No title

Chapter 27

Downloaded by UNIV OF ARIZONA on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch027

Recent Developments in the Use of N-Heterocyclic Carbenes: Applications in Catalysis Rebecca M. Kissling, Mihai S. Viciu, Gabriela A. Grasa, Romain F. Germaneau, Tatyana Güveli, Marie-Christiane Pasareanu, Oscar Navarro-Fernandez, and Steven P. Nolan* Department of Chemistry, University of New Orleans, 2000 Lakeshore Drive, New Orleans, LA 70148

Some recent work where N-heterocyclic carbenes (NHC) are used in catalysis is described. A family of [(NHC)PdCl ] complexes has been synthesized and is a storable amination catalyst that is tolerant of both moisture and air. Various NHC add to [Pd(allyl)(Cl)] , cleaving this dimer and forming a highly active, versatile pre-catalyst for the formation of C-N and C-C bonds. N-heterocyclic carbenes are also utilized as highly efficient transesterification catalysts converting vinyl and methyl substrates to more useful or desirable esters. 2 2

2

Introduction Imidazolium salts have garnered a great deal of attention in recent years as targets for greener chemistry. Many mixed alkyl imidazolium salts are liquids at or near room temperature, and thus these polar materials can serve as solvents for many chemical processes. The highly polar nature and concomitant high 1

2

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

323

324 vapor pressure of these salts generally leads to simple product recovery by extraction or distillation. These ionic liquids have been successfully employed as "solvents" in a number of transition metal catalyzed reactions. Interestingly, when BmimHX is utilized as a solvent under basic conditions the deprotonated salt, or N-heterocyclic carbene, binds the metal and thus modulates the chemical activity of the catalyst as would a tertiary phosphine. This reaction highlights another area of great interest regarding imidazolium salts, the development of heterocyclic carbenes as phosphine mimics for use in transition metal catalysis. 1

1

3

Early Studies of [(NHC)PdCl ] Mediated Catalysis Downloaded by UNIV OF ARIZONA on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch027

2 2

In palladium-mediated coupling studies, the (NHC)Pd fragment has been generated in situ by action of a base on the imidazolium salt precursor followed by addition of a palladium source. In these studies, the ratio of ligand to metal was optimized to 1:1. These observations led us to explore synthetic avenues leading to palladium complexes bearing a single NHC ligand per palladium; the number of such complexes is still rather limited. Some of these early studies on palladium/NHC systems revealed the Pd-carbene bond to be robust and stable to heat, indicating such a system might not require excess ligand to compensate for ligand metal bond lability. 4

5

f=\ (PhCN) PdCl 2

2

+

Ι γ Ν ν ^

THF, Toluene 2h,RT

Λ>

(1)

[(IPr)Pda ]2 2

We have investigated the reaction of 1 equiv IPr with PdCl (PhCN) in THF/toluene, eq 1. The carbene readily displaces the nitrile ligands generating the tan-orange [Pd(BPr)Cl ] 1. Single crystal X-ray analysis revealed 1 to be the dimer presented in Figure 1. The geometry at the metal centers is distorted square planar with all Pd and chlorine atoms coplanar, and the aryl groups of the trans-disposed IPr ligands canted normal to each other. The reaction requires dry solvents and an inert atmosphere, although the synthetic work up can be performed in air with little or no compromise to yield or purity of product. Complex 1 is soluble in polar solvents and sparingly soluble in hexanes and benzene. It is both air- and moisture-stable; and showed no deterioration after storage on the bench for 3 months. Complex 1 was used as a catalyst precursor in aryl amination. The amination reactions were performed (under optimized conditions) in DME with potassium im-amylate (KO'Am) as base, as described in Eq l . The palladium 2

2 2

6

7

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

2

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003. cm

Figure L ORTEP diagram ofl. Selected bond lengths (λ) and angles(deg): Pd(l)-Cl(3) 2.4029(9); Pd(l)-Cl(4) 2.2715(9); Pd(l)-C(27) 1.9553(3); Cl(3)~ Pd(iyCl(4) 91.26(3); Cl(3)-Pd(l)-C(27) 178.26(9); Cl(4)-Pd(l)-C(4) 90.48(9).

