ca01 only detailed ToC 1..24
ca01 only detailed ToC 1..24 ca01 only detailed ToC 1..24
172 Science of Synthesis 5.1 Germanium Compounds for the synthesis of derivatives such as bis(halodimethylgermyl)thiophene 15 synthesized from 2,5-dibromothiophene (14), [30] which can be difficult to synthesize using conventional organometallic procedures due to their tendency to participate in oligomerization i.e. reaction of 16 to give 17 (Scheme 5). [62] The major drawback with this methodology is the availability of 1,2-dichloro-1,1,2,2-tetramethyldigermane, [63] which is currently not commercially available and requires synthesis before use. Scheme 5 Palladium(0)-Mediated Coupling of Digermanes with Aryl Halides [30,61,62] PhI (Me2ClGe)2, Pd(PPh3)4 benzene, 120 o C Ph I Ge Me2 11 12 31% Br S 16 + Ph Ge Cl Me2 13 68% S Br (Me2ClGe)2, Pd(PPh3)4, benzene, 120 X = Br, Cl 56% after treatment with MeLi to give X = Me 14 15 oC X Ge Me2 S 1. BuLi, TMEDA 2. (Me2ClGe) 2 80% Me2Ge S GeMe2 S S S Me 2Ge GeMe 2 Iododimethylphenylgermane (12) and Chlorodimethylphenylgermane (13); Typical Procedure: [61] A soln of iodobenzene (11; 82 mg, 0.4 mmol)in benzene (0.8 mL)(CAUTION: carcinogen!) was treated with 1,2-dichloro-1,1,2,2-tetramethyldigermane (0.4 mmol)in the presence of Pd(PPh 3) 4 (23 mg, 0.02 mmol)at 1208C for 5 h. Purification by fractional distillation led to the isolation of 12; yield: 38 mg (31%)and 13; yield: 59 mg (68%)(yields calculated by GC). 5.1.22.3 Method 3: From Aryl Halides by Insertion of Dichlorogermylene Insertion of germylenes into aryl carbon–halogen bonds of simple aromatics generally proceeds in moderate yield using elevated temperatures (>150 8C)and sealed reaction vessels. [64,65] A modification of this procedure involving the use of the most readily accessible dichlorogermylene source, dichlorogermylene–dioxane complex, [66] and catalytic quantities of anhydrous aluminum trichloride allows these reactions to be conducted under relatively mild conditions (~80 8C, atmospheric pressure)giving insertion in excellent yields (Scheme 6). [4] The tolerance of this methodology towards other functionalities however remains to be established. Scheme 6 Germylene Insertion into Aromatic Halides [4] R 1 18 Br GeCl2 dioxane AlCl3, 80 oC R 1 = 4-Me 98%; (19/20) 64:32 17 R 1 19 GeCl 3 + R 1 20 X Ge Me2 GeBr 3
5.1.22 Aryl- and Heteroarylgermanes 173 Trichloro(4-tolyl)germane (19,R 1 = 4-Me) and Tribromo(4-tolyl)germane (20,R 1 = 4-Me); Typical Procedure: [4] A mixture of 4-bromotoluene (250 mL), GeCl 2 •dioxane (10 g, 43.2 mmol), and anhyd AlCl 3 (0.29 g, 2.17 mmol)was heated to 808C for 24 h with continuous stirring. After the mixture was cooled to rt, it was filtered, the dioxane removed in vacuo, and the unreacted 4bromotoluene removed by vacuum distillation (43 8C/5”10 –4 Torr)to leave the product as a white solid; yield: 13.33 g (98%); ratio 19/20 64:32 (by GC/MS). 5.1.22.4 Method 4: Heteroarylgermanes by Cycloaddition Cycloaddition reactions involving alkynylgermanes (Section 5.1.20)are generally analogous to those observed using nonmetalated alkynes. Hence, nitrogen-based heterocycles such as pyrazoles 22, triazoles 23, and pyridazines 24 can be readily accessed in high yield by treatment of alkynylgermanes 21 with diazomethane, [67] azides, [68] or tetrazines, [69] respectively (Scheme 7). Scheme 7 Nitrogen Heterocycles from Alkynylgermanes by Cycloaddition [67–70] R 2 21 GeR 1 3 CH 2N 2 R3N3 R1 = Me, Et; R3 R = Na, Ph, 4-Tol, 4-O2NC6H4 32−84% 2 = CH NR4 N N N N R1 = Me; R2 = H 80% R1 = Me; R2 = GeMe3 93% R 1 3Ge R 2 22 N N N N H R 1 3Ge R 2 R 2 23 24 GeR 1 3 4-(Trimethylgermyl)pyridazine (24, R 1 = Me; R 2 = H); Typical Procedure: [70] Tetrazine (82.0 mg, 1.0 mmol)and (ethynyl)trimethylgermane (21,R 1 =Me;R 2 = H; 300 mg, 1.80 mmol, contains approx. 