Abstract:In the reaction described by M. Murakami and M. Hasegawa on the following pages, the electronic effects are so dramatic that the sterically hindered substrate prefers to react by a more crowded pathway. This reaction will provide organic chemistry textbooks with a prime example of systems in which electronic effects are dominant over steric effects in determining the stereochemical outcome of a reaction.
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“…Usually for the ring opening of cyclobutenes, steric effects dominate the preference for inward versus outward rotation [ 15 ]. However, electronic effects can also dictate this rearrangement, as was reported very recently for the sterically hindered substrate 5 , which prefers to react via the more crowded inward rotatory pathway, leading mainly to butadiene 6 (Scheme 1 ) [ 16 , 17 ]. Scheme 1 Ring opening of cyclobutene 5 …”
The rearrangements for 2-phospha-4-silabicyclo[1.1.0]butane, analogous to the valence isomerization of the hydrocarbons bicyclobutane, 1,3-butadiene, and cyclobutene, were studied at the (U)QCISD(T)/6-311+G**//(U)QCISD/6-31G* level of theory. The monocyclic 1,2-dihydro-1,2-phosphasiletes are shown to be the thermodynamically preferred product, in contrast to the isomerization of the hydrocarbons, which favors the 1,3-butadiene structure. Furthermore, an unprecedented direct isomerization pathway to the 1,2-dihydro-1,2-phosphasiletes was identified. This pathway is competitive with the isomerization via the open-chain butadienes and becomes favorable when electron-donating substituents are present on silicon.
“…Usually for the ring opening of cyclobutenes, steric effects dominate the preference for inward versus outward rotation [ 15 ]. However, electronic effects can also dictate this rearrangement, as was reported very recently for the sterically hindered substrate 5 , which prefers to react via the more crowded inward rotatory pathway, leading mainly to butadiene 6 (Scheme 1 ) [ 16 , 17 ]. Scheme 1 Ring opening of cyclobutene 5 …”
The rearrangements for 2-phospha-4-silabicyclo[1.1.0]butane, analogous to the valence isomerization of the hydrocarbons bicyclobutane, 1,3-butadiene, and cyclobutene, were studied at the (U)QCISD(T)/6-311+G**//(U)QCISD/6-31G* level of theory. The monocyclic 1,2-dihydro-1,2-phosphasiletes are shown to be the thermodynamically preferred product, in contrast to the isomerization of the hydrocarbons, which favors the 1,3-butadiene structure. Furthermore, an unprecedented direct isomerization pathway to the 1,2-dihydro-1,2-phosphasiletes was identified. This pathway is competitive with the isomerization via the open-chain butadienes and becomes favorable when electron-donating substituents are present on silicon.
“…Murakami and Hasegawa studied the ring opening of trans-3,4-bis(trimethylsilyl)cyclobutene to better understand the importance of steric effects versus electronic effects on the reaction torquoselectivity. 31 Heating in n-decane at 110 uC yields a mixture of the products of inward and outward rotation, in a 78:22 ratio. This preference for inward rotation is borne out by B3LYP/6-31G* calculations.…”
Section: Methodsmentioning
confidence: 99%
“…Complementary calculations were performed using SCS-MP2/cc-pVTZ methodology, giving barriers of 21.5 and 32.5 kcal mol 21 for the parent phosphirane and hydrocarbon analog, respectively. Other sigmatropic behavior of 12 is observed; heating above 50 uC promotes the concerted [1,5] sigmatropic shift to the bicyclo[4.2.1]triene, which has a barrier of 24 kcal mol 21 based on both 31 P spectroscopy and calculations. This behavior is in contrast to the hydrocarbon, for which this process is known to occur through a diradical intermediate; the greater length of the C-P bond is thought to allow the concerted process to occur.…”
This review is a collection of the highlights in the field of pericyclic reactions published in the year 2004. Research in this fascinating topic continues to be centred on the understanding of the mechanisms of the three major classes of pericyclic reactions: cycloaddition reactions, electrocyclic reactions, and sigmatropic rearrangements, as well as into those of more unusual pseudopericyclic processes.
HighlightsThese featured papers published in 2004 involve fundamental questions in pericyclic reaction mechanisms. Baldwin and co-workers studied the forbidden disrotatory 4-electron ring opening of cis-bicyclo[4.2.0]oct-7-ene, and using secondary kinetic isotope effect measurements the authors conclude that the rearrangement occurs through a concerted disrotatory process; conrotation almost certainly occurs, but the strained product reverts to the reactant. 1,2 Doering et al. studied the sigmatropic [1,5] hydrogen migration in phenyl-substituted 1,3-pentadienes to determine whether this reaction, like the Cope rearrangement, has a chameleonic transition state, dependent on the position and nature of substituents. 3 Although substituent effects are small, the authors conclude that the results are consistent with the chameleonic transition state. Calculations of the same reaction were reported by Borden and co-workers and also show small substituent effects; also there are few structural differences between the contributing structures of the transition state but large energy differences. 4 Although the computational data do not support a chameleonic transition state, perhaps a different [1,5] rearrangement might. Gajewski published a second, updated edition of ''Hydrocarbon Thermal Isomerizations'', which includes many pericyclic rearrangements. 5 Two computational benchmarking studies of pericyclic reactions were published. Houk, Pulay and co-workers compared the use of the Handy/Cohen OLYP and O3LYP functionals in pericyclic reactions to B3LYP and CBS-QB3 methods. 6 Although the OLYP functional was found in a previous benchmarking study to be superior to BLYP in predicting thermochemistry and molecular geometry, it is comparable to BLYP for pericyclic reactions and O3LYP is comparable to B3LYP. Also, in a collaborative effort, Goumans et al. presented a new computational method for the inclusion of electron correlation. 7 This spin-component-scaled MP2 (SCS-MP2) method was tested on a variety of Diels-Alder reactions, [3,3] sigmatropic rearrangements and electrocyclic reactions, cases in which MP2 calculations fail. The results are in excellent agreement with experiment, are closer to G3 values than MP2 or B3LYP, and the results improve with the basis set size, unlike MP2 and B3LYP.
“…Torquoselectivity is more an electronic phenomenon than steric and at times one may over ride the other. 12 Following this, many works focused on torquoselectivity of silyl, 13 stannyl, 14 selenyl, 15 germanyl 16 and boryl 17 substituents at 3-position of cyclobutene. Recently, work relating to torquoselective ring opening of azetines, 18 substituted azetines 19 have substantiated interaction of lone pair of nitrogen and π*(LUMO) of cyclobutene.…”
The competition between the two electron donors (-OCH 3 and-OSiH 3) have been studied by positioning them at geminal and vicinal locations. We have also studied the cis and trans geometrical isomerism at the vicinal location. Thermodynamic and activation parameters such as free energies have been computed to understand the potential energy surface of the reaction. Our computed activation hardness supports the proposed torquoselectivity. Further, Nucleus Independent Chemical Shift (NICS) was calculated at the ring centre of the transition state (TS) which additionally supports the torquoselectivity. The delicate preferences for the pathways have been substantiated through NBO interactions.
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