The Chemistry of Highly Strained Oligospirocyclopropane Systems

Abstract

I. Introduction 93 II. Triangulanes: Nomenclature, Classification, and Stereoisomerism 94 III. Synthesis of Triangulanes 96 A. Synthetic Methods to Assemble Oligospirocyclopropanes 96 B. Synthesis of Unbranched Triangulanes (UTs) 97 C. Synthetic Approaches to Branched Triangulanes (BTs) 99 D. Cycloannelated Triangulanes (CATs) 102 E. Along the Route to Cyclotriangulanes (CTs) 103 F. Substituted Triangulanes 104 G. Heterotriangulanes 108 IV. Selected Physical Properties of Triangulanes 109 A. Spectral Data 109 B. Bonding Properties and Molecular Structures 110 C. Thermochemical Properties 115 V. Chemical Transformations of Triangulanes 117 A. With Retention of the Skeleton 117 B. With Ring Opening 121 VI. Triangulanes with Properties of Potential Practical Applicability 130 VII. Conclusions and Further Perspectives 133 VIII. Acknowledgments 136 IX. References 136 † Dedicated to Professor Wolfgang Lu ¨ttke on the occasion of his 80th birthday.

Linear and Branched Phospha[n]triangulanes

“…The two different PÀC bond lengths (av 1.796 and 1.846 ) of their terminal phosphirane rings are similar to the values of the W(CO) 5 -complexed smaller phospha [2]triangulane 4 b [4] with the distal bonds (remote to the spiro-carbon) being longer than the proximal ones (connected to the spirocarbon). This effect of spirofusion is similar but more pronounced than in the all-carbon triangulanes, [3,14] and likewise is due to rehybridization of the strained spiro-carbon, resulting in less s-character and thus elongation of the distal bonds.…”

“…Figure 1 shows the X-ray crystal structure for the less congested anti isomer 12 a that has the P-W(CO) 5 group anti to the terminal cyclopropane ring. The bond lengths of the phosphirane ring compare well with the values of the W(CO) 5 -complexed smaller phospha [2]triangulane 4 b [4] ( Table 1). Extending the number of cyclopropane rings to three, as in linear 10 or branched methylenetriangulane 11, gave the corresponding linear and branched phospha [4]triangulanes 13 and 14 (Scheme 1).…”

“…This effect of spirofusion is similar but more pronounced than in the all-carbon triangulanes, [3,14] and likewise is due to rehybridization of the strained spiro-carbon, resulting in less s-character and thus elongation of the distal bonds. [2] The PhPW(CO) 5 group also affects the C1-spirofused cyclopropane ring, which also has proximal C À C bonds of different length (av 1.503 and 1.468 ) and a 1.493 (av) distal bond. The C1ÀC2 bonds (av 1.498 ) of all the structures are of similar length as reported for the parent 1-phenylphosphirane 4 a.…”

“…[1] Embedding the threemembered ring into oligocycles by annelation or spirofusion with additional small rings augments the total strain to exceed that of the sum of the separate rings. This is well established not only for the ring-annelated bicyclic (1) and tricyclic propellane skeletons (2), but also for the spirofused linear and branched so-called [n]triangulanes (3). [2] The current record of a branched [15]triangulane [3] and linear [9]triangulane [3] exemplifies the accessibility of such extended arrays of spirofused cyclopropanes that are surprisingly stable in spite of their high overall strain.…”

“…[2] Compounds such as 1-phosphaspiro [2,n]alkanes 4 a-e, [4] the 1-phosphadispiroalkanes 5 a-d, [5,6] and 1,4-diphosphaspiropentane 6 [7] with a phosphorus atom, that is, a phosphirane ring, [8] became accessible by the [1 + 2] cycloaddition of the in situ generated carbene-like electrophilic phosphinidene complexes R-P= W(CO) 5 to the respective alkenes. [9,10] Abstract: Novel, highly stable, linear and branched mono-and diphospha[n]-triangulanes were synthesized in high yields by the CuCl-catalyzed phosphinidene addition to spirocyclopropanated methylenecyclopropanes and bicyclopropylidenes.…”