Palladium mediated synthesis of N-heterocycles by iminoannulation of allenes.

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UvA-DARE (Digital Academic Repository)

Diederen, J.J.H.

Publication date

2001

Link to publication

Citation for published version (APA):

Diederen, J. J. H. (2001). Palladium mediated synthesis of N-heterocycles by iminoannulation

of allenes.

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Chapter 1 Palladium mediated Synthesis of Carbo- and Heterocycles >

1.1 Synthesis of heterocycles by palladation of alkenes

Palladium has proven to be an excellent auxiliary element in the synthesis of a wide variety of heterocycles and carbocycles.[l] The tolerance of palladium reagents to many functional groups such as carbonyl and hydroxyl groups has greatly contributed to the utility of this transition metal. Pd-catalyzed reactions can be carried out without protection of these functional groups. Divalent palladium complexes are able to coordinate to alkenes with formation of jt-complexes. As a result of the electrophilic character of the palladium center, the electron density of the coordinated alkene is decreased and it may react with various nucleophiles.[l] The attack of nucleophiles with formation of a Pd-C bond is called the palladation reaction (Scheme 1). These palladium-alkene complexes may react with a variety of nucleophiles to give either nucleophilic substitution or nucleophilic addition (Scheme 1), depending on the reactants and conditions. Typical nucleophiles are water, alcohols, carboxylic acids, ammonia, amines, enamines and active methylene compounds.[2] The intramolecular version of this reaction is a useful way to synthesize heterocyclic compounds.

PdCI ₂ XH _{. / YH : Nurleonhilir addition . /}R R CIPd \ r / \ palladation - H C l CIPd X -HCl;-Pd(0) Y X - Pd(0) -HCl 1

Nucleophilic substitution via ß-hydrogen éliminât _ /R XH, YH = nucleophiles, H20 ,

^ ROH, RC02H, RNH2, CH2E2

A

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Palladium mediated Synthesis ofCarbo- and Heterocycles

The oxidation of ethylene by PdCl2 in water to give acetaldehyde has been known for

100 years, and is known as the Wacker Process.[3, 4] This reaction is a nice example of oxypalladation of ethylene, followed by ß-elimination to give a vinyl alcohol A, which stays bound to palladium. Rearrangement of the complex leads to B, which forms the aldehyde by ß-elimination (Scheme 2).

OH OH O—H O

PdC,2 - H C l C I P d ^ H ld H C | PdC, - M O )

A B Scheme 2

Oxidation of ethylene in alcohol with PdCl, in the presence of base gives an acetal and vinyl ether (eq 1).[5, 6]

R 2R'OH R R

>r

+

J=

(1)

T -

2HC1

-Pd(0) Ü H R'O ^R. R'O PdCI2 major

This method was applied to the synthesis of brevicomin in an intramolecular reaction (eq2).[7-9]

,0 cat. PdCl2, CuCl2

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OH DME, 45% V O

Besides alkyl alcohols, phenolic oxygen reacts similarly in an intramolecular oxypalladation. 2-Allylphenol reacts to give the exo cyclized benzofuran with Pd(OAc)2 (eq 3),

but in the case of PdCl2, the alcohol cyclizes in an endo fashion to give the benzopyran (eq 4).[10]

Pd(OAc),

(4)

Clmpter 1

PdCl,

(4)

^

O'

Unsaturated carboxylic acids afford lactones in a ring closure reaction, as shown by Larock et al. [11] The reaction is carried out in the presence of a stoichiometric amount of PdCl2(MeCN)2 to produce the isocoumarin (eq 5). If, however, the reaction is done with a

catalytic amount of Pd(OAc)2 in the presence of molecular oxygen and DMSO, a system that

was used by Hiemstra and coworkers,[12, 13] the Z-phthalide is obtained. Apparently, in this case a Tt-allylpalladium complex is involved (eq 6).[11]

