Synthesis of Phosphonium Salts by Metal-Catalyzed Addition

Chapter 22

Synthesis of Phosphonium Salts by Metal-Catalyzed Addition Reaction

Downloaded by UNIV OF ARIZONA on January 5, 2013 | http://pubs.acs.org Publication Date: July 7, 2007 | doi: 10.1021/bk-2007-0965.ch022

Mieko Arisawa and Masahiko Yamaguchi* Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan

Phosphonium salts can be synthesized by the transition-metalcatalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh , C H S O H , and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh ) directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)] reversed the regioselectivity (anti-Markovnikov mode). The Pd(PPh ) -catalyzed addition of the phosphine and C H S O H to alienes stereoselectively gave allylphosphonium salts: (E)Allylphosphonium salts were the thermodynamic product, and (Z)-salts the kinetic product. 3-Alkenylphosphonium salts were obtained by the RhH(PPh ) -catalyzed stereospecific addition of PPh and C F S O H to 1,3-dienes. From these results, the catalyzed addition to unactivated alkenes was achieved. The treatment of ethylene at atmospheric pressure, PPh , and T f N H in the presence of Pd (dba) CHCl gave an ethylphosphonium salt. 1-Alkylphosphonium salts were obtained by anti-Markovnikov addition to 1-alkenes. 3

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2

3

3

© 2007 American Chemical Society

In Recent Developments in Carbocation and Onium Ion Chemistry; Laali, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

477

478 Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 1

PPh RCH=CH

3

+

HX •

2

RCH CH PPh 2

2

3

X


PPh, •

RCH CH X 2

2

Scheme 1. This is part of our program to develop novel synthetic methods for organoheteroatom compounds, compounds of phosphorous, sulfur, selenium, and other elements. The syntheses of these compounds involve the process of bond formation between carbon and a heteroatom, and conventional methods in general employ a substitution reaction of organohalogen or organosulfonate compounds with heteroatom nucleophiles (Scheme 2). The use of highly reactive leaving groups is essential for this synthesis. Leaving groups are introduced in organic molecules typically by the activation of an alcohol, followed by replacement of heteroatom reagents. The method, however, has serious drawbacks from the standpoint of efficiency. The introduction of leaving groups requires multistep transformations, and such groups are eventually wasted, because they are not incorporated in the product. We considered the addition of heteroatom reagents from carbon-carbon multiple bonds to be preferable (Method I in Scheme 2), because a) unsaturated compounds are inexpensive and readily available compared with organohalogen compounds; b) the method does not waste leaving groups; c) unsaturated groups are generally inert under the conditions for various organic transformations, in which organohalogen compounds might be affected. Many heteroatom reagents, however, are inert to unsaturated carbon-carbon bonds, and catalysts are required for such transformation. Thus, we selected transition-metal complexes for this purpose. It has been thought that transition metals form strong complexes with heteroatom reagent, and are not suitable for the catalysis of heteroatom manipulation. We show here that rhodium and palladium complexes are effective for the addition reaction of heteroatom reagents. As a further extension of such organoheteroatom synthesis, we are also interested in the conversion of carbon-hydrogen bonds to carbon-heteroatom bonds (Method 2). 2

2

3

In the phosphonium salt synthesis, the addition reaction of tertiary phosphines to activated alkenes has been reported (Scheme 3). PPh is added to electrondeficient alkenes such as enones or enals at the β-position in the presence of acids. The reaction of styrenes with phosphine has recently been reported by Okuma, which gave Markovnikov adducts. Although no catalyzed reactions of 3

4

5


479 Method 1: Addition Se ψτ H - C • C H ( | ) ι C — CH2 ν, cat. ι ρ; < H - C • C H ® Elements Non-halogen 2

CX Substitution SY, -C-X ι

2

-C©

• X-Y

'

PY < - C < £ ) • X-Y Reactive Reagen\s Byproducts

Method 2: CH Substitution

r

-CTol) P (15mol%) T H F , 12 h, refl. 6

4

2

A?-C H 7

3

quant

Scheme 5.


