Addition reactions of carbocations with alkenes: Studies on the mechanism of carbocationic polymerization

In certain cases, Lewis acid catalyzed addition reactions of alkyl halides with alkenes terminate at the 1:1 product stage. This telomerization reaction has been employed to study the efficiency of initiating systems of carbocationic polymerization and the relative reactivities of alkenes towards carbenium ions. A direct determination of addition rate constants is possible in some cases.     

Kinetics of the reactions of halide anions with carbocations: quantitative energy profiles for s(n)1 reactions

Rate constants for the reactions of Laser flash photolytically generated benzhydrylium ions (diarylcarbenium ions) with halide ions have been determined in various solvents, including neat and aqueous acetonitrile as well as some alcohols. Substitution of the rate constants into the correlation equation log k = s(N + E) yields the nucleophilicity parameters N for the halide ions in different solvents. Linear correlations with negative slopes are found between the nucleophilicity parameters N for Cl(-) and Br(-) in different solvents and the solvent ionizing powers Y of the corresponding solvents. Increasing halide solvation reduces the rates of carbocation/chloride combinations by approximately half as much as it increases the rates of ionizations of benzhydryl chlorides. Comparison of the solvent dependent nucleophilicity parameters N of halide anions and the nucleophilicity parameters N(1) for solvents yields a quantitative prediction of common ion rate depression, as demonstrated by the analysis of a variety of literature reported mass-law constants alpha. Combination of the rate constants for the reactions of benzhydrylium ions with halide ions (k(-)()(1)) reported in this work with the ionization constants of benzhydryl halides (k(1)) and the recently reported rate constants for the reactions of benzhydrylium ions with solvents (k(2)) yields complete quantitative free energy profiles for solvolysis reactions. The applicability of Hammond's postulate for interpreting solvolysis reactions can thus be examined quantitatively.     

Revealing carbocations in highly asynchronous concerted reactions: The ene-type reaction between dithiocarboxylic acids and alkenes

The ene-type reaction between (dithio)carboxylic acids and alkenes has been studied computationally by DFT and topological (analysis of the electron localization function, ELF) methods. The reaction proceeds under kinetic control and the observed differences in regioselectivity are well-explained by the relative stability of the different transition structures. In agreement with experimental observations, electron-rich alkenes lead to Markownikoff adducts while electron-poor alkenes lead to Michael adducts. In all cases the reaction proceeds through an only transition structure (one kinetic step) although a different synchronicity was observed depending on the alkene electronics. The ELF analysis of the reactions corroborates the existence of a transient carbocation (hidden intermediate) in the reactions with electron-rich alkenes. On the other hand, electron-poor alkenes proceed through a more synchronous concerted mechanism. It can be predicted that with electron-rich alkenes bearing highly donating the transient carbocations might be captured by a nucleophile.     

Reactivities of carbocations and monomers in carbocationic polymerization and copolymerization

In carbocationic polymerization and copolymerization, a recent publication concluded that the substituent effect on carbocation reactivity is much larger than its effect on monomer reactivity, and this by a factor 10⁶ in the case of the rate constant k_12capp for p‐methylstyrene addition (monomer M₂) on, respectively, poly(p‐methoxystyrene)^± or poly(p‐methylstyrene)^± (M). This conclusion is disputed, as well as the assumption that the rate constants of capping (k_12capp) obtained in deactivation reactions of poly(p‐methoxystyrene)^± are identical with cross propagation rate constants in copolymerization (k_12copol). It is shown that the large calculated k_12capp are based on propagation constant values for p‐methylstyrene (k ≈ 10⁹) obtained by the diffusion‐clock method. They are 10⁴ times smaller as found for all styrenes, that is, between 10⁴ and 10⁵ when they are based on the ionic species concentrations. In such a case, the available data are still in agreement with an approximate compensation between the reactivities of a monomer and of the corresponding carbocation. It is also shown that copolymerization data for styrenes are not compatible with k values near to diffusion control, and that variations of log k_12capp and log k_12copol with the nucleophilicity parameter N of the monomers indicate a much lower selectivity of the monomers in the case of copolymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2666–2680, 2010     