C25A


326 loading, 1 mol%(0.5 mol % 1), is comparable to catalyst loadings in the best Pd/phosphine systems. 8,910

0.5 mol %

1

1.5 equiv K O Am

/λ\

1

Ar-X + 1.2 eq. HNRR'

-

-

Ar-NRR

(2)

80°C,DME

The role of the base is two-fold in this aryl amination system. NMR experiments indicate that first the alkoxide reacts with the dimer to form KC1 and a monomelic (IPr)Pd(O Am)(Cl) species transformed via a mechanism recently proposed by Hartwig. Results of the amination study using aryl and heteroaryl halides are listed in Table 1. We focused on the more difficult to activate aryl chlorides determining some overall steric and reaction condition limitations through reactions with aryl bromides. The reaction of 4-chlorotoluene with aniline proceeds to complete conversion in one hour at 50 °C (entry 1, Table 1); while we found only moderate conversion was achieved at room temperature (75% in 15 h). Activated aryl bromides and chlorides completely converted to products in just a few minutes at 80 degrees (entries 3 and 4). Electronically deactivated and sterically hindered substrates generally required longer reaction times of up to 3 h (entries 7, 8 and 9). Both primary amines and secondary anilines are capable coupling partners with aryl chlorides, giving high conversions to products in 2h at 80 °C. Chloropyridines are fully compatible with this catalyst system and lead to product in short reaction times (entry 10). Dehalogenation of ArX was only a minor side reaction in all cases (>2%) with the exception of entry 6 where thefluorobenzene/productratio was 22:78 as determined by GC analysis. Surprisingly, aryl triflates were not active amine coupling partners in the present system, nor were base sensitive substrates. Substitution of CS2CO3 or K3PO4 for KO'Am to address the latter limitation led to rapid formation of palladium black during the course of reactions. Indoles were inert to coupling in the present system; even at elevated temperatures coupling of indoles with aryl bromides did not occur. Current work is being carried out on these substrates. A remarkable aspect of aryl amination utilizing 1 is the tolerance of these reactions to both air and moisture. As evidence of the robust nature of 1, we were able to perform the amination reactions loaded on the bench top in reagent grade solvent (stored on the bench without measures to exclude air or water) under air (Table 2). There have only recently been reports of Pd-catalyzed coupling under similar conditions. In the present system many substrate pairs reacted on par with those of the air-free system (entries 2 and 3). Although some reactions involving aryl chlorides have markedly diminished activity when conducted in air (e.g. entry 9), moderate activity was found between 4chlorotoluene and morpholine using as little as 0.05 mol % of 1. Complex 1 shows good catalytic activity even under aerobic conditions. Exploration of the reactivity profile of 1 and related palladium complexes in coupling reactions as well as mechanism elucidation are ongoing. t


11

12

1314


327

Synthesis and Reactivity of (NHC)Pd(allyl)Cl Complexes Our recent success in generating Pd(II) complexes of type [(NHC)PdCl ] and the observed catalytic reactivity of these systems led us to explore the synthesis of monomelic palladium(II) complexes. Our synthetic approach makes use of a simple palladium (Π) source, [(allyl)PdCl] (2), and the optimal metal to ligand ratio previously described. Various (NHC)Pd(allyl)Cl complexes, where NHC is SIPr [iy;AT-bis(2,6-diisopropylpheny^ (3), IPr [iV^'-bisil.ô-diisopropylphenyyimidazoO-a-yUdene] (4), Mes {#ΛΓbis(2,4,6-trimethylphenyl) imidazol)-2-ylidene] (5) and fBu [N>N'-bis-tertbutyl-imidazol)-2-ylidene] (6), were synthesized in excellent yields using the reaction depicted in eq. 3. The reaction of most NHC with 2 can be performed in THF at room temperature in one hour. 2

2

2


15

The reaction of SIPr and 2 proceeds smoothly at -78°C in Et 0, affording 3 in 96% yield. Once the complexes are formed, the workup and recrystallization can be performed in air without reduction of yield. 2