15

1 5

481 of this reaction is the critical role of phenol as an acid, which led us to examine the effect of added acid on the transition metal catalyzed reaction. During our investigation, PPh and C H S 0 H were found to add to unactivated alkynes in the presence of a palladium or rhodium complex. The regiochemistry and stereochemistry could be controlled by a judicious selection of the metal catalyst. Alkenylphosphonium salts have various applications in synthetic chemistry, and the present method enables an easy access to the organophosphorous compounds using readily available starting materials. 1-Hexyne was treated with equimolar amounts of PPh and C H S 0 H in the presence of Pd(PPh ) (2.5 mof ) in refluxing T H F for 2 h. The counteranion was substituted for P F , and recrystallization gave a (l-hexen-2-yl)phosphonium salt in a quantitative yield (Scheme 6). The phosphine attacked the internal carbon of 1-hexyne regioselectively (Markovnikov mode). The palladium complex and C H S 0 H were essential, and no reaction occurred in the absence of either. With this catalyst, a turnover number of 1000 could be attained. The effect of the acid structure was small, and P h S 0 H , /?-tolS0 H, / ? - C l C H S 0 H , C F S 0 H , camphorsulfonic acid, and even H S 0 could be used equally as well. The catalytic activities of several palladium complexes were compared: Pd (dba) CHCl (99%), and Pd(OAc) (84%) were active, whereas PdCl (PPh ) was inactive. 3

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13

14

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3

/o

3

4


6

3

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2

2

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6

4

3

4

2

2

V

1) P d ( P P h ) ( 2 . 5 mol%) THF, refl., 2 h 2) LiPF EtOH 3

4

P*Ph

3

3

2

PFe

e

95% (R = n - C H ) 4

R

zzz

•

PPh

•

3

9

MeS0 H 3

1) [RhCI(cod)] (1.5mol%) 2

P Ph + p + l

Acetone, r. t., 12 h 2) LiPF EtOH 6i

PF -

3

6

79% (R = Ph(CH ) ) 2

2

Scheme 6. Scheme 6 shows a novel C-P bond forming reaction, in which the ligand coupling of an alkyne and a tertiary phosphine on palladium metal occurs. It was also considered very important that a C-P bond could be formed by the addition of a phosphorous reagent to unsaturated compounds. When the rhodium catalyst [RhCl(cod)] (cod = 1,5-cyclooctadiene) or RhCl(PPh ) was used, the observed regioselectivity of the reaction was opposite to that of the palladium-catalyzed reaction (^/-Markovnikov mode). The treatment of an equimolar mixture of 4-phenyl-l-butyne, PPh , and C H S 0 H with [RhCl(cod)] (1.5 mol%) in acetone at room temperature for 12 h yielded predominantly (£)-(4-phenyl-l-butenyl)phosphonium salt with a small amount of the regioisomer in a ratio of 10:1. The pure (£)-product was obtained by recrystallization of the P F salt in 79% yield (Scheme 6). The regioselectivity was shown to be under kinetic control: (4-Phenyl-l-butenyl)phosphonium salt did not isomerize to its internal derivative when treated with C H S 0 H in the presence of either [RhCl(cod)] under acetone reflux or Pd(PPh ) under THF reflux. 2

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2

6

3

2

3

3

4


3

3

482 The reactivity of the conjugated alkynes differed slightly from that of the aliphatic 1-alkynes. The addition of PPh to arylacetylenes was effectively catalyzed by RhH(PPh ) (Scheme 7), whereas Pd(PPh ) and [RhCl(cod)] were not effective. The reaction was applicable to phenylacetylenes with either electron-donating or electron-withdrawing groups giving aw/z-Markovnikov (£)adducts. RhH(PPh ) catalyzed the addition to l-undecen-3-yne yielding the Markovnikov adduct. 3

3

3

4

3

3 4

+ PPh

2

4

1 ) RhH(PPh ) (1.5 mol%) Acetone, refl.., 12 h Ph—=

4

Ph PFe

+ MeS0 H

3

3

P + P h

2) LiPF EtOH


6i

8

3

4

%

1)RhH(PPh ) (1.5mol%) Acetone, refl... 12 h n-CeH^^^r^^ 3 4

n-C H — 8

•

17

PPh

• MeS0 H

3

3

P*Ph

PFe'