Recent developments in living carbocationic polymerization of alkenes

The recent discovery of living cationic polymerization of alkenes has been followed up in three areas: 1) Demonstration of the validity of the DP_n = [M_o]/[I_o] relationship and the synthesis of high molecular weight polyisobutylenes M_n_>100 000) of narrow molecular weight distribution (M_w/M_n ≈︁1.1) 2) Discovery of electrophilic quenching and the use of this method for the preparation of allyl-telechelic polyisobutylenes by quenching with trimethylallylsilane and 3) Synthesis of novel linear and three-arm star radial thermoplastic elastomers comprising rubbery polyisobutylene central sequences connected to glassy polystyrene or a polystyrene derivative (i.e., p-methylstyrene, p-tert-butylstyrene, indene) outer sequences. Some physical-mechanical properties of these materials have been investigated.     

Kinetics, selectivity, and mechanism of the reactions of arenes with adamantyl carbocations in sulfuric acid

Data on the kinetics, selectivity, kinetic isotope effect, and the effect of the acidity of the medium on the rate of the reactions of benzene and alkylbenzenes in sulfuric acid (59–78 wt.% H₂SO₄) solutions of 1-adamantanol at 30 °C indicate that the direct reagents are the adamantyl carbocations (Ad⁺) that alkylate the arenes. The ortho positions of the benzene ring are not accessible on account of steric hindrances. The rate of attack by the Ad⁺ cation on the accessible para and meta positions of the ring is controlled by the formation of a σ complex. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 42, No. 1, pp. 14–18, January–February, 2006.     

Cycloaddition reactions of polysubstituted furans with oxyallyl carbocations

New and improved routes to polysubstituted furans are described, together with a survey of their reactivity in cycloaddition reactions with oxyallyl carbocations, producing polysubstituted 8-oxabicyclo[3.2.1]oct-6-en-3-ones.     

Biomimetic carbocationic polymerizations III: Investigation of isoprene polymerization initiated by dimethyl allyl bromide

Natural rubber (poly(1,4‐cis‐isoprene), NR) is a polymer of considerable industrial importance due to its exceptional properties. It is mostly produced from the cultivation of Hevea brasiliensis, and to a limited extent from Parthenium argentatum. Till date none of the synthetic equivalent of NR exists. Recently we suggested that the mechanism of NR biosynthesis is based on carbocationic polymerization, similarly to that of natural oligoisoprenoids forming by enzyme‐catalyzed prenylation. In this article we present proof of concept of a new bio‐inspired synthetic route towards the synthesis of polyisoprenes based on carbocationic polymerization initiated by dimethyl allyl bromide (DMABr)/TiCl₄. It is shown that using this strategy, 1,4‐oligoisoprene carrying a dimethyl allyl head group is produced in both cis and trans configurations, together with cyclized sequences. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2172–2180, 2009     

Synthetic approaches towards new polymer systems by the combination of living carbocationic and anionic polymerizations

This study summarizes recent efforts to obtain by combination of living carbocationic and anionic polymerizations block copolymers which are potential precursors for building new well-defined polymeric architectures with microphase separated morphology. Living carbocationic polymerization (LCCP) yields telechelic polyisobutylene (PIB) chains with a variety of useful endgroups, such as tert-chlorine, isopropenyl, primary hydroxyl, tolyl etc. When tolyl-ended PIB was used as precursor for macroinitiator of living anionic polymerization of 2-(tert-butyldimethylsilyloxy)ethyl methacrylate (tBuMe₂SiOEMA), mixtures of homopolymers and block copolymers were formed due to incomplete lithiation of this chain end. In another approach a new functionalization method was developed by end-quenching living PIB chains with 1,1-diphenylethylene (DPE). In the presence of BCl₃ a new telechelic PIB with 2,2-diphenylvinyl (DPV) endgroups was formed. A corresponding DPV model compound was synthesized from 2-chloro-2,4,4-trimethylpentane (TMPCl). Because of steric hindrance less than quantitative lithiation of this material occurred. Controlled deprotection of PtBuMe₂SiOEMA obtained by living anionic polymerization (LAP) was utilized to prepare a precursor network composed of partially deprotected PtBuM₂SiOEMA and hydroxyl-telechelic PIB by using a diisocyanate crosslinker. After network formation deprotection with HCl was completed and a new amphiphilic network (APN) containing PIB and poly(2-hydroxyethyl) methacrylate) (PHEMA) segments crosslinked by urethane linkages was obtained.     

