Organoboranes rearrangement

Step 2 The 7C complex rearranges to an organoborane Hydrogen migrates from boron to carbon carrying with it the two electrons m its bond to boron Development of the transition state for this process is shown m 2(a) and its transformation to the organoborane is shown m 2(b)... 

Hydroboration-oxidation (Sections 6 11-6 13) This two step sequence achieves hydration of alkenes in a ste reospecific syn manner with a regiose lectivity opposite to Markovnikov s rule An organoborane is formed by electro philic addition of diborane to an alkene Oxidation of the organoborane inter mediate with hydrogen peroxide com pletes the process Rearrangements do not occur... 

ESTERIFICATION OF HINDERED ALCOHOLS tert-BUTYL p-TOLUATE, 51, 96 Esters, from diazoketones and organoboranes, 53, 82 Esters, a-deuterio-, 53, 82 Esters, y [Pg.59]

Organoboranes react with ethyl 4-nitrobenzenesulphonyloxycarbamate under basic two-phase conditions in the presence of benzyltriethylammonium chloride or Aliquat to yield ethyl (V-alkylcarbamates [38]. The reaction probably proceeds via the initial formation of the nitrene, which reacts with the borane to form a B -N+ zwitterion. Subsequent rearrangement and solvolysis leads to the product. Aliquat is the better catalyst for the higher-molecular-weight boranes. 

If an internal olefin is treated similarly and the resulting branched trialkylborane heated at about 160° in diethylene glycol dimethyl ether, the molecule is rearranged and finally the terminal trialkylborane is produced. The isomerization of the organoborane proceeds through a series of eliminations to regenerate olefin and dialkylborane, followed by... 

This rearrangement can be also performed with acyclic tetrasubstituted olefins, enabling control of two adjacent carbon centers. Thus, hydroboration of Z- and E-2,3-dimethylstilbene (43) with BH3.THF and subsequent heating at 70 °C for 12 h furnishes stereo selectively the syn and anti organoboranes 44, which after oxidative workup provide the corresponding alcohols syn-45 and anti-45 in 90 % yield and dr > 99.5 % (Scheme 7) [7-9]. 

Table 1. Thermal rearrangement of organoboranes resulting from the hydroboration of tetrasubstituted cyclopentene derivatives of type 34.

Complexation is believed to be involved in the first step of the hydrobo-ration of alkenes and alkynes rearrangement reactions of organoboranes most likely involve intermediates of a rc-complex type. However, the stability of such complexes is generally too low to allow their isolation (112). However, evidence for 7t-complex formation has been obtained by the device of anchoring the alkene function to the metal atom in question by means of a... 

The B-G bond of allyl-, alkynyl-, aryl-, and benzylboranes is more easily cleaved than that of alkylboranes. The C-B bond of aromatic boronic acids is stable to water, but polyfluorinated phenylboronic acids are exceptionally sensitive to Me0H-H20-K0H and Me0H-H20-pyridine.488 Allylic organoboranes were hydrolyzed with water at room temperature with complete allylic rearrangement (Equation (102)).489,490 Addition of water resulted in the rapid hydrolysis of benzylic G-B bond of 329 due to the electron-withdrawing effect of Cr(CO)3 group (Equation (103)).491... 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

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