“…Values in the range 10.4-12.0 Hz have been reported for the trimethylsilyl analogue of 11, while trans-geometries result in higher values (15- 18 Hz) for the trans-vicinal olefinic coupling. 10,11 This geometry is also confirmed by the 13 C-NMR chemical shifts of C-1 (13.20 ppm) and C-4 (12.80 ppm) of the compound 11. These values would be at too high a field for two carbon atoms trans to each other.…”
Section: Resultsmentioning
confidence: 77%
“…In all cases IR spectroscopic monitoring of the reaction shows the initial conversion of Cr(CO) 6 into the respective Cr(CO) 4 (η 4 -1,3-diene) ( II ), presumably via a Cr(CO) 5 (η 2 -1,3-diene) intermediate ( I ), Scheme . The photochemical formation of complexes of the type I and II from group 6 hexacarbonylmetal(0) and dienes is known, − and the role of II or Cr(CO) 4 (η 2:2 -norbornadiene) as the precursor to the active catalyst in the photocatalytic hydrogenation of conjugated dienes or norbornadiene is well established. Expectedly, the hydrosilylation of 1,3-butadiene under continuous irradiation in the presence of Cr(CO) 4 (η 4 -1,3-butadiene) as catalyst instead of hexacarbonylchromium(0) gives the same result. 1 …”
Section: Resultsmentioning
confidence: 99%
“…These two hydrosilylation products are fully characterized by 1 H-and 13 C-NMR spectroscopy. This way, the main product 12a is identified as cis-1-(triethylsilyl)-2-pentene, which results from cis-1,4-hydrosilylation with attachment of the triethylsilyl group to C-1 of the diene.…”
Section: Resultsmentioning
confidence: 99%
“…The absence of any 1 H-NMR signals in the olefinic region eliminates alternative structures which would result from 1,2-addition of triethylsilane. Accordingly, the 13 Two 1,4-addition products, cis-1-(triethylsilyl)-2-methyl-2-butene (14a) and 1-(triethylsilyl)-3-methyl-2-butene (14b), are formed from the photocatalytic reaction of isoprene (4) with triethylsilane in the presence of hexacarbonylchromium(0), eq 4, in 76% and 22% yields, respectively, as determined by GC. Pure samples of both products are separated by distillation which, however, goes along with some loss of material.…”
Section: Resultsmentioning
confidence: 99%
“…The coupling pattern of H-5 and H-4 is indicative of an ethyl moiety, which in turn implies that in the 1,4-hydrosilylation the triethylsilyl unit is attached to the terminal carbon C-1 of the diene. In the 13 C-NMR spectrum of 15, one observes the signals of the methyl groups C-5 and C-6 at δ ) 14.59 and 26.41 ppm, respectively. The large difference between their chemical shifts is ascribed to the deshielding effect exerted by the olefinic carbon C-2 on C-6.…”
Chromium carbonyl photocatalyzed hydrosilylation of 1,3-butadiene, trans-1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-butadiene, trans-2-methyl-1,3-pentadiene, and 1,3-cyclohexadiene with triethylsilane yields the cis-1,4-adducts, 1-(triethylsilyl)-2-butene derivatives, as the main products which have been isolated by distillation or preparative GC and fully characterized by NMR spectroscopy. The proposed mechanism involves the initial conversion of Cr(CO) 6 into Cr(CO) 4 (η 4 -1,3-diene) followed by a further photolytic CO substitution by triethylsilane forming a Cr(CO) 3 (H)(SiEt 3 )(η 4 -1,3-diene) intermediate. Experiments with D-SiEt 3 lead to the conclusion that reversible addition of the hydride to the diene with formation of an η 3 -enyl intermediate occurs prior to the irreversible silyl transfer to the organic moiety. The 1,4-hydrosilylation adduct is then replaced by new substrates to complete the catalytic cycle.
