Leveraging Electron Push-Pull Effect for Catalytic Polymerization and Degradation of a Cyclobutane Monomer System

Ring-opening polymerization (ROP) offers a striking solution to solve problems encountered in step-growth condensation polymerization, including precise control over molecular weight, molecular weight distribution, and topology. This has inspired our interest in ROP of cycloalkanes with an ultimate goal to rethink polyolefins, which clearly poses a number of challenges. Practicality of ROP of cycloalkanes is actually limited by their low polymerizability and elusive mechanisms which arise from significantly varied ring size and non-polar C-C bonds in monomers. In this work, by using Lewis acid/Brønsted base/C(sp3)-H initiator system previously developed in our laboratory, we focus on cyclobutanes and explore the positional and electronic effects of substituents on the ring, namely electron push-pull effect, in promoting controlled polymerization to afford densely functionalized poly(cyclobutanes), as well as catalytic degradation of obtained polymers for upcycling. More importantly, experiments and DFT calculations unveil considerable population of Lewis-acid-induced thermostabilized 1,4-zwitterions, which distinguish cyclobutanes from cyclopropanes and others. All these findings would shed light on catalytic synthesis and degradation of saturated all-carbon main-chain polymers, as well as small molecule transformations of cyclobutanes.

2-Oxabicyclo[2.1.1]hexanes: Synthesis, Properties, and Validation as Bioisosteres of ortho- and meta-Benzenes

We have developed a general and practical approach towards 2-oxabicyclo[2.1.1]hexanes with two and three exit vectors via an iodocyclization reaction. The obtained compounds have been easily converted into the corresponding building blocks for use in medicinal chemistry. 2-Oxabicyclo[2.1.1]hexanes have been incorporated into the structure of five drugs and three agrochemicals, and validated biologically as bioisosteres of ortho- and meta-benzenes.

Biosynthesis of Strained Amino Acids Through a PLP-Dependent Enzyme via Cryptic Halogenation

Amino acids (AAs) are modular and modifiable building blocks which nature uses to synthesize both macromolecules, such as proteins, and small molecule natural products, such as alkaloids and non-ribosomal peptides (NRPs). While the 20 main proteinogenic AAs display relatively limited side-chain diversity, a wide range of non-canonical amino acids (ncAAs) exist that are not used by the ribosome for protein synthesis but contain a broad array of structural features and functional groups not found in proteinogenic AAs. In this communication, we report the discovery of the biosynthetic pathway for a new ncAA, pazamine, which contains a cyclopropane ring formed in two steps. In the first step, a chlorine is added onto the C4 position of lysine by a radical halogenase PazA. The cyclopropane ring is then formed in the next step by a pyridoxal-5'-phosphate-dependent enzyme, PazB, via an SN2-like attack onto C4 to eliminate chloride. Genetic studies of this pathway in the native host, Pseudomonas azotoformans, show that pazamine and its succinylated derivative, pazamide, potentially inhibit ethylene biosynthesis in growing plants based on alterations in the root phenotype of Arabidopsis thaliana seedlings. We further show that PazB can be utilized to make an alternative cyclobutane-containing AA. These discoveries may lead to advances in biocatalytic production of specialty chemicals and agricultural biotechnology.

Synthesis and Biological Evaluation of Cyclobutane-Based β3 Integrin Antagonists: A Novel Approach to Targeting Integrins for Cancer Therapy

The Arg-Gly-Asp (RGD)-binding family of integrin receptors, and notably the β3 subfamily, are key to multiple physiological processes involved in tissue development, cancer proliferation, and metastatic dissemination. While there is compelling preclinical evidence that both αvβ3 and αIIbβ3 are important anticancer targets, most integrin antagonists developed to target the β3 integrins are highly selective for αvβ3 or αIIbβ3. We report the design, synthesis, and biological evaluation of a new structural class of ligand-mimetic β3 integrin antagonist. These new antagonists combine a high activity against αvβ3 with a moderate affinity for αIIbβ3, providing the first evidence for a new approach to integrin targeting in cancer.

