A Stepwise [4 + 3] Cycloaddition Reaction of the 1,3-Diphenyl-2-azaallyl Anion

2-Azaallyl Anions, 2-Azaallyl Cations, 2-Azaallyl Radicals, and Azomethine Ylides

This review covers the use of 2-azaallyl anions, 2-azaallyl cations, and 2-azaallyl radicals in organic synthesis up through June 2018. Particular attention is paid to both foundational studies and recent advances over the past decade involving semistabilized and nonstabilized 2-azaallyl anions as key intermediates in various carbon-carbon and carbon-heteroatom bond-forming processes. Both transition-metal-catalyzed and transition-metal-free transformations are covered. Azomethine ylides, which have received significant attention elsewhere, are discussed briefly with the primary focus on critical comparisons with 2-azaallyl anions in regard to generation and use.

C-C bond formation and related reactions at the CNC backbone in (smif)FeX (smif = 1,3-di-(2-pyridyl)-2-azaallyl): dimerizations, 3 + 2 cyclization, and nucleophilic attack; transfer hydrogenations and alkyne trimerization (X = N(TMS)2, dpma = (di-(2-pyridyl-methyl)-amide))

Molecular orbital analysis depicts the CNC(nb) backbone of the smif (1,3-di-(2-pyridyl)-2-azaallyl) ligand as having singlet diradical and/or ionic character where electrophilic or nucleophilic attack is plausible. Reversible dimerization of (smif)Fe{N(SiMe3)2} (1) to [{(Me3Si)2N}Fe]2(μ-κ(3),κ(3)-N,py2-smif,smif) (2) may be construed as diradical coupling. A proton transfer within the backbone-methylated, and o-pyridine-methylated smif of putative ((b)Me2(o)Me2smif)FeN(SiMe3)2 (8) provides a route to [{(Me3Si)2N}Fe]2(μ-κ(4),κ(4)-N,py2,C-((b)Me,(b)CH2,(o)Me2(smif)H))2 (9). A 3 + 2 cyclization of ditolyl-acetylene occurs with 1, leading to the dimer [{2,5-di(pyridin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe3)2]2 (11), and the collateral discovery of alkyne cyclotrimerization led to a brief study that identified Fe(N(SiMe3)2(THF) as an effective catalyst. Nucleophilic attack by (smif)2Fe (13) on (t)BuNCO and (2,6-(i)Pr2C6H3)NCO afforded (RNHCO-smif)2Fe (14a, R = (t)Bu; 14b, 2,6-(i)PrC6H3). Calculations suggested that (dpma)2Fe (15) would favorably lose dihydrogen to afford (smif)2Fe (13). H2-transfer to alkynes, olefins, imines, PhN═NPh, and ketones was explored, but only stoichiometric reactions were affected. Some physical properties of the compounds were examined, and X-ray structural studies on several dinuclear species were conducted.

Cycloadditions of 2-Azaallyllithium Species with Conjugated Polyenes

2-Azaallyllithium species [R(1)CH(-)N=C(X)R(2)Li(+), where R(1) and R(2) are alkyl and X = OMe] were generated by tin-lithium exchange of (2-azaallyl)stannanes and underwent [pi4s+pi2s] and [pi6s+pi4s] cycloadditions with cyclic dienes and trienes, respectively, to generate novel bridged azabicyclic compounds in a highly diastereoselective endo fashion. The periselectivity using cycloheptatriene was modest, producing a 1:1 mixture of [pi6s+pi4s] and [pi4s+pi2s] adducts. The reactions of 2-azaallyllithium species with dienes proceeded by a [pi4s+pi2s] pathway. The cycloadducts derived from cyclic 2-azaallyllithium species possess the 7-azabicyclo[2.2.1]heptane (tropane) or 8-azabicyclo[3.2.1]octane ring system and have been elaborated into cocaine-like analogues.

Cycloadditions of nonstabilized 2-azaallyllithiums with cycloheptatriene

[reaction: see text] Transmetalation of tin-bearing cyclic imidates gave 2-azaallyllithiums that underwent [pi6s + pi4s] cycloadditions with cycloheptatriene to produce tricyclic adducts, which may be useful as analogs of cocaine. The peri- and stereoselectivity of this process are discussed.