CHE 118A Lecture Notes - Lecture 11: Structural Isomer, Nucleophile, Aryne
Document Summary
Regiochemistry of the addition of nucleophile to the triple bond http://en. wikipedia. org/wiki/aryne#cite_note-referenceb-36. There are two possible regioisomers of benzyne with substituent (y): triple bond can be positioned between c2 and c3 or between c3 and c4. Substituents ortho to the leaving group will lead to the triple bond between c2 and c3. Para y and lg will lead to regioisomer with triple bond between c3 and c4. Meta substituent can afford both regioisomers as described above. C2 and c3, electron withdrawing substituents (ewg) will direct the nucleophile addition to place carbanion as close as possible to ewg. However, electron donating substituents (edg) will provide little selectivity between products. In the regioisomer where triple bond is located between c3 and c4 the effect of substituent on nucleophile addition is diminished, and mixtures of para and meta products are often obtained. Also, sterics can influence regioselectivity as well (e. g. tert-butyl edg but not an equal mix, much more of product a)
