Conjugated diynes

The organoborate intermediates can also be generated from alkenylboronic esters and alkyllithium or Grignard reagents, or from ttialkylboranes and alkenyllithium compounds. Conjugated symmetrical and unsymmetrical diynes (289—291), stereochemically pure 1,3-dienes (292,293), and 1,3-enynes (294) including functionali2ed systems can be prepared (289,295). 

The addition of 1,3-dipoles to alka-l,3-diynes has been studied in less detail than that to conjugated alkadienes and alkenynes (80UK1801). Conjugated diynes get involved in [2- -3]cycloaddition at the unsubstituted acetylene bond. 

Diynes and triynes refer to alkynes containing two or three triple bonds poly-ynes contain multiple triple bonds. A conjugated triyne is a straight-chain hydrocarbon with triple bonds alternating with single bonds. An examples is... 

Would you expect a conjugated diyne such as 1,3-butadiyne to undergo Diels-Alder reaction with a dienophile Explain. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

In a similar reaction, symmetrical conjugated diynes, RC=C—C=CR, can be prepared by reaction of lithium dialkyldialkynylborates, Li" "[R2B(C=CR)2], with... 

Other conjugated systems, including trienes, enynes, diynes, and so on, have been studied much less but behave similarly. 1,4 Addition to enynes is an important way of making allenes ... 

Addition of ammonia or primary amines to conjugated diynes... 

Conjugated organoboron polymers were prepared by haloboration-phenylboration polymerization between diyne monomers and bromodiphenylborane (scheme 29).54 The polymerization was carried out by adding a slight excess of bromodiphenylborane to a tetrachloroethane solution of diynes at room temperature... 

The first examples of a consecutive radical 5 -exo-/dig-5-exo-dig cyclization of 1,5-diynes have been accomplished by the same researchers [43]. These authors were able to show that their cycloisomerization procedure provides access to strained semicyclic, conjugated dienes with a functionalized dioxatriquinane framework which occurs in the aglycones of steroidal cardiac glycosides, such as isogenine (3-96) [44] and C-norcardanolide (3-97) (Scheme 3.24) [45]. 

Similarly, conjugated dienes, enynes, and diynes such as ( , )-l,4-diphenylbuta-l,3-diene 162, fE)-l,4-diphenyl-but-l-en-3-yne 163, or l,4-diphenylbuta-l,3-diyne 164 on treatment with trithiazyl trichloride (2 equiv) afforded the bisthiadiazole 165 in moderate to good yield (40-60%), depending on the reaction conditions (Scheme 24) <1997CC1493, 1997J(P 1)3189, 1998CC1207>. 

Conjugated diynes can react analogously and give regio- and steroselective products for instance,... 

Before going into a detailed account of the chemistry of phanes, the author will touch on 3,4,7,8-tetrasilacycloocta-l,5-diyne briefly, since the compound illustrates the importance of a—it mixing. The ionization potential of the Si-Si bond is estimated by photoelectron spectroscopy to be 8.69 eV (9). Thus, the HOMO level of the Si-Si is comparable to most HOMOs of tt systems. Consequently, the Si-Si bond can conjugate efficiently with carbon-carbon double and triple bonds, benzene rings, and other tt systems. Most Si-Si bonds are stable enough to construct sophisticated structures by themselves and with organic molecules (10). 

The through-bond conjugation with strong ct-tt mixing for tetrasila-cycloocta-l,5-diyne is further demonstrated by photoelectron spectroscopic (15) and ab initio (3-21G basis set) studies (16). 

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