2-Carbomethoxy-1,3-butadiene: an electronically activated diene in [4 + 2] cycloadditions with electron-deficient dienophiles

Abstract

Cross Diels-Alder reactions between 2-carbomethoxy-l,3-butadiene 1 and electron-rich dienes were carried out. It was found that diene 1 had a higher reactivity than even the well-known Danishefsky diene in its Diels-Alder cycloaddition with electron-deficient dienophiles. In addition, (Z)-1-methylthio-3-carbomethoxy-l,3-butadiene 11 was found to have a higher reactivity than (Z)-I-methylthio-l,3-butadiene and (3-2-carbomethoxy-1,3-hexadiene, while both (a-and The facile dimerization of 2-carbomethoxy-l,3-butadiene 1 to give dimethyl mikanecate 4 has been known for some time (Scheme 1) (1, 2). Although the unusual ease with which this [4+2] cycloaddition takes place has been noted, a rationalization of this phenomenon has yet to be offered. A recent communication by Jung and Zimmerman provided the first kinetic data on this dimerization (3). They proposed that the high dienophilic character of the C1-C2 bond in 1 and the possibility that 1 may be more stable in its E-(s-cis) conformation could be major factors in accelerating this Diels-Alder reaction. W e provide evidence herein that another electronic factor plays a significant role in the rate acceleration of this dimerization and that in fact 1 should be regarded as an electronically activated diene.A 96% yield of dimer 4 was obtained by heating sulfolene 2 at 1 10°C (Scheme 1, method A) ( l a ) or by treating allylic bromide 3 with base at 25°C (Method B) ( I b). However, in the presence of the electron-rich diene 5 a (Y = Me) (4) and the very reactive diene 5 b (Y = OMe) (4b), either method led to a mixture of dimer 4 and cross cycloadduct 6 (Table 1). In most cases, the dimerization product prevailed unless a large excess of the electron-rich diene was used. Note that the lower temperature seems to favor dimerization over cross-cycloaddition (entries 3 , 4 , 5, and 8). These results indicate that diene 1 , as a dienophile, cycloadds more rapidly to itself, an electron-deficient diene, than it does to the electron-rich dienes 5 a and 5b. Note that the reactivity of diene 1 is underestimated here since diene 1 is generated slowly (3-4 h) and must be at low concentration relative to the other diene. W e ruled out the possibility of a local concentration effect, where molecules of diene 1 would dimerize faster because they were generated in proximity to one another, by stirring 1 equivalent each of sulfolene 2 and diene 5 b with '~u t h o r to whom correspondence may be addressed. excess methyl acrylate (10 equiv.) in refluxing toluene (Scheme 2). After 4 h, diene 1 had reacted completely to give 7 and Danishefsky's diene 5 b had reacted to the extent of 93% to give adduct 8 (gc analysis). Also detected were 6 b (<5%) and 4 (traces). Control experiments in which each diene was reacted separately with excess methyl acrylate confirmed that diene 1 reacted more rapidly with methyl acrylate than did Danishefsky's diene 56. We believe that diene 1 reacted as soon as it was generated since it (or dimer 4) was not detected in significant amount throughout the reactio...

The dimerisation of 2-methoxycarbonylbuta-1,3-diene: the importance of paralocalisation energy in assessing diene reactivity

“…We then proposed an early reorganisation of the π-network to explain the activation of 1. 8 We report herein a full account of this work which includes additional experimental results and ab initio calculations on the dimerisation of 1 supporting our hypothesis. We also extend our discussion of the rationale to include the concept of paralocalisation of electrons.…”

Mechanism and Regioselectivity of the Cycloaddition of Thiones Derived from Meldrum’s Acid, Malonates, or Other Dicarbonyls

“…However, one arrives at the correct prediction for the regiochemistry from FMO considerations (HOMO diene −LUMO thione ). We wanted to see the effect a C2-ester would have but we could not use 2-carbomethoxy-1,3-butadiene because it dimerizes too fast . We used instead 2-carbomethoxy-1-ethyl-1,3-butadiene ( 38 ).…”