3

2) LiPF EtOH 6i

89%

Scheme 7. In the presence of a palladium complex, trimethylsilylacetylene was converted to β-silylethenylphosphonium salt in 86% yield (Scheme 8). Phosphine attacked the terminal carbon in this case. When acetylene was used, l,2-bis(triphenylphosphino)ethane was obtained, which may be formed by the conjugate addition of PPh to the initially generated ethenylphosphonium salt. 3

SiMe

1)Pd(PPh ) (2.5 mol%) THF, refl.. 2 h

3

3 4

Me Si——H

•

3

PPh

• MeS0 H

3

3

2) LiPF EtOH

PF "

P*Ph

6>

86%

6

3

Pd(PPh ) (2.5 mol%) THF, refl.. 2 h 3 4

-H

•

PPh

3

• MeS0 H

|^P*Ph

f

3

P*Ph MeS0 -

3

P*Ph

3

3

2MeS0 -

3

3

75%

Scheme 8. The addition of a proton and a phosphine generally proceeded via cisaddition. When 1-octyne was treated with C H S 0 H and PPh in acetone-d in the presence of Pd(PPh ) , (Z)-l-deutero-l-octen-2-ylphosphonium salt was obtained (Scheme 9). A n H-D exchange occurred between sulfonic acid and the deuterated solvent generating deuterated sulfonic acid, which transferred deuterium to the alkyne. Two mechanisms are conceivable for the reaction. One involves the hydrometalation of the metal hydride at alkyne followed by reductive elimination. Alternatively, phosphinometalation followed by protiodemetalation can occur. To gain insight into the mechanism, a carbonylation 3

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4

15

16


6

483 P d ( P P h ) (2.5 mol%) 3

PPh

n-CeH-13 "

+

3

MeS0 H

4

• Acetone-d , refl.. 2 h

3

n-C H 6

1 3

6

P Ph

MeS0 " 3

+

3

95%, 7 8 % - d

Scheme 9. reaction was carried out. The reaction of 1-octyne, PPh , and C H S 0 H under carbon monoxide atmosphere gave the acylphosphonium salt, which upon treatment with butanol was converted to butyl ct-methyleneoctanoate (Scheme 10). Thus, palladium should be attached to the internal carbon of the alkyne, that undergoes carbonylation and phosphonylation. The result is consistent with the hydrometalation mechanism in the Markovnikov mode. 3


3

3

Pd(PPh ) , CO 3

n-C H 6

1 3

=

H

+

PPh

+

3

4

MeS0 H

n - C

3

THF

1

n-BuOH

n-CeH

H

1

3

^

P

+

Ο

l

Ph

3

MeS0 3

OBu 7f Ο

13

6

42%

Scheme 10 A possible mechanism therefore can be summarized as follows (Scheme 11). The oxidative addition of an acid to a low-valent metal complex provides a metal hydride, which undergoes hydrometalation with a 1-alkyne. Reductive elimination leads to an alkenylphosphonium salt and a low-valent metal species, which reacts with PPh . Strong electron-withdrawing groups such as CH S0 derived from the acid can facilitate the reductive elimination. The regioselectivity may be controlled at the hydrometalation step: Palladium hydride undergoes Markovnikov addition, whereas rhodium hydride antiMarkovnikov addition. 3

3

H

R M e S 0 " or R ^ H

W (Ph P)LnM ^H R

PPh

MLn

MeSOy

3

H

W (Ph P)LnM H MeS0 3 or MeSÔ M = Rh M = Pd anti-Markovnikov mode Markovnikov mode H

3

3

R

/

3

MLn(PPh )

3

3

3

"MeS0 H 3

MeS0 -MLn(PPh ) 3

R

=

3

H

Scheme 11.


484


Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, aliène formation was observed.

n-C?Hi5

Scheme 12.