// C{ >\ ((-Pd Pd-)) \ CI '/

2 NHC

//

£1

((-P4

v

THF, R. T. or ether -78° C

v>

(3) NHC

2 To confirm the structure of this family of complexes, a single crystal X-ray analysis of 3 (formed in Et 0 by slow cooling) was performed (Figure 2). The ORTEP of 3 reveals η ^coordination of the allyl fragment and distorted square planar coordination around the Pd center. With complexes 3-6 in hand, activation of this system towards catalysis was examined. In the formation of in situ catalysts from Pd(II) sources and imidazolium salts, the exact reaction pathway leading to Pd(0) species was not investigated. With these monomeric compounds, the reduction of Pd(II) to Pd(0) was examined with various bases used under catalytic conditions. The reaction of 6 with NaO Bu at room temperature led within minutes to a mixture of two different allyl species (60/40) as observed by H NMR spectroscopy. When this solution was warmed to 40°C for lh, one species was converted to the second species exclusively. The final allylic species is allyl-rm-butyl ether, confirmed by comparison with an authentic sample. In palladium-ally 1 systems, there are precedents for nucleophilic attack by an alkoxide base either at the allyl or at the palladium center. Regardless of the activation route, a NHC-Pd (0) complex is formed. The existence of such a complex was confirmed by a trapping experiment carried out in the presence of PCy (eq. 4). 2

l

l

16

17

18

19

3



3

Q

-

A

C — C I

O

H

q^)jH

F

α

4

—

o^yjH

C

N

3

^~^-NH

Amine

c(^JW

Me—^^-Cl

ArX

2

+ 1.2molNHRR'

2 M e O — C I

1

Entry

ImmolArX

3

Q

-

O

F C—^^-r^JD

m

~^Ty~\ )p

MeO—\^JP

>S

jO^^ '0'

e

Product

78(58)

80

1

10O(83)

"^

100(97)

80

50

9)

Yield*

50 100(86>

T, °C

ArNRR'

0.25 80

01

1

1

t,h

1.5 equiv KO'Amylate, DME

0.5 mol % of 1

Table I. Air-free aryl amination catalyzed by 1.



œnversion(isolated yield); average of 2 runs.


100(89)

78(71)

98(88)

95(92)

100(84)


5

4

3

2

1

Entry

-CI

-Br

Q^~^IH

NH

Me-Q -Br NH

h,

2

2

0.5 mol % of 1

jO*0

JTO jcfo

Product

1.5 equiv KO'Amylate, DME, 80 °C

NH2

Amine

-CJ °(^3

Me-Q--Br

"0

Me

ArX

1 mmol ArX + 1.2molNHRR'

0.6

0.5

0.1

1.3

0.3

Time

ArNRR'

Table 2. Aryl amination reactions catalyzed by 1 under aerobic conditions.


97(94)

99(87)

100(88)

100(81)

100(84)

Yield*


4,

Me

F

3C-^^-CI

cTAiH

M

° ~ ^ " ^ ~ \

Λ/Ύ\

H

^

Further conversion after 4 h.

FgO-^^-i^^)

e

MeC

^ J C conversion(isolated yield); average of 2 runs.

10

2

Λ - Ν Η HS

Me

^jC^f^X^l

: °~ JOCO

Λ-ΒΓ

\iH

v

9 M e O — C I

_(

Q^~\lH

M e

8 MeO—^~^-Br

7

6

ψ


4

0.5

0

0.5

4.0

15

100(95)

b

66(58)

100(89)

99(92)

100(85)

332 We postulate that the new (NHC)Pd(allyl)X species formed is the (NHQPdiallylXO'Bu) complex. The formation of this species would be the result of a simple metathesis between the (NHC)Pd(allyl)Cl and NaOR. Both alkoxide attack at the allyl position and metathetical alkoxide replacement would lead to the observed NHC-Pd species. The reductive elimination of ether from a palladium (II) complex would generate a catalytically active (NHC)Pd(O) species. Since a palladium (0) species is formed under basic conditions, the catalytic activity of the precatalyst, (NHC)Pd(allyl)Cl, in cross-coupling reactions of aryl chlorides with various substrates was examined. The palladium-mediated aryl amination reaction requires, with few exceptions, the use of a strong base (such as an alkoxide). Downloaded by UNIV OF ARIZONA on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch027

