Metathesis nitrile

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Cross Metathesis of Nitrile Rubber with 1-Hexene... 

W02005/080456 Al, Lanxess Inc. Process for the preparation of low molecular weight hydrogenated nitrile rubber EP2028194 Al, Lanxess Deutschland GmbH Metathesis of nitrile rubbers in the presence of transition metal complex catalysts Press release Lanxess - New York Conference, New York City, USA, Monday August 28, 2006, Address by Dr. Ulrich Koemm LANXESS Concepts in Rubber ... 

Polymeric isoxazolines were prepared by cycloaddition of nitrile oxides to norbomadiene followed by ring-opening metathesis polymerization (ROMP) <06PLM3292 06MM3147>. 

Three novel stereo- and regioselective schemes for the total synthesis of (+ )-brefeldin A 440 have been accomplished. Each of them exploit intermolec-ular nitrile oxide cycloaddition for constructing the open chain and introducing substituents, but differ in subsequent stages. The first (480) and the second (481) use intramolecular cycloaddition for the macrocycle closure. However, in the second scheme INOC is followed by C=C bond cis-trans-isomerization. In the third scheme (481) intermolecular cycloaddition is followed by ring closing metathesis as the key step. 

Unsaturated esters and silanes are not the only functionalised alkenes to have been employed as cross-metathesis substrates unsaturated alkyl chlorides [9], silylethers [10] and nitriles have all participated in metathesis reactions utilising... 

Like styrene, acrylonitrile is a non-nucleophilic alkene which can stabilise the electron-rich molybdenum-carbon bond and therefore the cross-/self-metathe-sis selectivity was similarly dependent on the nucleophilicity of the second alkene [metallacycle 10 versus 12, see Scheme 2 (replace Ar with CN)]. A notable difference between the styrene and acrylonitrile cross-metathesis reactions is the reversal in stereochemistry observed, with the cis isomer dominating (3 1— 9 1) in the nitrile products. In general, the greater the steric bulk of the alkyl-substituted alkene, the higher the trans cis ratio in the product (Eq. 11). 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

The following are some examples of reactions which have produced vinylidene complexes but are either not of general application or have not been further developed. Oxygen atom transfer occurs in reactions of NbH (t] — OC — CPh2) Cp 2 with nitriles or isonitriles to give isocyanates and Nb(=C=CPh2)Cp 2 [260]. Metathesis of Ph(R) C=C=NPh (R = Me, aryl) with W(=CHPh)(CO)5 proceeds via W C(NPh=CHPh)=C (R)Ph (CO)s, which is converted to W =C=C(R)Ph (CO)5 by treatment with BF3. OMe2 [261]. 

Nitrile rubber polymers, having lower molecular weight have been prepared by metathesis of nitrile butadiene rubber with ruthenium indenylidene complexes [65]. 

Where there is no spacer group between the C=C bond and the functional group, productive self-metathesis does not occur, but cross-metathesis reactions with other olefins are still possible. Recent impressive examples of this are the cross-metathesis reactions of acrylonitrile (equation 19). The reaction occurs with a wide variety of R groups. For 15 different compounds the yield of the new nitrile after 3 h at room temperature is 40-90%, with the cis isomer always strongly preferred (75-90%). Only minor amounts of RCH2CH=CHCH2R are formed, and no NCCH=CHCN182. The fact that acrylonitrile... 

The results of some cross-metathesis experiments for a series of nitriles CH2=CH(CH2) CN reacting with c -hept-3-ene are summarized in Table 4. No crossmetathesis occurs with acrylonitrile (n = 0). For n = 1, 2, 5, 8, 9 cross-metathesis products are formed in substantial amount, but for n = 3, 4 very little reaction occurs, an effect which is attributed to intramolecular coordination of the nitrile group to the metal centre in [Mt]=CH(CH2) CN (n = 3, 4), thereby reducing its metathesis activity or causing its destruction. With n > 5 the nitrile group has little influence on the reaction and its self-metathesis is preferred over that of hept-3-ene, whereas the reverse is true for n = 1,2. 

R. R. Schrock, M. L. Listemann, and L. G. Sturgeoff, Metathesis of Tungsten-Tungsten Triple Bonds with Acetylenes and Nitriles To Give Alkylidyne and Nitrido Complexes, J. Am. Chem. Soc. 104, 4291 1293 (1982). 

Nitriles, such as CH3CN, can produce metathesis reactions with dinuclear alkoxides such W2(OBu )6 to yield (Bu 0)3W=N and (Bu 0)3W=CMe. In contrast W2[OCMe2(CF3)]6 reacts reversibly to give an adduct, W2[OCMe2(CF3)]6(NCMe)2.87... 

The THT and SMe2 adducts have structures of the type (18-B-V). Their chemistry has been extensively studied and it is summarized in Fig. 18-B-7. The diverse, and in some cases unique, reactivity of these compounds includes substitution with preservation of the geometry or with conversion to (MX4)2(/t-X)2 species, oxidative-addition,53 cluster formation, splitting of C—N bonds,54 and above all coupling of the molecules with triply bonded carbon atom.55 They catalytically trimerize and polymerize terminal acetylenes, and dimerize nitriles and isonitriles with incorporation of the new ligand into the complex. Another remarkable reaction of M2C16L3 is the metathesis of M=M and N=N bonds into two M=N bonds upon reaction with azobenzene. 

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