Transition metals unactivated alkyne additions

In an interesting contrast to the analogous silicon and germanium systems, catalytic addition of the unactivated hexamethyldistannane to unsaturated substrate readily occurs with the common palladium catalysts Pd(PPh3)4 or Pd(dba)2. In fact, prior to 1991 this was the only distannane investigated for addition reactions. The first example of transition metal-catalyzed double stannylation of unsaturated molecules, palladium-catalyzed cis addition of hexamethyldistannane to 1-alkynes, was reported in 1983 by Mitchell and co-workers.66 More recently, the system has been extended to include other substrates and both the hexaethyl- and the hexabutyldistannanes. All examples reported involve the use of a Pd(0) complex as the catalytic species. 

Some of the most exciting reactions in organic chemistry are based on transition metals. How about these two for example The first is the Heck reaction, which allows nucleophilic addition to an unactivated alkene. Catalytic palladium (Pd) is needed to make the reaction go. The second, the Pauson-Khand reaction, is a special method of making five-membered rings from three components an alkene, an alkyne, and carbon monoxide (CO). It requires cobalt (Co). Neither of these reactions is possible without the metal. 

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. 

The transition metal-free addition of silylphosphines to styrenes, pyridines, and acrylates was promoted by a fluoride source (Scheme 4.17) [59]. The addition reaction occurred under extremely mild conditions and afforded moderate to excellent yields of the anti-Markovnikov addition prodnct. The reaction was proposed to proceed through initial formation of a phosphide anion that reacted with the alkenes through a phospha-Michael-type addition. The proton source for the generation of the final adduct was proposed to be adventitious moisture in the commercially available TBAF solution. While activated alkenes were quite amenable to this approach, unactivated alkenes were unresponsive. 2-Vmylpyridine was particularly reactive and was cleanly converted into the alkylphosphine (95%). While most of the substrates were terminal Michael acceptors, a number of internal alkenes as well as a tetrasubstituted alkene were converted into the alkylphosphines in moderate yields (Schane 4.18 and Example 4.17). The chemistry could be extended to terminal and internal alkynes, although a mixture of E- and Z-isomers were obtained with the EIZ> 1. Following this work, the authors were able take advantage of the need for another electrophile to complete the reaction and added an aldehyde to trap the intermediate carbanion [60]. This approach worked well and enabled the construction of gamma-hydroxyphosphonates in excellent yield (up to 88%). 

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