PETERSON, P. W.; ALABUGIN, I. V.; Chem. Rev. (Washington, DC, U. S.) 113 (2013) 9, 7089-7129, http://dx.doi.org/10.1021/cr4000682 ; Dep. Chem. Biochem., Fla. State Univ., Tallahassee, FL 32306, USA; Eng.) -S. Adam 45-229

The Baldwin rules constitute one of the clearest examples of the success which can be obtained through the application of stereoelectronic concepts to reaction design. With thousands of examples, the predictive power of these rules is inarguable. However, time has revealed a number of exceptions and gray areas within these rules, leading to extensions and revisions. In this review, we will present an overview of how subsequent studies of ring closure have clashed with several of Baldwin's predictions, leading to the revision of some classes of ring closure (alkyne cyclizations, electrophilic closures, etc.). We also discuss for which the original rules were vague (epoxides) or absent (promoted cyclizations), and the evidence revealed since Baldwin's work that has allowed for a better understanding of these ambiguities. With the concise summation of these amendments, this review aims to present an overview of the understanding of cyclization reactions to date. For further resources related to this article, please visit the WIREs website

Cycloaromatization reactions decouple two electrons by breaking two π bonds to form only one σ bond and illustrate one of the most common mechanistic dichotomies in chemistry, namely the two ways of breaking a chemical bond: Zwitterionic or diradical. With a suitable choice of reaction conditions, substitution patterns, and catalysts, cycloaromatization processes can be redirected from the usual formation of a diradical towards a variety of zwitterionic pathways. This review illustrates the practical approaches for directing zwitterionic cycloaromatization reactions and lays the mechanistic foundations for the further development of this emerging field.

The last missing example of the four archetypical cycloaromatizations of enediynes and enynes was discovered by combining a twisted alkene excited state with a new self-terminating path for intramolecular conversion of diradicals into closed-shell products. Photoexcitation of aromatic enynes to a twisted alkene triplet state creates a unique stereoelectronic situation, which is facilitated by the relief of excited state antiaromaticity of the benzene ring. This enables the usually unfavorable 5-endo-trig cyclization and merges it with 5-exo-dig closure. The 1,4-diradical product of the C1-C5 cyclization undergoes internal H atom transfer that is coupled with the fragmentation of an exocyclic C-C bond. This sequence provides efficient access to benzofulvenes from enynes and expands the utility of self-terminating aromatizing enyne cascades to photochemical reactions. The key feature of this self-terminating reaction is that, despite the involvement of radical species in the key cyclization step, no external radical sources or quenchers are needed to provide the products. In these cascades, both radical centers are formed transiently and converted to the closed-shell products via intramolecular H-transfer and C-C bond fragmentation. Furthermore, incorporating C-C bond cleavage into the photochemical self-terminating cyclizations of enynes opens a new way for the use of alkenes as alkyne equivalents in organic synthesis.

Chemoselective interaction of aromatic enynes with Bu3Sn radicals can be harnessed for selective cascade transformations, yielding either Sn-substituted naphthalenes or Sn-indenes. Depending on the substitution at the alkene terminus, the initial regioselective 5-exo-trig cyclizations can be intercepted at the 5-exo stage via either hydrogen atom abstraction or C-S bond scission or allowed to proceed further to the formal 6-endo products via homoallylic ring expansion. Aromatization of the latter occurs via β-C-C bond scission, which is facilitated by 2c,3e through-bond interactions, a new stereoelectronic effect in radical chemistry. The combination of formal 6-endo-trig cyclization with stereoelectronically optimized fragmentation allows the use of alkenes as synthetic equivalents of alkynes and opens a convenient route to α-Sn-substituted naphthalenes, a unique launching platform for the preparation of extended polyaromatics.

The Sonogashira/5-endo-dig/6-endo-dig cascade fuses a polycyclic aromatic backbone to the electron-rich furan subunit. The transformation proceeds in modest yields as a one-pot reaction. Efficiency of the full cascade is increased by removal of base prior to the addition of gold catalyst. Under these conditions, conversion to the full cascade products is achieved in nearly quantitative yields without purification of the intermediate products. Extension of the cascade toward triynes opens access to benzofuran-fused chrysene derivatives.

Radical cascades terminated by β-scission of exocyclic CC bonds allow for the formation of aromatic products. Whereas β-scission is common for weaker bonds, achieving this reactivity for carbon-carbon bonds requires careful design of radical leaving groups. It has now been found that the energetic penalty for breaking a strong σ-bond can be compensated by the gain of aromaticity in the product and by the stabilizing two-center, three-electron "half-bond" present in the radical fragment. Furthermore, through-bond communication of a radical and a lone pair accelerates the fragmentation by selectively stabilizing the transition state. The stereoelectronic design of radical leaving groups leads to a new, convenient route to Sn-functionalized aromatics.

Polycyclic aromatic hydrocarbons (PAHs) represent the link between resonance‐stabilized free radicals and carbonaceous nanoparticles generated in incomplete combustion processes and in circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars. Although these PAHs resemble building blocks of complex carbonaceous nanostructures, their fundamental formation mechanisms have remained elusive. By exploring these reaction mechanisms of the phenyl radical with biphenyl/naphthalene theoretically and experimentally, we provide compelling evidence on a novel phenyl‐addition/dehydrocyclization (PAC) pathway leading to prototype PAHs: triphenylene and fluoranthene. PAC operates efficiently at high temperatures leading through rapid molecular mass growth processes to complex aromatic structures, which are difficult to synthesize by traditional pathways such as hydrogen‐abstraction/acetylene‐addition. The elucidation of the fundamental reactions leading to PAHs is necessary to facilitate an understanding of the origin and evolution of the molecular universe and of carbon in our galaxy.