from Pfizer, was used as

supplied. Tris(dibenzylideneacetone)dipalladium, palladium acetate,

palladium on carbon, n-butyllithium (1.60 M in hexanes), and bromine

were purchased from Strem Chemical Company and used without

further purification. All other inorganic reagents were analytical reagent

grades purchased from Mallinckrodt and used as supplied.

species, followed by decomposition to a species which under-

goes facile one-electron oxidation. The oxidation pattern does

not rule out the tail-to-tail dimerization of the radical cation,

followed by dehydrogenation, but the product of this reaction

should be reduced easily to a benzidine derivative during the

reduction wave. Intramolecular C-C bond formation by the

odd-electron cation, followed by deprotonation and one-electron

oxidation to the carbazole, represents one possible explanation

for the observed behavior. Since the carbazole moiety is quite

difficult to oxidize,³⁸the product would contain an even number

of redox-active nitrogen atoms, and the formation of additional

carbazole units during a subsequent scan would not be expected.

Figure 9 illustrates the proposed carbazole formation; for

simplicity, only one product is shown, although the cyclization

could also occur in the middle of the chain.

Concluding Remarks

Using palladium catalysis and an orthogonal protective group

strategy, we have developed divergent-convergent and con-

vergent methods for the synthesis of well-defined, air-stable

oligoaniline precursors, soluble in a variety of common organic

solvents. These precursors are easily deprotected to form the

leucoemeraldine forms of the corresponding oligoanilines. The

synthetic methods are highly versatile, allowing the synthesis

of end-functionalized oligoanilines, and the preparation of even

or odd chain lengths.

The presence of electron-donating or electron-withdrawing

groups at the chain ends results in significant modifications of

the UV-vis spectra and electrochemical behavior of phenyl-

capped octaaniline. An increase in electron density results in

more intense electronic absorption by the emeraldine in the low

band gap region and stabilizes the electrochemically observed

nigraniline state. The electrochemistry of the heptamer, non-

amer, and decamer illustrates the importance of electron-pairing

in the redox behavior of these compounds. Oxidation of

oligoanilines occurs through even-electron transitions when

possible; thus, the decamer oxidizes in unequal steps, and the

odd-numbered oligomers generate radical cations only tran-

siently and at high potential. Our observations suggest that the

ability of polyaniline to stabilize an unpaired electron through

resonance or π-stacking is limited.

Experimental Section

Synthesis. N-(Diphenylmethylene)-4-bromoaniline (1). The method

of Taguchi and Westheimer³⁹was modified as follows: Benzophenone

(455 g, 2.50 mol) and 4-bromoaniline (473 g, 2.75 mol) were dissolved

in toluene (1.2 L, distilled) under argon in a 5 L flask, containing

molecular sieves (5 Å, 1.25 kg), fitted with a reflux condenser, rubber

septum, and pressure outlet. The mixture was heated to gentle reflux

General Information. Proton and carbon nuclear magnetic reso-

nance spectra (¹H NMR and ¹³C NMR) were recorded on Varian XL-

300, UN-300, or XL-500 spectrometers and referenced with respect to

residual solvent. The letter “a” before a multiplicity notation indicates

(38) Carbazole itself undergoes anodic oxidation at +1.16 V relative to

SCE: Ambrose, J. F.; Nelson, R. F. J. Electrochem. Soc. 1968, 115, 1159-

1164. Conjugation of the carbazole moiety with a protonated iminoquinone

might well raise the oxidation potential beyond the range investigated here.

(39) Taguchi, K.; Westheimer, F. H. J. Org. Chem. 1971, 36, 1570-

1572.

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4969

Figure 6. Cyclic voltammetry of phenyl-capped heptaaniline (28), octaanilines 27 (H, 27a, ;;; CN, 27b, s -; OCH₃, 27e, ‚‚‚), nonaaniline 29,

and decaaniline 30 on ITO (working electrode) in aqueous H₂SO₄(1.0 M). Proposed oxidation mechanisms at right.

and shaken occasionally; an intense yellow color soon developed.

Analysis by GC after 18 h showed that product formation was nearly

complete. The mixture was allowed to cool to room temperature, and

the yellow solution was decanted from the molecular sieves, which

were washed with diethyl ether until the filtrate was colorless. The

organic solutions were combined and concentrated to give an orange

oil, 900 mL. Methanol (ca. 80 mL) and a seed crystal of authentic

product were added. The product was allowed to crystallize at 0 °C

and collected by filtration. The mother liquor was further concentrated.

A second crop of crystals formed and was isolated by filtration.

Recrystallization of the combined product from methanol afforded the

title compound as yellow crystals (760 g, 90%): mp 82-83 °C; H

NMR (300 MHz, CDCl₃) δ 7.75 (dd, J ) 6.9, 1.6 Hz), 7.52-7.39 (m,

3H), 7.32-7.23 (m, 5H), 7.11 (dd, J ) 8.4, 1.9 Hz, 2H), 6.61 (dt, J )

1

4970 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Figure 9. Proposed carbazole formation in odd-numbered oligoanilines

at high potential.

Figure 7. Cyclic voltammetry of phenyl-capped 16-mer (31) and

phenyl-capped 24-mer (32), same conditions as above.

IR (neat, cm^-1) 3058, 3024, 1615, 1478. Anal. Calcd for C₁₉H₁₄BrN:

C, 67.87; H, 4.20. Found: C, 68.08; H, 4.28.

4-(Trimethylsilyl)aniline (2).²⁵Aryl bromide 1 (16.8 g, 50.0 mmol)

was dissolved in tetrahydrofuran (250 mL) in a dry Schlenk flask under

argon. The resulting solution was cooled with stirring to -78 °C. A

solution of n-butyllithium in hexanes (1.60 M, 31.5 mL, 50.4 mmol)

was added dropwise via syringe, causing the yellow solution to turn a

deep red color. The reaction mixture was stirred for 30 min at -78

°C. Chlorotrimethylsilane (6.5 mL, 51 mmol) was added dropwise

via syringe over 5 min, causing the red solution to turn a light orange

color. The reaction mixture was warmed to room temperature and

stirred for 45 min. Triethylamine (10 mL) and methanol (20 mL) were

added, resulting in a cloudy, pale yellow suspension. The suspensions

obtained from two reactions carried out in this manner were combined

and concentrated; the solid residue was taken up in diethyl ether (250

mL) and washed with brine (100 mL). The aqueous phase was

extracted with two 75-mL portions of diethyl ether. The organic

solutions were combined, dried over potassium carbonate, filtered, and

concentrated.

The yellow crystalline product was dissolved in methanol (200 mL).

Sodium acetate (16.4 g, 200 mmol) and hydroxylamine hydrochloride

(10.4 g, 150 mmol) were added with rapid stirring. After 5 min, solid

potassium bicarbonate (15 g, 150 mmol) was added, and the mixture

was stirred for 30 min. Diethyl ether (100 mL) was added, and the

mixture was filtered to remove precipitated salts. The collected solid

was dissolved in water (200 mL), and the resulting solution was

extracted with two 50-mL portions of diethyl ether. The combined

organic solutions were dried over potassium carbonate, filtered, and

concentrated. The residue was taken up in dichloromethane (20 mL),

cooled to -78 °C, and filtered to remove the precipitated benzophenone

oxime. The collected solid was suspended in dichloromethane to

dissolve adsorbed 2, and the mixture was cooled to -78 °C and filtered.

The filtrates were combined and concentrated, and the precipitation of

benzophenone oxime was repeated as described above. The crude

aniline was distilled from calcium hydride under high vacuum, affording

the title compound as a colorless oil (14.1 g, 85%): bp 44 °C/0.01

Figure 8. Irreversible oxidation of phenyl-capped heptaaniline (28)

and nonaaniline (29) at high potential, same conditions as above.

