Olefin complexes strained olefins

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

Oxidation of carbon ligands with concomitant insertion has been observed in the reaction of methallyl nickel complexes with norbornene or strained olefins in general and oxygen (example 3, Table IX). 

The alkyl-substituted titanium carbene complex 18 reacts with norbornene 24 to form a new titanacycle 25, which can be employed for the ROMP of 24 (Scheme 14.13). The titanacycle generated by the reaction of the Tebbe reagent with 24 is also used as an initiator for the same polymerization [23]. These preformed titanacyclobutanes also initiate ROMP of various other strained olefin monomers [24],... 

Recently, Rooney (80) expressed the view that the a,j8 exchange process involved tt olefin complexes and asserts that this explains the pattern of exchange on a palladium film of deuterium with 1,1-dimethylcyclo-butane. Its failure to exhibit appreciable multiple isotopic exchange was attributed to the difficulty of forming a tt olefin complex because of the strain in cyclobutene. 

The copolymerisation of CO and strained olefins such as norbornene and norbor-nadiene is effectively catalysed by Pd complexes with both phosphines and chelating diphosphines [43-49]. As previously mentioned, many model studies aimed at elucidating the elementary steps of alkene/CO copolymerisation have made use of cyclic alkenes in aprotic media because the products resulting from the insertion of... 

Other tris(olefin)platinum(0) complexes (where olefin represents a strained olefin such as bicyclo[2.2.1]heptene, dicyclopentadiene, or trans-cyclooctene) may be similarly obtained by direct displacement of 1,5-cyclooctadiene, often in quantitative yield.6... 

In contrast to 52 (Scheme 12), diastereomer 55 (Scheme 13), because of its more exposed and highly reactive strained olefin, undergoes rapid polymerization in the presence of 4a. The less reactive Ru complex 56 [23] can however be used under an atmosphere of ethylene to effect a tandem ROM/CM to generate 57. The resulting triene can be induced to undergo Mo-catalyzed ARCM (5 mol % 4a) to afford optically pure 58, the AROM/RCM product that would be obtained from 55. 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

Transition metal complexes of strained olefins have preferentially been prepared by two methods ligand exchange (Scheme 11) and rearrangement of appropriate transition metal complexes of polycyclo-alkyl-carbenes (Scheme 11). 

Unactivated double bonds do not react readily with nucleophiles. However, additions are observed if the olefin is coordinated to a transition metal many organometallic complexes are known which induce such reactions (263). While this aspect has not been explored with strained olefins, a few examples of direct nucleophilic additions to distorted double bonds have been reported. 

---