“…Values in the range 10.4-12.0 Hz have been reported for the trimethylsilyl analogue of 11, while trans-geometries result in higher values (15- 18 Hz) for the trans-vicinal olefinic coupling. 10,11 This geometry is also confirmed by the 13 C-NMR chemical shifts of C-1 (13.20 ppm) and C-4 (12.80 ppm) of the compound 11. These values would be at too high a field for two carbon atoms trans to each other.…”
Section: Resultsmentioning
confidence: 77%
“…In all cases IR spectroscopic monitoring of the reaction shows the initial conversion of Cr(CO) 6 into the respective Cr(CO) 4 (η 4 -1,3-diene) ( II ), presumably via a Cr(CO) 5 (η 2 -1,3-diene) intermediate ( I ), Scheme . The photochemical formation of complexes of the type I and II from group 6 hexacarbonylmetal(0) and dienes is known, − and the role of II or Cr(CO) 4 (η 2:2 -norbornadiene) as the precursor to the active catalyst in the photocatalytic hydrogenation of conjugated dienes or norbornadiene is well established. Expectedly, the hydrosilylation of 1,3-butadiene under continuous irradiation in the presence of Cr(CO) 4 (η 4 -1,3-butadiene) as catalyst instead of hexacarbonylchromium(0) gives the same result. 1 …”
Section: Resultsmentioning
confidence: 99%
“…These two hydrosilylation products are fully characterized by 1 H-and 13 C-NMR spectroscopy. This way, the main product 12a is identified as cis-1-(triethylsilyl)-2-pentene, which results from cis-1,4-hydrosilylation with attachment of the triethylsilyl group to C-1 of the diene.…”
Section: Resultsmentioning
confidence: 99%
“…The absence of any 1 H-NMR signals in the olefinic region eliminates alternative structures which would result from 1,2-addition of triethylsilane. Accordingly, the 13 Two 1,4-addition products, cis-1-(triethylsilyl)-2-methyl-2-butene (14a) and 1-(triethylsilyl)-3-methyl-2-butene (14b), are formed from the photocatalytic reaction of isoprene (4) with triethylsilane in the presence of hexacarbonylchromium(0), eq 4, in 76% and 22% yields, respectively, as determined by GC. Pure samples of both products are separated by distillation which, however, goes along with some loss of material.…”
Section: Resultsmentioning
confidence: 99%
“…The coupling pattern of H-5 and H-4 is indicative of an ethyl moiety, which in turn implies that in the 1,4-hydrosilylation the triethylsilyl unit is attached to the terminal carbon C-1 of the diene. In the 13 C-NMR spectrum of 15, one observes the signals of the methyl groups C-5 and C-6 at δ ) 14.59 and 26.41 ppm, respectively. The large difference between their chemical shifts is ascribed to the deshielding effect exerted by the olefinic carbon C-2 on C-6.…”
Chromium carbonyl photocatalyzed hydrosilylation of 1,3-butadiene, trans-1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-butadiene, trans-2-methyl-1,3-pentadiene, and 1,3-cyclohexadiene with triethylsilane yields the cis-1,4-adducts, 1-(triethylsilyl)-2-butene derivatives, as the main products which have been isolated by distillation or preparative GC and fully characterized by NMR spectroscopy. The proposed mechanism involves the initial conversion of Cr(CO) 6 into Cr(CO) 4 (η 4 -1,3-diene) followed by a further photolytic CO substitution by triethylsilane forming a Cr(CO) 3 (H)(SiEt 3 )(η 4 -1,3-diene) intermediate. Experiments with D-SiEt 3 lead to the conclusion that reversible addition of the hydride to the diene with formation of an η 3 -enyl intermediate occurs prior to the irreversible silyl transfer to the organic moiety. The 1,4-hydrosilylation adduct is then replaced by new substrates to complete the catalytic cycle.
In diesem Fortschrittsbericht wird ein neues Syntheseprinzip für Multimetall‐π‐Komplexe mit bisher nicht bekannten Strukturelementen diskutiert. Überraschenderweise reagieren die Olefin‐Nickel(0)‐Komplexe 1,5,9‐Cyclododecatriennickel(0) und Bis(1,5‐cyclooctadien)nickel(0) mit Hauptgruppenmetallen, insbesondere Alkalimetallen sowie deren Hydriden oder Organometall‐Verbindungen, zu Nickel‐Ligand‐Einheiten mit Überschußladung. Diese Einheiten sind bestrebt, die Überschußladung auf π‐Liganden wie Olefine oder Distickstoff weiterzugeben. Darüber hinaus konnten Multimetallkomplexe mit elektronenreichen Übergangsmetall‐π‐Ligand‐Einheiten aus Metallocenen, Alkalimetallen und ungesättigten Verbindungen erhalten werden. Synthesen, Strukturen und Reaktionen dieser neuen Substanzklassen werden zusammenfassend besprochen.
Den Synthesechemikern stehen heute viele Metallaktivierungsverfahren zur Verfugung. Nachdem mit der Einfuhrung der Rieke-Metalle zu Beginn der siebziger Jahre ein erster Durchbruch in der Metallaktivierung erzielt worden war, wurden in der Folge eine Reihe weiterer, zum Teil noch leistungsfahigerer Methoden entwickelt, mit denen nicht nur klassische metallinduzierte Umsetzungen substantiell verbessert, sondern auch vollig neuartige Reaktionen entdeckt werden konnten. Die einzelnen Aktivierungsverfahren werden in dieser Ubersicht diskutiert und, soweit es mit den verfugbaren Daten moglich ist, miteinander verglichen. Dabei sind vor allem die Metall-Graphit-Kombinationen durch meist unubertroffene Wirksamkeit bei gleichzeitig einfacher Herstellung und Handhabung hervorzuheben. Wie zahlreiche Anwendungen dieser Reagentien auf polyfunktionelle Substrate, besonders Naturstoffe, zeigen, schlieBen sich hohe Reaktivitat des Metalls und ausgezeichnete Selektivitat der Reaktion keinesfalls aus. Neben den rein praparativen Aspekten werden auch die bisherigen Erkenntnisse iiber Grundhgen und Grenzen der Metallaktivierung vorgestellt und uber Versuche zur Erfassung der Morphologie hochdisperser Systeme berichtet.
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