Isolable cyclotrisilanes"] owe their existence to the stabilization of the strained three-membered ring by bulky substituents. Not only their stability, however, but also their formation through metal-mediated coupling of dihalosilanes is founded on the steric demands of the substituents at the silicon centers:", 2l bulky groups force a conformation on the intermediate 3-halo-I-metallatrisilane in which the proximity of the termini of the acyclic trisilane favors an intramolecular ring closure (ThorpeIngold effect). Previously, bulky alkyl or aryl groups whose silyl substituents are blocked by alkyl groups in the ortlzo positions were used as suitable substituents for the synthesis of stable cyclotrisilanes. Against this background. the stability and the ready formation of the cyclotrisilane laL3] by transformation of the appropriate diaryldihalosilane with magnesium is astounding; the 2-(dimethylaminomethyl)phenyl substituent introduced by Corriu et al.I4] into the chemistry of highly coordinated silicon compounds has only a relatively moderate steric effect and should actually favor the formation of larger rings. Also unusual is the reactivity of la. which transfers all three diarylsilanediyl constituents of the ring to several substrates.[3. 51 la: R = 2-(Me,NCH2)C6H, 1 b: R = 2,6-Me,C6H3 R2Both the unexpected stability and the reactivity of l a made it desirable to investigate its structure in the crystal. An added goal was to gain more insight into the structural principles of 2-(diniethylaminomethyl)phenyl-substituted oligosilanes. We report here on the structures of the cyclotrisilane l a and the di-and tetrasilanes 2b and 2c, accessible from it. of molecule l a (A) small deviations from the ideal geometry of an equilateral triangle are present. in l a (B) the Si-Si distances are identical, within experimental error. The endocyclic bonds (between 235 and 237 pm) are distinctly shorter than most experimentally determined bonds in other cy~lotrisilanes.[~~ The average Si-Si bond length in lb,[*] which was the only perarylated cyclotrisilane whose structure in the crystal had been studied previously, is 240.7 pm. In contrast. the average Si-Si bond length of 236.2 pm in la approaches the value calculated for the parent compound (233-234 pm).[91 These observations show clearly that the 2-(dimethylaminomethyl)phenyl substituent does indeed have a relatively low steric demand. and class cyclotrisilane l a as a structurally hardly distorted member of this type of compound. The unusually facile cleavage of all three Si-Si bonds of l a cannot be predicted from its solid-state structure. Intramolecular stabilization of an intermediate silanediyl by the amino group of the aryl substituents cannot be excluded as reason for this reactivity.""] Cyclic trisilane la can be converted by treatment with lithium into the vicinal dilithiodisilane," 'I whose reactions with electrophiles make many other disilanes accessible. Thus the protonation of 2a with cyclopentadiene'"] yields the disilane 2b, [l3] which is moderately sen...
Isolierbare Cyclotrisilanel'] verdanken ihre Existenz der Stabilisierung des gespannten Dreiringes durch raumerfiillende Substituenten. Aber nicht nur ihre Stabilitat, auch ihre Bildung durch metallvermittelte Kupplung von Dihalogentrisilanen liegt im sterischen Anspruch der Substituenten an den Siliciumzentren begrundet ['* 'I: Voluminose Gruppen erzwingen im intermedihen 3-Halogeno-I metalla-trisilan eine Konformation, in der die raumliche Nahe der Termini des acyclischen Trisilans einen intramolekularen Ringschlull begunstigt (Thorpe-Ingold-Effekt) . Als geeignete Substituenten fur die Synthese stabiler Cyclotrisilane wurden bisher sperrige Alkyl-oder Arylgruppen, deren zum Siliciumzentrum ortho-standigen Positionen durch Alkylreste blockiert sind, verwendet. Vor diesem Hintergrund ist die Stabilitat ebenso wie die glatte Bildung des Cyclotrisilans 1 a"] durch Umsetzung des entsprechenden Diaryldihalogentrisilans rnit Magnesium erstaunlich, sollte doch der sterisch relativ anspruchslose 2-(Dimethylaminomethyl)phenyl-Substituent, der von Corriu et aI.I4] in die Chemie der hochkoordinierten Siliciumverbindungen eingefiihrt wurde, eigentlich die Bildung groDerer Ringe favorisieren. Ungewohnlich ist auch die Reaktivitat von 1 a, das unter milden thermischen Bedingungen alle drei ringkonstituierenden Diarylsilandiyl-Einheiten auf zahlreiche Substrate ubertragt13. ' I. Sowohl die unerwartete Sta-la: R = ~-(M~zNCH&H~ l b : R = 2,6-Me2C,H3 R2 SI ~~L ! s i , q * lc: R = H bilitat als auch die Reaktivitat von 1 a lieBen es wijnschenswert erscheinen, seine Struktur im Festkorper zu ermitteln, auch um mehr Einsicht in die Strukturprinzipien bei 2-(Dimethylamino-methy1)phenyLsubstituierten Oligosilanen zu erhalten. Wir berichten hier uber die Strukturen des Cyclotrisilans 1 a und der daraus praparativ zuganglichen Di-und Tetrasilane 2 b b m . 2 c. 2a: R = Li Ar2 R/s'. SI' 2b: R = H Ar2 2c: R = SiMe3 Ar = 2-(Me2NCH2)C6H, 2d: = 1 a kristallisiert rnit zwei unabhangigen Molekulen in der asymmetrischen Einheitr6]. Sowohl in 1 a (A) (Abb. 1) als auch in 1 a (B) besteht keine koordinative Wechselwirkung zwischen den Amino-und den Siliciumzentren ; dariiber hinaus schliel3t ["I Dr. J. B e h e r , Dip1.-Chem. H. Ihmels [**I Diese Arbeit wurde von der Deutschen Forschungsgemeinschaft und dem Fonds der Chemischen lndustrie gefordert. N. D. wurde durch ein ERASMUS-Austauschsripendium der Europaischen Union unterstiitzt. die 29Si-NMR-Verschiebung von 1 a, die mit b = -64.7 in dem fur perarylierte Cyclotrisilane erwarteten Bereich liegt, auch in Losung die Bildung hochkoordinierter Strukturen aus. Treten im Dreiring des Molekuls l a (A) noch kleine Abweichungen von der Idealgeometrie des gleichseitigen Dreiecks auf, so sind in 1 a (B) die Si-Si-Abstande innerhalb der Fehlergrenzen gleich. Die endocyclischen Bindungen sind rnit Werten zwischen 235.2 und 237.2 pm deutlich kurzer als die meisten experimentell in anderen Cyclotrisilanen ermittelten"'. So betragt die mittlere Si-Si-Bindungslange in 1 bF8], das bisher als einziges perarylier...
1995 structure structure (organic substances) K 9000 -039Structures of 2-(Me2NCH2)C6H4-Substituted Oligosilanes.-The structures of the silanes (I), (II), and (III) are confirmed by X-ray analysis. (I) is found to be almost undistorted. This indicates that the 2-(dimethylaminomethyl)phenyl substituent has a relatively low steric demand. (II) adapts an unexpected synperiplanar conformation about the Si-Si bond, while in (III) an approximately antiperiplanar arrangement of the substituents about the Si-Si bond is found. -(BELZNER, J.; DETOMI, N.; IHMELS, H.; NOLTEMEYER, M.; Angew.
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