Metal-heteroatom multiple bonds

SCHEME 11.40 Impact of d-electron count on metal-heteroatom multiple bonding (X = OR, NHR, etc.). 

As in the case of carbene complexes, 13C NMR spectroscopy is particularly useful in that the carbyne carbon typically resonates to low field (240 and 360 ppm), with heteroatom substituents shifting this to higher field. As noted above for carbene complexes, X-ray crystallography reveals that carbyne complexes have very short metal-carbon bonds, typically the shortest of any metal-carbon multiple bond, but lengthened if heteroatom substituents are present. 

The hydrosilylation of carbon-heteroatom multiple bonds had received little attention until it was found in 1972 that Rh(PPh3)3Cl is an extremely effective catalyst for the hydrosilylation of carbonyl compounds. This is a new and unique reduction method since the resulting silicon-oxygen bond can easily be hydrolyzed. Other transition metal complexes including platinum, ruthenium , and rhodium also have good catalytic activity in the selective and asymmetric hydrosilylation of carbonyl compounds "". 

As a facile method of carbon-carbon bond formation, the insertion of carbon-carbon multiple bonds into carbon-transition-metal bonds is a very important fundamental reaction in organotransition-metal chemistry. However, in contrast to the tremendous number of reports about the insertion of carbon-carbon multiple bonds into carbon-transition-metal bonds, direct insertion of carbon-heteroatom multiple bonds, such as carbonyl and nitrile groups, without using stoichiometric organometallic reagents, has received scant attention.111 A palladium(II)-catalysed cycliza-tion reaction of alkynes with carbon-heteroatom multiple bonds under mild conditions has been developed, using insertion of carbon-heteroatom multiple bonds into the carbon-palladium bond as the key step.[2]... 

This is a reaction with high atom economy and without the use of organometallic reagents, additives, or redox systems. Such an acetoxypalladation-initiated carbon-heteroatom multiple bond insertion-protonolysis system may extend the scope of transition metal-catalysed reactions pertaining to the insertion of carbon-heteroatom multiple bonds into metal-carbon bonds, and provide a new methodology in organic synthesis. The generality of the present catalytic system is shown in Table 10.2.[3]... 

Transition-metal-catalyzed hetero-[2 + 2 + 2]-cy-cloaddition of alkynes with carbon—heteroatom multiple bonds, such as isocyanides, carbon dioxide, nitriles, aldehydes, and ketones, provides heteroare-nes and unsaturated heterocycles. This reaction can be categorized into two groups one is the reaction of l,a>-diynes 397 with carbon—heteroatom multiple bonds, and the other is reaction of the alkynes 399, having a carbon—heteroatom multiple bond with alkynes as illustrated in Scheme 127. The reaction of 1,6 -diynes 397 proceeds through formation of the metalacyclopentadiene intermediate 398 followed by insertion of a carbon—heteroatom multiple bond, such as heterocumulenes (route a),189 nitriles (route b),190 and carbonyls (route c).191 On the other hand, the... 

Oxo and imido compounds also react as nucleophiles with compounds containing polar C=X bonds, such as aldehydes, ketones, imines, and heterocumulenes. These reactions generally occur by a formal [2+2] cycloaddition process to generate a metala-cycle that breaks down to the more thermodynamically stable combination of products containing metal-ligand and carbon-heteroatom multiple bonds. Three examples of such reactions are shown in Equations 13.90-13.92. In the first two cases, the four-membered metallacycle was isolated or characterized by low-temp6rature NMR spectroscopy. ... 

Hydrocyanation is the addition of HCN across carbon-carbon or carbon-heteroatom multiple bonds to form products containing a new C-C bond. The majority of examples from organometallic chemistry involve the addition of HCN across carbon-carbon multiple bonds, as shown in Equations 16.2 and 16.3. Lewis acids and peptides have been used to catalyze the enantioselective addition of HCN to aldehydes and imines to form cyanohydrins and precursors to amino acids.The addition of HCN to unactivated olefins requires a catalyst because HCN is not sufficiently acidic to add directly to an olefin, and the C-H bond is strong enough to make additions by radical pathways challenging. However, a large number of soluble transition metal compounds catalyze the addition of HCN to alkenes and alkynes. 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

The entries into transition metal catalysis discussed so far, required the presence of a specific bond (a polar carbon-heteroatom bond for oxidative addition or a carbon-carbon multiple bond for coordination-addition processes) that was sacrificed during the process. If we were able to use selected carbon-hydrogen bonds as sacrificial bonds, then we could not only save a lot of trouble in the preparation of starting materials but we would also provide environmentally benign alternatives to several existing processes. In spite of the progress made in this field the number of such transformations is still scarce compared to the aforementioned reactions. 

Germanium-carbon multiple bonds, formation, 3, 709 Germanium-chalcogen bonds, reactivity, 3, 745 Germanium complexes with alkali metal bonds, 3, 748 with Isis // -arcnc chromium heteroatoms, 5, 340 with chromium carbonyls, 5, 208 coupling reactions, 3, 711 with CpMoCO, 5, 463... 

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