Activation unsaturated substrates

The transfer of a carbamate moiety to olefins has been documented only in very few cases, using activated unsaturated substrates. For example, Yoshida and Inoue reported the selective (100%) formation of 1-ethoxyethyl N,N-dialkylcarbamate esters by the reaction of C02 (5 MPa) with ethyl vinyl ether and secondary amines R2NH (R = Me, Et) in the absence of any catalyst (Equation 6.11) [85]. 

Hence there is no direct reaction between the methyl manganese unit and the newly incoming carbon monoxide. Later, when we discuss the activation of a coordinated substrate molecule toward nucleophilic attack, the latter will turn out to be an alternative for the insertion/migration process. In the reaction involving nucleophilic attack at the coordinated, activated, unsaturated substrate, the anionic fragment is an uncomplexed species. There is, as yet, no proven example of an insertion of an uncomplexed unsaturated substrate into a metal-carbon a bond. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

H. Kuniyasu continues the discussion of the activation of group 16 element bonds with an overview on S(Se)-X additions to unsaturated substrates. For some time, it... 

The hydrogenations become analogous to those involving HMn(CO)5 (see Section II,D), and to some catalyzed by HCo(CN)53 (see below). Use of bis(dimethylglyoximato)cobalt(II)-base complexes or cobaloximes(II) as catalysts (7, p. 193) has been more thoroughly studied (189, 190). Alkyl intermediates have been isolated with some activated olefinic substrates using the pyridine system, and electronic and steric effects on the catalytic hydrogenation rates have been reported (189). Mechanistic studies have appeared on the use of (pyridine)cobaloxime(II) with H2, and of (pyridine)chlorocobaloxime(III) and vitamin B12 with borohydride, for reduction of a,/3-unsaturated esters (190). Protonation of a carbanion... 

Finally, the development of modified nanoparticles having better stability and a longer lifetime has involved interesting results in diverse catalytic reactions. Efficient activities are obtained with these transition-metal colloids used as catalysts for the hydrogenation of various unsaturated substrates. Consequently, several recent investigations in total, partial or selective hydrogenation have received significant attention. 

Col I(CN)-, 1 is readily formed under mild conditions from Co(CN)2, KCN and H2 [Eqs. (1) and (2)]. It is an active catalyst for the hydrogenation of a variety of unsaturated substrates, and in fact in the first documented examples of two-phase hydrogenations this catalyst was used [48, 49]. The catalysis suffers from several drawbacks such as rapid aging with a loss of activity, and the need to use highly basic aqueous solutions. 

Lewis acid-catalyzed cycloaddihon is also a powerful synthehc method, and various types of cycloaddihon have been reported. In parhcular, enantioselective variants using chiral Lewis acids have been comprehensively studied some of these were used as key reactions for natural product syntheses [5]. However, they generally require one or more heteroatoms in the substrates, such as enones or enoates, to which (chiral) Lewis acids can coordinate. In conhast, in the case of transition-metal-catalyzed cycloadditions, the metals coordinate direchy to the tt-electron and activate unsaturated motifs, which means that the heteroatom(s) are unnecessary. Moreover, the direct coordinahon to the reachon site can realize highly enantioselechve reachon using chiral transihon-metal complexes. 

In summary, the copper ion transfers an electron from the unsaturated substrate to the diazo-nium cation, and the newly formed diazonium radical quickly loses nitrogen. The aryl radical formed attacks the ethylenic bond within the active complexes that originated from aryldiazo-nium tetrachlorocuprate(II)-olefin or initial arydiazonium salt-catalyst-olefln associates and yields >C(Ar)-C < radical. The latter was detected by the spin-trap ESR spectroscopy. The formation of both the cation-radical [>C=C<] and radical >C(Ar)-C < as intermediates indicates that the reaction involves two catalytic cycles. In the other case, radical >C(Ar)-C < will not be formed, being consumed in the following reaction ... 

Disilanes connected via both the Si-Si bond and an organic or an organo-metallic linkage are activated toward reaction with unsaturated substrates to form cyclic bis(silyl) products. Reactions of 3,4-benzo-l,l,2,2-tetraethyl-1,2-disilacyclobutene with diphenylacetylene or benzaldehyde catalyzed by Ni(PEt3)4 proceed with addition across the multiple bond to form the ring-expanded product.54 A second product is formed in a lesser amount in the case of diphenylacetylene, with insertion into the Si-C bond [Eq. (13)]. 

In direct analogy to their work in the nickel-catalyzed double silylation of unsaturated substrates with 3,4-benzo-l,l,2,2-tetraethyl-l,2-disilacyclo-butene, Ishikawa and co-workers have studied the catalytic formation of arene-silicon bonds. The Ni(PEt3)4 catalyst activates the C-H bond of the arene to result in net Si-H and Si-Ar bond formation [Eq. (34)].96... 

Ritleng, V., Sirlin, C. and Pfeffer, M. (2002) Ru-, Rh-, and Pd-catalyzed C-C bond formation involving C—H activation and addition on unsaturated substrates reactions and mechanistic aspects. Chem. Rev., 102, 1731. 

The sesquiteipene cedrol (46) can be hydroxylated regio- and stereo-selectively with Beauveria sulfu-rescens (equation 13). This transformation serves to illustrate the general principle that substrates with an electron rich substituent, to serve as an anchor at, or close to, the active site of the hydroxylating enzyme system, generally are transformed with improved selectivity over those with no such anchor. For example, in the above system the unsaturated substrate cedrene (47) gives low yields of a mixture of products. ... 

While hydrocyanation, the addition of HCN to an unsaturated substrate, is not the only method of producing an organonitrile, it is often the easiest and most economical. The addition of HCN to aldehydes and ketones is readily accomplished with simple base catalysis, as is its addition to activated aUcenes (Michael addition). However, the addition of HCN to unactivated alkenes and dienes is best accomplished with a transition metal catalyst. The hydrocyanation of alkenes and dienes is the most important way to prepare nitriles and is the focus of this article. 

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