6-Endo-dig versus 5-exo-dig: Exploring Radical Cyclization Preference with First-, Second-, and Third-row Linkers using High-level Quantum Chemical Methods

As an expansion upon Baldwin rules, the cyclization reactions of hex-5-yn-1-yl radical systems with different first-, second-, and third-row linkers are explored at the CCSD(T) level via means of the SMD(benzene)-G4(MP2) thermochemical protocol. Unlike C, O, and N linkers, systems with B, Si, P, S, Ge, As, and Se linkers are shown to favor 6-endo-dig cyclization. This offers fundamental insights into the rational synthetic design of cyclic compounds. A thorough analysis of stereoelectronic effects, cyclization barriers, and intrinsic barriers illustrates that structural changes alter the cyclization preference by mainly impacting 5-exo-dig reaction barriers. Based on the high-level computational modeling, we proceed to develop a new tool for cyclization preference prediction from the correlation between cyclization barriers and radical structural parameters (e. g., linker bond length and bond angle). A strong correlation is found between the radical attack trajectory angle and the reaction barrier heights, i. e., cyclization preference. Finally, the influence of stereoelectronic effects on the two radical cyclization pathways is further investigated in stereoisomers of hypervalent silicon system, which provides novel insight into cyclization control.     

In search of efficient 5-endo-dig cyclization of a carbon-centered radical: 40 years from a prediction to another success for the Baldwin rules

Despite being predicted to be stereoelectronically favorable by the Baldwin rules, efficient formation of a C-C bond through a 5-endo-dig radical cyclization remained unknown for more than 40 years. This work reports a remarkable increase in the efficiency of this process upon beta-Ts substitution, which led to the development of an expedient approach to densely functionalized cyclic 1,3-dienes. Good qualitative agreement between the increased efficiency and stereoselectivity for the 5-endo-dig cyclization of Ts-substituted vinyl radicals and the results of density functional theory analysis further confirms the utility of computational methods in the design of new radical processes. Although reactions of Br atoms generated through photochemical Ts-Br bond homolysis lead to the formation of cyclic dibromide side products, the yields of target bromosulfones in the photochemically induced reactions can be increased by recycling the dibromide byproduct into the target bromosulfones through a sequence of addition/elimination reactions at the exocyclic double bond. Discovery of a relatively efficient radical 5-endo-dig closure, accompanied by a C-C bond formation, provides further support to stereoelectronic considerations at the heart of the Baldwin rules and fills one of the last remaining gaps in the arsenal of radical cyclizations.     

Thermodynamic and strain effects in the competition between 5-exo-dig and 6-endo-dig cyclizations of vinyl and aryl radicals

Electronic and structural factors controlling the competition between 5-exo-dig and 6-endo-dig cyclizations of sp2-radicals were analyzed using a combination of available experimental data and computation. Although the stereoelectronically favored 5-exo pathways usually has the lower activation energy, formation of a new aromatic ring not only makes the 6-endo process favorable thermodynamically in conjugated systems but also lowers its activation barrier to the extent where the 5-exo/6-endo selectivity is controlled by subtle factors such as the different sensitivity of the two pathways to strain effects in polycyclic systems. In particular, the stronger sensitivity of the 5-exo pathway to strain leads to a crossover in selectivity. The 6-endo cyclization is kinetically favored in smaller (and strained) cycles, whereas the 5-exo cyclization has lower barriers in the larger rings.     

Radical cascade transformations of tris(o-aryleneethynylenes) into substituted benzo[a]indeno[2,1-c]fluorenes

Oligomeric o-aryleneethynylenes with three triple bonds undergo cascade radical transformations in reaction with a Bu 3SnH/AIBN system. These cascades involve three consecutive cycle closures with the formation of substituted benzo[ a]indeno[2,1- c]fluorene or benzo[1,2]fluoreno[4,3- b]silole derivatives. The success of this sequence depends on regioselectivity of the initial attack of the Bu 3Sn radical at the central triple bond of the o-aryleneethynylene moiety. The cascade is propagated through the sequence of 5-exo-dig and 6-exo-dig cyclizations which is followed by either a radical attack at the terminal Ar substituent or radical transposition which involves H-abstraction from the terminal TMS group and 5-endo-trig cyclization. Overall, the transformation has potential to be developed into an approach to a new type of graphite ribbons.     

