2.3- dimethylbuta-1,3-diene conformations

On the contrary, a bulky substituent at 2-position as in 49 makes the trans conformation less stable and, hence, favors the 1,4-addition [17]. From a stereochemical perspective, some results agree with a concerted mechanism as found, for example, in the stereospecific formation of the above 21 or of the cw-cycloadduct 51 (isolated yield 85%) from 7 ,7i-l,4-dimethylbuta-1,3-diene 50 [37] (Sch. 25). 

It has been suggested that the ground-state conformation of 1,3-dienes could determine the structure of their photoadducts with benzene and also that the sym-cisoid conformation would be more reactive than the sym-transoid 6 Some evidence for this was obtained using 2,3-dimethylbuta-l, 3-diene and sym-cisoid 2,4-dimethylpenta-l,3-diene.4 Other workers have considered that the interpretation of these studies can be complicated by the use of acyclic dienes which are conformationally flexible, and have used the conformationally fixed dienes 1,2-dimethylenecyclohexane and 3-methylenecyclohexene.47 Irradiation of the fixed cw-diene in benzene yielded only the two products (30) and (31),... 

The reactivity of a particular diene depends on the concentration of the s-cis conformation in the equilibrium mixture. Factors that increase the concentration of this conformation make the diene more reactive. As an example of this effect, consider 2,3-dimethylbuta-l,3-diene (Scheme 4.8). 

The methyl groups on C-2 and C-3 cause the s-trans and s-cis conformations to have similar amounts of steric strain and, thus, more of the s-cis conformer is present at equilibrium for 2,3-dimethylbutal,3-diene than for buta-1,3-diene itself. Therefore, 2,3-dimethylbuta-1,3-diene reacts with faster rate than buta-1,3-diene in the Diels—Alder reaction. Moreover, presence of the electron-donating substituents (e.g., alkyl groups) further activates dienes. 

---