5-Membered rings reaction with carbenes

Fig. 2.30. Intramolecular acylation reactions with carbene-derived vinylketene complexes. Table 2.20. Preparation of five-membered rings by cyclization of vinylketene complexes.

An unusual system where the carbene carbon is part of a six-membered ring, but with two adjacent nitrogens, has been the subject of a short series of papers.Reactions of hypervalent sulfur-containing precursors with a platinum(O) complex take place under mild conditions, yielding new complexes 145 and 146 (Scheme 36). Little detail is given, but it is believed the displaced sulfur ends up as Ph3P=S. 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

Electronically rich 1,3-butadienes such as Danishefsky s diene react with chromium alkenylcarbene complexes affording seven-membered rings in a formal [4S+3C] cycloaddition process [73a, 95a]. It is important to remark on the role played by the metal in this reaction as the analogous tungsten carbene complexes lead to [4S+2C] cycloadducts (see Sect. 2.9.1.1). Formation of the seven-membered ring is explained by an initial cyclopropanation of the most electron-rich double bond of the diene followed by a Cope rearrangement of the formed divinylcyclopropane (Scheme 65). Amino-substituted 1,3-butadienes also react with chromium alkenylcarbene complexes to produce the corre-... 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

Barluenga and co-workers reported a novel [2 + 2 + 2 + 1 (-reaction based on the use of two metals, the nickel-mediated [2 + 2 + 2 + l]-reaction of Fischer carbenes with alkynes (Scheme 73).142 While, at present, this process requires the stoichiometric use of both metals, it is highly regioselective, affords good yields, and is a novel route to seven-membered rings. 

Obviously, the first intermediates in the syntheses with terminal alkynols are the vinylidene complexes [Ru(bdmpza)Cl(=C= CH(CH2) +iOH)(PPhg)] (n = 1, 2), which then react further via an intramolecular addition of the alcohol functionality to the a-carbon (Scheme 22), although in none of our experiments we were able to observe or isolate any intermediate vinylidene complexes. The subsequent intramolecular ring closure provides the cyclic carbene complexes with a five-membered ring in case of the reaction with but-3-yn-l-ol and with a six-membered ring in case of pent-4-yn-l-ol. For both products type A and type B isomers 35a-I/35a-II and 35b-I/ 35b-II are observed (Scheme 22, Fig. 22). The molecular structure shows a type A isomer 35b-I with the carbene ligand and the triphenylphosphine ligand in the two trans positions to the pyrazoles and was obtained from an X-ray structure determination (Fig. 25). 

The most important cheletropic reactions are those that involve the reaction of an electro-defficient species -as a carbene, nitrene or "atomic oxygen"- with an olefinic double bond and lead to three-membered rings. 

Intramolecular acylation reactions with ketene complexes, generated, for instance, by thermolysis or photolysis of carbene complexes, can also be used for the preparation of six-membered rings. Illustrative examples are shown in Table 2.23. 

Further methods for preparing seven-membered rings with the aid of heteroatom-substituted carbene complexes include the intramolecular acylation reactions with ketene complexes discussed in Section I.2.6.2. 

Intramolecular carbon-hydrogen insertion reactions have well known to be elTectively promoted by dirhodium(ll) catalysts [19-23]. Insertion into the y-position to form five-membered ring compounds is virtually exclusive, and in competitive experiments the expected reactivity for electrophilic carbene insertion (3°>2° 1°) is observed [49], as is heteroatom activation [50]. A recent theoretical treatment [51] confirmed the mechanistic proposal (Scheme 15.4) that C-C and C-H bond formation with the carbene carbon proceeds in a concerted fashion as the ligated metal dissociates [52]. Chemoselectivity is dependent on the catalyst ligands [53]. 

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