Intramolecular hydroarylation of alkynes catalyzed by platinum or gold: mechanism and endo selectivity

The cyclization of differently substituted aryl alkynes with PtII or AuI catalysts proceeds by endo-dig pathways. When AgI was used to generate reactive cationic AuI catalysts, 2H-chromenes dimerize to form cyclobutane derivatives by a AgI-catalyzed process. A DFT study on the cyclization mechanism shows a kinetic and thermodynamic preference for 6-endo-dig versus 5-exo-dig cyclizations in PtII-catalyzed processes. Calculations indicate that although Friedel-Crafts and the cyclopropanation processes via metal cyclopropyl carbenes show very similar activation energies, platinum cyclopropyl carbenes are the stationary points with the lowest energy.     

5-endo-dig radical cyclizations: "the poor cousins" of the radical cyclizations family

Kinetics and thermodynamics of 5-endo-dig radical cyclizations were studied using a combination of DFT computations and Marcus theory. When the reactant is stabilized by conjugation of the radical center with the bridge pi-system, the cyclization starts with reorientation of the radical orbital needed to reach the in-plane acetylene pi-orbital in the bond-forming step. This reorientation leads to loss of the above conjugative stabilization, increases the activation energy, and renders such cyclizations less exothermic. As a result, even when the radical needed for the 5-endo cyclization is formed efficiently, it undergoes either H-abstraction or equilibration with an isomeric radical. Only when the bridging moiety is saturated or when intramolecular constraints prevent the overlap of the bridge pi-orbital and the radical center, 5-endo cyclizations may be able to proceed with moderate efficiency under conditions when H-abstraction is slow. The main remaining caveat in designing such geometrically constrained 5-endo-dig cyclizations is their sensitivity to strain effects, especially when polycyclic systems are formed. The strain effects can be counterbalanced by increasing the stabilization of the product (e.g., by introducing heteroatoms into the bridging moiety). Electronic effects of such substitutions can be manifested in various ways, ranging from aromatic stabilization to a hyperconjugative beta-Si effect. The 4-exo-dig cyclization is kinetically competitive with the 5-endo-dig process but less favorable thermodynamically. As a result, by proper design of reaction conditions, 5-endo-dig radical cyclizations should be experimentally feasible.     

Synthesis and characterization of norbornanediol isomers and their fluorinated analogues

[reaction: see text] Fluorinated norbornene monomers exhibit the requisite properties for inclusion in 157 nm photoresists, but traditional addition and radical polymerizations with these monomers have failed. Norbornanediols provide an alternate route to these materials via condensation polymerization, and methods have been developed for the efficient synthesis of the exo-2-syn-7- and endo-2-exo-3-dihydroxynorbornanes. Synthesis of the fluorinated analogues is complicated by steric and electronic effects; however, a high-yielding synthesis of endo-2-exo-3-dihydroxynorbornane bearing a 5-endo-[2,2-bis(trifluoromethyl)hydroxyethyl] substituent is reported.     