1

mmHg (lit.²⁵102 °C/6 mmHg); H NMR (300 MHz, CDCl₃) δ 7.39

8.5, 2.0 Hz, 2H); ¹³C NMR (75 MHz, CDCl₃) δ 168.7, 150.4, 139.5,

136.1, 131.6, 131.0, 129.5, 128.8, 128.3, 128.2, 122.8, 116.3, 103.6;

(d, J ) 8.1 Hz, 2H), 6.75 (d, J ) 8.1 Hz, 2H), 3.75 (s, 2H), 0.29 (s,

9H).

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4971

Dimer 3. Aryl bromide 1 (25.15 g, 74.8 mmol), arylamine 2 (13.0

g, 78.6 mmol), sodium tert-butoxide (10.06 g, 105 mmol), Pd₂(dba)₃

(0.171 g, 0.187 mmol, 0.25 mol %), and S-BINAP (0.349 g, 0.560

mmol, 0.75 mol %) were dissolved in tetrahydrofuran (75 mL) in a

Schlenk flask under argon. The reaction mixture was heated to a gentle

reflux. Analysis by TLC after 17 h showed complete consumption of

aryl bromide 1. The reaction mixture was cooled to room temperature

and concentrated. The residue was taken up in dichloromethane (200

mL), washed with brine, dried over potassium carbonate, and concen-

trated. The crude product, 4-(dimethylamino)pyridine (1.635 g, 13.4

mmol, 20 mol %), and di-tert-butyl dicarbonate (21.90 g, 100 mmol)

were dissolved in tetrahydrofuran (67 mL) in a Schlenk flask under

argon. The resulting solution was heated to 60 °C with stirring. After

2 h the solution was cooled to room temperature and concentrated.

Crystallization of the product from methanol afforded dimer 3 as pale

yellow crystals (32.89 g, 84%): mp 123-124 °C: ¹H NMR (300 MHz,

CDCl₃) δ 7.75 (d, J ) 7.0 Hz, 2H), 7.49-7.40 (m, 5H), 7.28 (d, J )

6.2 Hz, 3H), 7.19 (d, J ) 8.2 Hz, 4H), 6.98 (d, J ) 8.5 Hz, 2H), 6.69

(d, J ) 8.7 Hz, 2H), 1.41 (s, 9H), 0.24 (s, 9H); ¹³C NMR (75 MHz,

CDCl₃) δ 168.8, 154.0, 149.5, 143.9, 139.8, 138.4, 137.0, 136.4, 133.8,

131.0, 129.7, 129.5, 128.8, 128.4, 128.1, 127.9, 125.3, 121.6, 81.1,

28.4, -0.9; IR (neat, cm^-1) 3059, 3022, 2954, 1711, 1500, 1327, 1162,

852. Anal. Calcd for C₃₃H₃₆N₂O₂Si: C, 76.11; H, 6.97. Found: C,

76.06; H, 7.18.

Dimer Amine 4. A Schlenk flask was charged with dimer 3 (3.64

g, 7.00 mmol), ammonium formate (5.297 g, 84.0 mmol), and palladium

on carbon (10%, 0.740 g, 0.70 mmol Pd) and purged with argon.

Methanol (100 mL) was added,⁴⁰and the resulting mixture was heated

with stirring to 60 °C. Analysis by TLC after 45 min showed complete

consumption of imine 3. The reaction mixture was cooled to room

temperature and concentrated. The residue was taken up in dichlo-

romethane, and the resulting solution was filtered through Celite and

concentrated. The white solid residue was triturated with hexanes (20

mL), cooled to 0 °C, and filtered to afford arylamine 4 as a white solid

(2.251 g, 90%): mp 108-109 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.42

(d, J ) 8.5 Hz, 2H), 7.20 (d, J ) 8.5 Hz, 2H), 7.00 (d, J ) 8.5 Hz,

2H), 6.64 (d, J ) 8.5 Hz, 2H), 3.66 (s, 2H), 1.45 (s, 9H), 0.24 (s, 9H);

¹³C NMR (75 MHz, CDCl₃) δ 154.3, 144.7, 144.1, 136.7, 134.1, 133.7,

128.8, 125.2, 115.4, 80.9, 28.4, -0.9; IR (neat, cm^-1) 3465, 3367, 3227,

2955, 1696, 1515, 1162, 852. Anal. Calcd for C₂₀H₂₈N₂O₂Si: C, 67.38;

H, 7.92. Found: C, 67.54; H, 7.99.

Dimer Bromide 5. Procedure A: A Schlenk flask was charged

with dimer 3 (7.291 g, 14.0 mmol) and sodium acetate (1.148 g, 14.0

mmol) and purged with argon. Tetrahydrofuran (100 mL) was added,

and the resulting mixture was cooled to -78 °C with stirring. Bromine

(1.50 mL, 29.1 mmol) was added dropwise, causing the mixture to

turn a deep green color. The mixture was stirred for 10 min at -78

°C and then warmed to 0 °C, causing the solution to turn a brown

color. Analysis by TLC after 20 min indicated complete consumption

of arylsilane 3. A solution of sodium bicarbonate (0.5 M) and sodium

sulfite (0.5 M) in water was added to the reaction mixture with vigorous

stirring, dispelling the brown color. The mixture was transferred to a

separatory funnel containing diethyl ether (50 mL). The phases were

separated, and the aqueous phase extracted with two 50-mL portions

of diethyl ether. The ether portions were combined, dried over

potassium carbonate, filtered, and concentrated, giving a yellow oil

which crystallized on standing. Recrystallization of the product from

a 4:1 mixture of hexanes and ethyl acetate afforded aryl bromide 5 as

pale yellow crystals (6.545 g, 89%): mp 161-162 °C; ¹H NMR (300

MHz, CDCl₃) δ 7.76 (d, J ) 8.7 Hz, 2H), 7.49-7.36 (m, 5H), 7.28 (d,

J ) 8.8 Hz, 3H), 7.13 (dd, J ) 7.8, 2.0 Hz, 2H), 7.05 (d, J ) 8.7 Hz,

2H), 6.96 (d, J ) 8.6 Hz, 2H), 6.70 (d, J ) 8.6 Hz, 2H), 1.41 (s, 9H);

¹³C NMR (75 MHz, CDCl₃) δ 168.9, 153.6, 149.6, 142.4, 139.5, 137.8,

136.2, 131.6, 131.0, 129.6, 129.5, 128.8, 128.4, 128.0, 127.7, 127.5,

121.6, 118.3, 81.3, 28.3; IR (neat, cm^-1) 3058, 2977, 1711, 1489, 1325,

1161, 697. Anal. Calcd for C₃₀H₂₇BrN₂O₂: C, 68.31; H, 5.16.

Found: C, 68.53; H, 5.35.

(95.7 mg, 0.105 mmol, 0.25 mol %), and S-BINAP (0.195 g, 0.314

mmol, 0.75 mol %) were dissolved in tetrahydrofuran (80 mL) in a

Schlenk flask under argon. The reaction mixture was heated to a gentle

reflux. Analysis by TLC after 24 h showed complete consumption of

the starting bromide. The mixture was cooled to room temperature,

taken up in ethyl acetate (80 mL), and washed with a 2.0 M aqueous

sodium hydroxide solution (80 mL), followed by brine (80 mL). The

organic phase was dried over sodium sulfate, filtered, and concentrated.

The residue was taken up in dichloromethane (88 mL), and tetra-n-

butylammonium tribromide (23.3 g, 48.3 mmol) was added in one

portion with stirring. After 30 min, a saturated aqueous solution of

sodium sulfite (80 mL) was added. The mixture was stirred for 10

min, and then 2.0 M aqueous sodium hydroxide solution (40 mL) was

added. The layers were separated, and the organic phase was washed

with brine (80 mL), dried over sodium sulfate, filtered, and concentrated.

