Destabilisierte 2‐Norbornyl‐Kationen

Abstract

ten quadratisch-pyramidalen Koordinationspolyeders liegt. Jeweils vier Sauerstoffatome ergeben rnit einer Winkelsumme von 320.1 o den basalen Ligandensatz. Die Winkel an den 0x0-Briicken betrapen im Mittel 89.3", und die Bindungslangen zwischen den Rhenium-und den Sauerstoffatomen fallen rnit 193-197 pm in den fur solche Strukturelemente bekannten Berei~h'~-'].Hinsichtlich ihrer Bindungsordnung deneit noch nicht schliissig interpretierbar sind die Metall-Metall-Abstande (274.7(2) pm), die der E n g e von Doppelbindungen oder von verkiinten Einfachbindungen ent~prechen~~l. Da das Komplexkation [(q'-CsMe5)3Re3(p-O)6]z+ von 2 rnit nur 46 Valenzelektronen eine Elektronenmangelspezies ist, verbleiben nach Beccksichtigung der neun Liganden fiir die Re3-Inselstruktur vier Bindungselektronen; formal ergibt sich damit eine Re-Re-Bindungsordnung von 2/3. Ob trotz der relativ kurzen Metall-Metall-Abstande iiberhaupt eine bindende Wechselwirkung zwischen den Metallatomen vorliegt, rnuR rnit einer MO-Rechnung gepriift werden. Die Metallatome des Clusterions haben formal die Oxidationsstufe + Sf.Trotz der einfachen Bildung von 2 ist der Reaktionsmechanismus unklar. Aufgrund von Kontrollexperimenten ist auszuschlieBen, daB der aus 1, PPh3 und wenig Sauerstoff entstehende Komplex 3[21 rnit iiberschiissigem 1 reagiert.Das elektronisch ungesattigte Clusterion von 2 ist nicht nur das erste Beispiel eine5 quadratisch-pyramidalen Komplexes vom Typ LRe04, es bietet auch eine in der Organometall-Chemie neuartige M306-Geriistgeometrie.

Thermal [1,7]-sigmatropic hydrogen shift versus electrocyclization of dienylallene sulfoxides

“…The observed major isomer 14d results from a hydrogen migration syn to the sulfoxide group. As mentioned earlier, this is opposite to what was previously observed for the corresponding [1,5] shift in vinylallene sulfoxides (Scheme II).5 In comparison to the isopropyl case where both modes of pericyclization were detected, it is suggested that the electrocyclization process is now completely inhibited because of steric factors; the bulky ieri-butyl group blocks carbon-carbon bond formation entirely.…”

“…The finding that the phenyl sulfoxide group is a syndirecting group in the [l,7]-sigmatropic shift (i.e., in the rearrangement of lOd to 13d plus 14d, the latter is preferred) nicely complements our previous finding that this same substituent is an anti-directing group in the [1,5]sigmatropic shift (i.e., in the rearrangement of 7 to 8a plus 8b, the former is preferred). This alternation in -facial selectivity between a 4n + 2 and 4n process exerted by a common substituent [PhS(O)] is only the second such case reported.4,18 However, the effect observed here is so small (2.3-2.6:l, symanti), especially when compared to the corresponding [1,5]-shift case (>50:1, anti:syn for analogous substituents), that steric effects may be adequate to explain the syn selectivity.…”

“…An investigation of the competitive [ 1,7]-sigmatropic hydrogen shift of the kind represented by the isomerization of 3 to geometric isomers 5a plus 5b, besides being of possible use in our vitamin A studies, would have been of considerable interest in connection with our attempts to evaluate the effect of substituents on 4n versus 4n + 2 pericyclic proce~ses.~ Such a process is closely related to the [1,5]-sigmatropic hydrogen shift of the allene sulfoxide 7 (prepared in a similar way from propargyl alcohol 6) to 8a plus 8b (Scheme II).5 In the latter study, it was dis- covered that the sulfoxide had a profound effect on rate and on the geometric course of the hydrogen shift. An allylic methyl hydrogen of 7 was shown to migrate to the central carbon of the allene preferentially in a manner anti to the sulfoxide, affording an excess of 8a over 8b (3:l to >98:2 depending on the nature of R).…”

“…The possibility that a substituent (sulfoxide) could induce the opposite *-facial selectivity when comparing a 4n + 2 ([1,5] shift) with a The possibility that a substituent (sulfoxide) could induce the opposite *-facial selectivity when comparing a 4n + 2 ([1,5] shift) with a…”

“…The former, 5b, results from migration of the allylic hy- drogen of 3 syn to the sulfoxide group. The possibility that a substituent (sulfoxide) could induce the opposite -facial selectivity when comparing a 4n + 2 ( [1,5] shift) with a 4n ( [1,7] shift) process has its origin in a single precedent reported by this laboratory.48 Although 3 was not observed to afford 5, we now find that a bulky substituent at the aliene terminus (terf-butyl group geminal to the PhSO as in lOd in Scheme III) of 3 diverts its rearrangement to [1.7] -shifted (e.g., 13 and 14) rather than electrocyclized products (e.g., 11 and 12). This latter result therefore allows a study of the effect of a substituent on the geometric course ( -facial selectivity6) of the [l,7]-sigmatropic shift.…”