Synthesis of Bis(amino acids) Containing the Styryl-cyclobutane Core by Photosensitized [2+2]-Cross-cycloaddition of Allylidene-5(4H)-oxazolones

The irradiation of 2-aryl-4-(E-3'-aryl-allylidene)-5(4H)-oxazolones 1 with blue light (456 nm) in the presence of [Ru(bpy)₃](BF₄)₂ (bpy = 2,2'-bipyridine, 5% mol) gives the unstable cyclobutane-bis(oxazolones) 2 by [2+2]-photocycloaddition of two oxazolones 1. Each oxazolone contributes to the formation of 2 with a different C=C bond, one of them reacting through the exocyclic C=C bond, while the other does so through the styryl group. Treatment of unstable cyclobutanes 2 with NaOMe/MeOH produces the oxazolone ring opening reaction, affording stable styryl-cyclobutane bis(amino acids) 3. The reaction starts with formation of the T₁ excited state of the photosensitizer ³[Ru*(bpy)₃]²⁺, which reacts with S₀ of oxazolones 1 through energy transfer to give the oxazolone T₁ state ³(oxa*)-1, which is the reactive species and was characterized by transient absorption spectroscopy. Measurement of the half-life of ³(oxa*)-1 for 1a, 1b and 1d shows large values for 1a and 1b (10-12 μs), while that of 1d is shorter (726 ns). Density functional theory (DFT) modeling displays strong structural differences in the T₁ states of the three oxazolones. Moreover, study of the spin density of T₁ state ³(oxa*)-1 provides clues to understanding the different reactivity of 4-allylidene-oxazolones described here with respect to the previously reported 4-arylidene-oxazolones.

Organophotocatalytic [2+2] Cycloaddition of Electron-Deficient Styrenes

A visible-light organophotocatalytic [2+2] cycloaddition of electron-deficient styrenes is described. Photocatalytic [2+2] cycloadditions are typically performed with electron-rich styrene derivatives or α,β-unsaturated carbonyl compounds, and with transition-metal-based catalysts. We have discovered that an organic cyanoarene photocatalyst is able to deliver high-value cyclobutane products bearing electron-deficient aryl substituents in good yields. A range of electron-deficient substituents are tolerated, and both homodimerisations and intramolecular [2+2] cycloadditions to fused bicyclic systems are available by using this methodology.

Sunlight-Induced In Situ Isomerization of Both Ligands in a Mixed-Ligand Coordination Polymer: From Photosalient to Photoinert Crystals

Reaction of Zn(NO₃ )₂  ⋅ 6H₂ O, maleic acid (H₂ mal) and trans-4-(1-naphthylvinyl)pyridine (trans-nvp) in the dark results in the formation of a one-dimensional coordination polymer (1D CP) [Zn(mal)(trans-nvp)] (1), which is photosalient in nature. The crystals of 1 pop violently under UV light and moderately in sunlight, and generate cyclobutane ligands. However, the same reaction mixture kept in visible light exhibits the rare example of in situ isomerization of both ligands: cis-trans transformation of maleate and trans-cis isomerization of the nvp ligands, and subsequent formation of another 1D CP [Zn(fum)(cis-nvp)₂ (H₂ O)₂ ] (2, H₂ fum=fumaric acid), which is found to be photoinert. Thus, altering the reaction condition from dark to visible light gives rise to photosalient to photoinert crystals.

Green-Chemistry-Inspired Synthesis of Cyclobutane-Based Hole-Selective Materials for Highly Efficient Perovskite Solar Cells and Modules

Hybrid lead halide perovskite solar cells (PSCs) have emerged as potential competitors to silicon‐based solar cells with an unprecedented increase in power conversion efficiency (PCE), nearing the breakthrough point toward commercialization. However, for hole‐transporting materials, it is generally acknowledged that complex structures often create issues such as increased costs and hazardous substances in the synthetic schemes, when translated from the laboratory to manufacture on a large scale. Here, we present cyclobutane‐based hole‐selective materials synthesized using simple and green‐chemistry inspired protocols in order to reduce costs and adverse environmental impact. A series of novel semiconductors with molecularly engineered side arms were successfully applied in perovskite solar cells. V1366‐based PSCs feature impressive efficiency of 21 %, along with long‐term operational stability under atmospheric environment. Most importantly, we also fabricated perovskite solar modules exhibiting a record efficiency over 19 % with an active area of 30.24 cm².

