Diene selective

When one of the reacting partners in the Wittig-Horner reaction, either the phosphine oxide or the carbonyl compound, has a double bond, the product is a diene. The Wittig-Horner reaction was utilized by Smith and coworkers in the total synthesis of milbemycin (equation 98)170. They found that when sodium hexamethyldisilazide was employed as a base, the desired E-diene selectivity is high (85%). Some examples from the literature where the Wittig-Horner reaction has been utilized for the construction of E-double bonds present in dienes and polyenes are given in Table 19171. 

Under basic conditions, obviously only one isomerization step takes place and thus a terminal alkyne will deliver 1,2-dienes selectively. With internal alkynes, on the other hand, selectivity can only be achieved when the alkyne is either symmetrical as in 14 [34] (Scheme 1.6) or has a tertiary center on one side as in 16 [35, 36] (Scheme 1.7). So, unlike potassium 3-aminopropylamide in 1,3-diaminopropane, where the Jt-bonds can migrate over a long distance by a sequence of deprotonations and reprotonations, here the stoichiometric deprotonation delivers one specific anion which is then reprotonated (in 16 after transmetalation). 

The iron carbonyl complex [Fe(CO)5] in basic media hydrogenated steroidal dienes selectively (equations 10-12)30. 

S. Verdier, B. Didillon, S. Morin, and D. Uzio, PdSn/Al203 catalysts from colloidal oxide synthesis II. Surface characterization and catalytic properties for buta-1,3-diene selective hydrogenation, J. Catal. 218, 288-295 (2003). 

Since differences in the hydrogenation rates of variously substituted monoalkenes were found over Pt-zeolite catalysts modified by CVD (see Section IV.A.l) these catalysts were anticipated to exhibit regioselectivities in the hydrogenation of dienes. Selective saturation of the less substituted double bonds of linear and cyclic dienes was indeed achieved over Pt-zeolite A and Rh-zeolite A catalysts treated with tetraethoxysilane78 or diethoxydiphenylsilane79,80 (Table 13). 

Fe(TPP)]jO Cycloalkanes + 02 Epoxides, alcohols, acids, ketones Solvent benzene X, = 350-440 nm TN 10 -103 (4233 for 1,5-dimethylcycloocta-1,5-diene) selective conversion of cyclooctane, 1-methylcyclooctane, and 1,5-dimethylcycloocta-1,5-diene to the corresponding epoxides [253]... 

Cycloaddition strategies have also featured widely in the synthesis of variously substituted 3,6-dihydro-2//-thiopyrans from thiocarbonyl-containing precursors and dienes. Selected H NMR data for 3,6-dihydro-2//-thiopyr-ans feature in Figure 20. 

Fig. 15.29. Regioselectivity and simple diastereoselectivity of a Diels-Alder reaction with a 1-substituted diene selectivity increase by way of addition of a Lewis acid.

Thioaldehyde S-oxides behave as dienophiles with 1,3-dienes [241]. Cycloaddition involves the carbon-sulfur double bond of the heterocumulene to furnish dihydro-2H-thiapyran S-oxides (Table 6, entry 1). The cis trans product ratio was found to depend upon the initial diene/sulfine ratio. A large excess of diene selectively provided the cis cycloadducts, as a result of a stereospecific reaction of the (Z) sulfine. With a produced proportion of diene, mixtures of cis trans isomers are obtained, with a preference for the cis compound. The Italian authors have demonstrated that the slower reaction allows a (Z) to (E) isomerisation of the sulfines prior to addition. 

The results above suggested that selective monooxymercuration of dienes is possible, and indeed this has been realized.2 In the case of symmetrical dienes such as 1,5-pentadiene, the yield of enol is lower than predicted for a statistical reaction (50 % enol) the statistical value is approached in longer chain dienes. Yields can be raised by using mercuric trifluoroacetate (2,195). In the case of unsymmetrical dienes selective hydration can be achieved. Thus limonene (1) can be converted to the enol (2) in 70% yield. 

The transformation of ditertiary 1,2-diols was found to be non-selective-a mixture of dienes and ketones is formed and diene selectivity often exceeds that for rearrangement products [14-16]. Increasing surface acidity and increasing temperature have been shown to shift the selectivity toward diene formation [17]. 

Diene syntheses with siloxy-1,3-dienes Selective cleavage of enoxysilanes to ketones... 

Predict the product of the following Diels-Alder reaction using Danishefsky s Diene. Select from the possible products shown and provide the correct stereochemistry for the structure. Explain how you determined both the regiochemistry and the stereochemistry. 

Tf2NH or B(C6F5)3. While B(C6F5)3 provided Diels-Alder products of a,P-unsaturated aldehydes and dienes selectively, Tf2NH gave a Diels-Alder product of a,P-unsatureted ketones and dienes preferentially. They proposed that this reversal of chemoselectivities arises from a combination of steric as well as electronic effects of the dienophiles (Table 2.20, Figure 2.7). 

Oligomerization of 1,3-dienes selectively yields four-, six- eight-, or twelve-mem-bered rings, depending on the nature of catalysts and reaction conditions [4,123]. Ni(COD)2-PPh3 catalyst has been shown to efficiently promote intramolecular cyclooctadiene formation in good yield from the corresponding bisdienes [124]. 

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