CO2H 1 equiv. PdCl2(MeCN)2 *-Na2CQ3 (5) ^ ^ C 0 2 H 10% Pd(OAc)2 NaOAc 02, DMSO (6)

Nitrogen nucleophiles are also able to react similarly with alkenes. An intermolecular reaction of aliphatic amines and alkenes in the presence of palladium salts is a difficult process because of the strong coordinating capacities of aliphatic amines towards divalent palladium. In a few exceptional cases, alkenes react in a so-called aminopalladation reaction. Treatment of 1-decene with methylamine in the presence of a stoichiometric amount of PdCl2, produces

aziridines. Initial complexation of the palladium to the double bond of 1-decene and subsequent nucleophilic attack of methylamine leads to a o-Pd complex. The secondary amine reacts subsequently intramolecularly to eliminate Pd(0) and the hydrochloride salt of methylamine (eq 7).[14] C«H 17 + MeNH2 PdClp C8H17 PdCI NHMe CsH-17 HCl Pd(0) ^ 7 N I Me (7)

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Unlike intermolecular reactions, intramolecular aminopalladation reactions proceed with greater ease. As an example, allylaniline cyclizes in an exo fashion to produce 2-methylindole which shows again that 5-membered ring formation is easier than 6-membered ring formation (eq 8).[15]

^ Y ~ ^ PdCl

2

(MeCN)

2

^ r ^ V A _

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^ ^ NH2 p-benzoquinone M

As was seen in the latter example of aminopalladation, nucleophilic attack takes place at the electronically favored position. Endo cyclization of 2-(3,3-dimethylallyl)aniline leads to dimethyl-l,2-dihydroquinoline, which shows again that regioselectivity is governed by electronic factors (eq 9).[16]

PdCl2(MeCN)2

p-benzoquinone ^ - ^ N'

H

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1.2 7t-Allylpalladium chemistry

In many cases, n-allylpalladium chemistry is involved in the formation of heterocycles and carbocycles, as is demonstrated by the great amount of publications.[1] 7t-AUylpalladium intermediates can be generated from alkenes, 1,2-dienes (aliènes) and 1,3-dienes starting from either Pd(II) complexes or Pd(0) complexes. 7t-Allylpalladium complex formation from alkenes takes place by the displacement of an allylic hydrogen of the alkene with Pd(II). The complex may then further react with a nucleophile (carbanion, heteroatom, organotin) with formation of Pd(0) (Scheme 3).

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Chapter 1 PdCl2 R fc--HCl PdCI/2 NuH

»•

-HCl - Pd(0) PdCl/ : Nu NuH -HCl

f^] - Pd(0)

R PdCI/2 Nu Scheme 3

Reaction of a 1,2-diene (aliène) with PdCl2 may produce thermodynamic product B,

formed by insertion of aliène into the Pd-Cl bond (Scheme 4).[17] The kinetic product A, formed by halide attack at the terminal carbon of the coordinated aliène, may be trapped by insertion of excess aliène into the palladium-vinyl bond to form a dimeric complex which reacts with chloride to give a dimeric structure. If B reacts with chloride, a monomeric structure is found.

1 CI" *- % / ^ n i = * = ,

r

ci

CI' - ^

Y^n

PdCl2 * PdCl/ 2 A kinetic adduct

r

ci

^

KxCi

* PdCl/ 2 A kinetic adduct PdCl/ 2 CI PdCl/ 2

or

- Pd(0) CI B th( ad ?rmodynamic duct Scheme 4

Reaction of 1,3-dienes with Pd(II)-complexes in the presence of a nucleophile leads to the introduction of the nucleophile at the terminal carbon atom of the conjugated diene (eq 10). When butadiene is treated with PdCl2, the 1-chloromethyl-it-allylpalladium complex is formed

by chloropalladation. Nucleophiles react at the unsubstituted end of the rc-allyl-palladium complex to produce allylic esters, halides or ethers.

