Continuing their studies of ene-yne cyclizations, the Schmittel group examined the apparent [2+2] cyclization of the allene-yne 1.1 They proposed that it first closed the diradical 2 and then in a second step the four-member ring is formed, giving 3.

a: R1=Ph, R2=R3=H
b: R1=Ph, R2=H,
c: R1=Ph, R2=POPh2,

Evidence supporting the intermediate diradical is that heating 1a in the presence of 1,4-cyclohexadiene gives 11% of the trapped species 4a. Interestingly, heating 1b gives 26% of 3b, while the reaction of 1c gives 72% of the ring closed product 3c.

Schmittel suggests the intermediate diradical 2b is planar, while 2c is not, and the radical centers are nicely position in the latter compound for quick closure to product.

UBLYP/6-31G(d) computations support the mechanism. The transition state taking 1b to 2b (TS1, shown in Figure 1) lies 20.2 kcal mol-1 above reactant. The intermediate diradical 2b is 7.9 kcal mol-1 above reactant 1b. The second transition state (TS2) for closing the four-member ring lies 27.8 kcal mol-1 above reactant, making it the rate determining step. The overall reaction is exothermic by -12.4 kcal mol-1. The transition state for a single step reaction, taking 1b directly into 3b (TS3) is very high, 49.0 kcal mol-1 above 1b, and is therefore non-competitive with the stepwise pathway. These computations suggest a reversible formation of the intermediate, followed by a rate limiting step to making the four-member ring, completely consistent with the experiments.





Figure 2. UBLYP/6-31G(d) optimized structures of 2b, TS1, TS2, and TS3.


1) Cinar, M. E.; Vavilala, C.; Fan, J.; Schmittel, M., "The thermal C2-C6/[2 + 2] cyclisation of enyne-allenes: Reversible diradical formation," Org. Biomol. Chem. 2011, 9, 3776-3779, DOI: 10.1039/C0OB01275K


1b: InChI=1/C21H20/c1-21(2,3)17-9-14-19-12-7-8-13-20(19)16-15-18-10-5-4-6-11-18/h4-8,10-14,17H,1-3H3/t9-/m0/s1

3b: InChI=1/C21H20/c1-21(2,3)20-17-13-15-11-7-8-12-16(15)19(17)18(20)14-9-5-4-6-10-14/h4-13,20H,1-3H3