Azidomercurations of alkenes: mercury-promoted Schmidt reactions

Azides bearing a suitably disposed alkene, when treated with either mercuric perchlorate or mercuric trifluoromethanesulfonate, produce bicyclic iminium ions. This new version of the Schmidt reaction proceeds by capture of the mercuronium ion intermediate by the azide to produce an aminodiazonium ion, which suffers a 1,2-shift to give an iminium ion (e.g., 10 --> 16 --> 17 --> 18). Reduction of the iminium ion may then be carried out to produce an amine. Compared to earlier work on the protic acid-promoted intramolecular Schmidt reaction of azido-alkenes, the mercury-promoted Schmidt reaction has several advantages. First, the acid-promoted Schmidt reaction of azido-alkenes requires that the intermediate carbocations be tertiary, allylic, benzylic, or propargylic. The mercury-promoted method has no such limitation; thus even 1,2-disubstituted alkenes may be used. Second, the mercury-promoted method is milder, allowing the presence of acid-sensitive functionality. The protic version, typically employing trifluoromethanesulfonic acid, is limited in its functional group tolerance. Third, whereas carbocation rearrangement is often observed prior to cyclization/rearrangement in the acid-promoted Schmidt reaction, the mercury-promoted method avoids this problem. Fourth, the presence of the mercurio group during the rearrangment may alter the regioselectivity of the 1,2-migration. Finally, the mercury-bearing iminium ions that are the result of the Schmidt reaction were found to be sensitive to protodemercuration, precluding their use in other transformations.     

Block copolymers of styrene and p-acetoxystyrene with polyisobutylene by combination of living carbocationic and atom transfer radical polymerizations

Successful combination of quasiliving carbocationic (QLCP) and atom transfer radical polymerizations (ATRP) was carried out by initiating ATRP with polyisobutylene (PIB) macroinitiators obtained by QLCP. It has been found that 1-chloro-1-phenylethyl-telechelic PIBs with M̄_n of 7 800 and 30 700 are efficient macroinitiators for ATRP of styrene in bulk and in xylene solution and for p-acetoxystyrene (pAcOSt) in xylene. Size exclusion chromatography (SEC) traces clearly indicated quantitative initiation and the formation of the desired polystyrene-block-polyisobutylene-block-polystyrene (PSt-PIB-PSt) and PpAcOSt-PIB-PpAcOSt triblock copolymers. Experiments also revealed absence of thermal self-initiation of styrene under ATRP conditions.     

Boron trifluoride promoted reactions of n-haloelectrophiles with alkenes

N-Haloelectrophiles react with alkenes in the presence of boron rifluoride etherate to give halofluorides and N-halo adducts.     

Insertion reactions of alkenes with diterpenoid chromium aminocarbenes

Insertion reactions of electron-deficient alkenes with chromium aminocarbenes derived from podocarpic acid generally give aryl ketone products derived from ring opening of an aminocyclopropane and subsequent enamine hydrolysis, the exception being alkenyl sulphones which give products derived from insertion of the carbene into the β-CH bond of the alkene. Increasing steric hindrance due to the substituents on the aminocarbene nitrogen appears to result in higher yields of the insertion products. However, other factors such as stabilisation of the intermediate tetracarbonylaminocarbene may explain why morpholinocarbenes give superior yields of the insertion products. Propenoic acid, propenal or nitropropene give a 13-formyl-substituted diterpenoid. Electron-rich alkenes do not undergo insertion reactions with these aminocarbenes at 110°C.     