The residual solid, 4-(dimethylamino)pyridine (0.536 g, 4.39 mmol,

11 mol %), and di-tert-butyl dicarbonate (1.054 g, 4.82 mmol) were

dissolved in tetrahydrofuran (50 mL). The resulting solution was heated

to reflux. After 3 h at reflux the solution was cooled to room

temperature and concentrated. Crystallization of the residue from

methanol afforded aryl bromide 5 as pale yellow crystals (18.7 g, 81%).

Spectroscopic data were identical to those reported above; mp 159-

160 °C. Anal. Calcd for C₃₀H₂₇BrN₂O₂: C, 68.31; H, 5.16. Found:

C, 68.52; H, 5.33.

Tetramer 6. Dimer amine 4 (2.353 g, 6.60 mmol), dimer bromide

5 (3.165 g, 6.00 mmol), sodium tert-butoxide (0.807 g, 8.40 mmol),

Pd₂(dba)₃(54.9 mg, 0.060 mmol, 1 mol %), and S-BINAP (89.7 mg,

0.144 mmol, 2.4 mol %) were dissolved in toluene (24 mL) in a Schlenk

flask under argon. The reaction mixture was heated with stirring to

80 °C. Analysis by TLC after 19 h indicated complete consumption

of the starting bromide. The mixture was cooled to room temperature

and taken up in dichloromethane (100 mL), washed with water (50

mL), dried over potassium carbonate, filtered, and concentrated. The

residual solid and 4-(dimethylamino)pyridine (0.1466 g, 1.20 mmol,

20 mol %) were dissolved in tetrahydrofuran (12 mL) in a Schlenk

tube under argon. A solution of di-tert-butyl dicarbonate in tetrahy-

drofuran (1.0 M, 8.0 mL, 8.0 mmol) was added, and the resulting

solution was heated with stirring to 60 °C. After 2 h the solution was

cooled to room temperature and concentrated. Crystallization of the

residual solid from methanol afforded tetramer 6 as pale yellow crystals

(4.63 g, 85%): mp 131-133 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.76

(d, J ) 7.2 Hz, 2H), 7.50-7.42 (m, 5H), 7.28 (m, 3H), 7.19 (d, J )

8.1 Hz, 2H), 7.13 (d, J ) 11.7 Hz, 10H), 6.98 (d, J ) 8.6 Hz, 2H),

6.69 (d, J ) 8.5 Hz, 2H), 1.469 (s, 9H), 1.453 (s, 9H), 1.408 (s, 9H),

0.26 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 169.0, 154.1, 154.0, 149.5,

143.5, 140.8, 140.6, 140.5, 139.8, 138.3, 137.8, 136.4, 134.0, 131.1,

129.8, 129.6, 129.0, 128.5, 128.2, 127.8, 127.5, 127.3, 127.2, 126.4,

126.2, 121.7, 81.6, 81.3, 28.4, -0.9; IR (neat, cm^-1) 3008, 2977, 1711,

1509, 1327, 1161, 851, 756. Anal. Calcd for C₅₅H₆₂N₄O₆Si: C, 73.14;

H, 6.92. Found: C, 72.79; H, 6.86.

Tetramer Amine 7. A Schlenk flask was charged with tetramer 6

(4.155 g, 4.6 mmol), ammonium formate (4.061 g, 64.4 mmol), and

palladium on carbon (5%, 0.979 g, 0.460 mmol Pd) and purged with

argon. Methanol (25 mL) and tetrahydrofuran (15 mL) were added,

and the resulting mixture was heated to 50 °C with stirring. Analysis

by TLC after 11 h indicated complete consumption of the starting imine.

The mixture was cooled to room temperature and concentrated. The

residue was taken up in dichloromethane (75 mL), and the resulting

mixture was filtered through Celite and concentrated. The white solid

residue was triturated with hexanes (30 mL), cooled to 0 °C, and

collected by filtration to afford amine 7 as a white solid (3.243 g,

1

95%): mp 190-192 °C; H NMR (300 MHz, CDCl₃) δ 7.45 (d, J )

5.0 Hz, 2H), 7.18-7.09 (m, 10H), 6.97 (d, J ) 5.0 Hz), 6.62 (d, J )

5.0 Hz, 2H), 3.67 (s, 2H), 1.46 (s, 9H), 1.44 (s, 18H), 0.25 (s, 9H); ¹³

C

NMR (75 MHz, CDCl₃) δ 154.3, 153.91, 153.88, 144.8, 143.4, 141.2,

140.4, 140.4, 139.6, 137.7, 133.9, 133.9, 129.01, 128.6, 127.4, 127.2,

127.1, 126.4, 126.2, 126.0, 115.4, 81.4, 81.4, 81.0, 28.4, 28.4, 28.4,

-0.9; IR (neat, cm^-1) 3472, 3366, 2978, 1708, 1508, 1331, 1161, 1055,

844. Anal. Calcd for C₄₂H₅₄N₄O₆Si: C, 68.26; H, 7.36. Found: C,

68.38; H, 7.52.

Procedure B: Aryl bromide 1 (14.1 g, 41.8 mmol), aniline (4.00

mL, 43.9 mmol), sodium tert-butoxide (5.63 g, 58.5 mmol), Pd₂(dba)₃

(40) This reaction is carried out under argon; we note that, under air,

palladium on carbon may ignite upon contact with methanol. Once started,

the imine hydrogenolysis reaction does not require rigorously air-free

conditions.

4972 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

Tetramer Bromide 8. A Schlenk flask was charged with tetramer

6 (2.50 g, 2.77 mmol) and sodium acetate (227 mg, 2.77 mmol) and

purged with argon. Tetrahydrofuran (28 mL) was added, and the

resulting mixture was cooled with stirring to 0 °C. Bromine (299 µL,

5.81 mmol) was added dropwise. The mixture was stirred for 20 min

at 0 °C, and then triethylamine (1.54 mL, 11.1 mmol) and a 1.0 M

aqueous solution of sodium sulfite (20 mL) were added with vigorous

stirring. The mixture was stirred for 5 min, and then partitioned

between ethyl acetate (60 mL) and a 2.0 M sodium hydroxide solution

(50 mL). The organic layer was washed with saturated aqueous sodium

chloride solution (50 mL) dried over anhydrous sodium sulfate, filtered,

and concentrated. Crystallization of the solid residue from methanol

afforded aryl bromide 8 as pale yellow crystals (2.13 g, 85%): mp 174-

176 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.75 (dd, J ) 8.5, 1.6 Hz, 2H),

7.50-7.35 (m, 6H), 7.27 (d, J ) 8.5 Hz, 2H), 7.12 (dd, J ) 7.7, 1.5

Hz, 2H), 7.02 (dd, J ) 9.0, 2.1 Hz, 2H), 6.94 (d, J ) 8.5 Hz, 2H),

6.69 (d, J ) 8.5 Hz, 2H), 1.39 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ

168.8, 153.5, 149.4, 142.3, 139.4, 137.8, 136.1, 131.5, 130.9, 129.5,

129.3, 128.7, 128.2, 127.9, 127.5, 127.4, 121.5, 118.1, 81.2, 28.2; IR

(neat, cm^-1) 1710. Anal. Calcd for C₅₂H₅₃BrN₄O₆: C, 68.64; H, 5.87.

Found: C, 68.38; H, 5.85

6.1 mmol), sodium tert-butoxide (0.8072 g, 8.40 mmol), Pd₂(dba)₃(54.9

mg, 0.060 mmol, 1.0 mol %), and S-BINAP (89.7 mg, 2.5 mol %)

were dissolved in toluene (20 mL) in a Schlenk flask under argon.

The reaction mixture was heated to 80 °C with stirring. Analysis by

TLC after 14 h indicated complete consumption of the starting bromide.

