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The reaction of tributylphosphane with a 7‐(2′‐formyl‐2‐biphenylyl)phosphanorbornadiene P‐W(CO)5 complex leads to the 5‐phosphaphenanthrene P‐W(CO)5 complex via an intramolecular phospha‐Wittig condensation. This complex is stable enough for detection by 31P NMR spectroscopy but cannot be isolated due to its high reactivity. It is trapped by addition of MeOH or cycloaddition with 2,3‐dimethylbutadiene or a nitrileimine. The adducts were characterized by X‐ray crystal structure analysis.

A DFT study suggests that the conversion of arsolylcarbenes into arsenines as described by Märkl proceeds via arsabenzvalenes. A similar conversion does not work with phospholylcarbenes. However, we have found that, in some cases,The discovery that phosphinines can play a significant role in homogeneous catalysis [1] has reignited the interest in their synthesis and chemistry. [2] In this context, our attention was drawn to an early work of Märkl describing the conversion of arsolylcarbenes into arsenines.[3] We were puzzled by the fact that this chemistry has no reported equivalent for nitrogen and phosphorus. As a first step, we decided to study this transformation by DFT computations at the B3LYP/6-311+G(d,p) level [4] in order to get a better understanding of the reaction pathway. The singlet arsolylcarbene has a planar structure with a As=C double bond (1.773 Å) and is closely related to the structure of phosphanylcarbenes.[5] Its structure forbids any interaction between the carbene and the diene orbitals. The triplet arsolylcarbene has a pyramidal arsenic atom (sum of the angles 293.5º) and a single As-C bond (1.888 Å). The plane of the carbene is orthogonal to the plane of the arsole. This triplet state lies only 3.7 kcal mol -1 above the singlet. This structure is similar to the structure of cyclopentadienylcarbene that yields benzvalene by intramolecular [1+4] carbene + diene cycloaddition.[6] Thus, we suggest that this easily accessible triplet is readily converted into arsabenzvalene. Computations show that this arsabenzvalene is a genuine local minimum (no negative frequency), more stable than the triplet arsolylcarbene by 55 kcal mol -1 . It is transformed into arsenine via a transition state (one negative frequency) that is 35.3 kcal mol -1 higher in energy. Both the arsa- benzvalene and the transition state are shown in Figure 1. Our data on arsabenzvalene (structure and energy) are close to those computed by Sastry.[7]Figure 1. Computed structure of arsabenzvalene I and the transition state II between I and arsenine. Significant distances [Å] and angles [º]: I: As-C1 2.168, As-C9 1.972, C1-C9 1.473, C4-C3 1.482, C3-C2 1.348; C1-As-C4 62.69, C1-As-C9 41.36. II: As-C1 2.023, As-C2 1.861, As-C3 2.836, C1-C2 2.082, C2-C3 1.483, C3-C4 1.353, C4-C5 1.434, C5-C1 1.367; C1-As-C2 64.67, C1-As-C3 60.85, C2-As-C3 28.16.The case of nitrogen is different. The triplet pyrrolylcarbene is higher in energy than the singlet by 21.8 kcal mol -1 and, thus, difficult to reach. Besides, its structure is different. The plane of the carbene is orthogonal to the plane of the ring, as in the arsenic case, but nitrogen is planar (sum of angles at N 359.8º), therefore blocking its transformation into azabenzvalene.As expected, the situation for phosphorus lies halfway between those for nitrogen and arsenic. The triplet phospholylcarbene is pyramidal at P (sum of angles 313.2º) and lies higher in energy than the singlet by 12.8 kcal mol -1 . It is, of course, difficult to predict whether its transformation into phosphabenzva...

As a result of their aromaticity, phosphinines play a central role in phosphorus heterocyclic chemistry and, since the demonstration of their extraordinary efficiency as ligands in the rhodium-catalyzed hydroformylation of olefins, [1] the need for new versatile synthetic methods that allow the tuning of their electronic and steric properties becomes even more evident. We have, some time ago, demonstrated that the chemistry of the easily available phospholes was governed for a large part by the readily attained equilibrium between 1H-and 2H-phospholes.[2] From a synthetic standpoint, a major consequence of this equilibrium is that 1-phosphanorbornadienes become easily available through the reaction of 2H-phospholes with alkynes [Eq. (1)].The versatility, the reaction conditions, and the efficiency of this cycloaddition chemistry heavily depend on the substitution scheme of the phosphole partner. By far the best phosphole in this respect is the readily made [3] 1-phenyl-3,4-dimethylphosphole. From another standpoint, the ready availability of 1-phosphanorbornadienes designated them as natural starting points for the synthesis of phosphinines by extrusion of the carbene bridge. The strain of the bridge, the relative weakness of the PÀC bonds, and the aromaticity of the phosphinine product suggested a rather easy transformation. Unfortunately, during a first series of attempts some time ago, we met a limited success in this direction. It appeared that only the 1-phosphanorbornadienes with a diphenylcarbene bridge made from 1,2,5-triphenylphosphole are susceptible to be transformed into phosphinines and, even in this case, the reaction conditions are drastic (230 8C) and the versatility is very limited.[4] Even worse, the 1-phosphanorbornadienes produced from 1-phenyl-3,4-dimethylphos-A C H T U N G T R E N N U N G pholes proved to yield only decomposition products upon thermolysis. We reasoned that this extrusion is so difficult because the ground state of carbenes is a triplet, [5] thus implying that the extrusion is not a concerted but a stepwise process. On the basis of this working hypothesis, we decided to completely change our synthetic approach.Our aim was to change the reaction pathway so that the extruded fragment would have a singlet ground state. Since there is no practical way to synthesize the 1-phosphanorbornadienes with aminocarbene bridges, we were led to investigate the oxidation of the strained P À C bridge bond as previously described in the literature.[6] This oxidation has two adverse effects for the kind of chemistry that we wanted to perform: it reduces the strain of the bridge and replaces a relatively weak PÀC by a stronger PÀO bond. Nevertheless, we were confident that the concerted extrusion of a carbonyl derivative would prove easier than the non-concerted extrusion of a carbene. This is indeed the case. Our preliminary experiments were carried out with compound 1 a, easily made from 1-phenyl-3,4-dimethylphosphole and diphenyl-A C H T U N G T R E N N U N G acetA C H T U N G T R E N N U N G...

Under mild conditions, iodine promotes the direct alkylation of 1,3-dicarbonyl compounds with benzylic alcohols or benzylic acetates.