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Palladium mediated Synthesis ofCarbo- and Heterocycle,

^

PdCl2 /

X-PdCl/ PdCl/ ; • Pd(0) Nu = Cl, OR, OAc

'X (10)

1,4-Difunctionalization with nucleophiles has wide synthetic applications. [18-20] The oxidative diacetoxylation of butadiene with Pd(OAc)2 affords l,4-diacetoxy-2-butene (A in eq

11) and l,2-diacetoxy-3-butene (B in eq 11). The latter may be isomerized to the former. An industrial process has been developed based on this reaction. 1,4-Butanediol and THF are produced commercially from l,4-diacetoxy-2-butene.

< ^ ^ AcOH _{•*• AcO}^ ^ ^ O A c +

Preparation of 7t-allylpalladium complexes by the oxidative addition of allylic compounds (esters, carbonates etc), and their reactions with nucleophiles are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with the allylic compounds (Scheme 5). The stereochemistry of the Pd-catalyzed allylation of nucleophiles has been studied extensively. [21, 22] In the first step, the n-allylpalladium complex is formed by

anti-attack of the palladium on the allylic carbon atom to give inversion of configuration. Then

subsequent reaction of soft carbanions, N- and O-nucleophiles, proceeds by inversion to give overall retention of configuration (Scheme 5). Reactions of hard carbanions of organometallic compounds (RMgX, RSnR'.,, RZnX, RB(OH)2) proceed via transmetallation. In this case overall

inversion is observed (Scheme 5).

Ri-OAc + Pd(0) inversion (OAc)/ 2 RM R1v -* Pd(OAc)/ 2 soft NuH -HOAc retention Pd(OAc)/ 2 -HOAc inversion Scheme 5 Nu

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Chapter 1

1.3 S y n t h e s i s of carbo- and h e t e r o c y c l e s via 7t-allylpalladium c h e m i s t r y i n v o l v i n g a l i è n e s

The 1,2-diene or aliène moiety is an interesting functionality because of the specific stereochemical features of its two orthogonal cumulated double bonds and the associated reactivity.[23] N u m e r o u s preparative methods for allenes, functionaliz»d or not, are available.[24] Because it is well known that organopalladium compounds will add to aliènes to produce 7t-allylpalladium complexes[25-27] (Chapter 1.2), many publications have appeared about the application of this very useful reaction. One of the pioneering groups who used aliènes as synthetic building blocks was that of Cazes and coworkers. Their papers are about the carbopalladation of aliènes leading to functionalized 1,3-dienic (or styryl) compounds in a three component reaction (Scheme 6).[28] Oxidative addition of a vinyl (or aryl) halide to a Pd(0) complex first generates a vinyl (or aryl) palladium complex I. In the second step, the aliène inserts into the newly formed a-Pd-C bond to generate a Pd-ir-allyl complex II. Finally, attack of the carbon nucleophile leads to the formation of functionalized dienic compounds III a n d / o r IV.

Pd(0)Ln

t

Pd(dba)2 + dppe Scheme 6

Tsuji's group described a related synthesis of dienic amines, based on the same methodology with pyrolidine as the nucleophile (eq 12).[29] The latter reaction was further extended with other amines and alkyl halides in our group. The major drawback in this type of reactions seems that nucleophilic attack of most amines on alkyl halides is a faster process than oxidative addition of the alkyl halide to Pd(0) resulting in alkylated amines. [30]

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O

NH

( ' \ ^

^ V

H'—\ 5% Pd(OAc)2 5% dppe MeCN, 100 °C, 3h

O

Grigg and coworkers showed that intramolecular carbocyclization of aliènes leads to the formation of carbo- and heterocycles and an excellent review has appeared about this methodology (Scheme 7,8 and 9).[31] Oxidative addition of the aryl halide to a Pd(0) complex, formed after in situ reduction of Pd(OAc)2 by PPh3, forms a Pd-aryl species (Scheme 7).

