4- Triangulane

The use of carbene additions to MCP derivatives has lately become a general strategy for obtaining spiropentanes and higher spiranic triangulanes. 

Gem-dibromospiropentane (620), obtained by the reaction of MCP with dibromocarbene, was used in the synthesis of BCP (3) (Scheme 90) [6a]. The reaction of the intermediate vinylidenecyclopropane 621 with a large excess of Simmons-Smith reagent gave exclusively BCP (3), but using modified Simmons-Smith procedures the [3]-triangulane (247) was also obtained in small yield (Scheme 91) [165],... 

The same triangulane 247 was also obtained by direct cyclopropanation of methylenecyclopropane 461 (Scheme 92) [166]. 

The synthetic procedure described by Arora and Binger [lb] to prepare 461 was extensively applied to the synthesis of unbranched 634 [172,6e] and branched 633, and 635—636 [173,172b] triangulanes. The sequence is based on the addition of a chloromethyl carbene to an appropriate methylenecyclo-... 

New procedures, in addition to the previous one, were developed to obtain higher branched triangulanes [174]. 

Fig. 5. Classification of the C-C bonds in triangulanes and bicyclopropylidenes in the general increment scheme (CIS) for the prediction of C-C bond lengths [87]...

The comparison of SEs for alkenes 1, 2, and methylenespiropentane 6 with those for cyclopropane and the linear triangulanes 96,97 is shown in Table 4. 

The additivity of strain energy increments appears to be a general feature for the triangulanes [95] and, most probably, also for their synthetic precursors [89], bicyclopropylidene (1) and spirocyclopropanated bicyclopropylidenes. As much as the strain energy of spiropentane (96) is more than twice that of cyclopropane, i. e., the spirojunction of two cyclopropane rings leads to an additional... 

On the other hand, as much as the increase in SE on going from methylenecyclopropane (2) to spiropentane (96) is substantial (23.4 kcal/mol), ASE is 21.1 kcal/mol on going from bicyclopropylidene (1) to [3] triangulane (97) (Table 4). As an expression of its increased SE, the double bond in 1 demonstrates a uniquely enhanced reactivity compared to the one in methylenecyclopropane (2) (see below). 

The addition of different types of carbenes onto bicyclopropylidene (1) is a common method for the preparation of [3]triangulane derivatives as well as branched trianguianes and normally proceeds without complications (for a review see [77]). Thus, the cyclopropanation under Gaspar-Roth [60] or modified Simmons-Smith [111] conditions gave dispiro[2.0.2.1]heptane ([3]triangulane, 97) in 80 [105] and 15% yield [5], respectively (Scheme 23). The palladium(II) acetate-catalyzed cycloprop anation of 1 with diazomethane, however, gave a number of products resulting from insertion of one or more than one methylene units into an initially formed palladacyclobutane 115 [112,113] (Scheme 23). 

Scheme 23. The cyclopropanation of bicyclopropylidene (1) and of enantiomerically pure ester (i )-42-Et as a key step in the preparation of enantiomerically pure (M)-[4]triangulane [(M)-119] [59,112,113]...

The addition of methylchlorocarbene onto bicyclopropylidene (1) (Scheme 26) and functionalized bicyclopropylidenes like 44a and 121 has been used as a key step in the synthesis of branched triangulanes [53,122-1241. This was applied... 

Scheme 26. Application of methylchlorocarbene addition onto 1 for the preparation of branched [n]triangulanes [65,122,123]...

Rotane (128) was also prepared by addition of cyclopropylidene liberated from in situ generated diazocyclopropane [62] onto 1 [65,105] (Scheme 27). This carbene adds also to bicyclopropylidenes 56 and 62 producing triangulanes 129 [33,65] (Scheme 27) and 67 [33,64,65] (Scheme 11). 

Scheme 27. Preparation of branched triangulanes 128,129 via carbene cycloadditions [33,65,105]...

It shows that the bicyclopropylidene ligand in 282 is remarkably bent out-of-plane by 40° at both termini of the double bond, i. e., this complex can be considered as a derivative of 7-cobalta[3]triangulane. 

It is not surprising that the synthesis of even more strained SPC (e.g. triangulanes, see Section VIII) has never been accomplished by using reductive cyclization, even if electrochemical methods were applied1 . The only exception is the preparation of 7-methylenedispiro[2.0.2.1]heptane II from dibromide 10 (equation 5)20. In this case, a participation of the double bond in the stabilization of an intermediate may account for the success of the cyclization. 

An improved method of preparation of this compound is based on cyclopropanation of 7-methylenedispiro[2.2. l]heptane (ll)19-53,54. This olefin was originally prepared by reductive cyclization of dibromide 10 (equation 5)20. A more convenient method is based on cyclopropanation of bicyclopropylidene 19 with chloromethylcarbene, followed by dehydrochlorination (equation 27)19. Additional information about [3]rotane, a member of the triangulane family, appears in Section VIII. 

Triangulanes are a unique class of polycyclic hydrocarbons constructed from spiroannulated cyclopropanes. Because of a stereochemical diversity, the number of isomeric triangulanes sharply increases with increasing the number of cyclopropyl fragments. Consequently, one may speak about the land of triangulanes . 

For classification purposes, triangulanes have been divided into three types—linear (43), branched (4) and cyclic (44)9. Compounds 45 and 46 can be considered as precursors to unknown, extremely strained [4]- and [5]cyclotriangulanesM 65. 

An application of graph theory to the problem of isomerism in linear triangulanes provided a program which allowed one to follow the development of a generation tree for each... 

---