Square network based upon the molecular salt of the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane and the sulfate anion

The formation and crystal structure of a hydrated molecular salt that results in a square network is reported. The crystalline solid is based upon the tetraprotonated photoproduct rtct-tetrakis(pyridin-4-yl)cyclobutane (4H-rtct-TPCB)⁴⁺ along with two sulfate anions (SO₄^2-) and eight waters of hydration, namely, 4,4',4'',4'''-(cyclobutane-1,2,3,4-tetrayl)tetrapyridinium bis(sulfate) octahydrate, C₂₄H₂₄N₄⁴⁺·2SO₄^2-·8H₂O. The fully protonated photoproduct acts as a four-connecting node within the square network by engaging in four charge-assisted N⁺-H...O hydrogen bonds to the sulfate anion. The observed hydrogen-bonding pattern in this square network is akin to T-silica, which is a metastable form of SiO₂. The included water molecules and sulfate anions engage in numerous O-H...O hydrogen bonds to form various hydrogen-bonded ring structures.

Antitumor Profile of Carbon-Bridged Steroids (CBS) and Triterpenoids

This review focuses on the rare group of carbon-bridged steroids (CBS) and triterpenoids found in various natural sources such as green, yellow-green, and red algae, marine sponges, soft corals, ascidians, starfish, and other marine invertebrates. In addition, this group of rare lipids is found in amoebas, fungi, fungal endophytes, and plants. For convenience, the presented CBS and triterpenoids are divided into four groups, which include: (a) CBS and triterpenoids containing a cyclopropane group; (b) CBS and triterpenoids with cyclopropane ring in the side chain; (c) CBS and triterpenoids containing a cyclobutane group; (d) CBS and triterpenoids containing cyclopentane, cyclohexane or cycloheptane moieties. For the comparative characterization of the antitumor profile, we have added several semi- and synthetic CBS and triterpenoids, with various additional rings, to identify possible promising sources for pharmacologists and the pharmaceutical industry. About 300 CBS and triterpenoids are presented in this review, which demonstrate a wide range of biological activities, but the most pronounced antitumor profile. The review summarizes biological activities both determined experimentally and estimated using the well-known PASS software. According to the data obtained, two-thirds of CBS and triterpenoids show moderate activity levels with a confidence level of 70 to 90%; however, one third of these lipids demonstrate strong antitumor activity with a confidence level exceeding 90%. Several CBS and triterpenoids, from different lipid groups, demonstrate selective action on different types of tumor cells such as renal cancer, sarcoma, pancreatic cancer, prostate cancer, lymphocytic leukemia, myeloid leukemia, liver cancer, and genitourinary cancer with varying degrees of confidence. In addition, the review presents graphical images of the antitumor profile of both individual CBS and triterpenoids groups and individual compounds.

Studies on the Lithiation, Borylation, and 1,2-Metalate Rearrangement of O-Cycloalkyl 2,4,6-Triisopropylbenzoates

A broad range of acyclic primary and secondary 2,4,6‐triisopropylbenzoate (TIB) esters have been used in lithiation‐borylation reactions, but cyclic TIB esters have not. We have studied the use of cyclic TIB esters in lithiation‐borylation reactions and looked at the effect of ring size (3‐ → 6‐membered rings) on the three key steps of the lithiation‐borylation protocol: deprotonation, borylation and 1,2‐metalate rearrangement. Although all rings sizes could be deprotonated, the cyclohexyl case was impractically slow, and the cyclopentyl example underwent α‐elimination faster than deprotonation at −78 °C and so could not be used. Both cyclobutyl and cyclopropyl cases underwent rapid borylation, but only the cyclobutyl substrate underwent 1,2‐metalate rearrangement. Thus, the cyclobutyl TIB ester occupies a “Goldilocks zone,” being small enough for deprotonation and large enough to enable 1,2‐migration. The generality of the reaction was explored with a broad range of boronic esters.

Reaction of 3-Oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate with Dimethylsulfoxonium Methylide

We have been interested in the reactivities of small-ring compounds and have reported reactions that proceed through cyclopropane intermediates starting from coumarin derivatives bearing an electron-withdrawing group at the 3-position or 2-oxo-2H-pyran-3-carboxylate derivatives and dimethylsulfoxonium methylide. This time, the reaction between 3-oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate and dimethylsulfoxonium methylide has been investigated. 3a,4,5,7a-Tetrahydro-7-hydroxybenzofuran-6-carboxylate and/or 2-hydroxybicyclo[4.1.0]hept-2-ene-3-carboxylate were obtained. The compounds were characterized using various spectral and X-ray crystallographic techniques. A plausible reaction mechanism has been discussed. This reaction was applied to some 3-oxa-2-oxobicyclo[4.2.0]oct-4-ene-1-carboxylate derivatives to clarify the generality.

