Silyloxyazadienes: one intermediate and two competitive pericyclic reactions

Area of Science:

• Organic Chemistry
• Computational Chemistry
• Reaction Mechanisms

Background:

• 3-trialkylsilyloxy-2-aza-1,3 dienes can undergo two distinct reaction pathways.
• These pathways, a [2+2] electrocyclic ring closure and a [4+2] hetero-Diels-Alder reaction, compete for the same diene intermediate.
• The competition between these mechanisms has not been previously analyzed.

Purpose of the Study:

• To investigate the competing [2+2] and [4+2] reaction mechanisms of 3-trialkylsilyloxy-2-aza-1,3 dienes.
• To understand the influence of temperature and substituents on the reaction outcome.
• To analyze the driving forces behind charge transfer in the transition states.

Main Methods:

• Density Functional computations were employed to study the reaction mechanisms.
• Analysis of transition states and reaction pathways was performed.
• Global electrophilicity values were calculated to understand charge transfer.

Main Results:

• The competition between the [2+2] and [4+2] mechanisms is sensitive to temperature and substituent effects.
• Entropy favors the [4+2] reaction at low temperatures and the [2+2] at high temperatures.
• Substituent modifications can alter the preferred pathway and even change the [4+2] mechanism to a two-step process.

Conclusions:

• Temperature and substituents are critical factors controlling the selectivity of these reactions.
• The observed competition provides new insights into the reactivity of aza-dienes.
• Charge transfer in the hetero-Diels-Alder transition states is governed by chemical hardness.