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The product formed in the following reaction is

A tetradeuterated 1,3-butadiene and cis-1,2-dideuterioethylene heated together

  • A.A 1:1 mixture of two hexadeuterated cyclohexenes, one with both ring deuteriums on the same side and one with them on opposite sides
  • B.Hexadeuterated cyclohexene with the two adjacent ring deuteriums on opposite faces (wedge and dash)
  • C.Hexadeuterated cyclohexene with the two adjacent ring deuteriums on opposite faces (dash and wedge)
  • D.Hexadeuterated cyclohexene with the two adjacent ring deuteriums both on wedge bonds, cis

Answer: D

The product is (D), with the two deuteriums from the dienophile cis to each other.

The Diels-Alder reaction is a concerted [4+2] cycloaddition that is suprafacial on the dienophile: both new bonds form on the same face of the alkene at the same time, so the relative stereochemistry of the dienophile is kept in the product. The dienophile here is cis-1,2-dideuterioethylene, so its two deuteriums end up cis on adjacent ring carbons, both drawn with wedges in (D). The deuteriums on the diene termini become the two CD2 groups.

(B) and (C) both show the two deuteriums trans, the product a trans dienophile would give. A 1:1 mixture of cis and trans (A) would require a stepwise reaction through an intermediate that can rotate, which the concerted cycloaddition does not have.

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