20

Figure 2. ORTEP diagram of(SIPr)Pd(allyl)Cl(3). Selected bond lengths (Â) ami angles (deg): Pd-C(l), 2.042(5); Pd-C(30) 2.098(6); Pd-C(29) 2.124(7); Pd-C(28) 2.210(6); Pd-Cl(l), 2.3757(14); C(l)-Pd(l)~C(29)> 137.4(2); C(l)Pd(l)-C(28), 169.0(2); C(30)-Pd(l)- C(28) 68.4(2); C(l)-Pd(l)-Cl(l) 92.36(14); C(29)-Pd(l)-Cl(l) 129.1(2). t

t

t

t

f


t

333 The aryl amination reactions were performed in DME with NaO^u and 3. In most cases, reactions were successfully performed at room temperature (Table 3,entries 1-4). Suzuki-Miyaura reactions have been conducted with a number of NHCpalladium systems. The most commonly used bases are inorganic carbonates, phosphates or fluorides. In our initial attempts at Suzuki-Miyaura coupling with 3, the use of Cs C0 , CsF, K P 0 or NaOAc alone led to no product conversion involving aryl chlorides and phenyl boronic acid. These bases failed to activate the catalyst. The use of NaO Bu led to a complete conversion (95% isolated) of the desired product (Table 4 Entry 1). The optimum conversions and reaction times were obtained utilizing a two base system: a catalytic amount of NaOUu was used to initiate the Pd(II) complex and Cs C0 was used as an operating base in Suzuki-Miyaura (Table 4 Entry 2). The coupling of simple ketones and aryl halides, despite its great synthetic importance, has been less explored. Strong bases are required to generate carbanions from ketones. NaO Bu is a convenient base in this system since it can activate the catalyst and deprotonate the ketone. Ketone arylation competes with condensation of two ketone molecules due to the presence of substrates with acidic protons and their conjugate bases. This side reaction can be minimized if a rapid oxidative addition of aryl chloride and a fast reductive elimination to the desired product are involved in the catalytic cycle. The size and donating properties of SIPr in 3 were found to be beneficial to this transformation. Aryl chlorides represent good coupling partners in ketone arylation (superior to bromides). In all cases examined (Table 5, Entries 1-4) byproducts were formed in less than 5 percent. 21

2

3

3

4


l

2

3

22

23

l

23

Based on the straightforward synthesis of a novel family of (NHC)Pd(allyl)Cl complexes ongoing efforts are aimed at exploring the scope of this facile activation protocol in a number of cross coupling and related reactions.

NHC- Catalyzed Transesterification The ester moiety is one of the most ubiquitous functional groups in chemistry, playing a paramount role in biology and serving both, as key intermediate or protecting group in organic transformations. As a consequence, highly efficient methods for the synthesis of different esters are potentially very useful. Base or Lewis acid-catalyzed acylation of alcohols by acetic anhydride can suffer from poor selectivity between primary and secondary 24

25

26



ArX

"Isolated yield; average of 2 runs.

Entry

Amine

1 mmol ArCI + l.2molNHRR'

Product

l

1.5equtvKO Bu, 4 ml DME

1 mol % (SIPr)Pd(Allyl)CI

Table 3. Palladium-mediated amination of aryl chlorides

T,°C Yield*

ArNRR'

t,h



ArX

+

b

Amine

2

1mmol PhB(OH)

2

3

c

l

Base is N a O B u .

Product

1.5 equiv base, 4 mL dtoxane

0-5 mo! % (SIPr)Pd(allyl)a

ïsolated yield; average of 2 runs. Base is C s C 0

Entry

ImmolArX

t,h

A r

.

T, °C

p h

Table 4. Palladium-mediated Suzuki-Miyaura reactions of aryl chlorides with phenyl boronic acid.


Yield

3


'Isolated yield; average of 2 runs.


337 alcohols or cleavage of acid-sensitive functional groups. Enol esters are convenient acylating agents since the tautomerization of the resulting enolate to the more favored aldehyde or ketone shifts the transesterification equilibrium in the desired direction. Although organometallic catalysts such as Cp* Sm(thf) and distannoxanes, or the very basic iminophosphoranes have been employed to this end, they either are limited to non acid-sensitive substrates or require high catalyst loading and long reaction times. On the other hand, readily available methyl esters require fairly harsh conditions to enable alcohol deprotection and, at the same time, do not easily undergo transesterification to higher homologues due to the reversibility of the reaction. Utilizing ΑΓ-heteroeyclic carbenes (NHC, imidazol-2-ylidenes) (Scheme 1) as nucleophilic catalysts in transesterification reactions leading to the synthesis of various esters. 27

2

28

2

29

30


31

Scheme 1. iV-Heterocyclic Carbenes used in this study.

