Linear carbenes

The olefins that undergo metathesis include most simple and substituted olefins cycHc olefins give linear high molecular-weight polymers. The mechanism of the reaction is beheved to involve formation of carbene complexes that react via cycHc intermediates, ie, metaHacycles. Industrial olefin metathesis processes are carried out with soHd catalysts (30). 

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Diazirines are in most cases more easily available than linear diazo compounds. Moreover, their decomposition via true carbenes is free of side reactions, whereas linear diazo compounds in presence of H-donors may react by a cationic pathway. Only where reactions of linear diazo compounds are optimized for carbene formation do they give the same products as do decomposing diazirines. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

Keywords Fischer carbene complex [3+2+1 ] -benzannulation reaction Asymmetric benzannulation Linear benzannulation... 

Kinetic studies using 1,9-decadiene and 1,5-hexadiene in comparison widi catalyst 14 and catalyst 12 demonstrate an order-of-magnitude difference in their rates of polymerization, widi 14 being the faster of the two.12 Furdier, this study shows diat different products are produced when die two catalysts are reacted widi 1,5-hexadiene. Catalyst 14 generates principally lineal" polymer with the small amount of cyclics normally observed in step condensation chemistry, while 12 produces only small amounts of linear oligomers widi die major product being cyclics such as 1,5-cyclooctadiene.12 Catalyst 12, a late transition metal benzylidene (carbene), has vastly different steric and electronic factors compared to catalyst 14, an early transition metal alkylidene. Since die results were observed after extended reaction time periods and no catalyst quenching or kinetic product isolation was performed, this anomaly is attributed to mechanistic differences between diese two catalysts under identical reaction conditions. 

Because of the absence of chain limiter, the catalyst itself may initially act as the chain limiter (Fig. 8.22). The catalyst reacts with the olefinic regions of the polymer backbone and causes chain scission to occur, forming two new chains. The reactive carbene which is produced then moves from chain to chain, forming two new chains with each scission until the most probable molecular weight distribution is reached (Mw/Mn = 2), producing linear chains end capped with [Ru] catalyst residues. 

An interesting carbene, 1-oxobutatrienylidene [25], having cumulated double bonds, has been found by IR spectroscopy in the photolysis (A>230nm) products of matrix-isolated l,2,3,4-pentatetraene-l,5-dione [26] (Maier et al., 1988) (in its turn the unstable dione [26] was generated by thermo- or photo-destruction of compound [27]). The second product was carbon monoxide. The linear structure of the carbene [25] has been suggested on the basis of two intense IR bands at 2222 cm and 1923 cm indicating respectively ketene and allene fragments. 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

A number of highly active ethylene polymerisation catalysts have resulted from the combination of functionalised NHC ligands with Ti, the first of these was the bis(phenolate)carbene ligated complex 3 [8], Upon activation with modified MAO (MMAO), this species gave an activity of 290 kg-mol bar h in the one test reported, making it one of the most active carbene-based olefin polymerisation catalysts known. In later work the same complex was evaluated with straight MAO activation, and activities of up to ca. 100 kg mol -bar" -h" were reported for linear polyethylene production [9],... 

The remainder of the work on Ni(II) complexes involves the use of chelating ligands in which the carbene is functionalised with pendant heteroatom donor(s). The picolyl-functionalised NHC dicationic complex 29 (Fig. 4.11) was tested for ethylene polymerisation after treatment with MAO [34]. This complex was found to be highly active in a preliminary test (330 kg moF bar h" ), giving predominantly linear polyethylene. Unfortunately this work does not seem to have been followed up. The same system was active for norbomene polymerisation (TOF = 24 400 h" over 1 h). Maximum activity was achieved at 80°C whereafter thermal deactivation became significant, although the nature of this deactivation was not studied. The phenoxide-functionalised carbene complex 30 (Fig. 4.11) was also... 