Controlling the regiochemistry of radical cyclizations

This review describes the results of our recent studies on the control of the regiochemistry of radical cyclizations. N-vinylic alpha-chloroacetamides generally cyclized in a 5-endo-trig manner to give five-membered lactams, whereas 4-exo-trig cyclization occurred when the cyclized radical intermediates were highly stabilized by an adjacent phenyl or phenylthio group to afford beta-lactams. The 5-exo or 6-exo cyclization of aryl radicals onto the alkenic bond of enamides could be shifted to the corresponding 6-endo or 7-endo mode of cyclization by a positional change of the carbonyl group of enamides. The 6-endo- and 7-endo-selective aryl radical cyclizations were applied to radical cascades for the synthesis of alkaloids such as phenanthroindolizidine, cephalotaxine skeleton, and lennoxamine. The 5-exo-trig cyclization of an alkyl radical onto the alkenyl bond of enamides could also be shifted to the 6-endo mode by a positional change of the carbonyl group of enamides. The 6-endo- selective cyclization was applied to the radical cascade to afford a cylindricine skeleton. Other examples of controlling the regiochemistry of radical cyclizations and their applications to the synthesis of natural products are also discussed.     

On the stereochemical course of the addition of allylsilanes to aldehydes

Model compounds 3 and 5 have been studied to determine the orientation of the reacting double bonds in the transition state of the allylmetal–aldehyde addition. These models were designed to remove any intrinsic steric bias for the formation of the bicyclic products that would obfuscate a stereoelectronic contribution to the transition states. Model system 3 revealed a modest preference for the synclinal transition state, albeit in very low yields. Model system 5 underwent selective and largely Lewis acid independent cyclization primarily via a synclinal transition state. The high proximal selectivity observed in these cyclizations likely reflects the selectivity of an unhindered allylmetal–aldehyde addition for the synclinal transition state and results from a stereoelectronic preference, not an intrinsic steric bias, for the synclinal arrangement of double bonds.     

Aziridines as a structural motif to conformational restriction of azasugars

In order to investigate the hypothesis that the glycosidase inhibitor isofagomine was bound to alpha- or beta-glucosidase in a 1,4B conformation, a number of bicyclic aziridines that adopt the 1,4B or B1,4 conformations were synthesised and investigated. (1R)-2-endo,3-exo-2,3-Dihydroxy-4-endo-4-hydroxymethyl-6- azabicyclo[3.1.0]hexane (5) and its N-methyl and N-benzyl analogues and (1S)-2-exo-3-endo-2,3-dihydroxy-4- endo-4-hydroxymethyl-6-azabicyclo-[3.1.0]hexane (6) were synthesised. The aziridines 5 and 6 were found to be weak or not inhibitors of alpha-glucosidase, beta-glucosidase and alpha-fucosidase.     

Face-selective Diels-Alder reactions between unsymmetrical cyclohexadienes and symmetric trans-dienophile: an experimental and computational investigation

A combined experimental and theoretical study of the Diels-Alder reactions between 2-trimethylsiloxy-1,3-cyclohexadienes (2-11) and (E)-1,4-diphenylbut-2-ene-1,4-dione (1) is reported. Two diastereomeric products, 5-endo-6-exo- (nx) and 5-exo-6-endo- (xn) dibenzoyl derivatives, are possible with symmetric trans-dienophile (1). While in many cases 5-endo-6-exo product is preferred over the corresponding 5-exo-6-endo product, the product ratio nx:xn is found to vary with the position of substituents on the diene. The density functional theory studies with the mPW1PW91/6-31G* as well as the B3LYP/6-31G* levels reveal that the electrostatic repulsion between the oxygen lone pairs on the diene and the dienophile is critical to the observed product selectivities. The optimized transition state geometries though appeared to involve secondary orbital interactions, careful examination of the frontier Kohn-Sham orbitals as well as calculations with the natural bond orbital (NBO) analyses confirm the absence of SOI in these transition states. In the case of methyl-substituted dienes, a cumulative effect of steric and electrostatic interactions between the diene and the dienophile is found to be the controlling element toward the observed selectivity.     

