Carbenes & Nitrenes Organic Chemistry.pptx

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1. Carbenes and Nitrenes  Carbenes are uncharged, electron deficient molecular species that contain a divalent carbon atom surrounded by a sextet of electrons.  Nitrenes are uncharged, electron deficient molecular species that contain a monovalent nitrogen atom surrounded by a sextet of electrons.

2. States of Carbenes and Nitrenes  Singlet state: carbocation- like in nature, trigonal planar geometry, electrophilic character  Triplet state: diradical- like in nature, linear geometry

3. Formation reactions of carbenes a) + _ R 2 C N N h  R C R (singlet) + N 2 b) 2 R C C O or  h  R C R (singlet) + CO c) 2 or  RCHX + base _ R CX 2 _ X _ R C X d) RCHX 2 + R'Li (where X = I & Br) R'X R C H (carbenoid) + LiX e) RCHX 2 + R'Li H R C Li X X R C Li X R'H R C X (carbenoid) + LiX (where X = F & Cl) f) CH I 2 2 Zn- Cu ether 2 I CH Zn I (Simmons- Smith reagent) a carbenoid species that reacts stereospecifically with alkenes to give cyclopropanes but does not insert into alkane C- H bonds

4. Typical Carbene Reactions a) Insertion into a  - bond: C H + CH 2 C CH 2 H b) Addition to a  - bond: C C + CH 2 C C C c) Ylide type rxn: R 2 C + X Y _ + R 2 C X Y an ylide R 2 C Y X d) Intramolecular insertion: C H 3 C H 3 C H C CH 3 C 3 H C H H 3 C CH CH 2 C 3 H C H 3 C CH 3 C H CH 3 CH 3 + + (5%) (90%) (5%) 1,2- insertion 1,2- insertion 1,3- insertion e) Cracking: C H + H C C H f) Dimerization: R 2 C + + R 2 C N N CR 2 R 2 C CR 2 + N 2 + N N + CR 2

5. Mechanistic Aspects of Carbene Chemistry Observations + N N A. Gas Phase Experiment at very low pressure: _ CH 2 + CH 3 CH 2 CH 2 CH 2 CH 3 h  ~49% CH 3 (CH 2 ) 4 CH 3 + CH 3 (I) CH 3 CH CH 2 CH 2 CH 3 (II) ~34% + CH 3 CH 2 CH 3 CH CH 2 CH 3 (III) ~17% Point of Information: As the pressure of the system is increased (nitrogen or argon added), the yields of (II) and (III) increase at the expense of (I). B. Liquid phase experiment : CH 2 + _ N N + H H 3 C CH 3 C C H h  Point of Information: When an inert diluent such as perfluoropropane is added to the starting materials, a mixture of cis- and trans-1,2- dimethylcyclopropanes is obtained. cis-1,2- dimethylcyclopropane (only organic product)

6. Mechanistic Aspects of Carbene Chemistry Explanation for Gas Phase Experiment: At very low pressure, reactive, singlet carbene inserts in the alkane C- H bonds in a completely random, statistical manner. However, upon addition of nitrogen or argon gas to the reaction, singlet carbene reverts to less reactive (more selective) triplet carbene via non- productive collisions with the inert diluent.Triplet carbene shows a preference for attacking tertiary and secondary hydrogen atoms rather than primary hydrogen atoms. Hence, formation of more tertiary and secondary abstraction products at the expense of primary abstraction product. Explanation for Liquid Phase Experiment: CH 2 H 3 C H H singlet + C C H 3 C CH 3 H H C 3 F 8 CH 3 H 3 C CH 3 H H triplet + C C CH 2 CH 2 H 3 C CH 3 H H cis only C 3 F 8 spin inversion CH 2 C C H 3 C CH 3 H H CH 2 C C H 3 C H CH 3 H rotation ring closure cis product + trans product ring closure C C diradical

7. NITRENE FORMATION REACTIONS a) h  or  R N + N 2 b) h  or  + _ R N N N R = alkyl,aryl, H R SO 2 N + N 2 h  or  + _ c) RO C N N N O RO C N + N 2 O d) O SO 2 NO 2 base NO 2 NO 2 2 _ RO C N + O SO O (nosylate) + _ R SO 2 N N N R = alkyl, aryl R = alkyl, aryl RO C NH O _ RO C N O SO 2 O

8. Mechanistic Aspects of Nitrene Chemistry Cycloaddition - Observations Liquid phase experiment + _ H 3 C H C C CH 3 H h  R N N N + N CH 3 H H 3 C H R N H 3 C H H CH 3 R + cis (predominant aziridine product) trans (minor aziridine product) Point of Information: When an inert solvent is added to the reaction mixture, more trans- product is obtained at the expense of the cis- product.

9. Mechanistic Aspects of Nitrene Chemistry Cycloaddition Explanation for Nitrene Cycloaddition: H 3 C H H H CH 3 H H 3 C R N CH 3 H 3 C CH 3 H H H 3 C CH 3 H H spin inversion C C H 3 C CH 3 H H C C 3 H C H CH 3 H rotation ring closure cis product + trans product ring closure C C diradical R N singlet + C C cis R N triplet + C C R N R N R N

10. Mechanistic Aspects of Nitrene Chemistry _ + + R N N N CH 3 CH 2 H C CH 2 CH 2 CH 3 CH 3 (+) S-3-methylhexane CH Cl 2 2 CH 3 CH 2 3 N R C CH 2 CH 2 CH 3 CH Insertion - Observations H (+) R- product Method of Generation Conc. of Alkane Obs. rot. of prod. Retention of Config R N 3  100% +1.69 o 99% 3 h  R N R N 3 h  R N 3 h  +1.71 o 100% 1.2% 26.8% +1.69 o 100% +1.68 o 99% 98% R = C OC 2 H 5 O

11. Explanation of Nitrene Insertion Observed rotation is independent of chiral alkane concentration. At less than 100% chiral alkane concentration (more inert solvent present), more triplet nitrene is formed at the expense of initially produced singlet nitrene. . The fact that higher concentration of triplet nitrene does not affect the observed rotation of chiral product is indicative of the inability of the triplet species to insert into alkyl C-H bonds.

12. Evidence of Singlet Nitrene C- H Insertion Selectivity + alkane R N alkane insertion products Alkane Relative reactivities (singlet) CH 3 CH 3 3 3 H C C C CH H H R N CH 3 CH 3 3 3 CH 3 CH 3 3 2 H C C C CH NH H H R H C C C CH + H NH R 67.0 : 1.0 3 H H H C C C CH H H 3 R N 3 3 H H H C C C CH H NH R 3 2 H H H C C C CH NH H H R + 9.0 : 1.0 Singlet nitrene C- H insertion selectivity: tertiary C-H > secondary C-H > primary C- H

13. Summary of Carbene/Nitrene Chemistry ✛ Singlet carbenes and singlet nitrenes add to C=C bonds in a one- step, stereospecific manner. ✛ Triplet carbenes and triplet nitrenes add to C=C bonds in a two- step, non- stereospecific manner . ✛ Singlet carbenes insert into alkyl C- H bonds randomly, whereas singlet nitrenes do so selectively. Both singlet species insert into alkyl C- H bonds with retention of configuration. ✛ Triplet carbenes insert into alkyl C- H bonds selectively, but not stereospecifically. ✛ Triplet nitrenes do not insert into alkyl C- H bonds.