Typical Experimental Procedures Under an argon atmosphere, a mixture of Pd(PPh ) (2.5 mol%, 29 mg), P P h ( l mmol, 262 mg), 1-hexyne (1 mmol, 0.12 mL), and C H S 0 H (1 mmol, 96 mg) in T H F (2 mL) was heated at reflux for 2 h. The solvent was removed under reduced pressure, and the residue was washed with ether. After dissolving the crude product in ethanol (2 mL), L i P F (1.5 mmol, 228 mg) was added, and the mixture was stirred at room temperature for 1 h. The precipitated solid was collected by filtration and was dissolved in CHC1 followed by filtration. The solution was concentrated, and the residue was recrystallized from CHC1 and ether (3:1) yielding (l-hexen-2yl)triphenylphosphonium hexafluorophosphate (468 mg, 96%) as a colorless solid. M p 148.5-149.0 °C. 3

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6

3

3

2. Addition Reaction of Phosphine to Aliènes Under the conditions where alkynes reacted, alkenes were inert. To extend this methodology to the less reactive substrate, the reaction of an activated olefin, aliène, was examined. 1-Phenyl-1,2-propadiene was treated with equimolar amounts of PPh and C H S 0 H in the presence of Pd(PPh ) (2.5 mol%) in refluxing THF for 12 h. The counteranion was exchanged with L i P F , and recrystallization gave (£)-(3-phenyl-2-propenyl)triphenylphosphonium hexafluorophosphate in 82% yield (Scheme 13). The phosphine attacked at the terminal carbon atom of the aliène regioselectively. The palladium complex and C H S 0 H were again essential for the addition reaction, and no reaction occurred in their absence. 17

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3

3

3

4

6

3

3


485 1) Pd(PPh ) (2.5 mol%) THF, refl., 12 h I • 2) LiPF EtOH 3

PhCH=C=CH

2

• pph

3

+ MeS0 H 3

^Ph

4

p .-

x

y

F{

6>

P

P n

1)Pd(PPh ) (2.5mol%) 3

PhCH CH=C=CH 2

2

+ PPh

3

T

+ MeS0 H

H

F

>

4

1 0

c

1

7

" 2) NaCI0 . EtOH

3

+

P

h

3

82%

C H2 2

n

4

f

) CI0 " 4

3 82%

P+Ph


Scheme 13.

When the reaction was conducted at -10 °C, the observed stereoselectivity of the reaction was opposite to that under refluxing conditions. The treatment of an equimolar mixture of 4-phenyl-l,2-butadiene, PPh , and C H S 0 H with Pd(PPh ) (1.5 mol%) in T H F at -10 °C for 17 h yielded predominantly (Z)-4phenyl-2-butenylphosphonium salt with a small amount of the (£)-isomer in a ratio of 13:1. The isomerically pure (Z)-compound was obtained by recrystallization of the perchlorate salt in 82% yield. A n isolated (Z)-product isomerized to the (£)-compound when treated with C H S 0 H and Pd(PPh ) under THF reflux. The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C = C H moiety of the aliène in the antiMarkovnikov mode from the opposite side of the substituent. This addition gives a π-allyl palladium complex with the (Z)-configuration, which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 3

3

3

3

4

3

3

3

4

2

18

MeS0

3

Pd-PPh

3

\ . P M

X PPh

e

S

°3*

+

P h

3

MeS0 3

E

3

Scheme 14.

Allylphosphonium salts are synthesized by substitution of allyl halides with PPh . The use of allyl alcohol, allyl acetate, or nitropropene with a palladium catalyst has also been reported. it is shown in this study that the organophosphorous compounds can be obtained by a palladium-catalyzed addition to an aliène. A notable aspect of this method is that it can control the stereochemistry of the phosphonium salt, and that (Z)-allylphosphonium salts have been obtained in pure form for the first time. 3

19


486 The Wittig reaction of (£)-allylphosphonium salts is notorious for not being stereoselective, and in accordance with this, the reaction of an (£)allylphosphonium salt with sodium hexamethyldisilazide in T H F followed by octanal gave 1:1 mixtures of (£,£)-isomers and (£,Z)-isomers (Scheme 15). It was considered interesting to compare the reaction of (Z)-isomers. The (Z)allylphosphonium salt gave a similar mixture of (Z,£)-isomer and (Z,Z)-isomers. The Wittig reaction of (£)- and (Z)-allylphosphonium salts exhibited similar behavior under the present reaction conditions. Although the stereochemistry of allylphosphonium salt was retained, a mixture of isomers was obtained with respect to the newly formed double bond.