Catalytic Enantioselective [2+2] Cycloaddition of α-Halo Acroleins: Construction of Cyclobutanes Containing Two Tetrasubstituted Stereocenters

A catalytic enantioselective formal [2+2] cycloaddition between α-halo acroleins and electronically diverse arylalkenes is described. In the presence of (S)-oxazaborolidinium cation as the catalyst, densely functionalized cyclobutanes containing two vicinal tetrasubstituted stereocenters were produced in high yields and high diastereoselectivities with excellent enantioselectivities. Mechanistic studies revealed that the cis isomer could be transformed into the trans isomer via an enantiocontrolled process. A gram-scale reaction of this catalytic method was used to demonstrate its synthetic potential.

An Enolate-Structure-Enabled Anionic Cascade Cyclization Reaction: Easy Access to Complex Scaffolds with Contiguous Six-, Five-, and Four-Membered Rings

Catalyst-free addition of ketone enolate to non-activated multiple C-C bonds involves non-complementary reaction partners and typically requires super-basic conditions. On the other hand, highly aggregated or solvated enolates are not reactive enough to undergo direct addition to alkenes or alkynes. Herein, we report a new anionic cascade reaction for one-step assembly of intriguing molecular scaffolds possessing contiguous six-, five-, and four-membered rings, representing a formal [2+2] enol-allene cycloaddition. Reaction proceeds under very mild conditions and with excellent diastereoselectivity. Deeper mechanistic and computational studies revealed unusually slow proton transfer phenomenon in cyclic ketone intermediate and explained peculiar stereochemical outcome.

[2+2] Photodimerization of Stilbazoles Promoted by Oxalic Acid in Suspension

In this study, a very simple technique to perform efficiently photodimerization of some vinylpyridines is reported. By irradiating a stirred mixture of several stilbazoles with solid oxalic acid dihydrate dispersed in a nonpolar (i.e., cyclohexane) or moderately polar (benzene, dichloromethane, dioxane) solvent, the corresponding dimeric cyclobutane adducts were obtained in high yields and excellent regio- and stereoselectivities. The strategy could also be applied successfully to oily, waxy, or even insoluble stilbazoles. Moreover, the oxalic acid loading could be lowered to substoichiometric amounts. When further optimizations were needed, our strategy was found to be highly flexible to identify other oligocarboxylic acids as alternative additives to improve, or even overturn, the regioselectivity. Oxalic acid and other oligocarboxylic acids were found to be capable of orienting more than 50 stilbazoles toward photodimerization under these conditions.

Exploring Diverse-Ring Analogues on Combretastatin A4 (CA-4) Olefin as Microtubule-Targeting Agents

Combretastatin-4 (CA-4) as a tubulin polymerization inhibitor draws extensive attentions. However, due to its weak stability of cis-olefin and poor metabolic stability, structure modifications on cis-configuration are being performed. In this work, we constructed a series of novel CA-4 analogues with linkers on olefin containing diphenylethanone, cis-locked dihydrofuran, α-substituted diphenylethanone, cyclobutane and cyclohexane on its cis-olefin. Cytotoxic activity of all analogues was measured by an SRB assay. Among them, compound 6b, a by-product in the preparation of diphenylethanone analogues, was found to be the most potent cytotoxic agents against HepG2 cells with IC₅₀ values of less than 0.5 μM. The two isomers of 6b induced cellular apoptosis tested by Annexin V-FITC and propidium iodide (PI) double staining, arrested cells in the G2/M phase by PI staining analysis, and disrupted microtubule network by immunohistochemistry study in HepG2 cells. Moreover, 6b-(E) displayed a dose-dependent inhibition effect for tubulin assembly in in vitro tubulin polymerization assay. In addition, molecular docking studies showed that two isomers of 6b could bind efficiently at colchicine binding site of tubulin similar to CA-4.