20% THF)in MeCN were heated to reflux for 90 min. The resulting pale yellow soln was concentrated in vacuo. The residual brown liquid was purified by column chromatography (silica gel, Et 2O)to afford the product as colorless oil (which turned yellow quite rapidly in air); yield: 158 mg (80%); 1 H NMR (CDCl 3, ä): 0.45 (s, 9H), 7.50 (dd, J = 4.9, 1.8 Hz, 1H), 9.06 (dd, J = 4.9, 1.4 Hz, 1H), 9.17 (dd, J = 1.8, 1.4 Hz, 1H); 13 C NMR (CDCl 3, ä): –2.43, 131.29, 142.43, 150.64, 154.98. N N NR 3 for references see p 175
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5.1.22 Aryl- and Heteroarylgermanes 173<br />
Trichloro(4-tolyl)germane (19,R 1 = 4-Me) and Tribromo(4-tolyl)germane (20,R 1 = 4-Me);<br />
Typical Procedure: [4]<br />
A mixture of 4-bromotoluene (250 mL), GeCl 2 •dioxane (10 g, 43.2 mmol), and anhyd AlCl 3<br />
(0.29 g, 2.17 mmol)was heated to 808C for 24 h with continuous stirring. After the mixture<br />
was cooled to rt, it was filtered, the dioxane removed in vacuo, and the unreacted 4bromotoluene<br />
removed by vacuum distillation (43 8C/5”10 –4 Torr)to leave the product as<br />
a white solid; yield: 13.33 g (98%); ratio 19/20 64:32 (by GC/MS).<br />
5.1.22.4 Method 4:<br />
Heteroarylgermanes by Cycloaddition<br />
Cycloaddition reactions involving alkynylgermanes (Section 5.1.20)are generally analogous<br />
to those observed using nonmetalated alkynes. Hence, nitrogen-based heterocycles<br />
such as pyrazoles 22, triazoles 23, and pyridazines 24 can be readily accessed in high yield<br />
by treatment of alkynylgermanes 21 with diazomethane, [67] azides, [68] or tetrazines, [69] respectively<br />
(Scheme 7).<br />
Scheme 7 Nitrogen Heterocycles from Alkynylgermanes by Cycloaddition [67–70]<br />
R 2<br />
21<br />
GeR 1 3<br />
CH 2N 2<br />
R3N3 R1 = Me, Et;<br />
R3 R<br />
= Na, Ph, 4-Tol, 4-O2NC6H4<br />
32−84%<br />
2 = CH NR4 N N<br />
N N<br />
R1 = Me; R2 = H 80%<br />
R1 = Me; R2 = GeMe3 93%<br />
R 1 3Ge R 2<br />
22<br />
N<br />
N<br />
N<br />
N<br />
H<br />
R 1 3Ge R 2<br />
R 2<br />
23<br />
24<br />
GeR 1 3<br />
4-(Trimethylgermyl)pyridazine (24, R 1 = Me; R 2 = H); Typical Procedure: [70]<br />
Tetrazine (82.0 mg, 1.0 mmol)and (ethynyl)trimethylgermane (21,R 1 =Me;R 2 = H; 300 mg,<br />
1.80 mmol, contains approx. 20% THF)in MeCN were heated to reflux for 90 min. The<br />
resulting pale yellow soln was concentrated in vacuo. The residual brown liquid was<br />
purified by column chromatography (silica gel, Et 2O)to afford the product as colorless<br />
oil (which turned yellow quite rapidly in air); yield: 158 mg (80%); 1 H NMR (CDCl 3, ä):<br />
0.45 (s, 9H), 7.50 (dd, J = 4.9, 1.8 Hz, 1H), 9.06 (dd, J = 4.9, 1.4 Hz, 1H), 9.17 (dd, J = 1.8,<br />
1.4 Hz, 1H); 13 C NMR (CDCl 3, ä): –2.43, 131.29, 142.43, 150.64, 154.98.<br />
N N<br />
NR 3<br />
for references see p 175
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