Alkanes, addition organometallics

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

Transition State Models. The stoichiometry of aldehyde, dialkylzinc, and the DAIB auxiliary strongly affects reactivity (Scheme 9) (3). Ethylation of benzaldehyde does not occur in toluene at 0°C without added amino alcohol however, addition of 100 mol % of DAIB to diethylzinc does not cause the reaction either. Only the presence of a small amount (a few percent) of the amino alcohol accelerates the organometallic reaction efficiently to give the alkylation product in high yield. Dialkyl-zincs, upon reaction with DAIB, eliminate alkanes to generate alkylzinc alkoxides, which are unable to alkylate aldehydes. Instead, the alkylzinc alkoxides act as excellent catalysts or, more correctly, catalyst dimers (as shown below) for reaction between dialkylzincs and aldehydes. The unique dependence of the reactivity on the stoichiometry indicates that two zinc atoms per aldehyde are responsible for the alkyl transfer reaction. 

Equation (1) depicts an early example of an intermolecular addition of an alkane C-H bond to a low valent transition metal complex [12], Mechanistic investigations provided strong evidence that these reactions occur via concerted oxidative addition wherein the metal activates the C-H bond directly by formation of the dative bond, followed by formation of an alkylmetal hydride as the product (Boxl). Considering the overall low reactivity of alkanes, transition metals were able to make the C-H bonds more reactive or activate them via a new process. Many in the modern organometallic community equated C-H bond activation with the concerted oxidative addition mechanism [10b,c]. 

R. A. Periana, and R. G. Bergman, Oxidative Addition of Rhodium to Alkane C—H Bonds Enhancement in Selectivity and Alkyl Group Functionalization, Organometallics 3, 508-510(1984). 

The activation of C-H bonds for direct C C bond formation reactions has the potential to become very important especially if it can be accomplished for sp C-H bonds, in methane or alkanes as these are the major feedstocks available. In addition, C-H bond activation of functionalized organic compounds for selective C-C bond formation has been and will continue to be a very important goal of organometallic catalysis. So far the use of transition metal complexes has led to interesting results which however are not yet industrially relevant. 

This has allowed us to identify, for the first time in solution at room temperature, organometallic noble gas complexes which are formed following irradiation of metal carbonyls in supercritical noble gas solution. We have found that these complexes are surprisingly stable and have reactivity comparable to organometallic alkane complexes. In addition, we have studied the co-ordination of COj to metal centres in supercritical CO2 (scCOj) and shown that v(C-O) bands provide a very sensitive probe for the oxidation state of the metal centre. We found evidence, albeit circumstantial, for the formation and reactivity of ri -O bound metal COj complexes in solution at or above room temperature and found these highly reactive COj complexes have similar reactivity to the analogous Xe complexes [11-12]. We have also used TRIR to examine the reactivity of CpMofCO), radicals in scCOj and found evidence for an interaction, possibly Lewis Acid/Base, between CpMo(CO), and scCO [13]. 

Besides carbonylation of cations, C-H activation and subsequent carbonylation of aromatic and aliphatic groups via organometallic complexes have experienced growing interest in the scientific community. In 1982 Janowicz and Bergman reported the first well-characterized example of oxidative addition of an unactivated alkane to a homogeneous permethylcyclopentadienyl-iridium complex [44]. 

The remainder of this section will focus on true SBMs, which have been the subject of vigorous research. Despite the electron deficiency of early transition metal, lanthanide, and actinide complexes, several groups reported that some of these d f" complexes do react with the H-H bond from dihydrogen and C-H bonds from alkanes, alkenes, arenes, and alkynes in a type of exchange reaction shown in equation 11.32. So many examples of SBM involving early, middle, and late transition metal complexes have appeared in the chemical literature over the past 20 years that chemists now consider this reaction to be another fundamental type of organometallic transformation along with oxidative addition, reductive elimination, and others that we have already discussed. 

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