氧杂双环烯烃开环反应的研究及其应用

有机金属试剂(其中包括镍、钯、铑、铁、铜)以及路易斯酸能选择性地催化氧杂双环烯烃的亲核开环反应,并建立多个手性中心。这种方法已经被用在一些生物活性分子的全合成过程当中,如甾体酮类似物等。     

Anionic amphiphilic end‐linked conetworks by the combination of quasiliving carbocationic and group transfer polymerizations

A series of amphiphilic end‐linked conetworks was synthesized by the combination of two “quasiliving” polymerization techniques, quasiliving carbocationic (QLCCP) and group transfer polymerizations (GTP). The hydrophobic monomer was polyisobutylene methacrylate synthesized by the QLCCP of isobutylene and subsequent terminal modification reactions. The hydrophilic monomer was methacrylic acid (MAA) introduced via the polymerization of 2‐tetrahydropyranyl methacrylate followed by acid hydrolysis after (co)network formation. The conetwork syntheses were performed by sequential monomer/crosslinker additions under GTP conditions. All the precursors and the extractables from the conetworks were characterized by gel permeation chromatography and ¹H NMR. The resulting polymer conetworks were investigated in terms of their degree of swelling (DS) in aqueous media and in tetrahydrofuran (THF) over the whole range of ionization of the MAA units and in n‐hexane for uncharged conetworks. The DSs in water increased with the degree of ionization (DI) of the MAA units and the hydrophilic content in the conetwork, whereas the DSs in THF increased with the reduction of the DI of the MAA units. The effective pK of the MAA units in the conetworks increased from 8.4 to 10.5 with decreasing MAA content. These findings can facilitate the design of similar unique conetworks with adjustable swelling behavior and composition‐dependent pK values. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 4289–4301, 2009     

(2-Chlorovinyl)chlorocarbenes: Generation and reactions with alkenes

The treatment of Z-1,1,3-trichloro-4,4-dimethyl-2-pentene (Ia) with t-C₄H₉OK in boiling hexane or benzene gave rise to (Z-2-chloro-3,3-dimethyl-1-butenyl)chlorocarbene (IVa), which reacted with alkenes to give the cyclopropane derivatives (V) in 44–57% yields. Dichloro-(2-chloro-1-alkenyl)methanes (Ib-d), which have a hydrogen atom at the C₃ position of the alkenyl substituent, were also used as carbene precursors under these conditions. These compounds gave rise to mixtures of the cyclopropanes (VI)–(VIII) (obtained in up to 57% yields) and the dienes (IX)–(XI) (yields up to 54%). The reaction of cis-2-butene with (2-chloro-1-cyclopentenyl)chlorocarbene (IVd) was found to be completely stereospecific, indicating that this carbene exists in a singlet ground state.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2552–2558, November, 1991.     

Chiral alpha-branched benzylic carbocations: diastereoselective intermolecular reactions with arene nucleophiles and NMR spectroscopic studies

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).     

Rate constants for reactions of perfluorobutylperoxyl radical with alkenes

Perfluorobutylperoxyl radicals were produced by radiolytic reduction of perfluorobutyl iodide in aerated methanol solutions. Rate constants for the reactions of this peroxyl radical with various organic compounds were determined by kinetic spectrophotometric pulse radiolysis. The rate constants for alkanes and alkenes were determined by competition kinetics using chlorpromazine as a reference. The results indicate that hydrogen abstraction from aliphatic compounds takes place with a rate constant that is too slow to measure in our system (<10⁵ M^?1 s^?1), and that abstraction of allylic and doubly allylic hydrogens is slow compared with addition. Addition to alkenes takes place with rate constants of the order of k = 10⁶ ? 10⁸ M^?1 s^?1. Good correlation was obtained between log k and the Taft substituent constants σ* for the various substituents on the double bond. Perfluorobutylperoxyl radical is found to be more reactive than trichloromethylperoxyl and other peroxyl radicals.