Cyclization takes place at the central carbon of the aliène to form a Pd-rc-allyl complex which reacts further with secondary amines, under basic conditions.[31] If K2C03 is used as a base

(condition 1), nucleophilic attack takes place at the sterically favored position, whereas in the case of Ag2CG*3 as base (condition 2), the amine reacts at the most substituted position of the

allyl terminus (kinetic product). The regioselectivity with Ag2C03 as the base is ascribed to a

cationic Pd(II) intermediate which reacts fast with the nucleophile to give the electronically favored kinetic product. The xc-allyl species generated by cyclization of aryl halides onto proximal aliènes can react with a wide range of nucleophiles (azides, sulphinates) to give functionalized heterocyclic products.

PdILn HN. J^ ^ O condition 1 NMe .... . „ „ _ or condition 1: K2C03 condition 2: Ag2CÖ3 Scheme 7

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Chapter 1

Allenes were used to create allylic amines (Scheme 8). Heck type cyclization of the Pd-aryl species creates a o-Pd-C bond which inserts allene to provide a Pd-rc-allyl species which is trapped by secondary amines. In this way it is possible to create 5- and 6-membered ring structures.

h

Pd(OAc)2 -Y pph3 PdLn = . = PdLn NR,R2 Scheme 8 HNRjRz

Grigg and coworkers found in certain reactions that, if aliènes are used and no nucleophile is present to trap the Pd-n-allyl species formed, the reaction ends with a ß-hydrogen elimination to produce a diene. They applied the ß-hydrogen elimination in a 3-component cascade reaction, terminating in a Diels-Alder reaction using N-methylmaleimide as dienophile (Scheme 9). R Pd(OAc)2 / PPh3 0 ;

J

\

[f NMe

0 , î ~ » ^ r - ' ' \"i _R' _("''_'\l _R'

"•V -

J^~

i ^ R

slow ß-elimination _^-

**x

_{. A ^ ,R}

1\

H

- H P d l - Ï / \

fS,

PdlLn Scheme 9

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Gallagher and coworkers described cyclization reactions on y-allenic amines leading to pyrolidines (eq 13). [32]

R2

J

U3)

NHR' Pd(PPh3)4 ^ N ;

K2C 03 R R

An elegant procedure to cyclize y-hydroxyallenes in the presence of aryl halide, a catalytic amount of Pd(PPh3)4 and base with incorporation of CO was published by Walkup et al. (eq 14).[33] Cyclizations without in situ carbonylation were described in an earlier paper by

the same group.[34]

10%(PPh3)4

V_x _ ^ £ ^

„

\—V CO (1 bar)

5equiv DMF, 55-60 °C

12-18 hr cis-trans (+/-25:75)

Allene-substituted lactams were cyclized to bicyclic enamides with unprecedented nucleophilic attack of the amide nitrogen atom on the central carbon of the aliène unit (eq 15).[35] 4 Phi 10%Pd(PPh3)4 r K2C03, TBAC1 MeCN NH

Liebeskind and coworkers published a method to synthesize various A'-carbapenems by a cyclization of 4-allenylazetidones in the presence of different activated olefins in excess, mediated by a stoichiometric amount of palladium chloride (Scheme 10).[36] It is presumed that these transformations proceed via palladium(II) induced N-C bond formation to form a palladium-vinyl species which reacts with the olefin in a Heck-type reaction. After ß-hydrogen elimination the A'-carbapenem is produced.

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Cliapter 1 TBDMSO | VÇ' TBDMSO Et3N - Et3N.HCl CH2C12

W^r

E

-

Et3N TBDMSO , . I H H /

o

E = COOEt, COCH3, CHO, CN TBDMSO PdCI ^ — N ^ -Et3N.HCl O - Pd(0) Scheme 10

Many intermolecular reactions of allenes and heteroatom or carbon nucleophiles were described by Larock and coworkers (eq 16 and 17).[37, 38] It was found that the regioselectivity was dependent on the ring size. Formation of 5-membered rings mainly involved cyclization at the more substituted end of the allene (eq 16) whereas 6-membered rings were formed from intramolecular cyclization of the nucleophiles at the less substituted end of the allene (eq 17). This chemistry was later extended to larger ring N-heterocycles (7- and 8-membered).[39] An enantioselective version of this reaction type was performed by choosing bisoxazoline ligands, developed by Pfaltz and others[40-42], to produce enantioselectively different hetero- and carbocycles with ee's up to 82%. [43]

