Con rotatory

For Woodward-Hoffm an allowed thermal reactions (such as the con rotatory ring opening of cyclobulan e), orbital symmetry is conserved and there is no change in orbital occupancy. Hven though bonds are made and broken, you can use the RHFwave fun etion. 

Would you expect the following reaction to proceed in a con rotatory or dis-votalory manner Show the stereochemistry of the cyclobutene product, and explain your answer. 

In con-rotatory motion C2 symmetry is maintained throughout while in disrotatory motion m symmentry is maintained. 

Now the 1,3-cyclopentanediyl diradical is constrained to cychze in a disrotatory fashion while the trimethylene species might well close in both disrotatory and con-rotatory ways. Were all other factors constant one could infer that the geometrical restrictions imposed on the 1,3-cyclopentanediyl diradical entailed no significant deduction in rate of cyclization, and thus that conrotatory cyclization of the trimethylene diradical is not strongly preferred under the given reaction conditions and circumstances. 

The ring closure of cis-butadiene to form cyclobutene can take place either in a con-rotatory or in a dis-rotatory way. In the first place the CH2 planes of the end groups rotate in the same direction keeping a C2 axis of symmetry ... 

Let us compare the linear reactions. In the disrotatory mode, the butadiene vacant orbital , interacts favorably with the nitrogen oz and n occupied orbitals. In the con-rotatory mode, only the interaction between 3 and nx is favorable and the interaction between the occupied orbitals 2 and oz is repulsive (Figure 4.11). Hence the disrotatory mode is expected to be favored. Note that if the six heavy atoms are coplanar, the initial overlaps of 3 with oz (Figure 4.11a), of 2 with cz and of 3 with nx (Figure 4.11b) are zero. However, as soon as the termini of butadiene start to rotate, these overlaps become positive. 

ElectrocycUzations involving eight electrons will proceed by a con-rotatory transition state. Houk examined the ring opening of three... 

The photochemically-exdted hexatriene system has one of its r-electrons promoted to the first excited r-orbitaL The terminal symmetry of this orbital (13) is the reverse of that depicted in (9), and requires bonding overlap to develop from a "con-rotatory displacement of end-groups, so that orbital envelopes on opposite faces of the molecule can interact [10]. This results in photochemical cyclisation of the B-seco-trienes (2) and (3) giving products with anti configurations about the C(9> C(io) bond, corresponding to ergosterol (9a,iOjS) and lumisterol (9/ ,ioa) respectively. The photochemical process... 

In the thermolysis of trans-a-ethylene-a -acetylenoxirane, four electronic con-rotatory openings occur and three products are formed via an intermediate ylide. The cis isomer also yields similar products (Eq. 392). In the gas- and liquid-phase reactions of substituted derivatives it has been verified that the initial product in these thermolyses is the oxacycloheptatriene. ... 

The value of the terms suprafacial and antarafacial is that, unlike disrotatory and con-rotatory, they can also be used to describe the way that 7r systems react in cycloadditions and sigmatropic rearrangements. Most importantly, when a 77 system reacts suprafacially, its out groups become cis in the product when it reacts antarafacially, they become trans. Note that in disrotatory electrocyclic reactions, the out groups become cis, and in conrotatory electrocyclic reactions, the out groups become trans. 

Note that for every electrocyclic reaction there are two con-rotatory and two disrotatory motions that may or may not be distinguishable. For example, the two conrotatory motions for trans 3,4-dimethylcyclobut-l-ene lead to cis-cis and trans-trans-1,4-dimethyl-butadiene ... 

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