Stannanes trialkyl— from

Stannanes from ethylene derivatives Trialkyl- from dialkyl-stannanes... 

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

In Pd(0) mediated addition of trialkyl[dimethyl(phenyl)silyl]stannane to acetylenes <89S218> the product obtained from l-hydroxybut-3-yne after normal workup contained the substituted 1,2,5-oxasilastannolene (354) in addition to the expected 2-tributylstannylalk-l-ene (Equation (32)). 

FIGURE 72. Calculated transition states at MP2 of the hydrogen abstraction reactions of trialkyl-stannane with hydrogen and alkyl radicals which are shown in Figure 71. Bond distances are in A, angles in deg. The calculated activation energies (kcal mol 1) at QCISD//MP2 (MP2 values in parentheses) refer to the forward reaction (A /i 1 i ) and the reverse reaction (A /i2 ), respectively. Reproduced by permission of The Royal Society of Chemistry from Reference 191... 

The StiUe couphng is the reaction of a trialkylaryl stannane or a trialkyl vinyl stannane (50) with an aromatic iodide, bromide, or triflate. Its mechanism differs from that of the Suzuki couphng by the fact that OH , RO, CO , or F is not required for the progression of the catalytic cycle (Scheme 9). 

Boronic acids derived from 3- and 5-substituted indoles are useful intermediates for palladium-catalyzed vinylation, arylation, and heteroarylation (Equation (47)) <92H(34)1395, 93TL2235>. As with the stannanes, a reverse process can be utilized. 6- and 7-Bromoindole can be coupled directly with arylboronic acids to give the 6- and 7-arylindoles, respectively a feature of the reaction is that protection of the indole nitrogen is unnecessary (Equation (48)) <94SL93>. A new route to 2-substituted indoles has been developed from trialkyl-(l-methylindol-2-yl)borates. The borate (153),... 

Enol stannanes of cyclohexanone and propiophenone have been indicated to take part in r/treo-selective aldol reactions with benzaldehyde at low temperatures e.g. —78 °C), but to be erythro-seAsciiwe at higher temperatures ca 45 °C). Two complementary methods have been described for stereoselection in aldol-type reactions. Whilst a-mercurio-ketones show eryr/wo-selection in their reactions with aldehydes in the presence of boron trifluoride diethyl etherate, pre-formed lithium enolates and aldehydes, in the presence of simple trialkyl-boranes, lead to mixtures that are rich in the more stable threo-d do product. Aldol-type products arise from 1,3-alkyl migrations of alk-l-enyl alkyl acetals and ketals, in a reaction that is catalysed by boron trifluoride diethyl etherate (Scheme 52). Diastereoselection is possible, since (.E)-alkenyl acetals give the... 

As one descends Group IVB of the Periodic Table from Si to Ge to Sn to Pb, the ease of addition of the hydrides (e.g. R3MH) to olefins increases markedly with the decrease in strength of the M—H bonds. Thus trialkyl-silanes require either heating to about 300X under olefin pressures of at least 300 atm, or UV irradiation, or catalysis by, for example, peroxides, certain metal salts or tertiary amines. Trialkylgermanes and trialkyl-stannanes add to activated terminal carbon-carbon double bonds at 120°C and 90 C respectively, and a catalyst is not necessary, e.g. 

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