Supramolecular Sandwiches: Halogen-Bonded Coformers Direct [2+2] Photoreactivity in Two-Component Cocrystals

The halogen-bond (X-bond) donors 1,3- and 1,4-diiodotetrafluorobenzene (1,3-di-I-tFb and 1,4-di-I-tFb, respectively) form cocrystals with trans-1,2-bis(2-pyridyl)ethylene (2,2′-bpe) assembled by N···I X-bonds. In each cocrystal, 2(1,3-di-I-tFb)·2(2,2′-bpe) and (1,4-di-I-tFb)·(2,2′-bpe), the donor molecules support the C=C bonds of 2,2′-bpe to undergo an intermolecular [2+2] photodimerization. UV irradiation of each cocrystal resulted in stereospecific and quantitative conversion of 2,2′-bpe to rctt-tetrakis(2-pyridyl)cyclobutane (2,2′-tpcb). In each case, the reactivity occurs via face-to-face π-stacked columns wherein nearest-neighbor pairs of 2,2′-bpe molecules lie sandwiched between X-bond donor molecules. Nearest-neighbor C=C bonds are stacked criss-crossed in both cocrystals. The reactivity was ascribed to the olefins undergoing pedal-like motion in the solid state. The stereochemistry of 2,2′-tpcb is confirmed in cocrystals 2(1,3-di-I-tFb)·(2,2′-tpcb) and (1,4-di-I-tFb)·(2,2′-tpcb).

Synthesis of Functionalized Tetrahydropyridines by SnCl4 -Mediated [4+2] Cycloaddition between Donor-Acceptor Cyclobutanes and Nitriles

Cycloadditions of strained carbocycles promoted by Lewis acids are powerful methods to construct heterocyclic frameworks. In fact, the formal [3+2] cycloadditions of donor-acceptor (DA) cyclopropanes with nitriles has seen particular success in synthesis. In this work, we report on the first [4+2] cycloaddition of nitriles with DA cyclobutanes by Lewis acid activation. Tetrahydropyridine derivatives were obtained in up to 91 % yield from various aryl-activated cyclobutane diesters and aliphatic or aromatic nitriles.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from β-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Construction of Cyclobutanes by Multicomponent Cascade Reactions in Homogeneous Solution through Visible-Light Catalysis

[2+2] Photocycloaddition of two olefins is a general method to assemble the core scaffold, cyclobutane, found in numerous bioactive molecules. A new approach to synthesize cyclobutanes through multicomponent cascade reactions by merging aldol reaction and Witting reaction with visible-light-induced [2+2] cycloaddition is reported. An array of cyclobutanes with high selectivity has been achieved from commercially available aldehydes, ketones (or phosphorus ylide), and olefins with visible-light irradiation of a catalytic amount of (fac-tris(2-phenylpyridinato-C₂ ,N)iridium) ([Ir(ppy)₃ ]) at room temperature. Control experiments and spectroscopic studies revealed that the triplet-triplet energy transfer from the excited [Ir(ppy)₃ ]* to enones, generated in situ from aldehyde and ketone or aldehyde and phosphorus ylide, is responsible for these simple and efficient muticomponent transformations.

In Situ Raman Monitoring of Silver(I)-Aided Laser-Driven Cleavage Reaction of Cyclobutane

The cyclobutane cleavage reaction is an important process and has received continuous interest. Herein, we demonstrate the visible laser-driven cleavage reaction of cyclobutane in crystal form by using in situ Raman spectroscopy. Silver(I) coordination-induced strain and thermal effects from the laser irradiation are the two main driving forces for the cleavage of cyclobutane crystals. This work may open up a new avenue for studying cyclobutane cleavage reactions, as compared to the conventional routes using ex situ techniques.

Photochemical dimerization of a fluorinated dibenzylideneacetone in chloroform solution

(1E,4E)-1,5-Bis(2,6-difluorophenyl)penta-1,4-dien-3-one, C17H10F4O, (I), dimerizes under sunlight in chloroform solution to form the corresponding cyclobutane derivative, (2E,2'E)-1,1'-[2,4-bis(2,6-difluorophenyl)cyclobutane-1,3-diyl]bis[3-(2,6-difluorophenyl)prop-2-en-1-one], C34H20F8O2, (II). The crystal structure of (I) explains why no topochemical dimerization can occur in the solid state. In the solid, molecules of dimer (II) show the `truxillic acid'-type arrangement of crystallographic centres of inversion, with half a molecule per asymmetric unit and cell dimensions closely related to those of the monomer. Intermolecular interactions in both solids are dominated by C-H···O and C-H···F contacts and also comprise interactions with aromatic systems (C-H···π and π-π).

