Hydrometallations of alkynes with HSiR3, HSnBu3and HBR2are useful reactions, because introduced metal groups MR in 75 are reactive and can be displaced with other functional groups to give the functionalized alkenes 76. Also hydro- heteroatom addition occurs using substrates containing H-S, H-Se, and H-P bonds.
75 76
E+ R
R + Pd(0)
H MR
H-SiR3, HSnBu3, H-BR2, H-hetero atoms
R R
H MR
H MR =
R R
H E
Compared with hydrosilylation of alkenes, less extensive studies have been car- ried out on hydrosilylation of alkynes. Mono- and dihydrosilylation occur depend- ing on conditions. Yamamoto found concomitant dimerization–hydrosilylation of 1-heptyne catalyzed by Pd-PPh3 catalyst to give a mixture of products 77, 78, and 79. Their ratios depend on the hydrosilanes used. The head–tail dimer 78 was the main product when HSiCl3 was used. The expected monohydrosilylation
took place to give 77with HSiMe2Cl. A small amount of tail –tail dimer 79was obtained with HSiCl3 and HSiMeCl2 [22].
Oshima found that hydrosilylation with less reactive triorganosilanes can be carried out by using electron-rich phosphines. Hydrosilylation of terminal alkyne 80 with HSiPh3 was carried out using PCy3 as a ligand at room temperature to provide81 regioselectively in high yield [23].
78
t-Bu
+ HSiX3
(h3-allyl-PdCl)2, PPh3 CH2Cl2, 50 °C
t-Bu
SiPh3
9 38 97
46 3 76
15 16 +
+
77 79
80 81
Pd2(dba)3-PCy3 HSiPh3
rt, 95%
n-C5H11
n-C5H11
SiX3
SiX3 SiX3
n-C5H11
n-C5H11 n-C5H11 n-C5H11
+
15 HSiMeCl2
HSiMe2Cl HSiCl3
Preparation of chiral benzylic alcohols has been achieved by asymmetric dihy- drosilylation of terminal arylalkynes, followed by oxidation [24]. The chiral diol 85with 95 % ee was obtained by direct Pd-catalyzed asymmetric dihydrosilylation of phenylacetylene (82) with HSiCl3using MOP [(R)-VI-18], followed by oxida- tion of 1,2-disilylated product84, but the yield of84was 33 % (the main product was 1-trichlorosilyl-2,4-diphenyl-1,3-butadiene). Better results were obtained by hydrosilylation using two catalysts. The first Pt-catalyzed monosilylation to give 83was followed by the second hydrosilylation catalyzed by the chiral Pd catalyst.
The diol85 with 95 % ee was obtained in 87 % overall yield by this method.
Ph
Ph
SiCl3
Ph
SiCl3 SiCl3
Ph
OH OH 82
84
83
+ (h3-allyl-PdCl)2-(R)-VI-18
87% from alkyne 95% ee (h3-allyl-PdCl)2-(R)-VI-18
85 33%
[PtCl2(C2H4)]2
HSiCl3
HSiCl3
Hydrostannation of alkynes proceeds smoothly to give alkenylstannanes, which are used for further transformations. The reaction is induced by transition metal catalysts and free-radical sources, giving different regioisomers 86 and 87 [25].
Pd complexes are effective catalysts for cis addition. Usually a mixture of the regioisomers 86 and 87 is obtained, and the ratio depends on the substituents of the alkynes and ligands. Hydrostannation of 82 afforded equal amounts of the regioisomers88 and89.
R R SnBu3
R SnBu3
89, 48%
+ Bu3SnH + HSnBu3
Pd(0)
86 87
Pd(Ph3P)4
82 88, 48%
Ph
Ph Bu3Sn
Ph
SnBu3
+
+
Several reports on regioselective hydrostannation have been published. In hydro- stannation of asymmetric diarylacetylene 90, stannation occurred regioselectively at the carbon close to theortho-substituted benzene to give91 [26]. Hydrostanna- tion of the serine-derived ynoate92at−10◦C afforded theα-stannyl ester93with high regioselectivity (16 : 1), and the reaction was applied to the total synthesis of asperazine [27].
90 Bu3Sn H
MeO THF, 94%
NBoc O
H
CH2OMe
NBoc O
H OMe
91
92 93
+ HSnBu3
CH2Cl2,−10°C 88%
+ HSnBu3
PdCl2(PPh3)2
Pd(PPh3)4
SnBu3
CO2Me
Usually, hydrostannation of propargyl alcohols is not regioselective. However, theβ-stannyl alcohol95was obtained with complete regioselectivity by hydrostan- nation of the propargyl alcohol 94 using P(o-Tol)3 as a ligand, and the reaction was applied to the total synthesis of zoanthamine [28].
While Pd-catalyzed hydrostannation of the conjugated enyne 96 afforded the alkadienylstannane 97 regioselectively, a radical-initiated reaction provided the terminal adduct98[29].
THF, rt, 84%
94 95
PdCl2[P(o-Tol)3]2
+ HSnBu3 MOMO
OH MOMO
OH
OTBDMS
SnBu3
OTBDMS
+ Bu3SnH
90°C, 55%
97
96
85%
98 AIBN
O O
O OMe
O OMe
SnBu3
O OMe
SnBu3 PdCl2(PPh3)2
Cyclization of the 1,6-enyne99with HSnBu3provided the cyclopentane 102at room temperature. Ligandless Pd(OAc)2was used. In this reaction, regioselective hydropalladation of the alkyne generates100at first, and subsequent alkene inser- tion yields 101. Finally reductive elimination gives rise to the cyclized product 102[30].
99 100
101 102
Pd(OAc)2, THF + HSnBu3
rt, 61%
O O
O O Pd
H Bu3Sn
H Pd Bu3Sn
O O
H Bu3Sn
O O
Hydrogermylation of 1-octyne with tri(2-furyl)germane10at room temperature in water provided the 1,3-dienylgermane104 in high yield [31].
(η3-allyl-PdCl)2,III-4
R3GeH= (2-furyl)3GeH
H2O, 91%
+ R3GeH
104 C6H13
C6H13
C6H13
GeR3 10
Hydrophosphinylation of 1-octyne with diphenylphosphine oxide (105) afforded the alkenylphosphine oxide106by internal attack with high regioselectivity. How- ever, addition of a small amount of Ph2P(O)OH changed the regioselectivity to provide the product107by terminal attack [32].
70°C, 92%
+
C6H13 C6H13 PPh2
106 O
107
105
C6H13 PPh2
O
[PdMe2(PPhMe2)2] Ph2P(O)H
The alkenylphosphonium salt 108 was prepared by the addition of PPh3 at the internal carbon of 1-alkyne in the presence of methanesulfonic acid. After anion exchange with LiPF6, the conjugated diene 109 was obtained by the reaction of 108with an aldehyde [33].
Ph
Ph
PPh3 + PPh3
Ph C7H15
1. Pd(PPh3)4,MeSO3H
1. (TMS)2NNa 2. n-C7H15CHO 78%
108
109
2. LiPF6, EtOH, 91% PF6−
Hydrothiolation of 1-alkynes with thiols gives vinyl sulfides with high regio- and stereoselectivities. Reaction of 1-octyne with benzenethiol catalyzed by Pd(OAc)2 gave the Markonikov-type product110. Isomerization of the double bond occurred and the isomer 111was obtained when PdCl2(PhCN)2was used [34].
+ PhS-H 110
111 PdCl2(PhCN)2
R
R 85%
R=n-C5H11 66%
SPh
R
SPh Pd(OAc)2