Norbomene/1-alkene

Several reports in which NHC-Pd complexes have been employed to catalyse the copolymerisation of alkenes with CO have appeared over the years. Herrmann and co-workers reported that the chelating dicarbene complex 38 (Fig. 4.14) is active for CO/ethylene [43], The highest TON [(mol ethylene + mol CO) mol Pd ] was 3 075 after a 4 h run. The modest TONs coupled with a very high molecular weight copolymer led the authors to conclude that only a small fraction of the pre-catalyst goes on to form an active species. Low molecular weight (M = 3 790) CO/norbomene copolymer resulted when complex 39 (Fig. 4.14) was tested by Chen and Lin [44]. The catalyst displayed only a very low activity, yielding 330 turnovers after 3 days. 

Since this initial report, there is only one other report for M-NHC catalysed copolymerisation of CO/alkenes [52]. Lin and co-workers synthesised the fcw-NHC complex dication 41, that copolymerises CO and norbomene. The copolymer is synthesised in 87% yield by employing 0.5 mol% 41, and 750 psi CO gas after 3 days at 60°C. The polymer formed contains 37 repeat units and = 4660 and M = 3790. 

Another convenient entry to fused cyclobutene-1,2-diesters was via site selective modification of the norbomene rt-bond in Smith s fe-alkene 49, e.g. treatment with 3,6-di(2 -pyridyl)-s-tetrazine 51 followed by DDQ oxidation afforded the cyclobutene-derivative 53 <97AA119>, while direct coupling with 3,5-f> (trifluoromethyl)-l,3,4-oxadiazo]e 54 furnished the tas(cyclobutene-l,2-diester) 55 (Scheme 6) <97SL196>. 

For carbenes 25 and 26 1,2-shifts of bonds a and b will lead to different bridgehead olefins, 17 and 27 for 26, and 1-norbomene 16 and bicy-clo[3.1.1]hept-l-ene (30) for 25, whereas carbene 24a will only give alkene 15. In accordance with experimental observations,19 calculations show that the more strained bond will migrate preferentially. 

Reactions of amines with alkenes have been reviewed298,299. Alkali metal amides are active homogeneous catalysts for the amination of olefins. Thus diethylamine and ethylene yield triethylamine when heated at 70-90 °C at 6-10 atm in the presence of lithium diethylamide and /V./V./V. /V -tetrarncthylcthylcncdiaminc. Solutions of caesium amide promote the addition of ammonia to ethylene at 100 °C and 110 atm to give mixtures of mono-, di- and triethylamines300. The iridium(I)-catalysed addition of aniline to norbomene affords the anilinonorbomane 274301. Treatment of norbomene with aniline... 

One could also imagine a complex that at one site will insert only one certain type of alkene due to high steric constraints (e.g. ethene) and a second alkene at the other site thus giving an alternating copolymer that may even contain stereoregularity, thus obtaining for instance an alternating norbomene/ethene copolymer [34],... 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

Involvement of a-elimination reactions for in situ prepared catalysts from WC16 and Me4Sn was demonstrated by the use of 13C in tetramethyltin. The norbomene polymers formed contained the 13CH2 alkene moiety as the end-group. Also unstable C14W=CH2 and Cl4W=13CH2 species were observed by H NMR spectroscopy [14],... 

Several arylations involving reactive alkenes such as norbomene or allenes have been reported. Togni and coworkers have shown that norbomene is selectively added to the ortho positions of phenols to produce a mixture of 30 and 31 in 69% and 13% yield, respectively, after 72 hours at 100°C (22) [108, 109]. 1,1-dimethylallene also reacts with aromatic carboxamides (33) to produce prenylation products (34) in the presence of cationic iridium complexes (23) [110]. In both cases, initial ortho C-H bond activation in arenes directed by coordinating groups followed by olefin insertion has been proposed. 

Insertion of aUcynes into aromatic C-H bonds has been achieved by iridium complexes. Shibata and coworkers found that the cationic complex [Ir(COD)2]BF4 catalyzes the hydroarylation of internal alkynes with aryl ketones in the presence of BINAP (24) [111]. The reaction selectively produces ort/to-substituted alkenated-aryl products. Styrene and norbomene were also found to undergo hydroarylation under similar condition. [Cp IrCl2]2 catalyzes aromatization of benzoic acid with two equivalents of internal alkyne to form naphthalene derivatives via decarboxylation in the presence of Ag2C03 as an oxidant (25) [112]. 

Since it is known that the cyclopentene ring of norbomene can be easily opened by methylidene carbene complex Ih, bicyclic compound 66 has been synthesized from norbomene derivative 65 having an alkene part in a sidechain in the presence... 

The most common alkenes employed in the Pd-catalysed synthesis of alternating polyketones are ethene, styrene, propene and cyclic alkenes such as norbomene and norbornadiene. Even though the mechanism does not vary substantially with the alkene, the reactions of the various co-monomers are here reported and commented on separately, starting with the ethene/CO copolymerisation, which is still the most studied process. As a general scheme, the proposed catalytic cycles are presented first, then the spectroscopic experiments that have allowed one to elucidate each single mechanistic step. 