R

2,4,6-trimethyIphenyI 2,6-di-/so-propyfphenyl fert-butyl adamantyl eyclohexyl

R*

IMes 2,4,6-trimethylphenyl SIMes IPr 2 6-di-/so-propylphenyiSIPr IBu lAd ICy f

NHC represent a class of ligands with a considerable stabilizing effect in organometallic systems as compared to the widely utilized tertiary phosphines. Our experience with NHC as ligands for transition metal catalyzed processes and the fact that transesterification of enol esters can be affected by basic tertiary phosphines such as PBu or iminophosphoranes, led us to attempt transesterification utilizing NHC. Indeed, as little as 0.5 mol% of the NHC M e s catalyzes the reaction of benzyl alcohol with vinyl acetate in THF, with almost quantitative conversion to benzyl acetate in 5 minutes at room temperature (Scheme 2). Based on this promising result, the acylation of a commercially available and more challenging substrate, methyl acetate, was tested with different nucleophiles. Two main factors were identified for biasing the reaction in the desired direction. First, 4Â molecular sieves are required to absorb the liberated methanol, leading to quantitative conversion of benzyl alcohol to benzyl acetate in 1 hr with 2.5 mol% of ICy (Table 6, entries 5 and 6). Second, the nature of the nucleophile also influences the efficacy of transesterification. 32

3


338 Scheme 2. IMes-catalyzed acylation of benzyl alcohol with vinyl acetate


97%

Under similar conditions (2.5 mol% catalyst, 1 ml methyl acetate, 1 hr, molecular sieves) IMes afforded the product in 93% conversion, while IPr led to only a moderate conversion (presumably due to the steric congestion provided by the carbene trending isopropyl groups) (Table 6, entries 1 and 2). The corresponding aryl-substituted imidazolin-2-ylidenes, SIMes and SIPr, afforded the products in only low conversion (entries 3 and 4) while the alkylsubstituted ICy, PBu and IAd performed much better in the model reaction affording the product quantitatively (Table 6, entries 5, 7 and 8) presumably owing to their higher nucleophilicity. Strongly basic species such as DMAP, DABCO and D B U are not effective catalysts for the transesterification of methyl acetate with benzyl alcohol. As expected, the strong inorganic bases NaH and KO*Bu led to high conversions (95%). However, the use of these bases may be problematic for more sensitive substrates. Having established that NHC are excellent catalysts for transesterification reaction of vinyl acetate/methyl acetate with benzyl alcohol, we investigated various substrates including nitro-containing ones to explore functional group compatibility. In all cases studies thus far excellent conversions are observed. Efficient selectivity of primary over secondary alcohols is important for acylation catalysts to be useful in organic synthesis. Selective protection of primary alcohols has been achieved using organometallic systems such as distannoxane/enol ester or Sc(OTf) /Ac 0. Transesterification selectivity under our standard reaction conditions with respect to primary alcohols was confirmed by the substantially lower activity of 2-butanol to acylation with vinyl acetate. Moreover, benzyl alcohol is almost exclusively acylated by vinyl acetate in the presence of 2-butanol (Scheme 3). Further explorations into the uses of this NHC catalyst family in organic transformations as well as mechanistic investigations focusing of the mode of action of the NHC catalyst in transesterification and related reactions are ongoing. 33

34

35

36

37

38

3

2


339 Table 6. Transesterification of methyl acetate with benzyl alcohol catalyzed by various nucleophiles

1


"'OMe

*

Η

Ο

" Ί η

C a t e l y 8 t ( 2

-

5 m

°

1 % )

1

4k M. S., r. t, 1 hr

i

^

S^CT

• CH,OH Ph

Entry catalyst yield(%y 1 Mes 93 2 IPr 45 3 SMes 21 4 SIPr 21 5 ICy 100 6 ICy 84 7 IAd 100 8 IBu 100 Reaction conditions: 1 mmol benzyl alcohol, 1 mL methyl acetate, 2.5 mol% catalyst, 0.5 g 4 A molecular sieves, room temperature, 1 hr. No molecular sieves were used. GC yield, average of two runs. b

a

C

Scheme 3. Mes-catalyzed selective acylation of benzyl alcohol

20

:

1

Acknowledgements We gratefully acknowledge partial support of the work described in this contribution by the National Science Foundation, the Petroleum Research Fund administered by the ACS and the Louisiana Board or Regents.

