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Negishi E. 2002 Handbook of organopalladium chemistry for organic synthesis / 0335 408

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III.2.6 Pd-CATALYZED ALKENYL–ARYL

385

D.iii. (Z)- -Substituted Alkenylmetals

Although stereoselective syn-hydrogenation and its equivalents via syn-hydrometallation of conjugated diynes[231] and enynes[232] have provided some prototypical selective routes to (Z,E )- and/or (Z,Z )-conjugated dienes, Pd-catalyzed alkenyl–alkenyl coupling has proved to be the method of choice for their preparation because of its general applicability, stereoselectivity, and favorable overall results including generally high product yields.

(Z,E )-conjugated dienes can, in principle, be prepared by either the reaction of (E )- alkenylmetals with (Z )-alkenylelectrophiles, as discussed in Sect. D.ii, or that of (Z )- alkenylmetals with (E )-alkenyl electrophiles. Choice between these two options depends on a number of factors including the relative accessibility of the two required reagents. The ready accessiblity of (Z )- -substituted alkenylcoppers via carbocupration[38] of ethyne makes Pd-catalyzed reaction of (Z )- -substituted alkenylcoppers a very attractive route to (Z,E )- and (Z,Z )-conjugated dienes.[28],[226],[227] Interestingly, the addition of ZnBr2 as a cocatalyst[18] has proved to have favorable effects on this reaction[226],[227]

(Scheme 52).

1.ZnBr2, THF

2.3% Pd(PPh3)4

I (CH2)9CH(OR)2 )

2 CuLi

1.MgCl2, ZnBr2, THF

2.3% Pd(PPh3)4

	)	I
			HO

ZO(CH2)10 2 CuLi 3. H+
ZO(CH2)10 2 CuLi 3. H+
			Scheme 52

CH(OR)2

74%, >99% Z,Z

72%, >99% Z,Z

As discussed in Sect. B.iii.b, (Z )- -substituted alkenylboranes are obtainable via 1-haloalkynes hydroboration – 1,2-hydride migration protocol.[36],[37],[96],[97] Their Pdcatalyzed reactions with (E )- and (Z )-alkenyl electrophiles provide (Z,E )- and (Z,Z )- conjugated dienes, respectively (Scheme 53).[48]

B(OiPr) +

67%

>98% E,Z

B(OiPr)

Hex

2 +

87%

Scheme 53 (Continued )

386

III

Pd-CATALYZED CROSS-COUPLING

t-Bu

Hex

81%

B(OPr)

t-Bu

>97% Z,Z

t-Bu

B(OiPr) +

t-Bu

79%

A = Pd(PPh), NaOEt,HC, 80°C

3 4

Scheme 53 (Continued )

At present, (Z)- -substituted alkenylmetals containing other metals are less readily accessible than those mentioned above, even though the Zr-catalyzed carboalumination of ethyne has been shown to produce (Z)- -substituted alkenylalanes.[228] They are generally prepared via metallation–transmetallation of (Z)- -substituted halides. Despite this drawback, (Z)- -substituted alkenylzincs generated by this procedure have been shown to be superior reagents in the subsequent Pd-catalyzed cross-coupling reaction[186],[198],[229] (Table 11).

TABLE 11. Pd-Catalyzed Coupling of (Z )- -Substituted Alkenylmetals with Alkenyl Electrophiles a

Type of

Yield

Alkenyl

(Selectivity)

Ref-

Electrophile

Conditions

erence

(E)-β-

C6H13

Pd(PPh3)4,

87(>99)

[22]

CH3

MgX

C6H6

C5H11

Pd(PPh3)4,

82(99)

[226]

C2H5

CuLi

ZnX , THF

Pd(PPh3)4,

85(97.6)

[226]

C2H5

CuLi

ZnX2, THF

Pd(PPh3)4,

C3H7

89(~100)

[186]

CuLi

( )

ZnX2, THF

Pd(PPh3)4,

C3H7

76(~100)

[186]

CuLi

( )

ZnX2, THF

B(OiPr)2

C6H13

Pd(PPh3)4,

70(>99)