The mixture was cooled to room temperature and taken up in diethyl

ether (75 mL). The resulting mixture was washed with brine (50 mL),

dried over potassium carbonate, filtered, and concentrated. The residue,

di-tert-butyl dicarbonate (1.53 g, 7.0 mmol), and 4-(dimethylamino)-

pyridine (0.131 g, 1.16 mmol, 20 mol %) were dissolved in tetrahy-

drofuran (15 mL) in a Schlenk flask under argon. The reaction mixture

was heated with stirring to 60 °C. After 3 h the solution was cooled

to room temperature and concentrated. The solid residue was crystal-

lized from ethanol. Recrystallization of the product from ethanol

afforded the title compound as pale yellow crystals (3.00 g, 81%): mp

149-151 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.75 (d, J ) 8.5 Hz, 2H),

7.48-7.38 (m, 3H), 7.33-7.24 (m, 5H), 7.21-7.16 (m, 3H), 7.14-

7.07 (m, 6H), 6.97 (d, J ) 8.8 Hz, 2H), 6.77 (d, J ) 8.8 Hz, 2H), 1.45

(s, 9H), 1.40 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 168.9, 154.0, 149.4,

143.0, 140.6, 140.0, 139.7, 138.2, 136.3, 131.0, 129.7, 129.5, 128.9,

128.4, 128.1, 127.7, 127.2, 127.0, 126.3, 125.9, 121.6, 81.4, 81.2, 28.4,

28.4; IR (neat, cm^-1) 2977, 2930, 1710, 1509, 1324, 1161, 758, 696.

Anal. Calcd for C₄₁H₄₁N₃O₄: C, 76.97; H, 6.46. Found: C, 77.16;

H, 6.70.

Octamer 9. Arylamine 7 (1.55 g, 2.10 mmol), aryl bromide 8 (1.82

g, 2.00 mmol), sodium tert-butoxide (0.269 g, 2.80 mmol), Pd₂(dba)₃

(37 mg, 0.026 mmol, 2 mol %), and S-BINAP (60 mg, 0.096 mmol,

4.8 mol %) were dissolved in toluene (15 mL) in a Schlenk tube under

argon. The reaction mixture was heated to 65 °C. Analysis by TLC

after 17 h indicated complete consumption of the starting bromide. The

reaction mixture was cooled to room temperature and taken up in

dichloromethane (75 mL). The resulting mixture was washed with brine

(50 mL), dried over potassium carbonate, filtered, and concentrated.

The residue, 4-(dimethylamino)pyridine (49 mg, 0.40 mmol, 20 mol

%), and di-tert-butyl dicarbonate (0.576 g, 2.64 mmol) were dissolved

in tetrahydrofuran (20 mL) in a Schlenk tube under argon. The resulting

solution was heated to 65 °C. After 3 h the solution was cooled to

room temperature and concentrated. Crystallization of the residual solid

from methanol afforded octamer 9 as pale yellow crystals (2.48 g,

N-Phenyl-N′-(4-aminophenyl)-N,N′-bis(tert-butoxycarbonyl)-1,4-

phenylenediamine (12). A Schlenk flask was charged with imine 11

(1.245 g, 1.95 mmol), ammonium formate (1.840 g, 29.2 mmol), and

palladium on carbon (5%, 0.414 g, 1.95 mmol Pd). Methanol (8 mL)

and tetrahydrofuran (4 mL) were added via syringe. The resulting

mixture was heated to 60 °C with stirring. Analysis by TLC after 90

min showed incomplete consumption of the starting imine. An

additional portion of palladium on carbon (5%, 0.414 g, 1.95 mmol

Pd) was added, and the solid ammonium formate which collected above

the mixture was periodically redissolved. Analysis by TLC after 2 h

showed complete consumption of the starting imine. The mixture was

cooled to room temperature and concentrated. The residue was taken

up in dichloromethane, and the resulting mixture was filtered through

Celite and concentrated. The residual white solid was triturated in

hexanes (30 mL), cooled to 0 °C, and collected by filtration, affording

the title compound as a white solid (0.884 g, 96%): mp 180-182 °C

1

74%): mp 169-171 °C; H NMR (300 MHz, CDCl₃) δ 7.76 (d, J )

8.4 Hz, 2H), 7.50-7.39 (m, 8H), 7.28-7.26 (m, 2H), 7.17 (d, J ) 8.4

Hz, 2H), 7.16-7.13 (m, 24H), 6.97 (d, J ) 8.4 Hz, 2H), 6.71 (d, J )

8.4 Hz, 2H), 1.45 (s, 9H), 1.43 (s, 45H), 1.39 (s, 9H), 0.25 (s, 9H); ¹³

C

NMR (75 MHz, CDCl₃) δ 168.6, 153.7, 153.7, 153.6, 149.2, 143.2,

140.5, 140.2, 140.1, 139.5, 138.0, 137.5, 136.1, 133.7, 130.8, 129.5,-

129.3, 128.6, 128.2, 127.9, 127.4, 127.2, 127.0, 126.8, 126.1, 125.8,

121.3, 81.3, 81.2, 80.9, 28.2, -1.1; IR (neat, cm^-1) 1712. Anal. Calcd

for C₉₉H₁₁₄N₈O₁₄Si: C, 71.28; H, 6.88. Found: C, 71.07; H, 7.00.

Octamer Amine 10. A Schlenk tube was charged with imine 9

(500 mg, 0.300 mmol), ammonium formate (568 mg, 9.00 mmol), and

palladium hydroxide on carbon (20%, 0.105 g, 0.150 mmol Pd) and

purged with argon. 2-Propanol (30 mL) was added, and the resulting

mixture was heated to 80 °C with stirring, causing a visible ef-

fervescence. After ca. 15 min the effervescence slowed, and an

additional portion of ammonium formate (568 mg, 9.00 mmol) was

added. Small portions of ammonium formate were added at 15 min

intervals until conversion to amine 10 was complete as judged by thin-

layer chromatography (ca. 1 h). The reaction mixture was cooled to

room temperature, taken up in dichloromethane (20 mL), and filtered

through Celite. The filtrate was diluted with dichloromethane (60 mL),

washed with a 2.0 M aqueous sodium hydroxide solution (50 mL),

dried over anhydrous sodium sulfate, filtered, and concentrated. The

white solid residue was recrystallized from a 5:1 mixture of 2-propanol

and water to afford arylamine 10 as white needles (425 mg, 94%):

mp 168-170 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.43 (d, J ) 8.4 Hz,

2H), 7.16 (d, J ) 8.4 Hz, 2H), 7.14-7.12 (m, 24H), 7.02 (d, J ) 8.4

Hz, 2H), 6.77 (d, J ) 8.4 Hz, 2H), 3.65 (bs, 2H), 1.44 (s, 9H), 1.43 (s,

54 H), 0.24 (s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 154.1, 153.6, 153.6,

144.6, 143.2, 141.0, 140.3, 140.3, 140.1, 140.1, 140.0, 139.3, 137.5,

133.7, 128.8, 128.4, 127.2, 127.0, 126.8, 126.2, 125.8, 115.1, 81.3,

81.2, 80.8, 28.1, -1.2; IR (neat, cm^-1) 3468, 3368, 1710. Anal. Calcd

for C₈₆N₁₀₆N₈O₁₄: C, 68.68; H, 7.10. Found: C, 68.43; H, 6.86.

N-(Diphenylmethylene)-N′,N′′-bis(tert-butoxycarbonyl)tera-

niline (11). Dimer bromide 5 (3.06 g, 5.80 mmol), aniline (0.56 mL,

1

with slow decomposition; H NMR (300 MHz, CDCl₃) δ 7.31 (dd, J

) 8.2, 3.9 Hz, 2H), 7.21-7.10 (m, 7H), 6.98 (dd, J ) 5.3, 1.2 Hz,

2H), 6.62 (dd, J ) 5.3, 1.2 Hz, 2H); ¹³C NMR (75 MHz, CDCl₃) δ

154.3, 154.0, 144.8, 143.0, 141.0, 139.8, 134.0, 129.1, 128.8, 128.7,

128.6, 127.2, 127.0, 126.4, 126.2, 125.8, 115.4, 81.3, 81.0, 28.4, 28.4;

IR (neat, cm^-1) 3460, 3366, 3037, 2978, 2919, 1702, 1508, 1337, 1161,

1055. Anal. Calcd for C₂₈H₃₃N₃O₄: C, 70.71; H, 6.99. Found: C,

70.84; H, 6.78.