References and Notes (1) (a)Welton. T. Chem. Rev. 1999, 99, 2071-2083. (b) Carlin, R.T.; Wilkes, J.S., In Advances in Nonaqueous Chemistry; Mamantov, G.; Popov, A . Eds.; VCH Publishing: New York, 1994. (2) (a)Olivier-Bourbigou, H. In Aqueous-Phase Organometallic Catalysis: Concepts and Applications; Cornils, B.; Herrmann, W.A., Eds. WileyVCH:Weinheim, 1998. (3) For example see: Matthews, C.J.; Smith. P.J.: Welton, T.; White, A.J.P.; Williams, D. J. Organometallics, 2001, 20,3848-3850.


340 (4) (a) Zhang,C.;Huang, J.; Trudell, M . T.; Nolan, S. P. J. Org. Chem. 1999, 64, 3804-3805. (b)McGuiness, D.S.; Cavell. K.J.; Skelton, B.W.; White, A.H.; Organometallics, 1999,18,1596-1605. (c) Stauffer, S. R.; Lee, S.; Stambuli, J. P.; Hauck, S. I.; Hartwig, J. F. Org.Lett,2000, 2, 1423-1426. (5) (a)Jackstell, R.; Andreu, G.A.; Frisch, Α.; Selvakumar, K.; Zapf, Α.; Klein, H.; Spannenberg, Α.; Rottger, D.; Briel, O.; Karch, R.; Beller, M . Angew. Chem., Int.Ed.,2002, 41, 986-989. (b) Viciu, M . S.; Kissling, R. M.; Stevens, E.D.;Nolan, S. P. Org. Lett., 2002, 4, in press. (6) Other alkoxide bases function well on the test reaction: KO Bu leads to the desired product in 78% yield in 1.5 hr, as does NaO Bu (75% in 1.5 hr). The increased solubility of the amylate base in DME may be responsible for the increased reactivity. (7) For these reactions, a stock solution of 1 and KO Am in DME were made and used over the course of several days experiments without apparent loss in activity. (8) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press: New York, 1985 (9) See for example: Harris, M . C.; Buchwald, S. L. J. Org. Chem. 2000, 65, 5327—5333. (10) Hartwig, J. F.; Kawatsura, M.; Hauk, S. I.; Shaughnessy, Κ. H.; AlcazarRoman, L. M . J. Org. Chem. 1999, 64, 5575—5580 (11) Alcazar-Roman, L. M.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 12905—12906. (12) Aerobic conditions: 50 mL vials were charged with 1 and KO Am in the glove box. On the bench DME, ArX and amine (used as received) were added to the open vials. The vials were capped to prevent evaporation and suspended in an 8 0 ¡C oil bath for the duration of the reaction. (13) (a) L i , G. Y.; Zheng, G.; Noonan, A.F. J. Org. Chem.2001,66,8677—8681. (b) Kwong, F. Y.; Klapars, Α.; Buchwald, S. L Org. Lett. 2002, 4, 581—584. (14) Hartwig (reference 4c) was able to load catalyst and excess base in air, evacuate the reaction vessel, and run amination under N with no deleterious effects to substrate conversion to products. (15) Galardon, E.; Ramdeehul,S.;Brown, J.M.: Cowley, Α.; Hii, K.K.; Jutand, A. Angew. Chem. Int. Ed., 2002, 41 1760-1763.. (16) Fellous, R.; Rabine, J.P.; Lizzam-Cuvelier, L.; Luft, R. Bull. Soc. Chim. Fr., 1974,11,2482-2484. (17) Tsuji,J.Transition Metal Reagents and Catalysts, John Wiley & Sons, 2000, pp. 109-168 and references therein. (18) Stanton, S.A.; Felman, S.W.; Parkurst, C.S.; Godleski S.A. J .Am. Chem. Soc. 1983,105,1964-1969. (19) The (IBu)Pd(PCy ) complex was identified by P NMR spectroscopy and compared to the previously isolated complex; Titcomb, L.R.; Caddick, S.; Cloke, F.G.N.; Wilson, D.J.; McKerrecher, D. Chem. Comm. 2001, 13881389. (20) (a) Wolfe, J.P.; Wagaw, S.; Buchwald S.L. J. Am. Chem. Soc., 1996, 118, 7215-7216. (b)Driver,M.S.;Hartwig, J.F. J.Am. Chem.Soc.,1996, 118, 7217-7218. (21) See for examples ref 4 a and 4b and B hm, V. P. W.; Gst ttmayr, C. W. K.; Weskamp, T.; Herrmann, W. A. J. Organomet. Chem.,2000, 595, 186-190. (22) (a) Kosugi.M.;Hagiwara,I.;Sumiya, T.; Migita, T. Bull. Chem.Soc.Jpn., 1984, 242-246. (b) Morgan, J.; Pinhey, J.T.; Rowe, B.A. J. Chem. Soc., Perkin Trans.1,1997, 1005-1008. (c) Hamann,B.C.;Hartwig. J. F.; J. Am. Chem.Soc.1997, 119. 12382-12383. (d) Palucki,Μ.;Buchwald, S.L. J.Am. Chem. Soc., 1997, 119, 11108-11109. (e) Satoh, T.; Kawamura. Y.; Miura, M., Nomura, M . Angew. Chem. Int.Ed.,1997,36, 1740-1742. (f) Culkin, D. Α.; Hartwig, J. F.J.Am.Chem. Soc., 2001, 124, 5816-5817. t