20

(CH ) Ph 1)(Me Si) NNa

^(CH ) Ph

2 2

/ I

3

2

^(CH ) Ph

2

2

2) n-C H CH0

P F E

7

P*Ph

15

83%(Z:E = 1 :1)

3

Ph(CH ) j 2

2N

*

3

C,

P h

Ph(CH )2. J 2

1)(Me Si) NNa

Γ p

2

2

4

°*

2

2)n-C H CH0 7

f|

15

Γ M

3 Π

"°

7 Η ι 5

' -

90% (Ζ: Ε» 2:1)

Scheme 15. Typical Experimental Procedures Under an argon atmosphere, a mixture of Pd(PPh ) (18 mg, 1.5 mol%), PPh (262 mg, 1 mmol), 4-phenyl-l,2-butadiene (1 mmol, 130 mg), and C H S 0 H (96 mg, 1 mmol) in T H F (2 mL) was stirred at -10 °C for 12 h. After stirring with a small amount of decolorizing charcoal for 30 min, insoluble materials were removed by filtration. The solution was concentrated under reduced pressure, and the residue was washed with ether. The crude product was dissolved in ethanol (2 mL), to which NaC10 (2 mmol, 244 mg) was added. The mixture was stirred at room temperature for 1 h, and the precipitated solid was collected by filtration. CHC1 was added to the solid, and insoluble C H S 0 L i was removed by filtration. The solution was concentrated, and the residue was recrystallized from acetone and ether (2:1) yielding (Z)-(4-phenyl-2-butenyl)triphenylphosphonium salt (403 mg, 82%) as colorless solid. M p . 180.0-181.0°C. 3

4

3

3

3

4

3

3

3

3 . Addition Reaction of Phosphine to 1,3-Dienes Before examining the reaction of deactivated alkenes, the phosphonium salt synthesis was applied to 1,3-dienes. When (£)-6-phenyl-l,3-hexadiene was treated with equimolar amounts of PPh and C F S 0 H in the presence of RhH(PPh ) (2.5 mol%) in T H F at 0 °C for 3 h, (£)-(6-phenyl-3hexenyl)triphenylphosphonium salt was obtained in 89% yield after anion exchange with L i P F and recrystallization (Scheme 16). The addition of phosphine and hydrogen occurred at the 1- and 2-carbon atoms of the 1,3-diene, respectively. The reaction of (7)-1,3-dienes was then performed for comparison. 21

3

3

3

3

4

6


487 The treatment of (Z)-4-(p-tolyl)-1,3-butadiene with PPh and CF3SO3H at room temperature for 5 h gave a 1,2-adduct, (Z)-4-(/7-tolyl)-3-butenylphosphonium salt, in 52% yield, which was accompanied by the formation of a 1,4-adduct (20%). 3

1)RhH(PPh ) (2.5 mol%) THF, 0 °C, 3 h 2) LiPF EtOH 3 4

CF3SO3H

2

PPh

Ph(CH ) 4^J. 2

2

p+ph3

pFfi

.

6l

RhH(PPh ) (2.5 mol%) 3 4

3

CF3SO3H

THF, 40 °C, 4 h +


P Ph

3

CF3SO3-

52% P*Ph CF S0 3

3

3

20%

Me

Scheme 16. (£)-l,3-Dienes reacted considerably faster than the (Z)-isomers, and this feature was utilized to separate the (Z)-isomer from the stereoisomeric mixture. When 6-phenyl-l,3-hexadiene (E/Z= 50/50) was treated with 0.6 molar amounts of PPh and C F S 0 H at 0 °C for 6 h, the (£)-phosphonium salt (53%) was formed. The unreacted (Z)-l,3-diene could be separated by ether extraction in 41% yield. This procedure provides easy access to (Z)-1,3-dienes. In general, transition metal-catalyzed addition reactions to 1,3-dienes gave 1,4-adducts via7i-allyl metal intermediates. The uw//-Markovnikov 1,2-addition mode of this reaction is therefore unusual (Scheme 17). It was noted that the configuration of the 3-olefin was retained with either (£)- or (Z)-1,3-dienes. The observation that the 3-olefin was unimportant for this reaction strongly suggests that the method could be applicable to "unactivated" alkenes. 3

3

3

22

23

Scheme 17. The difference between palladium andrhodium catalyses regarding regionselectivity is also noteworthy. The palladium complex provided Markovnikov