[98]

C4H9

NaOEt, C6H6

Pd(PPh3)4,

49(99)

[196]

C4H9

B(Sia)2

NaOEt, C6H6

C2H5

Pd(PPh3)4,

80(99)

[226]

C5H11

CuLi

ZnX2, THF

( )

Pd(PPh3)4,

C5H11

ZnBr

6 CO2CH3

92(~100)

[197]

THF

Pd(PPh3)4,

55(99)

[227]

tBu

Cu,MgX2

C5H11

ZnX2, THF

III.2.6

Pd-CATALYZED ALKENYL–ARYL

387

TABLE 11. (Continued )

Type of

Yield

Alkenyl

(Selectivity)

Ref-

Electrophile

Conditions

erence

SnBu3

C4H9

Cl2Pd(CH3CN)2,

[184]

DMF

(Z)-β-

C6H13

Pd(PPh3)4,

87(>97)

[22]

CH3

MgX

C6H6

Pd(PPh3)4,

C5H11

86(99)

[226]

ZnX2, THF

C2H5

CuLi

C2H5

ZnX

( )

Pd(PPh3)4,

88(99.5)

[229]

THF

8 OAc

B(Sia)2

( )

Pd(PPh3)4,

[200]

C3H7

NaOEt, C6H6

9 OH

( )

Pd(PPh3)4,

[200]

C3H7

B(Sia)2

NaOEt, C6H6

(Z)-β-

9 OH

Pd(PPh )

C5H11

CuLi

3 4

86(>99)

[226]

ZnBr2, THF

Pd(PPh3)4,

CO2Me

95(99)

[197]

C5H11

ZnBr

THF

( ) 7

C4H9

Cu,MgX2

C5H11

Pd(PPh3)4,

53(99)

[227]

ZnX2, THF

C4H9

Cl2Pd(MeCN)2,

[184]

SnBu3

DMF

α-

Pd(PPh3)4,

55(99)

[196]

C4H9

B(Sia)2

NaOEt

β,β-

CH3

Pd(PPh3)4,

94(~100)

[226]

C5H11

CuLi

THF

CH3

cis-α,β-

TfO

Pd(OAc)2, NMP

[230]

SnBu3

H3C

Pd(OAc)2, CH2Cl2

>90

[230]

Cl2Pd(MeCN)2,

SnBu3

[184]

DMF

aSee Table 10

388	III Pd-CATALYZED CROSS-COUPLING

D.iv. -Substituted Alkenylmetals

The -substituted alkenylmetals used in Pd-catalyzed cross-coupling have been mainly those containing Mg, Zn, B, and Sn, as shown in Table 12 as well as Schemes 54–57. Of these, -substituted alkenylmetals containing Mg and Zn can readily be prepared by direct oxidative metallation of 2-halo-1-alkenes[192] that are easily accessible by Markovnikov addition of HX to 1-alkynes (Scheme 54). -Substituted alkenyltin compounds have been prepared and used in the construction of bicyclic diene systems via intramolecular Stille coupling, as shown in Scheme 55.[234]

TABLE 12. Pd-Catalyzed Coupling of -Substituted Alkenylmetals with Alkenyl Electrophiles a

R	Type of						Yield
	Alkenyl						(Selectivity)	Ref-
M	Electrophile	X				Conditions	%	erence

