Formation of -diol

Figure 2. Mechanism of complex formation of diols by borates...

Scheme 5.13 provides several examples of reductive carbon-carbon bond formation, including formation of diols, alkenes, and acyloins. Entry 1 uses magnesium amalgam in the presence of dichlorodimethylsilane. The role of the silane may be to... 

Treatment of Fischer-type carbene complexes with different oxidants can lead to the formation of carbonyl compounds [150,253]. Treatment with sulfur leads to the formation of complexed thiocarbonyl compounds [141]. Conversion of the carbene carbon atom into a methylene or acetal group can be achieved by treatment with reducing agents. Treatment of vinylcarbene complexes with diborane can also lead to demetallation and formation of diols [278]. The conversion of heteroatom-substituted carbene complexes to non-heteroatom-substituted carbene complexes... 

The mechanism of the aldol-Tishchenko reaction has been probed by determination of kinetics and isotope effects for formation of diol-monoester on reaction between the lithium enolate of p-(phenylsulfonyl)isobutyrophenone (LiSIBP) and two molecules of benzaldehyde. ". The results are consistent with the formation of an initial lithium aldolate (25) followed by reaction with a second aldehyde to form an acetal (26), and finally a rate-limiting intramolecular hydride transfer (Tishchenko... 

Already in the first reports on olefin oxidation with the MTO/H2O2 system [3], it was noted that the formation of diols from the desired epoxides, caused by the Br0nsted acidity of the system, is a major drawback of this system. The solution for this problem was found in the same report by the addition of a nitrogen base. This method has been explored extensively since and has become an important factor in the MTO-catalyzed olefin epoxidation. 

Recent studies on the formation of diols such as 19 have shown that functional-group migration reactions are possible when very electron-rich aryl groups are used (Scheme 3.5). In the extreme case of 5-methoxythienyllithium, only the rearranged product 26 was isolated [31]. 

Lohray and coworkers investigated the formation of diol under AA conditions [56]. IR analyses of a mixture of osmium tetroxide and chloramine-T... 

This assumption was supported by the findings that substoichiometric amounts (10 mol%) of osmium tetroxide led to nearly equivalent amounts of the diol (12-14%) (Table 9, entries 1-3). Moreover, the formation of diol was reduced to 7% when the osmate ester 30 (Fig. 3) was used as catalyst. However, the... 

The transition state with the lowest barrier leads to the [3 + 2]-intermediate, and this should result in the formation of diols, at least as side products. There are still open questions to address, and it is expected that research in this area will continue. 

Figure 15 shows that the sol fraction is predicted to increase with increasing otp but to reach a steady value or even to pass through a minimum while continues to increase. This behaviour is similar to the effect of branching induced by degradation also here, every cut (formation of diol unit) prcxluces a new branch point (diester unit). 

Evidence for the formation of diol-periodate esters Diol-periodate esters are recognized as intermediates in the oxidation of 1,2-... 

The formation of diol-periodate esters is supported by physical evidence. The addition of ethane-1,2-diol to periodate solutions causes an initial rapid change in the uv absorption spectrum, followed by a slower change as the oxidation proceeds and lOJ is formed. Similar results are observed for other 1,2-diols except for highly substituted diols such as pinacol (Buist et al ). Buist and Bunton have shown that the cyclic periodate esters formed in alkaline solution from 1,2-diols and periodate can be detected by nmr. The initial fall in pH which occurs in the oxidations of ethane-1,2-diol and lightly substituted diols is also attributed to ester formation (Malaprade, Buist and Bunton ). Cyclic triesters, similar to the cyclic diesters formed from 1,2-diols, are formed from cyclic compounds containing the cis-l,2,3-triol system (Barker and Shaw , Dijkstra and from 1,2-0-isopropylidene-a-D-glucofuranose. In the latter case the presence of the triester has been demonstrated by nmr (Berlin and van Rudloff ). Monoesters of periodic acid have not been detected in any system, but they are postulated as intermediates in the formation of cyclic diesters from 1,2-diols (section 1.3.5). 

The evidence for carbocation intermediates in the pinacol rearrangement is compelling for some substrates and conditions. Bunton" found that pinacol itself that was recovered from O-enriched aqueous sulfuric acid had incorporated an appreciable amount of solvent oxygen. Under these conditions the reaction goes to completion, i.e. the incorporation cannot be due to reversible formation of diol from pina-... 

Additional examples, shown in Scheme 5.2.25, illustrate a-chelation control in the formation of 111 and 114,36 37 as well as the inclusion of chirality in the y-alkoxy substituent in 115 leading to 116,3 a role for coordination in the case of the a-amido substituent of 118, which yields selective formation of diol derivative 119 via 121.39... 

The discovery of iron complexes that can catalyze olefin czs-dihydroxylation led Que and coworkers to explore the possibility of developing asymmetric dihydroxylation catalysts. Toward this end, the optically active variants of complexes 11 [(1R,2R)-BPMCN] and 14 [(1S,2S)- and (lP-2P)-6-Me2BPMCN] were synthesized [35]. In the oxidation of frans-2-heptene under conditions of limiting oxidant, 1R,2R-11 was foimd to catalyze the formation of only a minimal amount of diol with a slight enantiomeric excess (ee) of 29%. However, 1P-2P-14 and 1S,2S-14 favored the formation of diol (epoxide/diol = 1 3.5) with ees of 80%. These first examples of iron-catalyzed asymmetric ds-dihydroxylation demonstrate the possibility of developing iron-based asymmetric catalysts that may be used as alternatives to currently used osmium-based chemistry [45]. 

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