488 products in the reaction of aliphatic 1-alkynes and aliènes, and antiMarkovnikov products were obtained via the rhodium-catalyzed reaction of aliphatic 1-alkynes and 1,3-dienes. This is a novel metal effect on regiochemistry. The phenomenon, however, has exceptions as exemplified by the reaction of "unactivated" 1-alkenes (vide infra): Both complexes gave antiMarkovnikov adducts. Typical Procedures for Separation of (Z)-1,3-Diene. Under an argon atmosphere, a mixture of RhH(PPh ) (290 mg, 2.5 mol%), PPh (6 mmol, 1.57 g), 6-phenyl-l,3-hexadiene (10 mmol, 1.58 g, E/Z= 50/50), and C F S 0 H (0.53 mL, 6 mmol) in T H F (20 mL) was stirred at 0 °C for 6 h. A small amount of activated charcoal was added, and the mixture was stirred for 30 min to adsorb the metal complex. Insoluble materials were removed by filtration, and the solution was concentrated under reduced pressure (53% yield of (£)-6-phenyl-2hexenylphosphonium salt by ^ - N M R ) . The residue was washed with ether, and the ether solution was washed with saturated N a H C 0 and brine. After drying over M g S 0 the solution was concentrated, and flash chromatography (hexane) over silica gel gave (Z)-6-phenyl-l,3-hexadiene (650 mg, 41%). The residue obtained by ether washing was dissolved in ethanol (10 mL), and LiPF (10 mmol, 1.52 g) was added to the resulted solution. After stirring at room temperature for 1 h, the precipitated solid was collected by filtration. CHC1 was added to the solid, and insoluble C F S 0 L i was removed by filtration. The solution was concentrated, and the residue was recrystallized from ethanol to give the (£)-phosphonium salt (586 mg, 21%) as a colorless solid. Mp.124.0-125.0 °C. 3

4

3


3

3

3

4

6

3

3

3

4. Addition Reaction of Phosphine to Alkenes From the results of the 1,3-diene addition reaction, the metal-catalyzed reaction of "unactivated" alkenes was examined, and it was found that the palladium complex effectively catalyzed the aw//-Markovnikov addition of triarylphosphines and bis(trifluoromethanesulfonyl)imide (Tf NH). Ethylene at atmospheric pressure (balloon) was treated with PPh and T f N H (1.1 eq.) in the presence of Pd (dba) CHCl (dba = dibenzylideneacetone) (1.25 mol%) in chlorobenzene at 65 °C for 5 h, and the ethyltriphenylphosphonium salt was obtained in 99% yield after recrystallization (Scheme 18). T f N H gave a better result than C F S 0 H . Catalyst loading could be reduced to 0.1 mol% without affecting the yield of the product. The use of a slight excess of T f N H over phosphine (molar ratio = 1.1:1) was critical, which suggests that the acid was not only a "proton source" required to add to 1-alkenes but also an activator of the palladium complex. RhH(PPh ) was less effective under the conditions used, and required a higher catalyst loading (10 mol%) for effective conversion. In addition, the Rh catalyzed reaction competed with phosphine oxidation to the oxide, presumably caused by a trace amount of dissolved oxygen. The reaction was applicable to 1-alkenes (Scheme 19). When propene at atmospheric pressure (balloon) was treated with PPh and T f N H (1.1 eq.) in the 24

2

3

2

2

2

3

3

3

3

2

3

4

3

2


489 Pd dba CHCl3 (0.1 mol%) 2

H C=CH 2

+

PPh

2

3

+

CH CH P Ph

(CF S0 ) NH

3

3

2

3

2

2

3

(CF S0 ) N3

2

2

C H C I , 65 °C. 5 h 6

5

1 atm PPh : (CF S0 ) NH = 3

3

2

2

1 : 0.85

N.R.

1 :1

30%

1 : 1.1

99%

Scheme 18.

presence of Pd (dba) CHCl (0.5 mol%) in chlorobenzene at 65 °C for 5 h, 1propyltriphenylphosphoniurn salt was obtained in 95% isolated yield. The antiMarkovnikov adduct was obtained exclusively, which indicates the essential role of metal catalysis rather than acid catalysis in C-P bond formation. The reaction of 1-butene (20 eq), PPh , and Tf NH (1.1 eq.) in the presence of Pd (dba) CHCl (1.0 mol%) in chlorobenzene at 65 °C for 8 h gave 1butyltriphenylphosphonium salt in 92% yield. The recovered butènes contained mainly 2-butene and a very small amount of 1-butene (2-butene: 1-butene > 20:1). Such equilibration of butènes was attained within 30 min under the conditions used, and a small amount of 1-butene must have reacted with PPh , which indicates a high activity of the catalyst.