Microflow photochemistry: UVC-induced [2 + 2]-photoadditions to furanone in a microcapillary reactor

[2 + 2]-Cycloadditions of cyclopentene and 2,3-dimethylbut-2-ene to furanone were investigated under continuous-flow conditions. Irradiations were conducted in a FEP-microcapillary module which was placed in a Rayonet chamber photoreactor equipped with low wattage UVC-lamps. Conversion rates and isolated yields were compared to analogue batch reactions in a quartz test tube. In all cases examined, the microcapillary reactor furnished faster conversions and improved product qualities.

Stereoselective preparation of cyclobutanes with four different substituents: total synthesis and structural revision of pipercyclobutanamide A and piperchabamide G

A general strategy was developed for the diastereo- and enantioselective synthesis of cyclobutanes with four different substituents. It consists of three transition metal-catalyzed reactions — a RhII-catalyzed cyclopropanation, a AgI-catalyzed regioselective and stereospecific ring expansion, and a RhI-catalyzed addition reaction. Structures of pipercyclobutanamide A and piperchabamide G were synthesized and revised.

Studies on the synthesis and reactivity of novel benzofuran-2-yl-[3-methyl-3-phenylcyclobutyl] methanones and their antimicrobial activity

Preparation in excellent yields of cyclobutyl benzofuran-2-yl- and naphthofuran- 2-yl-ketones, the corresponding ketoximes and thiosemicarbazones, ether derivatives of the ketoximes and thiazoles derived from the thiosemicarbazones are described. Two of the synthesized compounds have been tested against eight different microorganisms and found to be active against some of the species studied.

Isomerization Processes in Ions of the Empirical Formula C 4 H 8 +

Ions of the formulaC 4 H 8 + have been generated with different initial energies by ionizing ethylene ( C 2 H 4 + + C 2 H 4 → C 4 H 8 + , where theC 4 H 8 + ion is formed with an initial energy of > 11.51 eV), cyclobutane (initial energy ofC 4 H 8 + , > 10.84 eV), methylcyclopropane (> 10.15 eV), 1-C4H8 (> 9.58 eV), and i-C4H8 (> 9.06 eV) with 11.6-11.8 eV photons, and in some cases also with 10 eV photons and with gamma radiation. The structures of the ions have been determined from the structures of the C4H8 products formed in charge transfer reaction between the ions and charge acceptors such as dimethylamine and nitric oxide, as well as from the structures of the butanes formed inD 2 - transfer reactions with methylcyclopentane-d 12 (C 4 H 8 + + C 6 D 12 → C 4 H 8 D 2 + C 6 D 10 + ). At low pressures theC 4 H 8 + ions initially formed in ethylene, cyclobutane, and methylcyclopropane isomerize to the thermodynamically most stable configurations,i-C 4 H 8 + and2-C 4 H 8 + . The2-C 4 H 8 + structure predominates in all the experiments. As the pressure is raised, thei-C 4 H 8 + ion yield diminishes as that of2-C 4 H 8 + increases, indicating that when the precursor of thei-C 4 H 8 + ion is collisionally deactivated, it ends up as2-C 4 H 8 + . At high pressures,1-C 4 H 8 + ions are intercepted; their yield increases with increasing pressure, indicating that1-C 4 H 8 + is an intermediate which isomerizes further unless it is collisionally deactivated. The1-C 4 H 8 + ion formed in methylcyclopropane (initial energy > 10.15 eV) is more easily deactivated than that formed in cyclobutane (initial energy > 10.84 eV). That the isomerization of the1-C 4 H 8 + ion to lower energy structures such asi-C 4 H 8 + and2-C 4 H 8 + requires excess internal energy is demonstrated by the fact that in the photolysis with 10 eV photons, a negligible amount of isomerization is observed, but with 11.6-11.8 eV photons, more than half of the1-C 4 H 8 + ions isomerize to the2-C 4 H 8 + structure at a pressure of 2 torr. Isomerization of the low energyi-C 4 H 8 + ions formed in the photolysis of i-C4H8 to other structures is relatively unimportant at 11.6-11.8 eV. Taking the ratioi-C 4 H 8 + / 2 -C 4 H 8 + as an indicator of the amount of energy removed by collisions from the intermediateC 4 H 8 + species under conditions where only i- and 2 -C 4 H 8 + ions are intercepted, it is shown that the efficiency of energy transfer from the ions to helium, hydrogen, neon, krypton, xenon, nitrogen, and carbon dioxide is related to the polarizability of the added deactivator.