N-(Diphenylmethylene)-4-[4-methoxy-N-(tert-butoxycarbonyl)a-

nilino]aniline (13). Aryl bromide 1 (2.60 g, 7.74 mmol), p-anisidine

(1.00 g, 8.13 mmol), sodium tert-butoxide (1.04 g, 10.8 mmol), Pd₂-

(dba)₃(35.0 mg, 0.0387 mmol, 1.0 mol %), and S-BINAP (72.0 mg,

0.116 mmol, 1.5 mol %) were dissolved in tetrahydrofuran (25 mL) in

a Schlenk flask under argon. The reaction mixture was heated to reflux.

After 18 h, the mixture was cooled to room temperature. 4-(Dimethy-

lamino)pyridine (47.0 mg, 0.774 mmol, 10 mol %) and a solution of

di-tert-butyl dicarbonate in tetrahydrofuran (1.0 M, 11.6 mL, 11.6

mmol) were added, and the resulting mixture was heated to reflux.

After 3 h the reaction mixture was cooled to room temperature, taken

up in a 2:1 mixture of hexanes and ethyl acetate (25 mL), filtered

through Celite, and concentrated. Crystallization of the residual solid

from methanol afforded the title compound as yellow crystals (3.11 g,

1

84%): mp 148-149 °C; H NMR (300 MHz, CDCl₃) δ 7.73 (d, J )

7.1 Hz, 2H), 7.50-7.36 (m, 4H), 7.25 (d, J ) 6.0 Hz, 2H), 7.11 (d, J

) 7.1 Hz, 2H), 7.07 (d, J ) 9.1 Hz, 2H), 6.97 (d, J ) 8.8 Hz, 2H),

6.80 (d, J ) 9.1 Hz, 2H), 6.65 (d, J ) 8.8 Hz, 2H), 3.77 (s, 3H), 1.39

(s, 9H); ¹³C NMR (75 MHz, CDCl₃) δ 168.3, 157.0, 153.9, 148.4, 139.4,

138.5, 130.7, 129.4, 129.2, 128.5, 128.1, 127.8, 127.6, 126.8, 121.2,

113.7, 80.6, 55.4, 28.3; IR (neat, cm^-1) 1705, 1612. Anal. Calcd for

C₃₁H₃₀N₂O₃: C, 77.80; H, 6.32. Found: C, 77.77; H, 6.38.

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4973

N-(4-Methoxyphenyl)-N′-(4-bromophenyl)-N,N′-bis(tert-butoxy-

carbonyl)-1,4-phenylenediamine (14). A Schlenk flask was charged

with imine 13 (1.00 g, 2.09 mmol), ammonium formate (2.64 g, 41.8

mmol), and palladium hydroxide on carbon (20%, 0.291 g, 0.209 mmol

Pd). Ethanol (20 mL) was added, and the resulting mixture was heated

to 60 °C. After 30 min the reaction mixture was cooled to room

temperature, taken up in ethyl acetate (40 mL), and filtered through

Celite. The filtrate was diluted with ethyl acetate (60 mL), washed

with a 2.0 M aqueous solution of sodium hydroxide (100 mL) and

with brine (50 mL), dried over anhydrous sodium sulfate, filtered, and

concentrated. The residual white solid, 1,4-dibromobenzene (470 mg,

1.99 mmol), sodium tert-butoxide (268 mg, 2.79 mmol), Pd₂(dba)₃(18.2

mg, 0.0199 mmol, 1.0 mol %), and S-BINAP (37.2 mg, 0.0598 mmol,

3.0 mol %) were dissolved in tetrahydrofuran (10 mL) in a Schlenk

tube under argon. The reaction mixture was heated to reflux. After

24 h, the mixture was cooled to room temperature. 4-(Dimethylamino)-

pyridine (24.0 mg, 0.199 mmol, 10 mol %) and a solution of di-tert-

butyl dicarbonate in tetrahydrofuran (1.0 M, 3.0 mL, 3.0 mmol) were

added, and the resulting mixture was heated to reflux. After 3 h, the

reaction mixture was cooled to room temperature, taken up in a 2:1

mixture of hexanes and ethyl acetate (10 mL), filtered through Celite,

and concentrated. Crystallization of the residual solid from methanol

containing a small proportion of dichloromethane afforded the title

compound as white crystals (0.847 g, 75%): mp 169-170 °C; ¹H NMR

(300 MHz, CDCl₃) δ 7.40 (d, J ) 8.7 Hz, 2H), 7.17-7.06 (m, 8H),

6.84 (d, J ) 9.0 Hz, 2H), 3.80 (s, 3H), 1.44 (s, 18H); ¹³C NMR (75

MHz, CDCl₃) δ 157.7, 153.9, 153.4, 142.0, 141.0, 139.3, 135.6, 131.7,

128.5, 128.3, 127.0, 126.4, 118.9, 114.1, 81.6, 81.1, 55.4, 28.2, 28.2;

IR (neat, cm^-1) 1709. Anal. Calcd for C₂₉H₃₃BrN₂O₅: C, 61.16; H,

5.84. Found: C, 61.15; H, 5.81.

ethyl acetate (100 mL) and water (250 mL). The organic layer was

washed with brine (200 mL), dried over anhydrous sodium sulfate,

filtered, and concentrated. The residual solid was crystallized from a

mixture of chloroform and 2-propanol. The mother liquor was

concentrated, and the residue was taken up in 2-propanol. A second

crop of crystals formed and was collected by filtration. The crops were

combined and dried under vacuum to afford bis-imine 16 as yellow

crystals (10.1 g, 74%): mp 154-158 °C; ¹H NMR (300 MHz, CDCl₃)

δ 7.74 (d, J ) 7.0 Hz, 4H), 7.48-7.381 (m, 8H), 7.27-7.23 (m, 8H),

7.11 (s, 16H), 7.08 (s, 4H), 6.95 (d, J ) 8.4 Hz, 4H), 6.67 (d, J ) 8.4

Hz, 4H), 1.42 (s, 36 H), 1.38 (s, 18H); ¹³C NMR (75 MHz, CDCl₃) δ

168.5, 153.6, 153.5, 149.0, 140.5, 140.1, 140.0, 140.0, 139.4, 139.4,

137.9, 136.0, 130.7, 130.7, 129.4, 129.2, 128.6, 128.1, 127.8, 127.4,

126.9, 126.8, 126.0, 121.3, 81.3, 81.3, 81.0, 28.3; IR (neat, cm^-1) 1711.

Anal. Calcd for C₉₈H₁₀₂N₈O₁₂: C, 74.31; H, 6.49. Found: C, 74.36;

H, 6.54.

Octamer Diamine 17. A Schlenk flask was charged with bis-imine

16 (3.00 g, 1.89 mmol), ammonium formate (2.39 g, 37.9 mmol), and

20% palladium hydroxide on carbon (0.758 mmol). Tetrahydrofuran

(50 mL) and ethanol (25 mL) were added, and the resulting mixture

was heated to 70 °C, causing an effervescence which slowed after ca.