t


t

t

2

31

3



341 (23) March J. Advanced Organic Chemistry, Fourth Edition, John Wiley & Sons, 1992, pp. 468. (24) Otera, J. Chem. Rev. 1993, 93, 1449-1470 and references therein. (25) (a) Steglich, W.; Hofle, G. Angew. Chem., Int. Ed. Engl. 1969, 8, 981-983 (b) Shimizu, T.; Kobayashi, R.; Ohmori, H.; Nakata, T. Synlett. 1995, 650652. (c) D Sa, Β.; Verkade, J. G. J. Org Chem. 1996, 61, 2963-2966. (d) Vedejs, E.; Diver, S. T. J. Am Chem Soc. 1993, 115, 3358-3359. (26) (a) Ishihara, K.; Kubota, M.; Kurihara, H.; Yamamoto, H. J. Am. Chem. Soc. 1995, 117, 4413-4414. (b) Iqbal, J.; Srivastava, R. R. J. Org. Chem. 1992, 57, 2001-2007. (c) Miyashita, M.; Shiina, I.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1993, 66, 1516-1527. (d) Orita, Α.; Tanahashi, C.; Kakuda, Α.; Otera, J. J. Org. Chem. 2001, 66, 8926-8934. (27) Ishii, Y.; Takeno, M.; Kawasaki, Y.; Muromachi, Α.; Nishiyama, Y.; Sakagughi, S. J. Org. Chem. 1996, 61, 3088-3092. (28) Orita, Α.; Mitsutome, Α.; Otera. J. J. Org. Chem. 1998, 63, 2420-2421. (29) Ilankumaran,P.;Verkade, J. G. J. Org. Chem, 1999, 64, 9063-9066. (30) Ranu, B. C.; Dutta, P.; Sarkar, A. J. Org. Chem. 1998, 63, 6027-6028. (31) Recent related reports make use of NHC in polymerization of cyclic esters: Connor, E.F.; Nyce, G.W.; Myers, M.; Mock, Α.; Hedrick, J.L. J. Am. Chem. Soc. 2002, 124, 914-915. and in mediating the asymmetric benzoin condensation: Endeers, D.; Kalfass, U. Angew. Chem.Int. Ed. 2002, 41, 1743-1745. (32) (a) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am. Chem. Soc. 1999, 121, 2674-2678. (b) Huang, J.; Schanz, H.-J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370-2375. (33) Arduengo III, A. J.; Calabrese, J. C.; Davidson, F.; Rasika Dias, Η. V.; Goerlich, J. R.; Krafczyk, R.; Marshall, W. J.; Tamm, M.; Schmutzler, R. Helv. Chim. Acta 1999, 82, 2348-2364. (34) Kim, Y.-J.; Streitwieser,A. J. Am. Chem. Soc. 2002, 124, 5757-5761. (35) Aggrawal, V. K.; Dean, D. K.; Mereu, Α.; Williams, R. J. Org. Chem. 2002, 67, 510-514. (36) Aggrawal, V. K.; Mereu, A. Chem. Commum. 1999, 2311-2312. (37) Stanton, M . G.; Gagn, M . R. J. Org. Chem. 1997, 62, 8240-8242. (38) Procopiou, P. Α.; Baugh, S. P. D.; Flack, S. S.; Inglis, G. G. A. J. Org. Chem. 1998, 63, 2342-2347.


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