Reverse cope elimination of hydroxylamines and alkenes or alkynes: theoretical investigation of tether length and substituent effects

Quantum mechanical calculations have been used to study the intramolecular additions of hydroxylamines to alkenes and alkynes ("reverse Cope eliminations"). In intermolecular reverse Cope eliminations, alkynes are more reactive than alkenes. However, competition experiments have shown that tethering the hydroxylamine to the alkene or alkyne can reverse the reactivity order from that normally observed. The exact outcome depends on the length of the tether. In agreement with experiment, a range of density functional theory methods and CBS-QB3 calculations predict that the activation energies for intramolecular reverse Cope eliminations follow the order 6-exo-dig < 5-exo-trig < 5-exo-dig ≈ 7-exo-dig. The order of the barriers for the 5-, 6-, and 7-exo-dig reactions of alkynes arises mainly from differences in tether strain in the transition states (TSs), but is also influenced by the TS interaction between the hydroxylamine and alkyne. Cyclization onto an alkene in the 5-exo-trig fashion incurs slightly less tether strain than a 6-exo-dig alkyne cyclization, but its activation energy is higher because the hydroxylamine fragment must distort more before the TS is reached. If the alkene terminus is substituted with two methyl groups, the barrier becomes so much higher that it is also disfavored compared to the 5- and 7-exo-dig cyclizations.     

Cyclization of nonterminal alkynic beta-keto esters catalyzed by gold(I) complex with a semihollow, end-capped triethynylphosphine ligand

A cationic gold(I) complex with a semihollow-shaped trialkynylphosphine catalyzed 5-exo-dig and 6-endo-dig cyclizations of various internal alkynic beta-keto esters, showing a marked advantage over a gold(I)-PPh3 complex with respect to the rates of the reactions and the product yields. It is proposed that the gold-bound alkynic substrate in a catalytic pocket must be somewhat folded and that such a steric effect makes the carbon-carbon bond formation entropically more favorable.     

Primary Anion–π Catalysis of Epoxide‐Opening Ether Cyclization into Rings of Different Sizes: Access to New Reactivity

The concept of anion–π catalysis focuses on the stabilization of anionic transition states on aromatic π surfaces. Recently, we demonstrated the occurrence of epoxide‐opening ether cyclizations on aromatic π surfaces. Although the reaction proceeded through unconventional mechanisms, the obtained products are the same as those from conventional Brønsted acid catalysis, and in agreement with the Baldwin selectivity rules. Different mechanisms, however, should ultimately lead to new products, a promise anion–π catalysis has been reluctant to live up to. Herein, we report non‐trivial reactions that work with anion–π catalysis, but not with Brønsted acids, under comparable conditions. Namely, we show that the anion–π templated autocatalysis and epoxide opening with alcoholate–π interactions can provide access to unconventional ring chemistry. For smaller rings, anion–π catalysis affords anti‐Baldwin oxolanes, 2‐oxabicyclo[3.3.0]octanes, and the expansion of Baldwin oxetanes by methyl migration. For larger rings, anion–π templated autocatalysis is thought to alleviate the entropic penalty of folding to enable disfavored anti‐Baldwin cyclizations into oxepanes and oxocanes.     

A computational study of radical haloacetal cyclizations controlled by the acetal center

The stereochemical outcome of the radical haloacetal cyclization reaction (Ueno-Stork reaction) has been examined by ab initio and other molecular orbital techniques. It was found that the stereochemistry of 5-exo- and 6-exo trig cyclizations can be accurately predicted from calculations using moderate levels of theory (UHF/6-311G** or B3LYP/6-311G**). A simplified computational procedure, easily run on a standard desktop computer, has been developed that provides excellent predictive ability for the stereochemical outcome for the reactions in question. Interestingly, a novel twist transition state has been identified for the first time in 5-exo-trig radical cyclization reactions.     

Understanding the unusual regioselectivity in the nucleophilic ring-opening reactions of gem-disubstituted cyclic sulfates. Experimental and theoretical studies

The regioselectivity of the nucleophilic ring-opening reactions of three gem-disubstituted cyclic sulfates with sodium azide has been studied from both experimental and theoretical viewpoints. It is found that, depending on the substituent present in the cyclic sulfate, the reaction displays reversed regioselectivity, which allows one or another regioisomer to be obtained with selectivities greater than 4:1. The theoretical calculations show that, contrary to previous understanding, the intrinsic preference in all cases is azide attack at the less-substituted C(beta) position, a consequence of similar stereoelectronic effects in the three sulfates considered. The observed preference for C(alpha) attack in the case of the ester sulfate is explained in terms of differential solvent effects, which are in turn due to subtle differences in the charge transfer in the different transition structures.     

