Straightforward synthesis of five-membered metallacycloallenes: 1-Zirconacyclopenta-2,3-diene compounds derived from 1,3-enynes

Abstract

A five-membered metallacycloallene complex, 1-zirconacyclopenta-2,3-diene, was prepared from low-valent zirconocene and 2,4-bis(trimethylsilyl)but-1-en-3-yne. The complex reacted with carbonyl compounds to afford alkynyl alcohols after hydrolysis via C=O insertion into Zr-C bond. Similarly, nitriles inserted into the Zr-C bond to give the corresponding ketones after hydrolysis. The insertion of C=O and CN selectively took place at zirconium-sp 3 carbon of the metallacycle. nium.Recently, we and Erker's group independently reported stable five-membered cycloallenes, 1-metallacyclopenta-2,3-diene compounds (Fig. 2) [7][8][9][10][11][12]. These complexes are stable at room temperature (rt). It should be noted that replacement of one carbon atom with a transition metal made the strained compounds much more stable, and it must be interesting to study their reactivity. The fivemembered zirconacycloallene 1 was synthesized by transformation of a conjugated 1-zirconacyclopent-3-yne complex. The 1-zircona-and 1-hafnacyclopenta-2,3-dienes 2 were prepared from the corresponding bis(alkynyl)metallocenes via 1,1-hydroboration. The preparative methods of these complexes, however, were specific and unsuitable for large-scale synthesis. Therefore, more versatile synthetic routes have been desired in order to study the reactivity of five-membered metallacycloallenes. Bertrand and co-workers recently reported several bent allene compounds [13][14][15], including small cycloallenes. These compounds are of interest in relation to the metallacycloallenes, although the allene character of these is yet controversial [16,17]. Straightforward synthesis of unsaturated metallacyclesFor the syntheses of metallacyclic compounds such as 1-metallacyclpent-3-enes, 1-metallacyclopenta-2,3,4-trienes, and 1-metallacyclopent-3-ynes, low-valent metallocenes and 1,3-dienes, 1,3-diynes, and 1,2,3-trienes are employed (i-iii, Scheme 1) [8]. This methodology realized the synthesis of stable fivemembered metallacyclotrienes (ii) [18][19][20], as well as metallacycloalkynes (iii) [21][22][23][24], which have been thought to be highly reactive and unisolable. According to these protocols one must consider that 1,3-enynes would be possible starting compound for cycloallenes (iv, Scheme 1), as predicted by N. SUZUKI et al.

Synthesis of Five‐Membered Osmacycloallenes and Conversion into Six‐Membered Osmacycloallenes

“…The synthesis of small, strained metallacycloallenes has not been reported as frequently as the preparation of the related metallacycloallenes containing more than six atoms. [3] As shown in Scheme 1, the existence of four-(I, Scheme 1) [4] and five-membered metallacycloallenes (II) [5] has been confirmed. Examples of more strained metallacycles (for example, the five-membered metallacyclocumulene III) have been reported by Rosenthals group.…”

“…[7,10] Notably, the Os1-C1-C2 angle (137.8(2)8) deviates considerably from linearity but is much smaller than the corresponding angles found in the two reported six-membered metallacycloallenes (155.1(8)8 [7a] and 158.5(3)8 [7b] ). For comparison, the angles for the cumulative double bonds in the five-membered metallacycloallenoids range from 153.6(3)8 to 158.7(2)8 in II [5] and from 145.8(5)8 to 153.5(4)8 in III. [6] Strained five-or six-membered metallacycloalkynes, such as metallacyclopen-tynes, [11] metallapentalyne, [12] metallabenzynes, [13] and metallapyridyne, [14] display distorted angles at the carbyne carbon, which can be represented by resonance structures of metallacycloallenes.…”

Examples of Different Reactions of Benzylsulfanyl‐Substituted Alkynes with Selected Complexes of TiII and CoI

“…In general, the isolation of small unsaturated all-carbon metallacycles (Scheme 1, A-C, X = CH 2 , CMe 2 ) is challenging due to their high ring strain, [1][2][3][4][5] although an essential stabilisation effect has been attributed to interactions of metal d orbitals with remote π bonds. The formal substitution of ring CR 2 moieties by heteroatom groups leads to a variety of new species, the synthesis of which are motivated by their prospective preparative potential as well as an interest in investigating their stability.…”

Synthesis and Characterisation of Selected Group 4 Metallocene Complexes with 1,2‐Bis(4′,4′,5′,5′‐tetramethyl[1′,3′,2′]dioxaborolan‐2′‐yl)acetylene

“…[1][2][3][4][5] To gain deeper insight into the existence and stability of such three-membered, unsaturated metallacycles with different heteroatoms as substituents, the coordination chemistry of a series of neutraldonor-substituted acetylenes such as R 2 NCϵCNR 2 , [6] R 2 PCϵCPR 2 , [7] ROCϵCOR [8] and RSCϵCSR [9] with group 4 metallocene complexes was of high interest. were used.…”