CH3	(E)-β-
CH3		I				Pd(PPh3)4,
		I			C6H13	Pd(PPh3)4,	82(>97)	[22]
MgX					C6H13	C6H6	82(>97)	[22]
MgX						C6H6
CH3		Cl				Cl2Pd(PPh3)2,
		Cl			C5H11	Cl2Pd(PPh3)2,	55	[164]
MgX					C5H11	Et3N (8 equiv)	55	[164]
MgX						Et3N (8 equiv)
CH3					C5H11	Cl2Pd(PPh3)2,
		Cl			C5H11	Cl2Pd(PPh3)2,	62	[164]
MgX		Cl				Et3N (8 equiv)	62	[164]
MgX						Et3N (8 equiv)
iBu		Br				Pd(PPh3)4,
		Br			C6H13	Pd(PPh3)4,	84(>99)	[117]
BiBu2					C6H13	NaOH, H2O, THF	84(>99)	[117]
BiBu2						NaOH, H2O, THF
CF3		Br				Pd(PPh3)4,
		Br			Ph	Pd(PPh3)4,	87−91	[233]
ZnBr					Ph	THF	87−91	[233]
ZnBr						THF
	(Z )-β-
CH3		I		C6H13		Pd(PPh3)4,
		I		C6H13		Pd(PPh3)4,	84(>97)	[22]
MgX						C6H6	84(>97)	[22]
MgX						C6H6
iBu		Br			C6H13	Pd(PPh3)4,
		Br			C6H13	Pd(PPh3)4,	87(>99)	[117]
BiBu2						NaOH, H2O, THF	87(>99)	[117]
						NaOH, H2O, THF

	α -
CF3		Br				Pd(PPh3)4,
						Pd(PPh3)4,	92	[233]
ZnBr			Ph			THF	92	[233]
						THF

		III.2.6	Pd-CATALYZED ALKENYL–ARYL		389
TABLE 12. (Continued )

R	Type of			Yield
	Alkenyl			(Selectivity)	Ref-
M	Electrophile X		Conditions	%	erence

	cis-α,β-
CF3	I		Pd(PPh3)4,
			Pd(PPh3)4,	86	[233]
ZnBr			THF	86	[233]
			THF

	I
CF3			Pd(PPh3)4,	90	[233]
			THF	90	[233]
			THF

ZnBr

aSee Table 10.

Zn*

R = H, 91%

ZnBr

5% Pd(PPh3)4, THF

R = Me, 86%

Scheme 54

OTf

SnBu3

5% Pd(PPh3)4

R = H, 82%

THF

CO2Me

R = Me, 82%

Scheme 55

1. t-BuLi (2 equiv)

TMEDA(2 equiv)

OEt

Pent-n

OEt

2. ZnCl2

CH2

CHOEt

5% Pd(PPh3)4

ZnCl

Pent-n

74%

s-BuLi (1 equiv)

SEt

Pent-n

SEt

THF-HMPA

5% Pd(PPh3)4

Pent-n

CHSEt

CH2

1. s-BuLi (1 equiv)

THF-HMPA

SEt

Pent-n

SEt

5% Pd(PPh3)4

2. ZnCl2

ZnCl

Pent-n

81%

Scheme 56

390 III	Pd-CATALYZED CROSS-COUPLING
	OTf OR			OR	O
				OR	OH

				[O]
		MLn		[O]


	cat. Pd(0)
AcO		AcO	AcO
		R = Bu, MLn = ZnCl, 82%
		R = Me, MLn = SnBu3, 54%

Scheme 57

The Pd-catalyzed reactions of -heterosubstituted alkenylzincs containing alkoxy and thioalkoxy groups were developed with the goal of synthesizing heterosubstituted conjugated dienes for the Diels–Alder reaction[118] (Scheme 56). The use of the parent alkenyllithiums did not produce the desired dienes in detectable amounts. This reaction has been applied to the synthesis of steroidal -hydroxy enones[235] (Scheme 57). It should be noted that Zn has been shown to be decidedly superior to Sn as the countercation.

D.v. , -Substituted Alkenylmetals

A large number of natural products, such as carotenoids, contain conjugated diand oligo-ene moieties with at least one (E )-trisubstituted alkene unit, such as 1 and 2 shown in Scheme 58. Although all (E )-isomers are dominant, their stereoisomers are also known.

Scheme 58

Even today, carotenoids and other natural products represented by 1 and/or 2 are synthesized by using the Wittig and related carbonyl oleﬁnation reactions that are often not highly stereoselective, thus requiring delicate and tedious separations. Carbometallation reactions of alkynes,[38],[40] especially the Zr-catalyzed carboalumination discovered in 1978,[236],[237] used in conjunction with Pd-catalyzed cross-coupling[18] have provided a totally different carbometallation – cross-coupling tandem protocol for the synthesis of 1 and 2 (Scheme 59).

