Of pentadienyl cations

Vinylsilanes (8, 491 492) attylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C -O group (cncsilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopcntenone annelalion by l riedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of pentadienyl cations (9, 498-499).1... 

Electrocyclic closure of both pentadienyl cation and anion have been observed. Cations generated by protonation of dienones close in the predicted conrotatory manner as shown in Equation 12.55.99 The pentadienyl anion, a six-electron system, should close in the disrotatory sense a clear example is the rapid isomerization illustrated in Equation 12.56.100 Photochemical cyclization of pentadienyl cations has been observed Equation 12.57 shows an example in a cyclic system.101 The ready thermal reversion, which should be conrotatory and therefore difficult in the bicyclic system, may possibly occur by a stepwise path.102... 

Dienones, which because of the electron deficiency induced at the carbonyl carbon may be regarded as analogs of pentadienyl cations, are known to close photochemically.103 These closures occur readily in cyclic systems where the geometry requires the disrotatory mode, but the proposed intermediate product, formally a 1,3 diradical, is ordinarily stabilized through rearrangement.104 Woodward and co-workers have demonstrated the predicted photochemical... 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

We are not restricted to the reactions of neutral molecules. Allyl anions can react with olefins, allyl cations with dienes, and pentadienyl cations with olefins, as we can see from the examples in Fig. 4-6, in which we can also see why the reactions of allyl cations with olefins, of allyl anions with dienes, and of pentadienyl cations with dienes are not observed. 

Draw the pi MOs of pentadienyl cation. What is the HOMO and LUMO ... 

Computations on the cyclization of pentadienyl cations to cyclopentenyl cations has indicated increasing reactivity in the order X= NH2 < OH < H <... 

Sketch the nodal properties of the highest occupied molecular orbital of pentadienyl cation (CH2=CHCH=CHCH2 ). 

The stereospecificity of cyclization of pentadienyl cations 13-15 to the cyclopentenyl cations 16 and 17 in a variety of acidic media has been determined. The data are shown for 99% sulfuric acid at -5°C ... 

SCHEME 2.15 Thermal cyclization of pentadienyl cation in a conrotatory manner. 

Orbital symmetry considerations predict that ring closure of pentadienyl cations, which are 4n systems, should be controtatory. While some pentadienyl cations are sufficiently stable to be observed by NMR in acidic media, they do tend to cyclize to cyclopentenyl cations. The stereospecificity of cyclization of pentadienyl cations 12-14 to the cyclopentenyl cations 15 and 16 in a variety of acidic media has been determined. The data are shown for 99% sulfuric acid at —The observed stereochemical course is predominantly conrotatory, but the reactions are not as stereospecific as the others we have seen. This is attributed to the fact that... 

Conrotatory cyclization of a series of pentadienyl cations to cyclopentenyl cations has been investigated. Cyclizations proceed stereospecifically and at virtually the same rate in solvents of markedly different ionic strengths. Of the compounds examined the slowest cyclization rate was exhibited by (310) and the fastest by (311) the relative rate of cyclization of (311) with respect to (310) was ca. 2 x 10 at 25 °C. However, the rate constants obtained are not identical with those for the electrocyclic process as two equilibria are thought to be involved prior to the rate-determining step. This is shown in the general case... 

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