Organometallic compounds hydrogen bonding

For 1,2,3-triazole, the Ti (N ), i-ti (N, N ), and Ti fC) carbene coordination modes prevail in organometallic compounds. Benzotriazole has the same general pattern but often operates as a composite unit of benzotriazole and benzotriazolate linked by the hydrogen bond. In some organoosmium compounds, the i-ti (N, ) mode is realized, where the C -center refers to the annulated benzene ring. In some variations, together with the i-ti (N, N ) mode, the Ti fN" ) monodentate coordination occurs. The I-ti (N, N ) mode can also be traced. 

Halogenation of alkenyl organometallic compounds Addition of hydrogen halides to triple bonds Halogenation of alkynes or allenes Addition of alkyl halides to triple bonds Addition of acyl halides to triple bonds... 

There is almost no reliable information about the strength of metal-hydrogen bonds in organometallic compounds. Earlier (Section 2.2.) it was established that M-H bonds formed by (spontaneous) -elimination from metal alkyls should be stronger that their precursor, but this is an unsatisfactory and imprecise position. The dissociation... 

Most of the above reactions occur via a mechanism involving intermediates with a metal-silicon bond (i.e. silicometallics) and a metal-hydrogen bond, accompanied (or sided) only occasionally by compounds containing metal-carbon bonds (i.e. organometallics) that are characteristic of the key intermediates of transition-metal-catalyzed transformations of organic compounds (for recent reviews, see Refs [11, 13]). 

In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp2 carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides of sp2 carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C rr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes, conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /(-hydrogen. At the same time, the PdfO) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg. Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

The variety of bonding in organometallic compounds is enormous. We focus fairly narrowly here on the types of bonding found between transition metals and carbon and hydrogen atoms and on the interactions between metals and and n bonds between C atoms and C and H atoms. In particular, we seek to understand in some detail the process known as oxidative addition, as shown in reactions (13.1) and (13.2). 

Other additions, such as addition of alkyl halides and carbonyl compounds, are discussed in Chapter 5, whereas Chapter 7 covers addition reactions involving carbon monoxide (hydroformylation, carboxylations). Hydrogen addition is discussed in Chapter 11. The nucleophilic addition of organometallics to multiple bonds is of great significance in the anionic polymerization of alkenes and dienes and is treated in Chapter 13. 

The addition of metal-hydrogen bonds across carbon-carbon multiple bonds, called hydrometallations, are very important, versatile transformations in organic synthesis. First, they allow the synthesis of new organometallic compounds. The products thus formed may be further transformed into other valuable compounds. The two most important reactions, hydroboration and hydrosilylation, will be treated here in detail, whereas other hydrometallation reactions (hydroalanation, hydro-zirconation) will be discussed only briefly. Hydrostannation, a very important transformation of substituted unsaturated compounds, has no significance in the chemistry of hydrocarbons possessing nonactivated multiple bonds. 

Much effort has been devoted to activate hydrocarbons, particularly saturated compounds, through the formation of organometallic compounds and transformation of the latter to substituted derivatives. A number of transition-metal complexes have been found to insert into carbon-hydrogen bonds leading to stable alkyl metal hydrides ... 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

There are many addition reactions of a,(3-unsaturated aldehydes, ketones, and related compounds that are the same as the carbonyl addition reactions described previously. Others are quite different and result in addition to the alkene double bond. Organometallic compounds are examples of nucleophilic reagents that can add to either the alkene or the carbonyl bonds of conjugated ketones (see Section 14-12D). Hydrogen cyanide behaves likewise and adds to the carbon-carbon double bond of 3-butene-2-one, but to the carbonyl group of 2-butenal ... 

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