Metallotropic rearrangement

Simple diastereoselection in the reactions of 2-butenylboron compounds and aldehydes is critically dependent on the configurational stability of the reagentslb. As a general rule, most 2-bulenylorganometallics arc sensitive to sequential 1,3-metal shifts (1,3-metallotropic rearrangements) that result in E- to Z-olefin isomerization via the l-methyl-2-propenylmetal isomer. 

This chapter will cover only reactions in which the isomerization to the allene starts from a stable molecule and not from a reactive intermediate generated in situ by reactions which are not isomerizations, such as the Doering-Moore-Skattebol reaction or free carbenes. Metallotropic rearrangements also will not be covered many of these reactions can be found in Chapter 9. Furthermore, the allene should be the final product of the reaction and not only a transient species leading to other products (see, for example, Chapters 6 and 20). 

Metallotropic rearrangement, in mercury tri-azenide complexes, 30 41 Metals, see also Heterobimetallics specific element Transition metal complex alkoxides, 15 159-297 of actinides, 15 290-293 of alkali metals, 15 260-263 of alkaline earths, 15 264-266 of aluminium, 15 266-272 of beryllium, 15 264-266 double type, 15 293-294 of gallium, 15 266-272 of lanthanides, 15 290-293 of magnesium, 15 264-266 properties of, 15 260 of transition metals, 15 272-290 trialkylsilyloxides, 15 295-297 of zinc, 15 264-266... 

Haloalkylboron compounds in which the halogen atom is more distant than the f3-position are more stable than p-haloalkylboranes, while a-haloalkylboranes undergo ready rearrangement. p,7-Unsatu-rated organoboranes may show a tendency to undergo metallotropic rearrangement (Scheme 3). - ... 

Allyl metal compounds can exist in either the monohapto ti - or trihapto Ti -forms. Crotyl metal compounds that exist in the ti -form, including those classified as type I reagents, are generally sensitive to metallotropic rearrangements (sequential 1,3-shifts) which affect ( ) to (Z) isomerization via the intermediacy of the methallyl metal isomer (Scheme 6). Trihapto, or ir-bound, allyl metal reagents can exist in either of two forms the extended or ( )-isomer, and the U-shaped or (Z)-isomer. These ti -reagents can also isomerize if a pathway for interconversion with the ti -methallyl intermediate is energetically accessible. 

Early studies showed (194, 195) that trimethylstannylindene C was subject to an intramolecular metallotropic rearrangement proceeding via an apparent 1,3 shift ... 

Isomeric bis(trimethylstannyl) dihydropentalenes have been shown by H NMR spectra to exhibit suprafacial metallotropic rearrangements which... 

The pale yellow solid melts at 116°C with decomposition. Its NMR spectrum (In CDCI3) shows at 27°C a single broad resonance, 5 = 5.86 ppm, which broadens at lower temperature and below — 30°C splits into components which have been assigned to incompletely resolved AA BB X multiplets with 5(ppm) = 3.26 (H-1), 6.28 (H-3,4), and 6.63 (H-2,5) at —70°C (cf. Formula I). This behavior is consistent with rapid 1,2-metallotropic rearrangement above -30°C. 

Recently, we have estabhshed the satisfactory method of synthesis of each isomer of crotyl-trifluorosilanes with rather convenient procedure together with the stereochemistry of the reaction with various aldehydes in the presence of fluoride ions. The crotyltrifluorosi-lanes are configurationally very stable and storable for a couple of months at 0 °C without any change of the purity as expected from the low tendency to the metallotropic rearrangement. The present reagent system would thus fulfill almost all conditions for stereoselective crotylation, while chiral modification should be the next target. 

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