The time period bispericyclic response was famously coined by Caramella et al in 2002 to explain the weird options of the apparently innocuous dimerisation of cyclopentadiene. It reveals options of two paths for various pericyclic reactions, comprising a 2+4 cycloaddition within the early levels, however evolving right into a (degenerate) pair of [3,3] sigmatropic reactions within the latter levels. Houk (who additionally makes use of the time period ambimodal) has in recent times prolonged the variety of examples of such pericyclic sequences to trispericyclic (see right here) and even an ambimodel tetrapericyclic response, as reported on the latest WATOC occasion. Right here I present an instance of a brand new kind of bispericyclic response, comprising a 2+4 cycloaddition mixed with a electrocyclic ring opening.
The response begins with cyclopropanone. This species is reported to bear cycloaddition reactions with e.g. furan; under is proven the scheme with butadiene.
Calculations on the ωB97XD/Def2-TZVPP/SCRF=DCM had been undertaken (FAIR information DOI 10.14469/hpc/11246). As a result of it has been prompt that the “intermediate” ylid may need some biradical character, this was modelled utilizing the choice of a spin-unrestricted (UHF) technique and a beginning guess for the density matrix utilizing the Gaussian key phrase guess(combine), which makes use of a linear mixture of the HOMO and LUMO to destroy α-β and spatial symmetries. On the situated transition state, the spin expectation operator <S**2>= 0.4003 and it reaches a most worth of <S**2>= 0.7002, so this technique does certainly have some biradical character (the worth can be 0.000 for a closed shell system, and 1.000 for a pure biradical). The IRC is proven under.
<S**2> is non-zero solely between the vary -2.5 to +4.5, the remainder of the trail has the worth <S**2> =0.0. The central area of the trail (IRC ~0 to +4.5) reveals very small gradients, and will be characterised because the area akin to the ylid above appearing as a “hidden intermediate”.
The central area additionally coincides with a rise within the dipole second at first of the response and a lower on the finish indicating that the system additionally has a good quantity of zwitterionic character, as implied within the ylid construction proven above. However the dip in dipole second at round IRC = 2 corresponds to biradical character partially changing the charge-separated ionic mode.
The IRC animation proven under reveals that within the early levels of the response, electrocyclic ring opening (with disrotation) happens, however no specific ylid intermediate is fashioned. As an alternative the response continues with out pause to a cycloaddition response for completion.
One attention-grabbing query is what impression does the partial biradical character have on this consequence? Proven under are the IRC responses to a possible for which the wavefunction has <S**2> =0.0 throughout all the area.
The response reveals a completely totally different, however nonetheless bis-pericyclic response involving the identical electrocyclic ring opening to the ylid proven above, however then response as a 2+2 cycloaddition response by way of the oxygen quite than the carbon of the ylid. The dipole second now not reveals the biradical-induced dip within the center, however rises to a full most as a substitute.
So right here we see a sequential bispericyclic response wherein the product of an preliminary electrocyclic pericyclic response is so reactive that it instantly reacts with a diene to type the ultimate product, in an general concerted however asynchronous response. Modelling this appears to be very technique dependent, and the apparently appropriate product is simply obtained if an open-shell biradical wavefunction is used. Are there extra examples?
This submit has DOI: 10.14469/hpc/11247