The cyclic carbene complex shown in equation 3.4 was studied by X-ray diffraction [66], it shows a linear complex (angle C—Au—C 178.6(4)°) and the gold aryl bond distance is 1.993(10) A which is in accordance with such bonds in other known pentafluorophenyl complexes. The gold carbene carbon distance is 1.961(9) A, the dihedral angle between the planes formed by the two organic ligands is 5.35° and the shortest intermolecular Au—Au distance is 3.95 A. 

Synthesis and structural characterization of linear Au(l) N-heterocyclic carbene complexes New analogues of the Au(l) phosphine drug auranofin. Journal of OrganometaUic Chemistry, 690, 5625-5635. 

Laser flash irradiation of diazofluorene in perdeuterated matrices, in contrast, gave severely nonexponential decay of the carbene spectra. Analyses of the products formed in the low-temperature matrices showed that, as with the EPR studies, the carbene was not undergoing D-abstraction. LFP of the diazo compound 36 in CFCl3-CF2BrCF2Br glasses gave linear first-order decays, and linear Arrhenius plots, which were attributed to classical Cl and Br abstractions. 

When the cis/trans stereoselectivity of cyclopropanation with ethyl diazoacetate in the presence of CuCl P(0-z-Pr)3, Rh6(CO)16 or PdCl2 2 PhCN was plotted against that obtained with Rh2(OAc)4, a linear correlation was observed in every case, with slopes of 1.74,1.04 and 0.59, respectively (based on 22 olefins, T = 298 K) S9). These relationships as well as the results of regioselectivity studies carried out with 1,3-dienes point to the similar nature of the intermediates involved in Cu-, Rh-and Pd-catalyzed cyclopropanation. Furthermore, obvious parallels in reactivity in the transformations of Scheme 45 for a variety of catalysts based on Cu, Rh, Fe, Ru, Re and Mo suggest the conclusion that electrophilic metal carbenes are not only involved in cyclopropanation but also in ylide-forming reactions66. ... 

Heterocyclic carbenes have been applied as ligands to gold(i) and gold(m) with a wide variety of heteroatoms and substitution patterns.256,257 Gold(i) forms 1 1 and 1 2 complexes with a linear coordination geometry, while at gold(m) centers, carbenes appear only in combination with other ligands in a square-planar array. 

Linear coordination complexes are also observed with carbenes rather than carbonyl ligands. However, recent structural work carried out on bis(carbene) complexes indicates that coordination of the counteranion can lead to significant deviation of the Ccalbene-Hg-Gcalbene angle from linearity. In 70, for example, the two chloride anions are... 

Some diazirines, particularly the 3-trifluoromethyl-3-aryldiazirines, can rearrange upon photolysis to a linear diazo derivative, similar in structure to the photosensitive end of the crosslinker PNP-DTP (Chapter 5, Section 3.12). These isomerized products themselves can be photolyzed to the reactive carbene. 

Rate measurements are straightforward if the carbenes can be monitored directly. As a rule, the decay of carbene absorption is (pseudo) first-order, due to rearrangement and/or reaction with the solvent. In the presence of a quencher, the decay is accelerated (Eq. 1), and the rate constant kq is obtained from a plot of k0bs versus [Q], Curved plots were often observed with proton donors (HX) as quenchers, particularly for high concentrations of weakly acidic alcohols. Although these effects have been attributed to oligomerization of the alcohols,91 the interpretation of curved plots remains a matter of dispute.76 Therefore, the rate constants reported in Tables 2-4 are taken from linear (regions of) obs-HX plots, or refer to a specified concentration of HX. 

Linear dependencies of k0bS on [ROH] are also rare for the aryl(chloro)carb-enes 200 and 201 91 hence the ranking of these carbenes is somewhat uncertain. The aryl(oxy)carbenes 203, 154a, and 154c are clearly superior to their alkyl(oxy) counterparts (198, 199) as proton acceptors.76,77 The 4-OMe... 

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