Heteroatom-substituted 1,3-Dienes

In most of the successful Diels-Alder reactions reported, dienes containing no heteroatom have been employed, and enantioselective Diels-Alder reactions of multiply heteroatom-substituted dienes, e.g. Danishefsky s diene, are rare, despite their tremendous potential usefulness in complex molecular synthesis. Rawal and coworkers have reported that the Cr(III)-salen complex 15 is a suitable catalyst for the reaction of a-substituted a,/ -unsubstituted aldehydes with l-amino-3-siloxy dienes [21] (Scheme 1.28, Table 1.12). The counter-ion of the catalyst is important and good results are obtained in the reaction using the catalyst paired with the SbFg anion. 

Ab initio calculation of Diels-Alder reactions of a series of 5-heteroatom substituted cyclopentadienes Cp-X (65 X = NH, 50 X = NH, 64 X = NH3, 67 X = O", 54 X = OH, 68 X = OH3% 69 X = PH, 51 X = PH, 70 X = PH3% 71 X = S, 55 X = SH, 72 X = SH/) with ethylene at HF/6-31++G(d)//HF/6-31-i i-G(d) level by BumeU and coworkers [37] provided counterexamples of the Cieplak effect. The calculation showed that ionization of substituents has a profound effect on the n facial selectivity deprotonation enhances syn addition and protonation enhances anti addition. The transition states for syn addition to the deprotonated dienes are stabilized relative to those of the neutral dienes, while those for anti addition are destabilized relative to those of the neutral dienes. On the other hand, activation energies for syn addition to the protonated dienes are similar to those of the neutral dienes, but those for anti addition are very much lowered relative to neutral dienes (Table 6). 

Heteroatom-substituted vinylcarbene complexes are also excellent dienophiles, which usually undergo Diels-Alder reactions under very mild reaction conditions (see Experimental Procedure 2.2.7). Unfortunately the outcome of reactions between Fischer-type vinylcarbene complexes and dienes is difficult to predict ([256] compare, e.g.. Experimental Procedures 2.2.7 and 2.2.9). The course of... 

Heteroatom-substituted vinylcarbene complexes react particularly well with donor-substituted 1,3-butadienes to yield the corresponding cycloheptadienes [264]. Some of these reactions proceed at room temperature. The intermediate divinylcyclopropanes have occasionally been isolated [264]. As indicated in Figure 2.37, for donor-substituted dienes in particular the formation of zwitterionic intermediates... 

J Non-Heteroatom-Substituted Carbene Complexes Table 3.21. Preparation of alkenes and dienes by cross metathesis. 

The stereochemistry and regiochemistry of cyclofunctionalization reactions in the following sections are discussed in terms of the above mechanistic concepts. Emphasis is placed on the relationship between the products and the heteroatom-substituted substrates thus, examples of electrophilic heterocycli-zation involving two sequential additions to nonconjugated dienes are not discussed. These reactions have been covered in a recent review. ... 

These results can be interpreted as being due to the reversible formation of the 3-substituted dehydrohydantoins by way of the 5-carbonium ions. Reaction of the dehydrohydantoins or the ions with dienes is slower than reaction with methanol but is irreversible. This system is useful for the preparation of certain cycloadducts of dehydrohydantoins. Its limitations result from the presence of methanol and acid, which preclude observation or isolation of the dehydrohydantoin and the utilization of most heteroatom-substituted dienes. 

Cyclopentadienyl rings, in chromocenes with heteroatom substitutions, 5, 330 with pentalene rings, 5, 331 with substitutions, 5, 329 Cyclopentadienyl ruthenocenes, synthesis, 6, 639 Cyclopentadienyl—silylamido complexes, with Zr(IV) and Hf(IV) and monodentate ligands, 4, 852 Cyclopentadienyl—silylamido dienes, with Zr(IV) and Hf(IV),... 

Dienes, including cyclobutadiene and heteroatom-substituted dienes, e.g. vinyl ketones... 

Table 6 Influence of High Pressure on the Diels-Alder Addition of Heteroatom-substituted Dienes to Acrylates...

Much attention has been directed to the chemistry of p-heteroatom-substituted allylic cations (5), especially oxyallyl cations (7), from both synthetic and mechanistic viewpoints. One of the characteristic reactions of oxyallyls (7) is their [4 -i- 3] 7 cycloadditions with conjugated dienes, which provide a... 

The 3-heteroatom-substituted allylic cations shown in Section 5.1.2 serve as active three-carbon units toward 1,3-dienes to afford the corresponding seven-membered ring compounds selectively, though five-membered rings are obtained in some cases. - ... 

There are a few reports of hetero-Diels-Alder Reactions promoted by LPDE. Intriguing stereoselectivity is observed for the [4 + 2] cyclization between Danishefsky s diene 77 and a-heteroatom-substituted aldehydes. For example, reaction of 77 with N-Boc-protected a-aminoaldehyde with 76 gave the threo isomer selectively, a result in keeping with a chelation-controUed process. In contrast, the threo diastereoselectivity observed could be reversed by changing the amino protecting group from A-Boc to A,A(-dibenzyl. In this instance, the erythro isomer was generated exclusively via a non-chelation-controlled transition state (Sch. 38) [89]. 

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