2

3

3

3

2

3

2

3

3

Me-CH=CH

2

•

PPh

•

3

(CF S0 ) NH 3

3

2

1 atm

Ρ

^ )

(0-5 mol%)

3

^

%

C H C I , 65 ° C , 5 h 6

^

ρ

^

(CF S0 ) N-

5

3

3

2

Q

5

%

Scheme 19.

Higher 1-alkenes reacted more effectively with ( p - O C H ) P than with PPh or (p-tolyl) P. The reaction of 1-pentene (5 eq), T f N H (1.1 eq), and (/?C1C H ) P in the presence of the palladium complex (1.0 mol%) gave the 1pentylphosphonium salt in 91% yield (Scheme 20). The use of 5 eq of 1-pentene was sufficient for this reaction, despite the volatile nature of this compound (bp 30 °C), and the rapid olefin migration to form a very small amount of 1-pentene. The reaction of 1-hexene also proceeded effectively giving the 1hexylphosphonium salt in 88% yield. 6

3

3

6

4

e

q

3

3

„ ^ , ^.. R-CH=CH 5

4

2

Pd (dba) (1.0 mol%) ^ * , ' C H C I , 65 °C, 24 h 2

2

•

(p-CIC H ) P + 6

4

3

(CF S0 ) NH 3

3

2

6

3

5

»

+

R"CH CH P (p-CIC H ) (CF S0 ) N2

3

2

3

6

2

91%

R = n-C H

7

88%

R = n-C H

9

3

Scheme 20.


4

4

3

490 Because olefin migration was very rapid under the conditions used, internal alkenes could be used. Thus, treatment of (£)-2-pentene or (£)-3-pentene with T f N H (1.1 eq) and (p-ClC H ) P in chlorobenzene at 65 °C for 24 h both gave the 1-pentylphosphonium salt in 91% yields (Scheme 21). The use of mixtures of alkene regioisomers and stereoisomers for the reaction may have a synthetic advantage. 2

6

4

3

Pd (dba) (1.0 mol%) c H CI. 65 «C, 24 h 2

(P-CIC H ) P * 6

4

(CF S0 ) NH

3

3

3

2

6

+

3

5

n-C H P (p-CIC H ) (CF S0 ) N" 6

'

13

6

3

3

4

3

2


91%

Scheme 21.

It was confirmed that the Wittig reaction proceeded using the phosphonium salt (Scheme 22).

Tf N 2

1)BuLi +

C H C H P P h (CF S0 ) N3

2

3

3

2

^

2

/?-C H CH=CHCH 9

19

2) n - C H C H O 1

9

1 9

3

82%(E:Z=1:1)

Scheme 22. Typical Experimental Procedures In a two-necked flask equipped with a reflux condenser were placed PPh (5 mmol, 1.31 g), Pd dba «CHCl (0.1 mol%, 5.2 mg), and T f N H (5.5 mmol, 1.55 g) in chlorobenzene (6 mL) under an argon atmosphere. After substituting an argon balloon for ethylene, the solution was heated at 65 °C for 5 h. It was then concentrated, and the residue was washed with ether/hexane. Recrystallization from ethanol gave the ethylphosphonium salt (2.79 g, 98%) as a colorless solid. M p 97.0-97.5 °C. 3

2

3

3

2

5. Conclusion Phosphonium salts can be synthesized from unsaturated compounds by addition of a triarylphosphine and an acid in the presence of a palladium or rhodium catalyst. Transition metal catalysis turned out to be effective for the synthesis of organophosphorous compounds.

Acknowledgments This work was supported by JSPS (Nos. 16109001 and 17689001). M . A . expresses her thanks to the Grant-in-Aid for Scientific Research on Priority Areas, "Advanced Molecular Transformation of Carbon Resources" from M E X T (No. 18037005).


491

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1.

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Synthesis of Phosphonium Salts by Metal-Catalyzed Addition

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