30 min. An additional portion of ammonium formate (2.39 g, 37.9

mmol) was added. Ammonium formate was added in portions every

60 min until conversion to the diamine was complete as judged by

TLC analysis. The mixture was cooled to room temperature, taken up

in ethyl acetate (40 mL), and filtered through Celite. The filtrate was

diluted with a 2:1 mixture of hexanes and ethyl acetate (40 mL). The

resulting solution was washed with a 2.0 M aqueous solution of sodium

hydroxide (40 mL) and with brine (40 mL), dried over anhydrous

sodium sulfate, filtered, and concentrated. Crystallization of the residual

solid from a mixture of hexanes and 2-propanol afforded diamine 17

as white crystals (2.03 g, 86%): mp 169-172 °C; ¹H NMR (300 MHz,

CDCl₃) δ 7.16-7.07 (m, 20H), 6.96 (d, J ) 8.4 Hz, 4H), 6.60 (d, J )

8.4 Hz, 4H), 3.65 (bs, 4H), 1.43 (s, 54 H); ¹³C NMR (75 MHz, CDCl₃)

δ 154.1, 153.6, 153.6, 144.6, 141.0, 140.3, 140.1, 140.0, 139.4, 137.8,

133.7, 128.4, 126.9, 126.9, 126.8, 126.2, 115.1, 81.2, 81.2, 80.7,

28.2, 28.1; IR (neat, cm^-1) 3460, 3369, 1702. Anal. Calcd for

C₇₂H₈₆N₈O₁₂: C, 68.88; H, 6.90. Found: C, 68.68; H, 6.84.

Tetramer Diamine 15. 1,4-Phenylenediamine dihydrochloride (4.53

g, 25.0 mmol), aryl bromide 1 (17.0 g, 50.5 mmol), sodium tert-butoxide

(10.8 g, 113 mmol), Pd(OAc)₂(56.1 mg, 0.250 mmol, 1.0 mol %),

and S-BINAP (234 mg, 0.375 mmol, 1.5 mol %) were dissolved in

toluene (200 mL) in a Schlenk flask under argon. The reaction mixture

was heated to 80 °C with stirring. After 24 h, the mixture was cooled

to room temperature. 4-(Dimethylamino)pyridine (305 mg, 2.50 mmol,

10 mol %), a solution of di-tert-butyl dicarbonate in tetrahydrofuran

(1.0 M, 87.5 mL, 87.5 mmol), and tetrahydrofuran (50 mL) were added.

The resulting mixture was heated to 80 °C with stirring. After 24 h

the hot reaction mixture was poured into hot ethanol (400 mL). Heating

was discontinued and the mixture was allowed to stand for 6 h. The

yellow powder which formed was collected by filtration. The crude

product and hydroxylamine hydrochloride (4.34 g, 62.5 mmol) were

suspended in pyridine (8.1 mL, 100 mmol), chloroform (400 mL),

tetrahydrofuran (100 mL), and ethanol (50 mL). The suspension was

stirred for 3 h and then treated with triethylamine (34.8 mL, 250 mmol).

After an additional 3 h the reaction mixture was concentrated. The

residual solid was heated in 2-propanol (600 mL), chloroform (120

mL), and water (60 mL) for 10 min, then allowed to cool to room

temperature, and to stand for 12 h. The precipitated product was

collected by filtration, washed with water followed by 2-propanol, and

dried in vacuo to afford diamine 15 as a white powder (11.1 g, 91%):

mp 208-211 °C; ¹H NMR (300 MHz, DMSO-d₆) δ 7.06 (s, 4H), 6.81

(d, J ) 8.4 Hz, 4H), 6.49 (d, J ) 8.4 Hz, 4H), 5.11 (s, 4H), 1.33 (s, 18

H); ¹³C NMR (75 MHz, DMSO-d₆) δ 153.4, 146.9, 140.2, 131.1, 128.0,

125.9, 113.8, 79.6, 27.8; IR (neat, cm^-1) 3460, 3364, 1707. Anal. Calcd

for C₂₈H₃₄N₄O₄: C, 68.55; H, 6.99. Found: C, 68.57; H, 7.05.

General Procedure for the Conversion of Octamer Diamine 17

to Octamers 18a-d. Diamine 17 (1.26 g, 1.00 mmol), aryl bromide

(2.30 mmol), sodium tert-butoxide (240 mg, 2.50 mmol), Pd₂(dba)₃

(18.7 mg, 0.0204 mmol, 2.0 mol %), and S-BINAP (38.1 mg, 0.0613

mmol, 6 mol %) were dissolved in tetrahydrofuran (10 mL) in a Schlenk

tube under argon. The reaction mixture was heated to reflux. After

48 h, the mixture was cooled to room temperature. 4-(Dimethylamino)-

pyridine (12.0 mg, 0.100 mmol, 10 mol %) and a solution of di-tert-

butyl dicarbonate in tetrahydrofuran (1.0 M, 3.5 mL, 3.5 mmol) were

added, and the resulting mixture was heated to reflux. After 3 h, the

mixture was cooled to room temperature, taken up in a 2:1 mixture of

hexanes and ethyl acetate (10 mL), and filtered through Celite. The

filtrate was concentrated and the residue was crystallized.

Phenyl-Capped Octamer 18a. Obtained as pale yellow crystals

from a 6:1 mixture of methanol and chloroform in 77% yield: mp

171-173 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.30 (t, J ) 8.7 Hz, 2H),

7.20-7.15 (m, 4H), 7.13 (s, 32H), 1.44 (s, 18H), 1.43 (s, 54H); ¹³C

NMR (75 MHz, CDCl₃) δ 153.7, 153.6, 153.6, 142.8, 140.5, 140.2,

140.0, 128.7, 127.0, 125.7, 81.3, 81.3, 81.2, 28.2; IR (neat, cm^-1) 1711;

HRMS (FAB) m/z 1606.8043 (1606.8037 calcd for C₉₄H₁₁₀N₈O₁₆, M⁺).

Anal. Calcd for C₉₄H₁₁₀N₈O₁₆: C, 70.22; H, 6.90. Found: C, 70.25;

H, 6.91.

Octamer Bis-Imine 16. Diamine 15 (4.23 g, 8.63 mmol), dimer

bromide 5 (9.56 g, 18.1 mmol), sodium tert-butoxide (2.32 g, 24.2

mmol), Pd(OAc)₂(116 mg, 0.518 mmol, 6.0 mol %), and S-BINAP

(376 mg, 0.604 mmol, 7.0 mol %) were dissolved in tetrahydrofuran

(43 mL) and triethylamine (11 mL) in a Schlenk flask under argon.

The reaction mixture was heated to 90 °C. After 48 h, the mixture

was cooled to room temperature. 4-(Dimethylamino)pyridine (105 mg,

0.863 mmol, 10 mol %), tetrahydrofuran (20 mL), and a solution of

di-tert-butyl dicarbonate in tetrahydrofuran (1.0 M, 34.5 mL, 34.5

mmol) were added. The resulting mixture was heated to 67 °C. After

24 h the mixture was cooled to room temperature. Ethyl acetate (100

mL) and a 2.0 M aqueous solution of sodium hydroxide (60 mL) were

added. The mixture was stirred for 15 min and then partitioned between

r,ω-Bis(cyano)phenyl-Capped Octamer 18b. Obtained as pale

yellow crystals from methanol in 79% yield: mp 163-166 °C; ¹H NMR

(300 MHz, CDCl₃) δ 7.55 (d, J ) 9.0 Hz, 4H), 7.31 (d, J ) 9.0 Hz,

4H), 7.21 (d, J ) 8.7 Hz, 2H), 7.16-7.13 (m, 24H), 7.08 (d, J ) 8.7

Hz, 2H), 1.44 (s, 36H), 1.43 (s, 36H); ¹³C NMR (75 MHz, CDCl₃) δ

153.6, 153.5, 151.6, 147.0, 140.2, 140.1, 140.1, 140.1, 136.9, 132.5,

128.2, 128.0, 127.3, 127.0, 126.7, 125.6, 82.6, 82.2, 81.3, 28.2, 27.9;

IR (neat, cm^-1) 1713; HRMS (FAB) m/z 1656.7952 (1656.7945 calcd

for C₉₆H₁₁₄N₈O₁₆, M⁺). Anal. Calcd for C₉₆H₁₁₄N₈O₁₆: C, 69.55; H,

6.57. Found: C, 69.24; H, 6.68.

4974 J. Am. Chem. Soc., Vol. 120, No. 20, 1998

Sadighi et al.

