Carbenes cyclic unsaturated

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

Based on these assumptions many different heteroatom-substituted carbenes have been synthesized. They are not limited to unsaturated cyclic di-aminocarbenes (imidazolin-2-ylidenes Scheme 3, A) [17-22] with stericbulk to avoid dimerization like 1 l,2,4-triazolin-5-ylidenes (Scheme 3, B), saturated... 

A sequence of reactions for conversion of acyclic and cyclic ketones into a,(3-unsaturated ketones with insertion of a =CHCH3 unit has been developed. The method uses l-lithio-l,l-dichloroethane as a key carbenoid reagent. The overall sequence involves three steps, one of them before and one after the carbenoid reaction. By analysis of the bonding changes and application of your knowledge of carbene reactions, devise a reaction sequence that would accomplish the transformation. 

Various cyclic compounds are prepared by the reaction of these carbene complexes with various unsaturated compounds [78-80]. The metallacyclobutane 246 is generated by [2+2] cycloaddition with electron-rich alkenes, and its reductive elimination affords the cyclopropanes 247. 

The cyclic ylide intermediate 366, as a 1,3-dipole, is generated by intramolecular reaction of Rh-carbene with the ketone in 365, and undergoes cycloaddition with n-bonds to give the adduct 367 [121]. When a-diazocarbonyls have additional unsaturation, domino cyclizations occur to produce polycyclic compounds. The Rh-carbene method offers a powerful tool for the construction of complex polycyclic molecules in short steps, and has been applied to elegant syntheses of a number of complex natural products. 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

Titanocene(n) species promote the conversion of unsaturated thioacetals to cyclic compounds. This cyclization proceeds with the loss of the terminal alkene carbon. Treatment of the thioacetal 83 with the low-valent titanium species Cp2Ti[P(OEt)3]2 (3 equiv) in refluxing THF afforded benzoxocines 86 and 87 (by isomerization of 86) in 61% yield (Scheme 14) <1999SL354>. Using 4 equiv of the titanocene(n), the yield is higher (70%) but the selectivity is lower (the ratio 86 87 becomes 82 18). The mechanism or the reaction probably involves the formation of the titanium carbene complex 84, its intramolecular reaction with the double bond to form titanocyclobutane 85, and the subsequent elimination of methylidenetitanocene <1999SL354>. 

A transition metal catalyzed synthesis of ethers by carbene insertion into the O—bond has been reported. Not only saturated but also unsaturated alcohols can be utilized in this catalytic process. ° Intermolecular and intramolecular oxirane ring opening reactions by alkoxides and phenoxides also provide efficient and stereospecific preparations of acyclic and cyclic ethers. The procedures have been surveyed in detail. ... 

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