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. States of Carbenes and Nitrenes
Singlet state: carbocation-like in nature, trigonal planar geometry, electrophilic character Triplet state: diradical-like in nature, linear geometry Formation reactions of carbenes
_ + hν a) R2C N N R C R + N2 or (singlet) Δ hν b) R2C C O R C R + CO or (singlet) Δ _ _ _X c) RCHX2 + base R CX 2 R C X
H R'X d) RCHX2 + R'Li R C Li R C H + LiX (where X = I & Br) X (carbenoid) X R'H e) RCHX2 + R'Li R C Li R C X + LiX (where X = F & Cl) X (carbenoid)
Zn-Cu I CH Zn I (Simmons-Smith reagent) f) CH2I2 ether 2 a carbenoid species that reacts stereospecifically with alkenes to give cyclopropanes but does not insert into alkane C-H bonds Typical Carbene Reactions
a) Insertion into a σ-bond: C H + CH2 C CH2 H
b) Addition to a π-bond: C C + CH2 C C C
_ + Y c) Ylide type rxn: R2C + X Y R2C X Y R2C X an ylide
d) Intramolecular insertion: CH3 H3C H H3C H H3C C C + C + H3C C CH3 H3C CH CH2 H3C C CH3 CH3 H (5%) (90%) (5%) 1,2-insertion 1,2-insertion 1,3-insertion
e) Cracking: C H + H C C H
+ + + R C CR + N f) Dimerization: R2C N N + CR2 R2C N N CR2 2 2 2 Mechanistic Aspects of Carbene Chemistry
Observations
A. Gas Phase Experiment at very low pressure: CH + _ 3 hν CH2 N N + CH3CH2CH2CH2CH3 CH3(CH2)4CH3 + CH3 CH CH2CH2CH3 (I) (II) ~49% ~34% CH3 + CH3CH2 CH CH2CH3 (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 :
+ _ H3C CH3 hν CH2 N N + C C cis-1,2-dimethylcyclopropane H H (only organic product) 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. 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:
C3F8 CH2 CH2 CH2 CH2
H H C3F8 H H singlet triplet C C spin C C + + H C CH inversion H C CH 3 diradical 3 3 3 H H H H C C C C rotation H3C CH3 H3C CH3 ring closure CH2
H CH3 cis product C C H C H H H + 3 H3C CH trans product cis only 3 ring closure NITRENE FORMATION REACTIONS
_ + hν R N N N R N + N a) or 2 R = alkyl,aryl, H Δ
+ _ hν b) R SO N N N R SO N + N 2 or 2 2 R = alkyl, aryl Δ
+ _ c) RO C N N N hν RO C N + N or 2 O Δ O R = alkyl, aryl d) RO C NH O SO NO 2 2 _ O RO C N + O SO NO base 2 2 _ O (nosylate) RO C N O SO2 NO2 O Mechanistic Aspects of Nitrene Chemistry Cycloaddition - Observations
Liquid phase experiment R R N N + _ H3C CH3 R N N N + C C hν + H H H3C CH3 H3C H H H H CH3 cis trans (predominant aziridine product) (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. Mechanistic Aspects of Nitrene Chemistry Cycloaddition
Explanation for Nitrene Cycloaddition: R R R N R N N N
H H H H singlet triplet C C spin C C + + H C CH inversion H C CH 3 diradical 3 3 3 H H H H C C C C rotation H3C CH3 H3C CH3 ring R closure N
cis product R H CH3 + C C N H3C H trans product H H ring closure H C CH 3 cis 3 Mechanistic Aspects of Nitrene Chemistry Insertion - Observations H H N R + _ R N N N + CH3CH2 C CH2CH2CH3 CH3CH2 C CH2CH2CH3 CH2Cl2 CH3 CH3 R = C OC2H5 (+) S-3-methylhexane (+) R-product O
Method of Generation Conc. of Alkane Obs. rot. of prod. Retention of Config o R N 100% +1.69 99% 3 Δ o R N 1.2% +1.71 100% 3 h ν o R N 26.8% +1.69 99% 3 hν o R N 100% +1.68 98% 3 hν 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. Evidence of Singlet Nitrene C-H Insertion
Selectivity R N + alkane alkane insertion products (singlet)
Alkane Relative reactivities
CH3 CH3 CH3 CH3 CH3 CH3 R N H3C C C CH3 H3C C C CH3 + H3C C C CH2 NH H H H NH R H H R 67.0 : 1.0
H H H H H H R N + H3C C C CH3 H3C C C CH3 H3C C C CH2 NH H H H NH R H H R
9.0 : 1.0
Singlet nitrene C-H insertion selectivity: tertiary C-H > secondary C-H > primary C-H 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.