N-Alkenyl-2-aziridinylmethyl radicals and N-alkenylaminyl radicals in cascade cyclizations to pyrrolizidines and indolizidines

[reaction: see text] The radical cascade cyclizations of N-alkenyl-2-aziridinylmethyl radicals to pyrrolizidines and indolizidines were examined using density functional theory (DFT) calculations. A large preference for cyclization to pyrrolizidines was found. These predictions corroborated very well with experimental results, leading to an efficient synthesis of pyrrolizidines. No radical cascade cyclization to indolizidines could be performed in practice as only ring opening of N-alkenyl-2-aziridinylmethyl radicals to N-allyl-N-alkenylamines occurred.     

Regio- and stereoselectivity of the 3-membered ring opening of some 3,4-epoxytetrahydropyrans and of the corresponding epibromonium ions

The reactions of 3,4-epoxytetrahydropyran and of its cis- and trans-2-methyl derivatives with hydrogen halides and with lithium aluminum hydride have been investigated in order to assess the influence of an O atom in the β position on the regioselectivity of the epoxide ring opening. All these reactions exhibit a high preference for nucleophilic attack at position 4, which decreases moderately only when the inductive effect of the O atom and the stereoelectronic requirements of the attack act in opposite directions. Similar trends are observed in the reactions of the 5,6-dihydro-2H-pyrans with NBA, which occur with preferential nucleophilic attack by water at position 4 of the intermediate epibromonium ions. A remarkably high preference (96%) for electrophilic attack syn to the 2-Me group is observed in the latter type of reaction, in accordance with a previous proposal of a mechanism in which the nucleophilic step is rate determining.     

Primary Anion–π Catalysis of Epoxide-Opening Ether Cyclization into Rings of Different Sizes: Access to New Reactivity

The concept of anion–π catalysis focuses on the stabilization of anionic transition states on aromatic π surfaces. Recently, we demonstrated the occurrence of epoxide-opening ether cyclizations on aromatic π surfaces. Although the reaction proceeded through unconventional mechanisms, the obtained products are the same as those from conventional Brønsted acid catalysis, and in agreement with the Baldwin selectivity rules. Different mechanisms, however, should ultimately lead to new products, a promise anion–π catalysis has been reluctant to live up to. Herein, we report non-trivial reactions that work with anion–π catalysis, but not with Brønsted acids, under comparable conditions. Namely, we show that the anion–π templated autocatalysis and epoxide opening with alcoholate–π interactions can provide access to unconventional ring chemistry. For smaller rings, anion–π catalysis affords anti-Baldwin oxolanes, 2-oxabicyclo[3.3.0]octanes, and the expansion of Baldwin oxetanes by methyl migration. For larger rings, anion–π templated autocatalysis is thought to alleviate the entropic penalty of folding to enable disfavored anti-Baldwin cyclizations into oxepanes and oxocanes.     

First isolation of eclipsed vic-disulfoxide: 7,8-dithiabicyclo[4.2.1]nona-2,4-diene 7-exo,8-exo-dioxide

[reaction: see text] The oxidation of 7,8-dithiabicyclo[4.2.1]nona-2,4-diene 7-exo-oxide with dimethyldioxirane (DMD) provided the 7-exo-8-exo-dioxide, the structure of which was determined by X-ray crystallography [S-S 2.341(2) A and 90 degree angle O-S-S-O 4.1(3) degrees ]. The exo attack of DMD to give the exo,exo-dioxide was kinetically more favorable than the endo attack to give the endo,exo-dioxide. DFT calculations showed that the exo,exo-dioxide is thermodynamically more stable than the other stereoisomers.