Since the preparation of , -substituted alkenylmetals as the ﬁrst-generation organometals has been achieved mostly by Zr-catalyzed carboalumination[40] and carbocupration,[38] the exploitation of Protocol I has largely resorted to these two carbometallation reactions, as indicated by the results summarized in Table 13. Moreover, since it is impractical to carry out methylcupration of alkynes requiring several days at 25 °C,[238] the synthesis of natural products represented by 1 and 2 by the use of Protocol I has mostly been limited to those cases where Zr-catalyzed carboalumination is

III.2.6 Pd-CATALYZED ALKENYL–ARYL

391

Me3Al

R1C

cat. Cp2ZrCl2

AlMe2

cat. PdLn, ZnX2

Protocol I

cat. PdLn

Protocol II

HMLn

1. BuLi

2. ZnX2

ZnX

cat. PdLn

Protocol IV

Various

Protocol I, II, or IV

MLn

options

using XR2 or MR2

R1, R2 = C groups. X = halogen, etc. M = metal countercations.

Scheme 59

TABLE 13. Pd-Catalyzed Coupling of , -Substituted Alkenylmetals with Alkenyl Electrophiles a

R1	M	Type of				Yield
	M	Alkenyl				(Selectivity)	Ref-
		Alkenyl				(Selectivity)	Ref-
R2		Electrophile X			Conditions	%	erence
C5H11		Vinyl
C5H11	AlMe2				Pd(PPh3)4, ZnCl2	73(>97)	[18]
	AlMe2		Br		Pd(PPh3)4, ZnCl2	73(>97)	[18]
		AlMe2	Br		Pd(PPh3)4, ZnCl2	70(>98)	[18]
ZO		AlMe2	Br		Pd(PPh3)4, ZnCl2	43(94)	[251]
H3C		(E )-β-
H3C	)		I
	2 CuLi		I		Pd(PPh3)4, THF	96(>99)	[226]
CH3				C5H11	Pd(PPh3)4, THF	96(>99)	[226]
CH3
H3C	Cu.MgX2		I
	Cu.MgX2		I		Pd(PPh3)4, ZnCl2	78(>99)	[227]
C2H5				C5H11	Pd(PPh3)4, ZnCl2	78(>99)	[227]
C2H5
H3C	Cu.MgX		Br
	Cu.MgX				Pd(PPh3)4, THF	74(99)	[227]
	2				Pd(PPh3)4, THF	74(99)	[227]
C2H5				Ph
C2H5

(Continued )

392

III Pd-CATALYZED CROSS-COUPLING

TABLE 13. (Continued )

Type of

Yield

Alkenyl

(Selectivity)

Refe-

Electrophile

Conditions

rence

C5H11

Pd(PPh3)4, ZnCl2

65(>97)

[18]

AlMe2

C4H9

CH3

H3C

Cu.MgX2

C5H11

Pd(PPh3)4, THF

70(>99)

[227]

C5H11

C6H13

Pd(PPh3)4,

BBr2

C6H13

52(99)

[122]

C4H9

LiOH, H2O

TMS

BBr2

C6H13

Pd(PPh3)4,

81(99)

[122]

LiOH, H2O

C6H13

(Z )-β-

H3C

)

C5H11

Pd(PPh3)4, ZnX2

92(>99)

[226]

2 CuLi

H3C

CH3

Cu.MgX2

C5H11

Pd(PPh3)4

64(>99)

[227]

H3C

C2H5

Cu.MgX2

C5H11

Pd(PPh3)4

70(>99)

[227]

iC3H7

(Z )-β-

TMS

Pd(PPh3)4,

C4H9

[122]

BBr2

LiOH, H2O

C6H13

Cu.MgX2

C2H5

Pd(PPh3)4, THF

70(>99)

[227]

H3C

C2H5

β,β-

CH3

)

2 CuLi

Pd(PPh3)4, ZnX2

94(>99)

[226]

CH3

H3C

Cu.MgX2

C4H9

Pd(PPh3)4, THF

55(>99)

[227]

C2H5

CH3

H3C

SnBu3

cis-α,β-

TfO

Pd(PPh3)4,

[188],

CH3

LiCl, THF

[189]

aSee Table 10.

