Cycloheptatriene 1,7 -hydrogen shift

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. 

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-... 

Despite the numerous investigations into [1,5] sigmatropic hydrogen shifts in cycloheptatriene systems, the stereospecificity of this reaction apparently is still seldom used as a means for generating new stereogenic carbon centers in this seven-membered ring skeleton. 

As an example, the suprafacial [1,7] sigmatropic H-shift will be considered that occurs on irradiation of 1,3,5-CHT with formation of bicyclo[3.2.0]hepta-2,6-diene (BHD) as a minor product. In 1-substituted CHT this [1,7]-H shift is regiose-lective in 1-cyano-cycloheptatriene (CN-CHT) the hydrogen atom moves exclusively to the unsubstituted terminal carbon of the heptatriene moiety, whereas in 1-methyl-cyclohep-tatriene (Me-CHT) only 2% of the product exhibits this regio-chemistry and 98% corresponds to a hydrogen shift toward the substituted carbon, as shown in Scheme 6.1. Different models have been proposed to explain this regioselectivity. ... 

Fig. 10.35. Structures and relative energies of isomeric mediyl-l,3-cycloheptatrienes and TS for [l,5]-sigmatropic hydrogen shift between them. Data from Int. J. Quantum Chem.,...

Tropilidene (1,3,5-cycloheptatriene) is a non-planar, tub-like molecule with only a 6.3 kcal/mol barrier to ring inversion. It is converted to toluene upon heating with log = 13.54 - 51 100/2.3/ T or log = 13.9 - 52200/2.3/ r. Themost reasonable course of this rearrangement is disrotatory electrocyclization to norcaradiene followed by homolytic cleavage of an external cyclopropane bond to a 1,3-biradical and subsequent vicinal hydrogen shift (Scheme 8.8). 

Heating cycloheptatriene at roughly 100°C results in 1,5-hydrogen shifts this is the so-called skin rearrangement to distinguish it from the carbon skeletal or bones rearrangement. With 7-deuteriocycloheptatriene log =11.2 — 31 500/2.37 r for formation of 3-deuterio material which gives the other deuterium isomers (Scheme 8.9). ... 

In order to answer the question posed in Scheme 8.8 as to whether or not external cyclopropane bond homolysis was reversible in norcaradiene, Willcott and Berson examined the pyrolysis of 3,7,7-trimethyl-cycloheptatriene. " Remarkably, the 2,7,7- and 1,7,7-isomers were formed in a 10 1 ratio, respectively, along with small amounts of m- and p-cymene. Importantly, substantial quantities of l-methyl-3-isopropenyl-1,4-cyclohexadiene and 2-methyl-3-isopropenyl-1,4-cyclohexadiene were also found, and these reverted to the cycloheptatrienes under the reaction conditions. Therefore, the interconversion could be the result of biradical formation or reversible homodieny 1-1,5-hydrogen shifts (Scheme 8.10). 

Also of interest is the finding that the hydrogen shift to give toluene occurs only from the transition state that involves retention at all levels of theory. Further, calculations on the cycloheptatriene-to-norcaradiene equilibrium at the B3LYP(RHF) level come closest to experiment, namely 6.5 kcal/mol for... 

Finally, the activation energies (without zero point corrections) calculated for the 1,5-hydrogen shift in cycloheptatriene by MROPT2, CASSCF, and B3LYP are 38.7, 60.2, and 40.6 kcal/mol, respectively, and a zero point correction would lower these energies by about 4 kcal/mol. Clearly, the CASSCF method without dynamic correlation is suspect just as in the Cope rearrangement (see Chapter 7, Section 4.1). 

Bicyclo[2.2.1]heptadiene (norbornadiene) gives cycloheptatriene upon heating with log k = 14.68 — 50 610/2.3/ r. Also formed in the reaction is cyclopentadiene and acetylene, the retro Diels-Alder products with log k = 14.68 — 51900/2.3/ r and toluene with log k = 14.23 — 53 A0/23RT. Most likely, the initial reaction proceeds via cleavage of the C1-C7 bond to give a biradical which can form norcaradiene and then cycloheptatriene or undergo a hydrogen shift to toluene, but the retro 4 -h 2 reaction must result from C1-C2 (and C3-C4) bond fission (Scheme 8.18). 

A stepwise 1,7-vinyl shift was proposed to account for the reaction after carbene addition to make the cyclopropene. Deuterium labeling studies were consistent with this pathway. Interestingly, upon heating to 145°C the phenyl-substituted 5.2.0 tetraene isomerized to 2-phenylindene in what appears to be a cyclization of the cycloheptatriene moiety followed by opening of the bicyclo[2.1.0]pentene and a 1,5-hydrogen shift (Scheme 10.9). 

The reaction appears to be a 1,5-shift of a cycloheptatriene (the retro-electrocyclization product of the norcaradiene) ring carbon over the cyclopentadiene system which is followed by a 1,5-hydrogen shift. 

Two possible pathways were envisioned for the reaction (a) cyclopropane to propylene-like rearrangement followed by 1,5-hydrogen shifts, that can equilibrate C4 and C6 as well as C3 and Cl with a slower 1,5-deuterium shift (due to the primary isotope effect) and (b) the second pathway would involve a retro-electrocyclization to a cycloheptatriene destroying the aromatic it system in the process, and this undergoes a 1,5-deuterium shift to the 1,2-benzocycloheptatriene which subsequently undergoes a 1,5-hydrogen shift to equilibrate C4 and C6 but also must equilibrate C3 with Cl. Further, a 1,5-deuterium shift in the 7-deuterio material gives the isomer from path (a) (Scheme 12.8). 

The scrambling of deuterium in 3,4-benzo starting material was ascribed to a 1,5-hydrogen shift followed by reversible norcaradiene-cycloheptatriene electro-cyclization, and subsequent 1,5-hydrogen shifts could account for the 1,2-benzo isomer (Scheme 12.10). 

Butenenyl)cycloheptatriene gives a variety of intramolecular Diels-Alder adducts subsequent to 1,5-hydrogen shifts in the cycloheptatrienyl moiety at 215" C after 4 days (Scheme 12.30). " Similar results were obtained with allylcyclohepta-trienyl ether and with 2-(3-butenyl)dihydrocycloheptatrienone. ... 

In cycloheptatriene, an antarafacial [1,7] hydrogen shift is impossible. Consequently, [1,7] hydrogen shifts within this system must be initiated photochemically. For example, the interconversion of isomers of l,4-di(cycloheptatrienyl)benzene involves the sequence of thermal [1,5] and photochemical [1,7] sigmatropic hydrogen shifts (Scheme 3.8). 

Methyl-cycloheptatriene 34 on heating undergoes slow [l,5]-suprafacial hydrogen shift rather that [l,7]-antarafacial H-shift to yield a mixture of methyl-substituted isomers [9]. 

Photochemical [l,7]-suprafacial alkyl and hydrogen shifts have been observed in the conversion of cycloheptatriene derivative 50 into its isomers [26]. 

An explanation for the orbital symmetry-forbidden stereochemistry of the [1,51-sigma-tropic shift of substituted norcaradienes has been reported, and ab initio methods have been used to study the [l,5]-hydrogen shift in cycloheptatriene and the [1,51-carbon shift in norcaradiene. A degenerate rearrangement consisting of a formal... 

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... 

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