Cycloheptatriene 1,5-shifts

Reaction of iron atoms with cycloheptatriene to form [Fe( r) -C7H7)-(t7 -C7H9)] was confirmed by another group 15) these workers determined the crystal structure of the species, demonstrating a sandwich structure with the open faces of the two 7j -systems skewed to each other. The temperature-dependent NMR spectrum of this species (16) indicated two types of fiuxional behavior in solution. Evidence for a 1,-2-shift mechanism of the l-5-i7-cycloheptatrienyl moiety in the structure shown. 

The UV spectrum closely resembles that of tropone and exhibits a batho-chromic shift over those of cycloheptatriene (unconjugated). Although the "B signal at 53.6 ppm is not much shifted over that in 1-phenyl-4,5-dihydroborepin (54.6 ppm), the borepin ring protons occur at markedly... 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

In the case of 1,3-butadiene, the chemical shifts of inner (H2, H3) protons and outer (HI, H4) is large, while in the case of cycloalkadienes (e.g. 1,3-cyclopentadiene and 1,3-cyclohexadiene), the difference is very small. It is interesting to note that in 1,3,5-cycloheptatriene, the chemical shifts of three kinds of olefinic protons are very diverse. The effect of the ring size and in the chemical shifts of radialenes was also included. 

TABLE 4. 13C chemical shifts of cycloheptatriene and norcaradiene compounds... 

It was shown that [1,5]-hydrogen shift occurs in this case about 30 times more quickly than that in the cycloheptatriene, while the butadienylcyclopropane rearrangement proceeds 3 x 10 9 slower than the Cope rearrangement of the isomeric 2,5-diene 11054. 

For cycloheptatriene and a series of its derivatives various thermal unimolecular processes, namely conformational ring inversions, valence tautomerism, [1,5]-hydrogen and [l,5]-carbon shifts, are known. An example of such multiple transformations was described65 which can provide a facile approach to new polycyclic structures by a one-step effective synthesis (yields up to 83%) of the two unique ketones 156 and 157. The thermolysis of the neat ether 151 at 200 °C for 24 h gives initially the isomeric allyl vinyl... 

Thermal 1,5 hydrogen shift of cyclopentadiene and 1,3,5-cycloheptatriene, and methyl shifts in the corresponding methyl-substituted derivatives and in methyl-1,3-... 

The diamagnetic susceptibility measurements are consistent with the NMR properties of cycloheptatrienes. Pikulik and Childs compared the H NMR chemical shifts of the C(7) proton of 7-substituted cycloheptatrienes and the corresponding 1,4-cycloheptadienes and showed that there was a considerable upfield shift of the resonance of the former protons... 

The forbiddenness of a suprafacial [ 1,7] shift is shown by the absence of the same type of reaction in cycloheptatrienes 5.15. Heating these compounds, when a substituent is present to reveal the presence of a reaction, interconverts all the mono-substituted isomers, just as it did with the... 

Thus [6+2] cycloaddition of alkene with complex 306, bearing an optically active side chain, under irradiation at room temperature afforded the bicyclic compound 307 in 98% de [73]. According to the Woodward-Hoffmann rule, the [6+2] cycloaddition proceeds by irradiation, and is thermally forbidden. However, the cycloheptatriene complex 308 underwent 1,5-hydride shift, followed by [6+2] cycloaddition by heating, to give the tricyclic compound 309 in 90% yield [74], The cycloaddition was applied to the synthesis of /f-cedrene [75]. 

The study above has some applications. Normally, the equilibrium between cyclo-heptatriene (51, R = H) and norcaradiene (52, R = H) is strongly displaced in favor of the cycloheptatriene. To favor the norcaradiene, the electrocyclic ring opening must be inhibited by strengthening the 1-6 bond. Our analysis shows that this may be achieved by incorporating acceptor groups R. Donor R groups shift the equilibrium to the left.82... 

The two pathways, electrocyclic closure to the bicyclo[3.2.0]heptadiene and the [l,7]-shift of hydrogen, are in competition. This makes the experimental work more difficult, because the four possible isomers of the cycloheptatriene interconvert during the photolysis. Nevertheless the pattern of reactivity is clear, and the observation is that the better the donor substituent, the more it favours the formation of the bicyclo[3.2.0]heptadiene at the expense of the [l,7]-shift. This too is reasonable, since the hydride migration towards the donor-substituted atom 8.80 leads to a less stable cation, and ought to be slow. 

The diamagnetic susceptibility measurements are consistent with the NMR properties of cycloheptatrienes. Pikulik and Childs compared the NMR chemical shifts of the C(7)... 

Kraka and Cremer have calculated the and C NMR chemical shifts and magnetic susceptibility as a function of interaction distance of both the cycloheptatriene and norcaradiene systems They point out that both the magnetic susceptibility and the shift difference between the endo and exo protons at C(7) are at a maximum at the transition state for the valence tautomeric rearrangement between the two systems. The transition state is characterized by a C(l)-C(6) distance of 1.864 A and an almost complete equalization of C—C bond lengths, bond orders, atomic charges and shifts of C(2)—C(5). 

At present, there are only NMR data available for 7r-arene and TT-cycloheptatriene complexes of chromium, molybdenum, and tungsten (80,158,161) (see Table XXV). The chemical shifts of the complexed... 

NMR Chemical Shifts of Some Arene and Cycloheptatriene Ligands and Their Complexes with M(C0)3, M = Cr, Mo, W ... 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

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