III.2.6 Pd-CATALYZED ALKENYL–ARYL

393

involved. Some prototypical examples are shown in Schemes 60–62. Particularly noteworthy is the development of a highly efﬁcient and stereoselective iterative and convergent method for the synthesis of carotenoids and retinoids shown in Schemes 61 and 62.[240] It should be noted that the reactivity of , -substituted alkenylalanes can be signiﬁcantly enhanced by the addition of ZnBr2 or ZnCl2.[18]

Despite the high efﬁciency and stereoselectivity associated with Protocol I shown in Scheme 59, its full development as a method for the synthesis of carotenoids, retinoids, and

			I				OZ
Me		Al	cat. Pd(PPhCl)
		3	2			3 2		OZ
		2 2						OZ
	cat. CpZrCl		+2n-BuLi
				[239]				vitamin A(Z=H), 60%
				[239]				vitamin A(Z=H), 60%
					Scheme 60
					Me3Al			AlMe2
				cat. Cp2ZrCl2
				cat. Cp2ZrCl2

cat. Cl2Pd(PPh3)2 + DIBAL, ZnBr2

[240]

		β-carotene, 68%
	Scheme 61
	1. Me3Al
	cat. Cp2ZrCl2
	2. Pd(PPh3)4, ZnBr2			Br
	Br
	I		85%	1
	[240]		85%	1
	[240]
A		A
	82%			74%
	82%			74%

1.Me3Al

cat. Cp2ZrCl2

2.Pd2(dba)3, TFP, ZnBr2, 1

γ-carotene, 53%
A = (1). Me3Al, cat. Cp2ZrCl2; (2). Pd2(dba)3, TFP, ZnBr2,		TMS	; (3). TBAF



	Br

Scheme 62

394	III Pd-CATALYZED CROSS-COUPLING

other natural products has been made only within the past few years. Consequently, it has not yet been widely utilized. Although more circuitous, Protocols II and IV have been more widely exploited in the synthesis of a variety of natural products, such as rapamycin,[241],[242] caliculin A,[243]–[245] indanomycin,[246] sanglifehrin A,[247] vitamin A,[239],[248] restrictinol,[249] and - and -carotenes,[240] as exempliﬁed by the results shown in Schemes 63 and 64. The structures of these natural products are shown in Table 3 in Sect. III. 2.18.

[240]

1. LDA

1. BuLi

100%

2. ClPO(OEt)2

2. ZnCl2

3. LDA

)

[239]

87%

A = (1) Me3Al, Cp2ZrCl2; (2) I2

B = Pd(PPh3)4,

68%

OZ vitamin A (Z = H)

				I	OZ
C = (1) Pd2(dba)3, TFP,			TMS; (2) TBAF D = (1) Me3Al, Cp2ZrCl2 ; (2) n-BuLi; (3) (CHO)n



	BrZn
				Scheme 63
		1. Pd(PPh3)4, TlOH
I	O			B(OH)2	O
I				B(OH)2
	OMe	2. TBAF			OMe
	OSiEt3			restrictinol, 69%	OH

Scheme 64

Haloboration of 1-alkynes followed by chemoselective cross-coupling can provide, -substituted alkenylborons that undergo Pd-catalyzed coupling with alkenyl halides to give stereodeﬁned trisubstituted alkenes.[122] The use of other , -substituted alkenylmetals has also been demonstrated. For example, exocyclic alkenylmetals containing Zn and Sn have been used in Pd-catalyzed cross-coupling approach to the vitamin D skeleton[250] as shown in Scheme 65. It should be noted that the alkenylzinc appeared to be distinctly superior to the corresponding alkenyltins.

				R
		I
R	R		H	M = ZnBr	95%



1. t-BuLi
1. t-BuLi		OTBS		M = SnMe3	<33%
2. MX		OTBS		M = SnMe3	<33%
				M = SnBu3	0%

	H		Pd(0)
	H		H
Br		M	OTBS
			Scheme 65

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