“…Recently, Matzinger and Bally reported the complete UV/VIS and IR spectra of both PC and CHTE . Interestingly, to date, direct evidence has not been obtained for the existence of bicyclo[4.1.0]hepta-2,4,6-triene ( BCT ), although there have been several product studies characterizing the reactivity of putative BCT and related precursors .…”
Section: Introductionmentioning
confidence: 99%
“…Studies of benzannelated derivatives, though, do provide direct evidence for the analogous intermediates in the rearrangement of naphthylcarbenes . As well, Miller and Gaspar demonstrated the participation of a BCT analogue (as either a short-lived intermediate or transition state) in the rearrangement of cyclohexenylmethylene 12c…”
The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functional theory. It is found that substituents have a large effect on both the thermochemistry and activation energy of these rearrangements. For instance, para-substitution yields a range of overall activation energies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH(2) and NO(2) substituents, respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTE species are more exothermic than that of the parent PC. The proximity of the substituent to the carbene center can also affect the overall chemistry as in the case of ortho-substituted species. Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures, can compete with the ring-expansion process. Also, as calculated herein, the ortho substituents can, by a combination of mesomeric and steric interactions with the carbene center, affect the overall barrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens (F and Cl) raise the activation barrier to ring expansion by approximately 5 kcal/mol. This is reminiscent of the effect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.
“…Recently, Matzinger and Bally reported the complete UV/VIS and IR spectra of both PC and CHTE . Interestingly, to date, direct evidence has not been obtained for the existence of bicyclo[4.1.0]hepta-2,4,6-triene ( BCT ), although there have been several product studies characterizing the reactivity of putative BCT and related precursors .…”
Section: Introductionmentioning
confidence: 99%
“…Studies of benzannelated derivatives, though, do provide direct evidence for the analogous intermediates in the rearrangement of naphthylcarbenes . As well, Miller and Gaspar demonstrated the participation of a BCT analogue (as either a short-lived intermediate or transition state) in the rearrangement of cyclohexenylmethylene 12c…”
The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functional theory. It is found that substituents have a large effect on both the thermochemistry and activation energy of these rearrangements. For instance, para-substitution yields a range of overall activation energies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH(2) and NO(2) substituents, respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTE species are more exothermic than that of the parent PC. The proximity of the substituent to the carbene center can also affect the overall chemistry as in the case of ortho-substituted species. Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures, can compete with the ring-expansion process. Also, as calculated herein, the ortho substituents can, by a combination of mesomeric and steric interactions with the carbene center, affect the overall barrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens (F and Cl) raise the activation barrier to ring expansion by approximately 5 kcal/mol. This is reminiscent of the effect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.
“…Interestingly, he did not obtain evidence for either BCT or 1 CHT to occur as intermediates in any of these cases, although the benzannelated analogs of these two intermediates are clearly involved in the chemistry of the naphthylcarbenes. ,,, Trapping studies further support the involvement of CHTE . , The currently accepted mechanism for the interconversion of simple phenylcarbenes involves the (reversible) ring-expansion of a carbene to a cycloheptatetraene. The intimate details of this reactionwhether it occurs as a direct process or via an undetected BCT intermediateare not well-understood. ,, …”
Section: Introductionmentioning
confidence: 99%
“…The intimate details of this reactionswhether it occurs as a direct process or via an undetected BCT intermediatesare not well-understood. 3,22,23 Questions concerning the structure of cycloheptatrienylidene (CHT) and its role in arylcarbene rearrangements remain unresolved. Most π-substituted carbenes (aryl carbenes, vinyl carbenes, acetylenic carbenes) possess triplet ground states unless the carbene carbon bears a heteroatom substituent or is constrained to a 3-or 4-membered ring.…”
The geometries and force fields of phenylcarbene (PC)
and cycloheptatrienylidene (CHT) in their
singlet
and triplet electronic states as well as of cycloheptatetraene
(CHTE) and bicyclo[4.1.0]heptatriene
(BCT) and the
transition states for the formation and decay of the latter were
evaluated by various methods. Relative single point
energies were calculated at the CCSD(T)/cc-pVDZ//BLYP/6-31G*
level. Finally, the effects of extending the basis
set to triple-ζ quality were estimated by (R)MP2 calculations
and carried over proportionally to CCSD(T).
These
calculations show that CHTE which has a strongly distorted
allenic structure is the most stable species on that part
of the C(CH)6 surface which was examined in the present
study, followed by planar
3PC. The
strained BCT is
found to be nearly degenerate in energy with
1PC, but the high activation energy for its
formation from
1PC together
with the low activation energy for ring-opening to CHTE
suggests that this species cannot persist under the
experimental conditions employed for production of CHTE.
In analogy to the case of cyclopentadienylidene,
CHT
exists in the form of a closed shell singlet
(1A1) and two related pairs of open shell
singlet and triplet states (1,3A2
and 1,3B1) which correspond to the
Jahn−Teller distorted structures of the cycloheptatrienyl radical.
The relative
energies and the nature of the different CHT stationary
points depend on the method of calculation, but it appears
that the decrease in electron repulsion lowers the
1A2 state slightly below the
1A1 state so that the open shell
species
serves as a planar transition state for enantiomerization of
CHTE with an estimated activation energy of ∼20
kcal/mol. The two triplets are very close in energy with the higher
lying being either a transition state or a shallow
minimum. The 1B1 state is an excited state
of the open-shell singlet. The calculated IR spectra of the three
most
stable isomers were compared to those published previously by Chapman
et al. whereby the assignment of the
photoproduct of UV photolysis of phenyldiazomethane to CHTE
was confirmed. A full study of the force fields of
PC and CHTE is under way.
“…The [2+2] cycloaddition of 9 was highly selective as described above. The regioselectivity is significantly higher than that of the bridgehead olefins and is similar to those of distorted monocyclic allenes. , Adamantene ( 1 ) was reported to afford head-to-head cycloadduct 2 and head-to-tail cycloadduct 3 in a 1:2 ratio (Scheme ), and 1,2-cycloheptadiene ( 17 ) was described to produce predominantly head-to-head cycloadduct 18 (Scheme ) 2 A drawing of HOMO for 9 optimized by B3LYP/6-31G(d) calculations. 4 …”
Dehalogenation of 3-bromo-4-iodo-4-homoadamantene with n-BuLi gave rise to 3,4-homoadamantadiene, a novel tricyclic bridgehead allene, which readily dimerized to head-to-head and head-to-tail [2+2] cycloadducts in a ratio of 96:4. Trapping with 1,3-diphenylisobenzofuran was successful to yield the corresponding Diels-Alder adduct.
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