XH 1 (or Br) R N a2C 03 TBAC1 or LiCl cat. Pd(0) DMF, A (16)

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Palladium mediated Synthesis ofCarbo- and Heterocycles XH I (or Br)

==ƒ

N a2C 03 TBAC1 or LiCl cat. Pd(0) DMF, A X = O, NR, COO, C(C02Et) (17)

Other groups used the same methodology for the synthesis of heterocyclic products. Mérour and coworkers published a palladium catalyzed heteroannulation with aliènes and 3-iodo-2-aminopyridines yielding 3-methylene-pyrrolo[2,3-b]pyridine derivatives (eq 18).[44]

,R2 ~ ^ T v ' -NHR N a2C° 3 A N H H 1 5 mol% Pd(OAc)2 5 mol% PPh3 DMF X = N, CH VX N Ri (18)

The group of Alper showed that Larock's reactions of o-iodophenols could be extended with an extra carbonylation step producing l-benzopyran-4-ones (eq 19).[45]

^ _ / ,

2

0 H Na2C03or(i-Pr)2NEt

5% Pd(OAc)2 ;5% dppb

benzene

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Benzoquinolizinium derivatives were synthesized in the group of Pfeffer by reacting cyclopalladated pyridines with 1,1-dimethylallene (eq 20).[46]

Î ^ <

^ N PdCI/2 CH2C12 ; A - Pd°

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Clmpter 1

Bäckvall's group reported on the 1,2-oxidation of aliènes by LiBr in the presence of a catalytic system based on Pd(OAc)2 and p-benzoquinone in HOAc leading to 1,2-dibromides

(Scheme 11).[17] In the mechanism, the aliène first coordinates to the electrophilic palladium center. Then nucleophilic attack of the bromide on the central carbon of the aliène produces a n-allylpalladium complex with p-benzoquinone as a stabilizing ligand. Nucleophilic attack of a second bromide on the n-allylpalladium complex produces the dibromide together with reduced palladium complex. The palladium is subsequently reoxidized by p-benzoquinone in the presence of HOAc to the catalytically active Pd(II)-species.

Pd(ll)

Scheme 11

Extension of the latter reaction to an intramolecular version was also reported by the group of Bäckvall. y-Allenic acids cyclized under similar reaction conditions to y-lactones with mainly Z stereochemistry (eq 21).[47, 48]

n OH cat. Pd(OAc)2 LiBr, LiOAc p-benzoquinone HOAc, 40 °C (21)

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Recently, azetidines and tetrahydropyridines were synthesized via a palladium catalyzed cyclization of enantiopure allenes (eq 22). [49]

^ ^ .*R R!X Pd(PPh3)4 R1 -|R or H f \ k '/ N -N- vR K2CO3 I I M e C N P (22)

1.4 Purpose of the investigation

Many reactions of amines, alcohols or carboxylic acids as nucleophiles with K-allylpalladium complexes or activated alkenes are known. The main goal of the investigation described in this thesis is to probe, whether imines can be used as nucleophiles in an intramolecular annulation reaction to produce N-heterocycles, which may be further functionalized.

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Chapter 1

1.5 Outline of this thesis

In Chapter 2 the synthesis of several iminium salts from cyclopalladated a-tetralone ketimines and 1,2-dimethylallene or vinylidene cyclohexane will be described. Iminium salts 4, synthesized from cyclopalladated a-tetralone ketimines and 1,2-dimethylallene with high regioselectivities, reacted with KOH to enamines 8, or with NaBH4 to amines 9 (Scheme 12).

F

PF6 -,+

9?