Ring strain enhances alkene reactivity. Norbomene, for example, undergoes rapid addition at 0°C.12... 

As is true for most reagents, there is a preference for approach of the borane from the less hindered side of the molecule. Because diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered molecules. Table 4.6 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result both from the low regioselectivity and from addition to both faces of the double bond. Even the quite hindered 7,7-dimethyl-norbomene shows only modest preference for endo addition with diborane. The selectivity is enhanced with the bulkier reagent 9-BBN. 

In addition to the role of substituents in determining regioselectivity, several other structural features affect the reactivity of dipolarophiles. Strain increases reactivity. Norbomene, for example, is consistently more reactive than cyclohexene in 1,3-dipolar cycloadditions. Conjugated functional groups also usually increase reactivity. This increased reactivity has most often been demonstrated with electron-attracting substituents, but for some 1,3-dipoles, enamines, enol ethers, and other alkenes with donor substituents are also quite reactive. Some reactivity data for a series of alkenes with a few 1,3-dipoles are given in Table 6.3. Scheme 6.5 gives some examples of 1,3-dipolar cycloaddition reactions. 

Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

The absence of a second cyclopentadienyl ring coupled with the short bridge gives a very open environment at the metal site. This allows easier access for bulky monomers, including 1-alkenes and norbomene, compared to polymerization with metallocenes. CpA initiators yield ethylene copolymers not easily available with metallocenes. Copolymers containing significant amounts of comonomers such as styrene, norbomene, and a-olefins from 1-hexene to 1-octadecene are easily obtained with CpA, but not with metallocene or traditional Ziegler-Natta initiators. 

The meso-ionic l,3-dithiol-4-ones (134) participate - in 1,3-dipolar cycloaddition reactions giving adducts of the general type 136. They show a remarkable degree of reactivity toward simple alkenes including tetramethylethylene, cyclopentene, norbomene, and norbor-nadiene as well as toward the more reactive 1,3-dipolarophilic olefins dimethyl maleate, dimethyl fumarate, methyl cinnamate, diben-zoylethylene, A -phenylmaleimide, and acenaphthylene. Alkynes such as dimethyl acetylenedicarboxylate also add to meso-ionic 1,3-dithiol-4-ones (134), but the intermediate cycloadducts are not isolable they eliminate carbonyl sulfide and yield thiophenes (137) directly. - ... 

Some typical epoxidations are listed in Table 3.1. The first Ru-catalysed epoxida-tion was reported in 1983 by James et al., in which RuBrlPPh XOEPl/PhlO/CHjClj was used to epoxidise styrene, norbomene and c/x-stilbene in low yields trans-stilbene was not oxidised [588]. It was later noted that tranx-RulOl lTMPl/Oj/C H aerobically oxidised cyclic alkenes, and a catalytic cycle involving Ru 0(TMP) was proposed, in which there is disproportionation of Ru(0)(TMP) to Ru(TMP) and fran -Ru(0)2(TMP), the latter epoxidising the alkene and the former being oxidised back to the latter by (Fig. 1.26) [46, 583]. Stilbene, tranx-styrene and norbomene were efficiently epoxidised by trani-RulOl lTMPl/lCl pyNOl/CgH [589], as was epoxidation of exo-norbomenes catalysed by trani-RulOl lTMPl/Oj/ CgHg [590]. 

As stoich. [Ru(0)(bpy)(tmtacn)]VCH3CN it functioned as a competent (sic) epoxidant for alkenes, though the products were often contaminated with by-products (e.g. fran -stilbene gave fran -stilbene oxide and benzaldehyde cw-stilbene gave cis- and trans- epoxides). Kinetics of the epoxidation of norbomene and styrene were reported, with activation parameters measured and discussed [682]. Kinetics of its non-stereospecific, stoicheiometric epoxidation of aromatic alkenes in CH3CN were studied, and the rates compared with those of oxidations effected by other Ru(IV) 0x0 complexes with N-donors, e. g. [Ru(0)(tmeda)(tpy)] ", trans-[Ru(0)(Cl3bpy)(tpy)] " and [Ru(0)Cl(bpy)(ppz )] + [676]. 

Ru(C. 35C(0)CH2C(0)C. j5)3] , a substituted (acac) complex, is made from the 1,3-diketone C j COCH COC Fu with RuClj (quoted as RuCl in the paper) in ethanol with KCHCOj). In biphasic solvents [Ru(C jjC(0)CH2C(0)C j3)3]7per-fluorodecaUn-toluene/O (1 atm)/65°C oxidised aldehydes to ketones, disubstituted alkenes (cyclo-octene, norbomene) to epoxides and sulfides to sulfoxides or sul-fones [821, 822],... 

The first Ru-catalysed epoxidation was reported in 1983 by James et al. using RuBr(PPh3)(OEP)/PhlO/CH2Cl2 with styrene, norbomene and aT-stilbene in low yields cf. mech. Ch. 1 [23]. Eater work showed that fran -Ru(0)2(TMP)/02/CgHg catalysed aerobic alkene epoxidation of cyclo-octene, cis- and trans- -methylstyrenes and norbomene (Fig. 1.26) [24],... 

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