r,ω-Bis(tert-butyl)phenyl-Capped Octamer 18c. Obtained as pale

chloroform afforded the title compound as white crystals (3.98 g,

1

yellow crystals from a 10:1 mixture of ethanol and chloroform in 82%

70%): mp 174-176 °C; H NMR (300 MHz, CDCl₃) δ 7.41 (d, J )

1

yield: mp 172-176 °C; H NMR (300 MHz, CDCl₃) δ 7.30 (d, J )

8.6 Hz, 4H), 7.12 (s, 4H), 7.07 (d, J ) 8.6 Hz, 4H), 1.43 (s, 18H); ¹³

C

9.0 Hz, 4H), 7.12 (s, 28H), 7.10 (d, J ) 9.0 Hz, 4H), 1.44 (s, 18H),

1.43 (s, 54H), 1.29 (s, 18H); ¹³C NMR (75 MHz, CDCl₃) δ 153.8,

153.6, 153.6, 148.6, 140.6, 140.2, 140.1, 139.9, 127.0, 126.3, 125.6,

81.3, 81.3, 81.0, 24.4, 31.3, 28.2; IR (neat, cm^-1) 1713; HRMS (FAB)

m/z 1718.9275 (1718.9292 calcd for C₁₀₂H₁₂₆N₈O₁₆, M⁺). Anal. Calcd

for C₁₀₂H₁₂₆N₈O₁₆: C, 71.22; H, 7.38. Found: C, 71.02; H, 7.27.

r,ω-Bis(n-dodecyl)phenyl-Capped Octamer 18d. Obtained as pale

yellow crystals from a 10:1 mixture of ethanol and chloroform in 82%

NMR (75 MHz, CDCl₃) δ 153.3, 141.9, 140.1, 131.8, 128.4, 127.1,

119.1, 81.7, 28.2; IR (neat, cm^-1) 2976, 1711, 1510, 1488, 1322, 1159.

Anal. Calcd for C₂₈H₃₀Br₂N₂O₄: C, 54.39; H, 4.89. Found: C, 54.15;

H, 4.79.

Octamer Dibromide 21. Diamine 17 (2.03 g, 1.62 mmol), 1,4-

dibromobenzene (955 mg, 4.05 mmol), sodium tert-butoxide (404 mg,

4.20 mmol), Pd(OAc)₂(14.5 mg, 0.0648 mmol, 4.0 mol %), and BINAP

(48.4 mg, 0.0778 mmol, 5 mol %) were dissolved in toluene (15 mL)

and triethylamine (3 mL) in a Schlenk flask under argon. The reaction

mixture was heated to 90 °C. After 48 h, the mixture was cooled to

room temperature. 4-(Dimethylamino)pyridine (20.0 mg, 0.162 mmol,

10 mol %) and a solution of di-tert-butyl dicarbonate in tetrahydrofuran

(1.0 M, 5.7 mL, 5.7 mmol) were added. The resulting mixture was

heated to 67 °C. After 3 h, the mixture was cooled to room temperature

and partitioned between ethyl acetate (50 mL) and a 2.0 M aqueous

solution of sodium hydroxide (25 mL). The organic layer was washed

with brine (30 mL), dried over sodium sulfate, filtered, and concentrated.

Crystallization of the residual solid from 2-propanol afforded dibromide

1

yield: mp 172-175 °C; H NMR (300 MHz, CDCl₃) δ 7.16-7.08

(m, 36H), 2.57 (t, J ) 8.0 Hz, 4H), 1.65-1.53 (m, 4H), 1.43 (s, 72H),

1.32-1.20 (m, 36H), 0.88 (t, J ) 7.3 Hz, 6H); ¹³C NMR (75 MHz,

CDCl₃) δ 153.8, 153.7, 153.6, 140.6, 140.3, 140.2, 139.8, 128.7, 127.0,

126.9, 126.8, 81.3, 81.3, 81.0, 35.4, 31.9, 31.3, 29.6, 29.6, 29.6, 29.5,

29.3, 29.3, 28.2, 27.9, 22.6, 14.1; IR (neat, cm^-1) 1712. Anal. Calcd

for C₁₁₈H₁₅₈N₈O₁₆: C, 72.88; H, 8.19. Found: C, 72.71; H, 8.24.

r,ω-Bis(methoxy)phenyl-Capped Octamer 18e. Aryl bromide 14

(300 mg, 0.527 mmol), diamine 15 (123 mg, 0.251 mmol), sodium

tert-butoxide (60 mg, 0.627 mmol), Pd₂(dba)₃(4.6 mg, 0.00502 mmol),

and S-BINAP (9.4 mg, 0.0151 mmol) were dissolved in toluene (3

mL) in a Schlenk tube under argon. The reaction mixture was heated

to 80 °C. After 48 h, the mixture was cooled to room temperature.

4-(Dimethylamino)pyridine (3.1 mg, 0.0251 mmol, 10 mol %) and a

solution of di-tert-butyl dicarbonate in tetrahydrofuran (1.0 M, 0.88

mL, 0.88 mmol) were added, and the resulting mixture was heated to

reflux. After 3 h the reaction mixture was cooled to room temperature,

taken up in a 2:1 mixture of hexanes and ethyl acetate (6 mL), filtered

through Celite, and concentrated. Crystallization of the residual solid

from a mixture of 2-propanol and water afforded 18e as white crystals

(0.301 g, 72%): mp 173-176 °C; ¹H NMR (300 MHz, CDCl₃) δ 7.12

(s, 28H), 7.11 (d, J ) 9.0 Hz, 4H), 6.84 (d, J ) 9.0 Hz, 4H), 3.79 (s,

6H), 1.43 (s, 72H); ¹³C NMR (75 MHz, CDCl₃) δ 157.6, 153.9, 153.6,

140.8, 140.2, 139.7, 135.8, 128.4, 127.0, 126.9, 114.1, 81.3, 81.2, 81.0,

55.4, 28.2; IR (neat, cm^-1) 1711; HRMS (FAB) m/z 1666.8244

(1666.8251 calcd for C₉₆H₁₁₄N₈O₁₈, M⁺). Anal. Calcd for

C₉₆H₁₁₄N₈O₁₈: C, 69.13; H, 6.89. Found: C, 69.28; H, 7.11.

1

21 as white crystals (2.15 g, 75%): mp 145-147 °C; H NMR (300

MHz, CDCl₃) δ 7.41 (d, J ) 7.8 Hz, 4H), 7.12 (s, 28H), 7.07 (d, J )

7.8 Hz, 4H), 1.43 (s, 72 H); ¹³C NMR (75 MHz, CDCl₃) δ 153.9, 153.8,

153.6, 153.4, 145.1, 142.0, 140.6, 140.4, 140.2, 139.9, 139.7, 137.6,

133.1, 131.8, 128.4, 128.3, 127.9, 127.0, 124.1, 119.0, 81.6, 81.4, 81.2,

81.1, 28.2; IR (neat, cm^-1) 1712. Anal. Calcd for C₉₄H₁₀₈Br₂N₈O₁₆:

C, 63.94; H, 6.16. Found: C, 63.76; H, 5.93.