Scheme 12

In Chapter 3 , a new iminoannulation catalyzed by palladium will be described. Different aryl- or vinylimines were reacted with mono-substituted aliènes in the presence of a palladium catalyst and base to produce various pyridines and isoquinolines (eq 23). In the case of N -benzyl substituted imines, the N-heterocyclic products were deprotected by hydrogenolysis of the benzyl group. Tert-butyl groups could be removed under basic conditions and high temperatures via a ß-elimination step. However, the latter reaction was found to be

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difficult to perform, therefore we introduced a propane nitrile group on the imine, which enormously facilitated the deprotection step.

.R Pd cat. X = Br, I R = Bn, i-Bu, CH2CH2CN R' = n-Bu, f-Bu, Ph, Cy (23)

In Chapter 4 reactions of o-allyl-substituted arylimines with palladium salts will be described. Extension of this reaction towards vinyl imines was impossible due to our inability to introduce an allyl moiety at the vinylic position. Furthermore, intramolecular annulation reactions of o-allenylmethylarylimines or o-allenylmethylbenzylalcohol in the presence of an electrophile (RX) catalyzed by palladium will be described leading to tetrahydroisoquinolinium salts or tetrahydropyrans. The iminium salts reacted with a variety of Grignard reagents and NaBH4 to produce different tetrahydroisoquinolines (Scheme 13).

RX Pd cat. M+ X" Rj = H, Me RX Pd cat. base Scheme 13 Parts of this work have been published.[50-51]

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Chapter 1

1.6 R e f e r e n c e s

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[2] J. Tsuji, Ace. Chem. Res. 2 (1969) 144.

[3] W. Hafner, R. Jira, J. Sedlmeier, J. Smidt, Chem. Ber. 95 (1962) 1575. [4] R. Jira, J. Sedlmeier, J. Smidt, Liebigs Ann. Chem. 693 (1966) 99.

[5] 1.1. Moiseev, M. N. Vargaftik, Y. K. Syrtik, Dokl. Akad. Nauk SSSR 133 (1960) 377. [6] E. W. Stern, M. L. Spector, Proc. Chem. Soc. (1961) 370.

[7] N. T. Byrom, R. Grigg, B. Kongkathip, Chem. Commun. (1976) 216.

[8] N. T. Byrom, R. Grigg, B. Kongkathip, J. Chem. Soc. Perkin Trans. (1984) 1643. [9] K. Mori, Y. B. Seu, Tetrahedron 41 (1985) 3429.

[10] T. Hosokawa, S. Miyagi, S. Murahashi, A. Sonoda, J. Org. Chem. 43 (1978) 2752. [II] R. C. Larock, T. R. Hightower, ƒ. Org. Chem. 58 (1993) 5298.

[12] R. A. T. M. Benthem van, H. Hiemstra, W. N. Speckamp, ƒ. Org. Chem. 57 (1992) 6083. [13] R. A. T. M. Benthem van, H. Hiemstra, J. J. Michels, W. N. Speckamp, ƒ. Chem. Soc. Chem.

Commun. (1994) 357.

[14] L. S. Hegedus, K. Siirala-Hansen, J. Am. Chem. Soc. 97 (1975) 1184.

[15] L. S. Hegedus, Comprehensive Organic Synthesis, Vol. 4, Pergamon Press, Oxford 1991. [16] L. S. Hegedus, J. M. McKearin, J. Am. Chem. Soc. 104 (1982) 2444-51.

[17] J. E. Bäckvall, C. Jonasson, Tetrahedron Lett. 38 (1997) 291-94. [18] J. E. Bäckvall, Ace. Chem. Res. 16 (1983) 335.

[19] J. E. Bäckvall, Pure Appl. Chem. 55 (1983) 1669. [20] J. E. Bäckvall, Neiv J. Chem. 14 (1990) 447.

[21] B. Âkermark, A. Jutand, ƒ. Organomet. Chem. 217 (1981) C41. [22] B. M. Trost, L. Weber, ]. Am. Chem. Soc. 97 (1975) 1611.