Phenyl-Capped Heptamer 22. Arylamine 12 (0.799 g, 1.68 mmol),

dibromide 19 (0.326 g, 0.764 mmol), sodium tert-butoxide (0.2153 g,

2.24 mmol), Pd₂(dba)₃(14.0 mg, 0.0153 mmol, 2 mol %), and S-BINAP

(22.8 mg, 0.0366 mmol, 4.8 mol %) were dissolved in toluene (6 mL)

in a Schlenk tube under argon. The reaction mixture was heated to 80

°C with stirring. After 27 h the mixture was cooled to room temperature

and taken up in dichloromethane (75 mL). The resulting mixture was

washed with brine (50 mL), dried over potassium carbonate, filtered,

and concentrated. The residue and 4-(dimethylamino)pyridine (46.4

mg, 0.38 mmol, 25 mol %) were dissolved in tetrahydrofuran (10 mL)

in a Schlenk tube under argon. A solution of di-tert-butyl dicarbonate

in tetrahydrofuran (1.0 M, 2.0 mL, 2.0 mmol) was added, and the

resulting solution was heated to 60 °C with stirring. After 6 h the

solution was cooled to room temperature and concentrated. The residual

solid was crystallized from ethanol containing a small proportion of

chloroform and recrystallized from a mixture of ethanol and toluene,

to afford heptamer 22 as white crystals (0.647 g, 60%): mp 168-170

°C with slow decomposition; ¹H NMR (500 MHz, CDCl₃) δ 7.31 (at,

4H), 7.21-7.16 (m, 4H), 7.14 (ad, 26H), 1.45 (s, 27H), 1.44 (s, 36H);

¹³C NMR (125 MHz, CDCl₃) δ 153.9, 153.9, 153.9, 143.0, 140.6,

140.4, 140.2, 128.9, 127.2, 127.2, 125.9, 81.6, 81.5, 81.4, 28.4; IR (neat,

cm^-1) 2977, 2931, 1711, 1509, 1327, 1161, 1057, 757. Anal. Calcd

for C₈₃H₉₇N₇O₁₄: C, 70.37; H, 6.90. Found: C, 70.15; H, 6.98.

r,ω-Bis(trimethylsilyl)phenyl-Capped Nonamer 23. Arylamine

7 (1.301 g, 1.76 mmol), dibromide 19 (0.3417 g, 0.800 mmol), sodium

tert-butoxide (0.2153 g, 2.24 mmol), Pd₂(dba)₃(14.7 mg, 0.016 mmol,

2 mol %), and S-BINAP (23.9 mg, 0.0384 mol, 4.8 mol %) were

dissolved in toluene (7 mL) in a Schlenk tube under argon. The reaction

mixture was heated to 80 °C with stirring. After 27 h the mixture was

cooled to room temperature and taken up in dichloromethane (75 mL).

The resulting mixture was washed with brine (50 mL), dried over

potassium carbonate, filtered, and concentrated. The residue and

4-(dimethylamino)pyridine (48.9 mg, 0.40 mmol, 25 mol %) were

dissolved in tetrahydrofuran (10 mL) in a Schlenk tube under argon.

A solution of di-tert-butyl dicarbonate in tetrahydrofuran (1.0 M, 2.0

mL, 2.0 mmol) was added. The resulting solution was heated to 60

°C with stirring. After 6 h the solution was cooled to room temperature

and concentrated. The residual solid was crystallized from ethanol

containing a small proportion of chloroform, and recrystallized from a

mixture of ethanol and toluene, to afford nonamer 23 as white crystals

(0.970 g, 62%): mp 183-185 °C with slow decomposition; ¹H NMR

(500 MHz, CDCl₃) δ 7.45 (d, J ) 8.3 Hz, 4H), 7.18 (d, J ) 8.3 Hz,

4H), 7.14 (s, 32H), 1.46 (s, 18H), 1.44 (s, 63H), 0.25 (s, 18H); ¹³C

N-(tert-Butoxycarbonyl)-4,4′-dibromodiphenylamine (19). Diphe-

nylamine (4.231 g, 25.0 mmol) was converted to 4,4′-dibromodiphe-

nylamine by the method of Berthelot et al.²⁸The crude product and

4-(dimethylamino)pyridine (0.611 g, 5.00 mmol, 20 mol %) were

dissolved in tetrahydrofuran (20 mL) in a Schlenk flask under argon.

Di-tert-butyl dicarbonate (neat, 6.30 mL, 27.5 mmol) was added via

syringe, and the resulting solution was heated to reflux. After 1 h the

mixture was cooled to room temperature and concentrated. Crystal-

lization of the residual solid from methanol afforded the title compound

as white crystals with a faint pink cast (8.65 g, 81% based on

1

diphenylamine): mp 113-115 °C; H NMR: (300 MHz, CDCl₃) δ

7.44 (d, J ) 8.7 Hz, 4H), 7.08 (d, J ) 8.7 Hz, 4H), 1.46 (s, 9H); ¹³C

NMR (75 MHz, CDCl₃) δ 153.3, 141.9, 132.1, 128.7, 119.5, 82.2, 28.4;

IR (neat, cm^-1) 2977, 1712, 1488, 1322, 1160, 1072, 1011, 824. Anal.

Calcd for C₁₇H₁₇Br₂NO₂: C, 47.80; H, 4.01. Found: C, 48.02; H, 3.87.

N,N′-Bis(4-bromophenyl)-N,N′-bis(tert-butoxycarbonyl)-1,4-phe-

nylenediamine (20). 1,4-Phenylenediamine (1.00 g, 9.25 mmol) 1,4-

dibromobenzene (4.58 g, 19.4 mmol), sodium tert-butoxide (2.31 g,

24.0 mmol), Pd₂(dba)₃(0.085 g, 0.093 mmol, 1.0 mol %), and S-BINAP

(0.173 g, 0.278 mmol, 3.0 mol %) were dissolved in tetrahydrofuran

(20 mL) in a Schlenk flask under argon. The reaction mixture was

heated to a gentle reflux. Analysis by TLC after 15 h indicated an

incomplete reaction. Additional portions of Pd₂(dba)₃(0.020 g, 0.022

mmol, 0.24 mol %) and S-BINAP (0.040 g, 0.064 mmol, 0.69 mol %)

were added. Analysis by TLC after a further 15 h at reflux indicated

a complete reaction. The mixture was cooled to room temperature.

Di-tert-butyl dicarbonate (7.07 g, 32.4 mmol) and 4-(dimethylamino)-

pyridine (0.226 g, 1.85 mmol, 20 mol %) were added. The resulting

solution was heated to a gentle reflux. After 3 h the reaction mixture

was cooled to room temperature and filtered through a plug of silica

gel and Celite, which was then washed with a 1:1 mixture of hexanes

and ethyl acetate. The filtrate was concentrated. Crystallization of

the residual solid from methanol containing a small proportion of

Palladium-Catalyzed Synthesis of Oligoanilines

J. Am. Chem. Soc., Vol. 120, No. 20, 1998 4975

NMR (125 MHz, CDCl₃) δ 153.9, 153.9, 143.4, 140.5, 140.3, 140.3,

137.7, 133.9, 127.5, 127.2, 126.1, 81.6, 81.5, 81.5, 81.4, 28.4, 28.4,

-0.9; IR (neat, cm^-1) 2976, 2932, 1713, 1509, 1327, 1161, 1057, 851,

756. Anal. Calcd for C₁₁₁H₁₃₉N₉O₁₈Si₂: C, 68.60; H, 7.21. Found:

C, 68.57; H, 7.13.

Chain-Length Confirmation for 16-mer and 24-mer by ¹H NMR.

The oligomer and hexamethylbenzene were weighed into a vial,

dissolved in CD₂Cl₂(0.75 mL), and transferred to an NMR tube. Three

samples were prepared for each oligomer. Each spectrum was recorded

with 16 scans and a relaxation delay of 20 s. Relative integration of

the resonances for the internal standard, the BOC groups, and the

trimethylsilyl endgroups yielded the ratios of repeat units to end groups;

for each oligomer, the average ratio of the three runs was taken. The

spectra are available as Supporting Information.

r,ω-Bis(trimethylsilyl)phenyl-Capped Decamer 24. Arylamine

7 (1.301 g, 1.76 mmol), dibromide 20 (0.495 g, 0.800 mmol), sodium

tert-butoxide (0.2153 g, 2.24 mmol), Pd₂(dba)₃(14.7 mg, 0.016 mmol,

2 mol %), and S-BINAP (23.9 mg, 0.0384 mol, 4.8 mol %) were

dissolved in toluene (8 mL) in a Schlenk tube under argon. The reaction