[23] S. Landor, The chemistry of aliènes, Academic Press, London 1982.

[24] L. Brandsma, H. D. Verkruijsse, Studies in Organic Chemistry 8, Synthesis of Acetylenes,

Aliènes and Cumulenes, Elsevier, Amsterdam 1981.

[25] D. Medema, R. Helden van, C. F. Kohll, lnorg. Chim. Acta 3 (1969) 255. [26] R. Medema, R. Helden van, Reel. Trav. Chim. Pays-Bas 90 (1971) 304.

[27] R. Helden van, C. F. Kohll, G. Medema, T. Verberg, T. Jonkhoff, Reel. Trav. Chim.

Pays-Bas 87 (1968) 961.

[28] B. Cazes, Pure & Appl. Chem. 62 (1990) 1867-78. [29] I. Shimizu, J. Tsuji, Chem. Lett. (1984) 233-236.

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Palladium mediated Synthesis o/Carbo- and Heterocycles

[31] R. Grigg, V. Sridharan, ƒ. Organomet. Chem. 576 (1999) 65-87. [32] I. W. Davies, D. I. C. Scopes, T. Gallagher, Synlett (1993) 85.

[33] R. D. Walkup, L. Guan, Y. S. Kim, S. W. Kim, Tetrahedron Lett. 36 (1995) 3805-8. [34] R. D. Walkup, M. D. Guan, S. W. Mosher, S. W. Kim, Y. S. Kim, Synlett (1993) 88-90. [35] W. F. J. Karstens, F. P. J. T. Rutjes, H. Hiemstra, Tetrahedron Lett. 38 (1997) 6275-78. [36] J. S. Prasad, L. S. Liebeskind, Tetrahedron Lett. 29 (1988) 4257-60.

[37] R. C. Larock, Y. He, W. W. Leong, X. Han, M. D. Refvik, J. M. Zenner, J. Org. Chem. 63 (1998) 2154-60.

[38] R. C. Larock, N. G. Berrios-Pena, ƒ. Org. Chem. 56 (1991) 2615-17. [39] R. C. Larock, C. Tu, P. Pace, ƒ. Org. Chem. 63 (1998) 6859-66.

[40] R. E. Lowenthal, A. Abiko, S. Masamune, Tetrahedron Lett. 31 (1990) 6005-8. [41] D. Muller, G. Umbricht, B. Weber, A. Pfaltz, Helv. Chim. Acta 74 (1991) 232-40. [42] G. Helmchen, A. Krotz, K. T. Ganz, D. Hansen, Synlett (1991) 257-8.

[43] R. C. Larock, J. M. Zenner, ƒ. Org. Chem. 60 (1995) 482-83. [44] E. Desarbre, J.-Y. Mérour, Tetrahedron Lett. 37 (1996) 43-46. [45] K. Okuro, H. Alper, ƒ. Org. Chem. 62 (1997) 1566-67.

[46] J. Chengebroyen, C. Sirlin, M. Pfeffer, Tetrahedron Letters 37 (1996) 7263-66. [47] C. Jonasson, J.E. Bäckvall, Tetrahedron Lett. 39 (1998) 3601-4.

[48] C. Jonasson, A. Horvâth, J.E. Bäckvall, J. Am. Chem. Soc. 122 (2000) 9600-9.

[49] F. P. T. J. Rutjes, K. C. M. F. Tjen, L. B. Wolf, W. F. J. Karstens, H. E. Schoemaker, H. Hiemstra, Org. Lett. 1 (1999) 717-20.

[50] J. J. H. Diederen, M. Pfeffer, H.-W. Frühauf, H. Hiemstra, K. Vrieze, Tetrahedron Lett. 39 ( 1998)4111-4.

[51] J. J. H. Diederen, R. W. Sinkeidam, H.-W. Frühauf, H. Hiemstra, K. Vrieze, Tetrahedron